text: Evolution temporelle du champ magnétique lunaire Camille Lepaulard
  To cite this version: Camille Lepaulard. Evolution temporelle du champ magnétique lunaire. Planète et Univers [physics]. Aix-Marseille Université, 2018. Français. tel-02135982
  HAL Id: tel-02135982 https://hal.archives-ouvertes.fr/tel-02135982 Submitted on 21 May 2019
  
   UNIVERSITE D'AIX-MARSEILLE ED 251 – Sciences de l'environnement CEREGE / UMR 7330 Thèse présentée pour obtenir le grade universitaire de docteur Spécialité : Géosciences de l'environnement
  Camille LEPAULARD Evolution temporelle du champ magnétique lunaire Temporal evolution of the lunar magnetic field Soutenue le 28/11/2018 devant le jury : Mme France LAGROIX Mme Sonia TIKOO Mme Mireille PERRIN M. Pierre ROCHETTE M. Mark WIECZOREK M. Jérôme GATTACCECA
  IPGP, CNRS, Paris Rutgers University, N.J, USA CEREGE, CNRS, Aix-en-Pce CEREGE, AMU, Aix-en-Pce OCA, CNRS, Nice CEREGE, CNRS, Aix-en-Pce
  Rapporteur Rapporteur Examinatrice Examinateur Examinateur Directeur de thèse
  Numéro national de thèse/suffixe local : 2018AIXM0414/039ED251 « That's one small step for man, one giant leap for mankind » Neil Amstrong July 21th, 1969
  Evolution temporelle du champ magnétique lunaire 1 2
  Evolution temporelle du champ magnétique lunaire Remerciements
  Remerciements Je crois que le fait d'écrire mes remerciements me fait enfin prendre conscience que ma thèse prend fin. Le temps est passé si vite que je suis déjà nostalgique que cette aventure prenne fin et de quitter ce laboratoire. Je crois que les pages qui suivront contiendront le plus grand nombre de « merci » jamais lu en dehors d'une thèse, donc merci au lecteur qui aura pris le temps de lire ces remerciements. Je souhaite remercier toutes les personnes qui ont participé de près ou de loin à mes travaux de thèse durant ces trois années et que j'aurai omis de mentionner (ne m'en voulez pas, j'ai fait une liste pourtant). Mes touts premiers remerciements vont aux astronautes des missions Apollo, et à toute l'équipe derrière ces missions, qui ont permis le retour des échantillons lunaires sur Terre, et qui ont constitué le coeur de ma thèse. Sachez également que nous célébrons cette année 2018 les 60 ans de la NASA, institut à l'origine de ces missions (merci !). Je ne pourrais continuer mes remerciements sans mentionner mon directeur de thèse, Jérôme Gattacceca, sans qui tout ceci n'aurait eu lieu. Merci pour ta confiance, ta patience, tes conseils, ta bonne humeur, tes nombreuses corrections d'articles, de manuscrit, de PowerPoint (et ce n'est pas fini !). Merci de m'avoir formé pendant 3 ans, d'avoir mesuré pour moi 2 échantillons au MIT et de m'avoir confié ces bouts de Lune (je crois que je ne réaliserai jamais vraiment avoir tenu des roches Apollo dans ma main, via un gant). Je te remercie de m'avoir permis de voyager autant pendant ces 3 années (Houston, la NouvelleOrléans, Münster, Rutgers, Cargèse, Toulouse, Grenoble), ce fut de super moments et de belles opportunités. Je me rappellerai toujours cette journée où tu m'as dit que l'on irait à la NASA pour analyser les échantillons lunaires, comme si tu me parlais de la météo. Aller à la NASA fut l'un des moments les plus inoubliables et remarquables de ces 3 ans. Merci 1 969 fois (les initiées comprendront). Je tiens également à remercier mes deux rapporteurs, Sonia Tikoo et France Lagroix, qui ont accepté de relire et de juger avec attention ce manuscrit. Merci aux autres membres de mon jury de thèse Mireille Perrin, Mark Wieczorek et Pierre Rochette d'avoir accepté de faire partie de ce jury, de juger et critiquer mon travail, mais aussi pour leurs conseils à l'issue de cette soutenance. Mes analyses au laboratoire n'auraient été aussi efficaces sans l'aide de François Demory. Merci de m'avoir expliqué maintes fois le fonctionnement des appareils, de m'avoir conseillé, écouté et d'avoir relu mon travail. Merci pour ces nombreuses pauses café qui vont me manquer et pour ces moments de rigolades. Merci également à Yoann Quesnel et Minoru Uehara pour leurs collaborations, leurs conseils. Merci à Gwenaël Hervé pour ses conseils, ses relectures, nos discussions. Merci à Quentin Simon pour nos discussions à la Nouvelle Orléans et dans les couloirs du labo. Encore merci à Pierre Rochette qui, je me rappelle en fin de stage de Licence 3, m'avait dit : « tu reviendras en thèse ». Je n'aurais jamais cru boucler la
  Evolution temporelle du champ magnétique 3 Remerciements boucle au CEREGE. Merci à Nicolas Thouveny et à toute l'équipe de magnétisme de m'avoir accueillie au laboratoire. Je souhaite remercier l'Université Aix Marseille et l'ANR MagLune pour les financements de ma thèse. Un grand merci au personnel administratif (Aurore, Isabelle, Noémie, Patricia, Hadja, Hajer) pour leur aide au long de ces trois ans. Merci également à Bertrand Devouard et Julien Longerey pour m'avoir aidé à réaliser les cartes des échantillons au MEB, merci à Thibaut de Garidel-Thoron et Corinne Sonzogni de m'avoir faite participer à la fête de la science, ce fut une super expérience. Encore merci à Sonia Tikoo pour son très bon accueil dans son laboratoire de magnétisme à Rutgers et nos discussions sur les échantillons de Lune. Merci de m'avoir fait découvrir les environs et de m'avoir donné envie de revenir. Merci à Ryan Zeigler et Andréa Mosie pour leur accueil au Johnson Space Center, (la NASA), le prêt des échantillons Apollo, l'accès à la salle de stockage des échantillons et la visite du JSC. Ma thèse a été associée à l'ANR Maglune, ce qui m'a permis de rencontrer une fois par an les autres membres de cette ANR. Merci à Mark Wieczorek pour sa participation dans mon comité de thèse, en plus d'être dans mon jury ; merci à David Cébron pour ses conseils sur la dynamo lunaire et les éclaircissements apportés à ce propos et pour cette collaboration d'article. Merci à Ben Weiss pour son aide et pour la datation indirecte des échantillons. Merci également à Joana Oliveira et Mathieu Laneuville pour nos discussions lors des réunions MagLune (Joana, mon chat va toujours aussi bien). Lors de ma seconde année de thèse, j'ai pu faire un contrat de valorisation scientifique. Merci à Frank Torre pour cette année originale, à Fy Raveloarisoa et Ana Torquet pour le côté administratif et logistique. Grâce à ce projet, j'ai pu participer aux cordées de la réussite à Barcelonnette. Merci à Olivier Bellier pour cette jolie collaboration, à Fabrice Jestin pour le logement à Séolane et aux enseignants du lycée de Barcelonnette pour leur participation à ce projet. Au cours de ces 3 ans, j'ai croisé la route de nombreuses personnes. Merci à Christina Verhagen de ta visite chez nous et de ta bonne humeur, vivement que l'on se revoit. Un grand merci à Cécile Cournède, 1ère initiatrice au magnétisme, encadrante de mon 1er stage de Licence 3 au CEREGE. Merci pour tous tes conseils, nos discussions depuis 5 ans (déjà !) et de m'avoir transmis ce truc qui a fait que je ne me suis jamais vue faire autre chose que du magnétisme depuis ce printemps 2013. J'ai hâte de te revoir de nouveau. Merci à Myriam Kars que j'ai rencontré à l'AGU, française au Japon et qui a vécut à 5 min de chez moi. Merci pour tes conseils et pour ces moments très sympa à NOLA. Ne faisant plus partie du CEREGE, je peux remercier Amandine Sartégou ici. Un grand merci pour notre collaboration aux cordées et nos ballades à Barcelonnette (et arrêts à la fromagerie, au parc animalier de Serre Ponçons et la rencontre avec Dakota), nos visites à NOLA et la découverte du Jazz, des
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  Evolution temporelle du champ magnétique Remerciements spécialités créoles, du brunch-Jazz, des beignets, du Bayou, du Mississippi, du fameux meeting AGU, de l'alligator, du French Quarter et j'en passe. Je tiens à remercier mes co-bureaux passés et actuels : Nicolas (le plus ancien) jamais stressé (j'ai presque stressé pour toi), qui m'a supporté ces 3 années mais on aura bien rigolé ; Gabrielle, ATER de passage pendant 1 an au bureau et Brina pour ton soutien « entre filles » dans cette équipe très masculine, on aura aussi bien rigolé. Merci à Lisa, petite nouvelle arrivée début Octobre avec qui j'ai bien rigolé en cette fin de thèse. Nous avons reformé le bureau de filles pendant 2 mois ! Merci de ta gentillesse et de ton soutien. Merci aux doctorants qui ont partagé de près ou de loin ces 3 années de thèse : Amandine et Marie avec qui j'ai fait ces 3 ans (courage pour la fin), pour nos soirées, nos sorties cinéma, nos visites de la région, nos discussions et soutien mutuel (surtout au yoga), nos rigolades (« il faut rafraîchir les maquignons »). Ce fut 3 années riches en émotions mais rendues très agréables surtout grâce à vous 2. On aura formé un super trio et vous m'avez soutenue comme jamais ! Merci aussi à Frank, Céline, Hamed qui ont déjà fini. Jonathan, Benjamin, Nicolu, Clément, Agathe, Sébastien, Solène qui finissent bientôt (si, si, ça passe vite) et Didier (déjà permanent). Je voudrais remercier mes amies de toujours Anaïse, Clarisse, Sylviane pour leur soutien depuis toujours (le lycée mais c'est tout comme). Ce fut dur de ne plus se voir aussi souvent durant ces 3 ans mais vous avez toujours été là et je vous en remercie. Merci à Chloé que l'on voit pour nos visites à Paris ou sur Tours à Noël, à Marseille quand c'est toi qui viens. Merci à Steph et Laure pour votre soutien, nos rigolades, nos soirées, nos longues discussions même si vous êtes parties loin voire revenues. Merci à ma famille pour votre soutien de près ou de loin, les relectures de dernière minute (il en reste encore !) ; à ma bellefamille pour leur soutien et leur aide ; à mon chat Kaori pour ses câlins et ses conversations en miaou (je parle officiellement 3 langues). Enfin je remercie mon futur époux, Guillaume, pour tout. Ton soutien, ton amour, tout ce qui a été et sera. Merci.
  Evolution temporelle du champ magnétique 5 Remerciements
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  Evolution temporelle du champ magnétique Résumé
  Résumé Il est établi que la Lune a eu par le passé un champ généré par une dynamo interne. Cependant, les mécanismes à l'origine et permettant le maintien de la dynamo sont encore mal connus. La durée de ce champ magnétique est encore débattue. Mon travail de thèse a consisté tout d'abord à une caractérisation magnétique (aimantation naturelle et susceptibilité magnétique) d'une grande partie de la collection Apollo avec l'étude des masses principales de 161 roches. Le contrôle de la susceptibilité magnétique par la lithologie via l'incorporation du métal météoritique a été confirmé. Le rapport aimantation naturelle sur susceptibilité a été utilisé comme indicateur grossier de la paléointensité. Ces résultats, cohérents avec les deux grandes époques du champ magnétique lunaire (époque de fort champ avant ~3.5 Ga et champ de quelques μT seulement ensuite), ont permis de faire une sélection d'échantillons pour des analyses paléomagnétiques détaillées en laboratoire qui ont constitué la seconde partie de mon travail. J'ai étudié en laboratoire l'aimantation naturelle de 25 échantillons Apollo et 2 météorites lunaires. Différentes techniques ont permis d'obtenir 8 valeurs de paléointensités (entre 1 et 47 μT) et 7 limites supérieures de paléointensités (entre 10 et 30 μT), pour des roches d'âge varié. Ces données ont été couplées aux âges radiométriques (existants et nouvellement acquis) pour retracer l'évolution du champ de surface lunaire au cours du temps. Les résultats corroborent l'existence d'une période de champ fort entre 4 et 3.5 Ga et prolonge cette période jusqu'à environ 3 Ga. Les paléointensités > 1 μT que nous obtenons à 1 Ga et jusqu'à et 0.5 Ga indiquent un arrêt très tardif de la dynamo. D'autre part, quelques paléointensités <10 μT sont également obtenues dans l'époque de champ fort, suggérant une valeur de champ moyen plus faible que proposé précédemment dans la littérature. Cette étude permet ainsi de mieux contraindre l'évolution temporelle du champ magnétique lunaire.
  Evolution temporelle du champ magnétique 7 Résumé
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  Evolution temporelle du champ magnétique Résumé
  Abstract It is admitted that the Moon used to have a magnetic field, generated by an internal dynamo. However, the mechanisms responsible for the dynamo and its preservation are still poorly known today. The lifetime of the magnetic field is also debated. My thesis was focused first on the magnetic characterization (natural magnetization and magnetic susceptibility) of a large part of the Apollo collection, with the study of 161 main masses of Apollo rocks. Control of the magnetic susceptibility by the lithology was confirmed, caused by the incorporation of meteoritic metal into the rock. In the second part, i used the ratio of the natural magnetization to the magnetic susceptibility to obtain an approximate indicator of paleointensity. Results of this ratio were coherent with the two major epochs determined in the lunar magnetic field (high field epoch before ~3.5 Ga and a weak field epoch after) and allowed me to select samples for detailed paleomagnetic analyses in another part of my thesis. Then, I studied in laboratory the natural magnetization of 25 Apollo samples and 2 lunar meteorites. Different methods were used to obtain 8 paleointensities values (between 1 and 47 μT) and 7 upper limits of paleointensity (between 10 and 30 μT), at different times of the magnetic field evolution. These data were coupled with radiometric ages to trace the evolution of the lunar surface field over time. These results corroborate the existence of a strong field epoch between 4 and 3.5 Ga and extend this epoch until ~3 Ga. Paleointensities of values > 1 μT obtained at 1 Ga and until 0.5 Ga indicates a very late interruption of the dynamo. On the other hand, weak paleointensities were obtained in the high field epoch, suggesting a value of average field lower than previously proposed in literature. This study allows to better constrain the temporal evolution of the lunar magnetic field.
  Evolution temporelle du champ magnétique 9 Résumé
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  Evolution temporelle du champ magnétique Tables
  Table des matières Remerciements  3 Résumé  7 Table des matières  11 Table des figures  13 Table des tableaux  15 Introduction générale  17
  Chapitre 1 : La Lune, notre satellite  21 1.1 1.1.1 1.1.2 1.1.3 1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.3 1.3.1 1.3.2 1.3.3
  L'exploration lunaire  21 Observations et explorations 21 Retour d'échantillons  22 Les missions Apollo  25 La Lune aujourd'hui 26 Caractéristiques générales  26 Structure interne  26 Structure externe  28 Géologie et classification 30 Evolution de la Lune  34 Formation et différenciation de la Lune  34 Chronologie de la surface lunaire  35 Discussion sur la stratigraphie  37
  Chapitre 2 : Le champ magnétique lunaire 41 2.1 2.2 2.2.1 2.2.2 2.2.3 2.3 2.3.1 2.3.2 2.4 2.5
  Généralités  41 Origine du champ magnétique lunaire  44 Champ magnétique par dynamo  44 Champ magnétique par impacts  45 Champ magnétique crustal  45 Mécanismes à l'origine de la dynamo lunaire  47 Origine mécanique  47 Origine convective  48 Evolution du champ magnétique et incertitudes  49 Conclusion  55
  Chapitre 3 : Matériel d'étude et méthodes en magnétisme  57 3.1 3.1.1 3.1.2 3.1.3 3.1.4 3.2 3.2.1 3.2.2
  Échantillons lunaires  57 Description des échantillons analysés au Johnson Space Center  58 Description des échantillons analysés en laboratoire  59 Datation des échantillons  60 Préparation des échantillons en laboratoire  61 Magnétisme des roches lunaires  62 Propriétés magnétiques de l'atome à la roche  62 Minéraux magnétiques lunaires  64 Evolution temporelle du champ magnétique lunaire 11 Tables
  3.2.3 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.3.6 3.4 3.4.1 3.4.2 3.4.3 3.4.4
  Etude du magnétisme des roches lunaires  67 Aimantation rémanente naturelle des roches  68 Aimantation thermorémanente  68 Aimantation rémanente de choc  68 Aimantation rémanente visqueuse  69 Aimantation rémanente chimique  69 Aimantation gyro- rémanente 69 Aimantation rémanente isotherme  70 Détermination des paléointensités 70 Evaluation de la qualité des échantillons  70 Protocoles R et REM'  70 Protocoles de Thellier-Thellier  71 Cas des échantillons sans composante stable  75
  Chapitre 4 : A survey of the Natural Remanent Magnetisation and the magnetic susceptibility of Apollo whole rocks  79 Article 82
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire 123 5.1 Cas des météorites  123 5.2 Etude des échantillons lunaires  127 Article 128 Conclusion générale  161 Bibliographie  163 Annexes  175
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  Evolution temporelle du champ magnétique lunaire Tables
  Table des figures Figure 1.1 : Carte de la face visible de la Lune avec localisation des sites d'alunissages des missions Apollo, Luna, Surveyor et Lunokhod.  p.24 Figure 1.2 : Hypothèse actuelle de structure interne lunaire.  p.28 Figure 1.3 : Exemple de l'activité volcanique au sein d'un bassin d'impact.  p.29 Figure 1.4 : Différents types de roches lunaires, classés selon leur source géologique.  p.30 Figure 1.5 : Relation entre la température d'équilibre melt-clast et le taux de fusion de la roche, après un impact.  p.32 Figure 1.6 : Coupe schématique d'un cratère d'impact montrant la localisation possible des différents types de brèches  p.32 Figure 1.7 : Interprétation possible de la différenciation de la Lune à 4.4 et 3 Ga.  p.35 Figure 1.8 : Grandes étapes de l'évolution de la Lune.  p.36 Figure 2.1 : Les composantes du champ magnétique.  p.41 Figure 2.2 : Modèle en harmoniques sphériques du champ magnétique total à 30 km de la surface p.46 Figure 2.3 : Compilation des données de paléointensités de la littérature.  p.49 Figure 2.4 : Bilan des divers mécanismes permettant le maintien de la dynamo dans le temps, associés aux données récentes de paléointensités.  p.51 Figure 3.1 : Présentation des conditions de mesures au JSC.  p.59 Figure 3.2 : Répartition des masses des roches Apollo analysées selon leur lithologie.  p.60 Figure 3.3 : Exemple de découpe de la brèche de régolithe 10018.  p.62 Figure 3.4 : Préparation d'un échantillon pour analyse.  p.63 Figure 3.5 : Répartition des domaines dans une particule avec le champ généré en bleu et le spin en rouge.  p.64 Figure 3.6 : Différents types d'alignement des spins dans le ferromagnétisme sensu largo.  p.65 Figure 3.7 : Observation de lames minces de certains échantillons Apollo au microscope optique. . p.66 Figure 3.8 : Cycle d'hystérésis de l'échantillon 10018 après correction de la pente.  p.68 Figure 3.9 : Description du protocole de désaimantation thermique IZZI pour l'estimation de paléointensités.  p.72 Figure 3.10 : Système de chauffe sous oxygène contrôlé installé sur le four de paléointensité.  p.73 Figure 3.11 : Calibration du flux de gaz utilisé dans la désaimantation thermique sous atmosphère contrôlée.  p.74
  Evolution temporelle du champ magnétique lunaire 13 Tables
  Figure 4.1 : ß (avec les barres d'erreur) en fonction de l'âge des roches étudiées.  p.81 Figure 5.1 : Résultats paléomagnétiques pour les météorites LAP02205 (a) et NWA10782 (b).  p.124 Figure 5.2 : Résultats des paléointensités des météorites LAP02205 (a) et NWA10782 (b).  p.125 Figure 5.3 : Evolution du champ magnétique (paléointensités) en fonction du temps (Ga) et des mécanismes de dynamo associés.  p.127
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  Evolution temporelle du champ magnétique lunaire Tables
  Table des tableaux Tableau 1.1 : Détail des missions avec retour d'échantillons.  p.23 Tableau 1.2 : Unités stratigraphiques depuis la formation de la Lune avec chronologie des bassins d'impact majeurs, des cratères d'impact principaux et du volcanisme avec formation des mers.  p.36 Tableau 2.1 : Récapitulatif des différents types de dynamo lunaire.  p.48 Tableau 2.2 : Récapitulatif des données de paléointensités de la littérature et leurs âges respectifs. p.43 Tableau 3.1 : Mélanges de gaz utilisés pour les expériences de désaimantation de roches lunaires. . p.75 Tableau 5.1 : Données magnétiques des deux météorites LAP05505 et NWA10782.  p.123
  Evolution temporelle du champ magnétique lunaire 15 Tables
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  Evolution temporelle du champ magnétique lunaire Introduction générale
  Introduction générale La Lune a toujours fasciné les hommes, de part sa proximité avec la Terre, sa surface étrangement blanche et cratérisée mais aussi de part les phénomènes naturels qu'elle engendre sur la Terre. Longtemps inscrite dans le folklore médiéval, la Lune a ensuite suscité l'intérêt scientifique et est rapidement devenue le principal objectif spatial à atteindre dès le début de la guerre froide. Les premières données acquises lors des missions lunaires spatiales, dans les années 60, ont notamment montré que, contrairement à la Terre, la Lune ne possédait pas de champ magnétique global mais uniquement un faible champ crustal. Dès le retour des échantillons Apollo, les analyses paléomagnétiques préliminaires réalisées sur les roches lunaires ont montré que certains échantillons avaient enregistré des paléointensités significatives (plusieurs dizaines de μT), indiquant l'existence d'un ancien champ magnétique lunaire. Dès lors, de nombreuses études ont eu lieu afin de comprendre l'origine de ce champ magnétique passé mais aussi de comprendre comment et pourquoi il a prit fin. De nombreuses hypothèses ont été formulées dans la littérature afin de comprendre l'origine de ce champ magnétique, ainsi que les processus permettant son maintien. Ce champ était-il engendré par des impacts d'astéroïdes ? Une dynamo lunaire a-t-elle fonctionné, comme pour la Terre ? Si les données acquises ont permis de retracer l'évolution globale du champ magnétique lunaire en fonction du temps, les mécanismes proposés restent insuffisants pour expliquer cette évolution. De plus, le manque de données à certaines époques ne permet pas de contraindre avec exactitude l'évolution du champ au cours du temps, en particulier son arrêt. Les enjeux de ce travail de thèse sont donc d'acquérir de nouvelles données de paléointensités afin de mieux comprendre l'évolution du champ magnétique lunaire au cours du temps. Cela permettra ainsi de mieux comprendre quels mécanismes étaient à l'origine de ce champ magnétique. Pour cela, un descriptif sur les caractéristiques générales de la Lune est fait dans le premier chapitre afin de comprendre sa géologie, sa stratigraphie et sa structure interne. Les missions Apollo sont décrites afin de situer la lithologie et l'origine géographique des échantillons lunaires rapportés sur Terre. La formation et la différenciation de la Lune seront abordées afin de comprendre comment la Lune est devenue telle que nous la connaissons aujourd'hui. Les notions de champ magnétique sont abordées dans le Chapitre 2. Après un bref rappel sur le champ magnétique terrestre et les domaines d'application de son étude, le champ magnétique lunaire est abordé. L'origine du champ magnétique lunaire est détaillée ainsi que les mécanismes possibles qui pourraient avoir alimenté une dynamo lunaire. Un bilan sur les données de la littérature est réalisé afin de comprendre l'évolution temporelle du champ magnétique lunaire et les incertitudes que nous avons encore.
  Evolution temporelle du champ magnétique lunaire 17 Introduction générale Pour répondre à la problématique, deux études ont été réalisées, sur deux jeux d'échantillons. Les échantillons analysés dans ce travail sont décrits dans le Chapitre 3. Ce chapitre introduit les notions de base du magnétisme des roches et notamment l'aimantation naturelle rémanente des roches lunaires. Les minéraux magnétiques des roches lunaires et leurs caractéristiques sont décrits. Puis les méthodes de déterminations de paléointensités sont présentées à la fin de ce chapitre. Le Chapitre 4 est consacré au descriptif de la première étude, qui a consisté en l'analyse de l'aimantation rémanente naturelle et de la susceptibilité magnétique de 161 roches Apollo stockées dans le Lunar Sample Facility au Johnson Space Center (NASA, Houston, USA). Cette analyse a permis de tester un critère de détermination grossière de paléointensités et de sélectionner neuf échantillons pour des analyses détaillées en laboratoire. Les deux instruments de mesures développés spécialement pour ces analyses sont présentés. Après un résumé détaillé de l'étude, ce chapitre est ensuite présenté sous la forme d'un article, soumis à Physics of the Earth and Planetary Interiors. Le Chapitre 5 est lui consacré à la seconde étude de cette thèse. Deux météorites sont analysées en détail dans une première partie afin de montrer l'intérêt et la validité de l'étude magnétique de météorites lunaires puis la seconde partie porte sur l'étude en laboratoire de vingt-cinq roches lunaires de la collection Apollo, dont les neuf échantillons sélectionnés à partir de l'étude précédente. L'analyse de ces échantillons a permis d'obtenir treize valeurs de paléointensités et six valeurs de limite supérieure. Les autres échantillons n'ont pas donné de résultats. Après un résumé détaillé de l'étude, cette partie est présentée sous la forme d'un article, en cours de préparation pour Earth and Planetary Science Letters.
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  Evolution temporelle du champ magnétique lunaire Chapitre Premier La Lune, notre satellite
  Descente de l'astronaute Buzz Aldrin sur la surface lunaire, le 16 Juillet 1969 Copyright © Lunar and Planetary Institute
  Evolution temporelle du champ magnétique lunaire 19 20
  Evolution temporelle du champ magnétique lunaire Chapitre 1 : La Lune, notre satellite
  Chapitre 1 La Lune, notre satellite Ce premier chapitre présente les différents aspects de la Lune. De son observation à son exploration avec retours d'échantillons, les missions Apollo et Luna y sont également détaillées. Sa géologie, sa structure interne et externe et sa stratigraphie y sont décrites. Un retour sur sa formation et sa différenciation est fait pour comprendre comment la Lune est devenue telle que nous la connaissons aujourd'hui.
  1.1
  L'exploration lunaire
  1.1.1 Observations et exploration Les premières observations lunaires remontent au XVII siècle, avec les scientifiques Thomas Harriot (1609) puis Galilée (1610) qui firent les premiers dessins lunaires à partir d'observations à l'oeil nu puis à la lunette astronomique (Neal, 2009). Cette cartographie primitive, appelée Sélénologie, permit de détailler avec précision les mers et les continents lunaires. Une des oeuvres les plus célèbres est l'ouvrage Sidereus Nuncius, publiée par Galilée en 1610 qui sera suivie par la publication de Almagestum Novum (traité de sélénographie) de Riccioli et Grimaldi en 1651. Ce dernier pose alors les fondements de la nomenclature lunaire. Il faudra attendre le XXème siècle et les premiers satellites pour analyser et étudier la surface lunaire, notamment la face cachée de la Lune avec la mission soviétique Luna 3 en 1959. Ces missions avaient pour but de répondre aux questions longtemps débattues : Existeil de la vie sur la Lune ? ; Quelle est la nature des roches en surface ? La sonde Luna 1, envoyée par l'URSS en 1959, marque la première étape de la conquête lunaire en passant à 5 995 km de sa surface (Neal, 2009). Ses missions étaient variées ; on pourra retenir comme mission la mesure des champs magnétiques de la Lune et de la Terre (Shelton et al., 1969). Ces mesures ont ainsi montré une absence de champ magnétique lunaire global actuel. Suivront ainsi entre 1959 et 1969 une succession de missions spatiales (missions de mises en orbite autour de la Lune et d'alunissages de modules) menées par l'URSS (Luna, Zond) et par les EtatsUnis (Pioneer, Ranger, Surveyor, Lunar Orbiter, Apollo 1,6-10). C'est à partir de 1969, avec la mission Apollo 11, que l'exploration lunaire prend un autre tournant : le retour d'échantillons est possible grâce à l'envoi d'astronautes sur la surface lunaire (Table 1.1). Des mesures en surface ont été réalisées au cours de la mission Apollo 16 (déploiement d'un magnétomètre portable, d'un sismomètre passif et d'un équipement de flux de chaleur). Le rover lunaire apporté avec la mission Apollo 17 a permis le déplacement des astronautes sur plusieurs kilomètres (contre 100 m lors de la mission Apollo 11) afin de récolter diverses séquences de roches (110.5 kg de roche rapportés lors de cette mission). D'autres missions lunaires se succèdent, sans retour d'échantillons, avec l'implication du Japon (missions Hiten, Hayabusa, SELENE) dès 1990, de la Chine (missions AsiaSat, Chang'E) dès 1997 et de l'Inde (mission Chandrayaan) en 2008. e
  Evolution temporelle du champ magnétique lunaire 21
  1 1
  Chapitre 1 : La Lune, notre satellite
  1.1.2 Retour d'échantillons L'étude des caractéristiques de la Lune en laboratoire se fait de manière directe (analyses d'échantillons) et de manière indirecte (données par satellites – Lunar Reconnaissance Orbiter, GRAIL, Chang'E, SMART-1, Kaguya, Chandrayaan-1). Les échantillons proviennent de deux sources principales : les météorites lunaires (environ 230 kg répertoriés pour ~140 météorites, la plus massive NWA 11787 faisant 23.5 kg) et les échantillons prélevés sur la surface lunaire (de quelques grammes à plusieurs kilogrammes). Les météorites lunaires ont principalement été découvertes lors d'expéditions privées dans les déserts chauds (68% dans les déserts Nord Africains) et en Antarctique par des équipes scientifiques (6%). Le prélèvement d'échantillons lunaires (voir détail Table 1.1) devient possible avec le début des missions Luna (16, 20 et 24, 1970-1976) et Apollo (11 à 17, 1969-1972); totalisant 320 g et 380 kg respectivement d'échantillons de lithologies et tailles différentes (Heiken et al., 1991).
  Mission Apollo 11 Apollo 12 Luna 16 Apollo 14 Apollo 15 Luna 20 Apollo 16 Apollo 17 Luna 24
  Localisation Mare Tranquilitatis Oceanus Procellarum Mare Fecunditatis Fra Mauro (Mare Imbrium) Hadley Rille/ Monts Appenine Apollonius Highlands Descartes Highlands Mare Serenitatis Mare Crisium
  Masse (kg) 21.6 34.3 0.1 42.3 77.3 0.03 95.7 110.5 0.17
  Date de mission Juillet 1969 Novembre 1969 Septembre 1970 Février 1971 Août 1971 Février 1972 Avril 1972 Décembre 1972 Août 1976
  Lithologies Basaltes et Highlands Basaltes Brèches Basaltes et Highlands Brèches Basaltes et brèches
  Tableau 1.1 : Détail des missions avec retour d'échantillons. D'après Fuller et Cisowski (1987) et Heiken et al. (1991). De futures missions avec retour d'échantillons sont programmées par la Chine, à partir de 2018. La mission Chang'E-5, troisième étape du Programme d'Exploration Lunaire Chinois, prévoit un alunissage près du Mont Rümker (situé près de Mare Imbrium) qui, de par sa géologie complexe, est un site idéal pour un nouveau retour d'échantillons (le retour d'environ 2 kg de régolithes est prévu) et pour la réalisation d'une nouvelle carte géologique. Cette zone est également intéressante de par la présence de dômes basaltiques jeunes, indiquant une activité volcanique possible jusqu'à l'Eratosthène (2-1 Ga) (Zhao et al., 2017). Les échantillons rapportés de la Lune ne proviennent que de la face visible de la Lune, comme illustré en Figure 1.1, et de son pourtour équatorial, zone la plus facilement accessible et la mieux documentée (Heiken et al., 1991). Ils ont été numérotés selon la mission dont ils proviennent. Pour les missions Apollo, la numérotation débute par 10 pour Apollo 11 ; 12 - Apollo 12 ; 14 - Apollo 14 ; 15 - Apollo 15 ; 6 - Apollo 16 et 7 - Apollo 17. Pour les missions Luna, les deux premiers nombres correspondent à la mission : 16 pour Luna 16 ; 20 - Luna 20 et 24 - Luna 24 (Heiken et al., 1991).
  22
  Evolution temporelle du champ magnétique lunaire Chapitre 1 : La Lune, notre satellite
  Figure 1.1 : Carte de la face visible de la Lune avec localisation des sites d'alunissages des missions Apollo, Luna, Surveyor et Lunokhod (Lunar landing site chart, Lunar and Planetary Institute, 1967).
  1.1.3 Les missions Apollo Les météorites ayant été l'unique source d'échantillons extra-terrestres pendant longtemps, il avait été supposé que la Lune était elle-même un corps primitif condensé à partir de la nébuleuse solaire. Les missions Apollo 6 à 10 ont surtout consisté en des vols en orbite autour de la Lune, avec des mesures de champ magnétique à distance par exemple. Des tests sur les modalités de futurs alunissages ont été faits (Apollo 10) mais aucun retour d'échantillons n'a eu lieu. La diversité des roches rapportées par les missions Apollo 11, 12, 14, 15, 16 et 17 a permis de prouver que la Lune n'était pas un corps primitif mais un corps évolué ayant subi de grands bombardements météoritiques (> 3.8 Ga) ainsi que des transformations internes (différenciation, volcanisme, sismicité) (Heiken et al., 1991). Site Apollo 11 Le module de la mission Apollo 11 s'est posé au sud-ouest du bassin Mare Tranquillitatis dans une zone relativement plate. Ce bassin est essentiellement composé d'anciennes laves
  Evolution temporelle du champ magnétique lunaire 23
  1 1
  Chapitre 1 : La Lune, notre satellite
  basaltiques (de 3,88 à 3,57 Ga) riches en titane. Des éjectas des highlands environnants (situés à 40 km du bord du cratère) sont retrouvés dans le bassin d'impact. Site Apollo 12 Ce site d'alunissage est localisé dans le bassin Oceanus Procellarum, à 180 m de celui du module Surveyor 3. Cette région se compose de basaltes plus cratérisés que ceux trouvés sur le site Apollo 11. Les échantillons récoltés montrent au moins trois groupes de basaltes de composition différente datés entre 3,29 et 3,08 Ga par les méthodes isotopiques. Les basaltes KREEP (voir partie 1.2.4 pour une définition) ont pu être récoltés lors de cette mission et sont sombres et riches en verre volcanique. Site Apollo 14 Cette mission s'est située au nord de la région du cratère Fra Mauro dans les highlands. De part sa localisation, des éjectas du bassin Imbrium ont pu être rapportés. Les roches récoltées sont essentiellement des brèches (voir détails dans la partie 1.4) polymictes fragmentées, des impacts melt breccia et des brèches à composition basaltiques ou riches en KREEP. Ces brèches se seraient formées entre 3,9 et 3,8 Ga. Site Apollo 15 Ce site est dans la région de Hadley Rille dans le bassin Mare Imbrium. Ce site a surtout été choisi pour échantillonner les monts et highlands qui forment l'anneau du bassin Imbrium ainsi que les basaltes de Palus Putredinis. Les basaltes collectés sont issus de deux groupes minéralogiques (riches en quartz ; riches en olivine) mais datés de la même période (3,3 Ga). Les roches récoltées des highlands sont plus diversifiées : anorthosites, roches plutoniques riches en magnésium, impact melt et granulites. Deux autres matériaux ont été découverts : un basalte alumineux riche en KREEP ; et du verre volcanique vert de composition ultramafique. Site Apollo 16 Cette mission s'est localisée près du cratère Descartes et des plaines de Cayley, dans les highlands afin d'en étudier la morphologie. Les échantillons rapportés sont des brèches polymictes, des impact melts et de rares anorthosites. Les dépôts d'éjectas de brèches localisées au niveau du site semblent provenir du bassin d'impact Nectaris (3.92 Ga). Site Apollo 17 Le dernier module Apollo s'est posé dans la vallée Taurus-Littrow, près de la bordure du bassin Serenitatis. Il a été montré que la vallée est composée de basaltes riches en titane datés de 3,8 à 3,7 Ga. Le fond de la vallée est également recouvert d'une couche de dépôts noirs datés à 3,64 Ga, riches en fragments pyroclastiques noirs et oranges. Le massif qui surplombe la vallée est composé d'impact melt breccia (daté à 3,87 Ga) et de roches plutoniques. Les brèches semblent refléter l'âge du bassin Serenitatis mais leur origine n'est pas connue.
  24
  Evolution temporelle du champ magnétique lunaire Chapitre 1 : La Lune, notre satellite 1.2
  1
  La Lune aujourd'hui
  1.2.1 Caractéristiques générales La Lune est l'unique satellite naturel de la Terre. D'un rayon 6 fois inférieur à celui de la Terre (1 738 km), elle est le cinquième plus grand satellite de notre système solaire. Dépourvue d'atmosphère, sa température de surface varie entre 107 et -153°C. Elle évolue à une distance moyenne de 384 467 km de la Terre, ce qui induit une influence gravitationnelle sur la Terre par des marées par exemple. Cette influence est réciproque. Les effets de marées provoquent un ralentissement de la rotation terrestre, qui induit un éloignement progressif de la Lune, de quelques centimètres par an, afin de maintenir le moment angulaire du système Terre-Lune constant (Heiken et al., 1991). C'est au cours des missions spatiales vers la Lune qu'un traité de l'espace a été signé le 10 Octobre 1967, afin de considérer la Lune comme un espace international.
  1.2.2 Structure interne La structure interne de la Lune a pu être caractérisée grâce aux échantillons des missions Apollo et Luna ; mais aussi grâce à l'analyse des données géophysiques récoltées par les satellites (données GRAIL, LLR, LOLA, Kaguya). Sa structure interne (Figure 1.2) est semblable aux planètes terrestres différenciées : une croûte, un manteau et un noyau (externe et interne). La croûte La croûte lunaire est complexe. Son épaisseur et sa composition varient latéralement et verticalement. La partie supérieure de la croûte se compose principalement d'anorthosites tandis que la partie inférieure serait plutôt de composition noritique à troctolitique (Arai et al., 2008 ). Wieczorek et Zuber (2001a) montrent grâce aux données géophysiques que la croûte supérieure est composée de 88 ± 4 % de plagioclases (qui correspond à 29-32 wt.% Al O ) tandis que la croûte inférieure, plus mafique, se compose de 65 ± 8% de plagioclases (soit 1825 wt.%). Ces teneurs sont cohérentes avec l'hypothèse d'une croûte formée par cumulas de matière, en flottaison sur le magma océanique lunaire. Yan et al. (2015) estime l'épaisseur moyenne de la croûte entre 30 et 50 km, à partir de l'analyse des données de topographie (LRO). L'analyse des données GRAIL a permis de déduire la densité moyenne des highlands de la croûte, qui est estimée vers 2250 ± 18 kg.m . Ces faibles densités, bien inférieures à celles utilisées pour les anorthosites crustales dans les modèles géophysiques (2800 et 2900 kg.m habituellement), semblent causées par de nombreuses fractures de la roche et par les brèches en surface (Wieczorek et al., 2013). De plus, la porosité moyenne de la croûte est estimée à 12% mais varie régionalement de 4 à 21%, en raison des impacts par exemple (Wieczorek et al., 2013). Ces deux facteurs (densité et porosité) ont ainsi permis de proposer une croûte lunaire non plus de 50 km mais plutôt de 34-43 km (Wieczorek et al., 2013). 2
  3
  -3
  -3
  Les analyses sismiques ont montré que les highlands sont les zones les plus épaisses, jusqu'à 110 km d'épaisseur, et les bassins d'impacts les plus fins (< 5 km) (Ishihara et al., 2009). La croûte est aussi asymétrique entre sa face visible et sa face cachée, les highlands de la face cachée étant en moyenne 10-15 km plus épais que ceux de la face visible (Wood, 1973). Une discontinuité sismique a pu être observée vers 20 km de profondeur, au niveau des sites Apollo 12, 14 et 16. En effet, la vitesse des ondes sismiques augmente progressivement de la
  Evolution temporelle du champ magnétique lunaire 25 1
  Chapitre 1 : La Lune, notre satellite
  surface jusqu'à 20 km puis reste constante jusqu'à la base de la croûte. Cette caractéristique serait le résultat d'un changement de composition pétrologique (Wieczorek et al., 2006). Le manteau Les analyses géophysiques sur les sites Apollo ont permis de mettre en évidence une stratification du manteau. Il n'y aurait pas de couche franche de transition entre la base de la croûte et le manteau (Matsuyama et al., 2016) mais une discontinuité majeure a été montrée par la sismique entre 500 et 560 km de profondeur (Wieczorek et al., 2006). La forte atténuation des ondes sismiques vers la base du manteau solide et silicaté suggère la présence d'une couche de manteau partiellement fondue d'une centaine de kilomètres d'épaisseur à 1000 km de profondeur. Cette zone marque la limite avec le noyau externe. (Wieczorek et al., 2006 ; Weber et al., 2011 ; Matsuyama et al., 2016). Le noyau Les caractéristiques principales du noyau sont connues grâce aux contraintes de masse, de moment d'inertie, de facteur de qualité de la dissipation tidale, sismiques et de champ magnétique récoltées ces dernières années (Matsuyama et al., 2016). Les rayons précis et les compositions chimiques sont encore mal contraints.
  Noyau externe La présence d'un noyau externe liquide a été montrée grâce à la réflexion des ondes sismiques qui ont été enregistrées sur la Lune lors des missions Apollo. Ce noyau dense liquide serait composé de fer, de silicates riches en titane, de soufre et de nickel (Wieczorek et al., 2006). Les concentrations en éléments sidérophiles du manteau et retrouvés dans les roches Apollo permettent d'estimer l'abondance en nickel du noyau par exemple, qui est estimée entre 8.3 et 55 wt% selon les études (Wieczorek et al., 2006). Son rayon est estimé entre 380 ± 40 km (Garcia et al., 2011) et 330 ± 20 km (Weber et al., 2011).
  Noyau interne Le noyau interne de la Lune a été mis en évidence suite à l'analyse des données sismiques récoltées lors des missions Apollo. Weber et al. (2011) propose une composition mixte de fer et de soufre (6-8 wt % de soufre), d'après les modélisations de dynamo lunaire et l'analyse des données sismiques. Righter et al. (2017) utilise les données géochimiques (température, pression) pour estimer la proportion de soufre et de carbone contenue dans ce noyau, en plus du fer et du nickel. Il en résulte qu'une proportion de 0.5 wt% de soufre et 0.375 w% de carbone (pour un liquidus estimé à 1550°C et un solidus estimé à 1250°C) est la plus probable pour correspondre avec les données géochimiques et géophysiques. Ces taux ont été pris d'après les proportions de soufre et carbone trouvées dans les verres des roches Apollo. Le rayon du noyau solide est estimé entre 240 ± 10 km (Weber et al., 2011) et 140 ± 20 km (Laneuville et al., 2014), ce qui représente 40 à 70% du noyau total.
  26
  Evolution temporelle du champ magnétique lunaire Chapitre 1 : La Lune, notre satellite Face visible
  Nearside
  Manteau
  Farside
  Face cachée
  Shallow Moonquakes supérieur ? A15
  Deep Moonquake Source Region
  Croûte anorthositique Anorthositic Crust Manteau inférieur ~587-km radius partiellement fondu Zone of Partial Melt (rayon ~587 km) (Lower Mantle)
  ~350-km radius Noyau externe liquide (rayon ~350Core km) Fluid Outer
  A12/14 A16 Manteau moyen Middle Mantle
  Noyau interne solidifié ~160-km radius 17% du noyau cristallisé Solid Inner Core (rayon10% ~160of km) (assuming the
  core has crystallized)
  ? Discontinuité (~560 km) 560 km Discontinuity
  Upper Mantle
  South Pole-Aitken Basin Bassin pôle sud Aitken
  Figure 1.2 : Hypothèse actuelle de structure interne lunaire. La discontinuité est supposée à 560 km de profondeur ici. De même, les rayons proposés ici sont moyennés de la littérature (d'après Wieczorek et al., 2006). Les points noirs sont les séismes profonds et les blancs les séismes superficiels. A15, A12/14 et A16 correspondent aux localisations des sites Apollo (15, 12, 14 et 16 respectivement). Modifié d'après Wieczorek et al. (2006).
  1.2.3 Structure externe La surface lunaire est composée de cratères d'impacts plus ou moins érodés, d'étendues volcaniques (appelées mers) et de reliefs (formés par les highlands). Elle est recouverte d'une couche de fines particules, qui compose le sol lunaire. Trois autres processus sculptent sa surface : les impacts hypervéloces, le volcanisme et l'activité tectonique. Les impacts hypervéloces C'est le processus de surface lunaire dominant. Les structures d'impacts sont divisées en 3 groupes : cratère simple, cratère complexe et bassin d'impact. Ainsi, les roches en surface et celles excavées lors d'un impact ont subi des transformations pétrographiques, physiques et chimiques. Ce sont les bassins d'impact qui ont modelé la majorité de la topographie de la Lune. La plupart de ces anciens bassins d'impact ont été recouverts par les laves basaltiques, formant les mers actuelles (Mare Imbrium, Oceanus Procellarum pour ne citer qu'eux) (Heiken et al., 1991). Le volcanisme La source de ce volcanisme provient de la fusion des roches mantelliques associées ou non à des roches de la croûte. Cette source est formée à 150-200 km de profondeur pour
  Evolution temporelle du champ magnétique lunaire 27
  1 1
  Chapitre 1 : La Lune, notre satellite
  une température supérieure à 1100°C. Les laves ont recouvert 17 % de la surface lunaire, particulièrement les bassins d'impact (Head, 1976). Ce sont des laves basaltiques de faible viscosité, remontées en surface par des fractures, qui ont rempli successivement ces bassins (Figure 1.3). Par exemple, le bassin Imbrium s'est rempli sur 500 Ma, formant Mare Imbrium. Le volcanisme lunaire a été actif entre ~3.9 et ~1.2 Ga mais l'essentiel de l'activité volcanique a pris fin vers 3 Ga. L'étude de ces basaltes permet de comprendre la composition du manteau lunaire dont ils sont issus (Heiken et al., 1991). Grabens
  Grabens
  Coulées de lavesCoulées successives de laves successives
  Dikes intrusifs
  Dikes intrusifs
  Croûte Manteau
  Croûte Manteau
  Figure 1.3 : Exemple de l'activité volcanique au sein d'un bassin d'impact. Coupe schématique d'un bassin d'impact recouvert de coulées volcaniques et illustrant la localisation des cheminées volcaniques, la croûte et le manteau lunaire. Chaque coulée de lave (numérotées de I à III selon leur chronologie) est remontée depuis le manteau jusqu'à la croûte, formant des dikes (en noirs) puis des grabens en surface. Ce schéma témoigne de trois épisodes de volcanisme. Modifié d'après Heiken et al. (1991). L'activité tectonique Divers types de déformations tectoniques sont visibles sur la Lune: des failles, des plis et des chaînes de montagne près de la surface. L'analyse de ces structures permet de comprendre la structure interne de la Lune, son évolution mais aussi ses propriétés mécaniques et thermiques. Les forces externes à l'origine de l'activité tectonique lunaire proviennent des impacts de météorites et des forces tidales (générées par le champ de gravité de la Terre). Les forces internes à l'origine de cette activité sont les contraintes thermiques et les intrusions volcaniques (Heiken et al., 1991).
  1.2.4 Géologie et classification La compréhension de la structure interne de la Lune s'est considérablement améliorée grâce au retour et à l'analyse d'échantillons lunaires rapportés des missions Apollo et Luna. La Lune est formée de 3 grands groupes géologiques : les roches ignées
  28
  Evolution temporelle du champ magnétique lunaire Chapitre 1 : La Lune, notre satellite (volcaniques et plutoniques), les brèches et le sol lunaire, que nous détaillerons ci-dessous. Elles proviennent de trois sources géologiques (Figure 1.4) : formées à partir des impacts (les brèches) ; formées à partir des séries magmatiques (les roches volcaniques et certaines plutoniques) ou formées à partir de l'océan magmatique primitif lunaire (les roches plutoniques). Roches lunaires
  Sources
  Océan magma2que primi2f
  Croûte inférieure Manteau mafique
  Mg-suite
  Séries magma2ques
  Anorthosites
  Alkali-suite
  urKREEP
  Basaltes KREEP
  Roches d'impact
  Brèches
  Basaltes
  Roches fondues
  Roches pyroclas2ques
  Figure 1.4 : Différents types de roches lunaires, classés selon leur source géologique. Les roches en vert correspondent aux roches plutoniques, en bleu les roches d'impact et en rouge les roches volcaniques. Les roches Mg-suite sont des roches riches en magnésium (norites, gabbros, troctolites, dunites, etc) et les Alkali-suite les roches riches en sodium (anorthosites alcalines par exemple). Modifié d'après Kring et al. (2012). Le sol lunaire Le sol lunaire est issu de débris non consolidés et fragmentés (granulométrie 40 à 800 μm) de roches basaltiques et anorthositiques qui couvrent la surface (Heiken et al., 1991). Sa composition chimique varie grandement selon la composition des roches mères dont il est issu. Le sol se compose de 0.5% de fer métallique et de moins de 2% de composants météoritiques. Ce taux de fer est supérieur à celui contenu dans les basaltes mais il reste similaire à la teneur en fer métallique des brèches de régolithes. Il existe deux types de sols lunaires, classés selon leur teneur en fer Fe : les sols des mers basaltiques et les sols des Highlands. Ces derniers possèdent des particules de fer (> 100 μm) ayant pour source principale le fer météoritique (Fuller et Cisowski, 1987). 2+
  Les roches ignées Les roches volcaniques Les basaltes lunaires sont issus du manteau et recouvrent 17% de la surface. Ils ont notamment rempli les bassins d'impact, formant les mers actuelles. Ces basaltes sont souvent classés selon leur composition minéralogique (basalte à olivine, à ilménite, etc). Celle-ci est semblable à celle des basaltes terrestres ; elle dépend de la viscosité et de la température de la lave dont ils proviennent. Leur composition chimique dépend directement de la composition chimique des laves (basaltes KREEP, roches pyroclastiques et basaltes classiques notés « mare basalts ») à partir desquelles ils ont cristallisé. Certaines éruptions étaient de type « firefountain », formant les verres volcaniques et les dépôts pyroclastiques dont la majorité sont
  Evolution temporelle du champ magnétique lunaire 29
  1 1
  Chapitre 1 : La Lune, notre satellite
  issus de la fin de l'Imbrien entre 3.2 et 3.7 Ga. Les basaltes KREEP sont des basaltes riches en potassium (K), éléments rares (REE) et en phosphore (P). Ils proviennent de sources profondes du manteau et ont été mis en évidence lors de la mission Apollo 11 (Heiken et al., 1991). Les roches plutoniques Ces roches ignées forment les highlands. Elles ont été relativement peu modifiées par les impacts météoritiques survenus sur la surface lunaire. Ces roches sont classées en trois groupes principaux en fonction de leur composition chimique : les anorthosites ferreuses, les roches riches en magnésium et les autres roches. ● Anorthosites ferreuses : ce type de roche est le plus commun des roches ignées des highlands. Elles sont blanches ou de couleur claire de par leur forte teneur en plagioclases riches en calcium et aluminium. Elles contiennent en plus faible proportion des pyroxènes et olivines. Leur granulométrie grossière indique une formation près de la surface avec un refroidissement lent. Étant en général des roches très bréchifiées, l'origine géographique des anorthosites est souvent difficile à identifier. La majorité des anorthosites lunaires collectées ont été rapportées lors de la mission Apollo 16 (Heiken et al., 1991). ● Roches riches en magnésium : ces roches sont enrichies en magnésium et se distinguent par leurs minéraux majeurs avec une forte proportion d'olivines, de feldspaths plagioclases et de pyroxènes qui constituent 99% de la roche. Il s'agit de norites et de gabbros (enrichies en pyroxènes et plagioclases), de troctolites (riches en olivines et plagioclases) et de dunites (riches en olivines). La plupart de ces roches sont bréchiques et proviennent de la cristallisation d'un magma mafique dans la croûte primitive lunaire (Heiken et al., 1991). ● Autres types : ce groupe représente les roches ignées présentes en minorité sur la Lune, comme les granites, les urKREEP ou les anorthosites alcalines. Ces dernières contiennent 85% de plagioclases enrichis en sodium (contrairement aux anorthosites ferreuses), le reste étant du pyroxène. Les granites, de texture fine, sont enrichis en feldspath potassique mais ne contiennent pas de minéraux hydratés (comme le mica ou l'amphibole), contrairement aux granites terrestres. Les urKREEP (le préfixe « ur- » signifie « primitif ») sont des roches riches en éléments incompatibles. Ils sont issus de la cristallisation du dernier pour cent de magma restant de l'océan magmatique primitif (voir détails sur la différenciation lunaire partie 1.3.1). Ils seraient présents sous forme de patchs de 2 km d'épaisseur entre la croûte et le manteau mais n'ont jamais été échantillonnés jusqu'à présent (Heiken et al., 1991). Les brèches Les brèches d'impact sont, à l'origine, des roches ignées ou d'anciennes brèches qui ont été modifiées suite à un ou plusieurs impacts météoritiques. Les clastes peuvent être des fragments de roche ignée, des fragments de brèches ou des fragments métamorphisés par choc ou fondus de ces derniers. La matrice a une texture fine et se décline en trois types : matrice fondue solidifiée (glassy ou crystalline), matrice clastique (fragmental) ou un mélange des deux métamorphisé. Sous ces conditions, les systèmes isotopiques utilisés pour les datations de ces brèches peuvent être remis à zéro. Ces roches contiennent également des débris météoritiques, ce qui permet de les différencier des autres types de roches. La quantité de matériel fondu et la température d'équilibre entre les clasts et le matériel fondu permettent de caractériser les brèches, comme le détaille la Figure 1.5.
  30
  Evolution temporelle du champ magnétique lunaire Lunar Rocks 241 Chapitre 1 : La Lune, notre satellite
  1
  Température d'équilibre melt – clasts (°C)
  GAZEUX
  et Dimict breccia
  Fragmental et Regolith breccia Glassy melt breccia
  EFFRITAGE
  FRACTURES AMBIANT
  Taux de fusion iniEale (%) Fig. 6.39. Generalized scheme showing the relations between petrographically distinct impact breccias Figure 1.5: Relation la température d'équilibre melt-clast et le taux de originally fusion depresent la roche, produced and lithified entre in a single impact event as a function of the amount of melt and après the equilibrium temperature attained between clasts and melt (figure modified from Simonds et al., 1976). un impact. La texture des brèches obtenues est contrôlée par l'équilibre thermique entre les clastsThe et la textures of resulting breccias are assumed to be controlled by the initial process of rapid thermal equilibration roche fondue. La localisation des liquidus, solidus et des autres transformations peut varier avec between clasts and the enclosing melt. Subsequent cooling rates will have some, but much less, effect on la matrix textures. in which breccias occur on the diagram are approximate, and the locations of composition de Fields la roche mère. different Modifiée d'après Heiken et al., 1991. liquidus, solidus, and other transformation boundaries will vary with bulk composition. Dimict breccias, granulitic breccias, and regolith breccias do not plot as classes on this diagram. Regolith breccias could be classified as glassy melt breccias or fragmental breccias, but they differ in that they are commonly lithified from Elles sont classées en by deux catégories : les brèches seule lithologie loose fragments of lunar soil an impact event that is separate frommonomictes (and later than) (une those that produced the assemblage of fragments. Similarly, lithification of granulitic breccias is usually the result of recrystallization impactée) et les brèches polymictes (plusieurs lithologies de roches impactées), d'après la long assembly of the fragments. Dimict breccias can be considered a special case of crystalline melt 242after Lunar Sourcebook classification la plus communément utilisée de Heiken et al., (1991). Leur répartition au sein breccias.
  d'un cratère d'impact est illustrée ci-dessous, sur la Figure 1.6. Bordure du cratère (1981) identified such genetically-related melt 10% are olivine and pyroxene (Minkin et al., 1977). fragments in breccia 67016, as did Marvin et al. Fragments of cataclastic anorthosite make up about Mélange de12% Glassy of et this rock, granulites about 8%, and Pic (1987) in breccia 67015. impact central Fragmental The Apollo 16 fragmental breccias consist mainly of melts about 8%. Couches d'Impact melt breccias breccias etHowever, de de clast-poor impact melts plagioclase feldspar. They contain three dominant the et Apollo 16 fragmental breccias are not par:cules fondues types of rock fragments: (1) cataclastic anorthosite, (2) all the same. Sample 67015 contains a more diverse granulitic breccia, and (3) fragment-rich feldspathic assemblage of fragments, including (1) anorthositic Fragmental breccias impact-melt rocks (James, 1981; Stöffler et al., 1981, norites that are related to ferroan anorthosites and (2) 1985; R. Warner et al., 1976a; and others). The less aluminous KREEPy impact melts (Marvin and Fragmental breccias impact-melt clasts are dark, while the anorthosites Lindstrom, 1983; Marvin et al., 1987; see section 6.3). and granulites are light colored. Stöffler et al. (1985) studied the modal and textural The impact-melt clasts have significant variations in characteristics of many of these Apollo 16 fragmental mineral and chemical composition. In sample 67455, breccias from North Ray Crater and found that the a fairly typical specimen, half the mineral grains relative proportions of different rock types in the lithic larger than 40 μm are plagioclase and about fragments vary Monomict breccias
  Dimict breccias Fig. 6.40. Cross-section of an ideal lunar crater showing the relations of different polymict breccia types and possible monomict breccias to the geological environment of the crater du (adapted frommagnétique Stöffler, 1981). Numbers Evolution temporelle champ lunaire indicate actual lunar samples that are examples of the different breccia types. In reality, most lunar polymict breccias are the result of a series of impacts, and in almost all cases the target for each new breccia-producing impact is already a polymict breccia.
  31 1
  Chapitre 1 : La Lune, notre satellite
  Figure 1.6: Coupe schématique d'un cratère d'impact montrant la localisation possible des différents types de brèches, modifié d'après Stöffler et al., 1981. Les brèches monomictes Ces brèches sont plutôt rares car elles ne sont composées que d'une seule roche mère ignée, qui s'est cassée et déformée (Warren et Wasson, 1980). Ne s'étant pas mélangés avec d'autres types de roches, ses clastes peuvent conserver leur texture originelle. Ces roches sont donc utiles pour témoigner de la formation de la croûte lunaire. Les brèches polymictes Elles sont divisées en 7 sous-groupes, en fonction de leur texture et de leur degré de cristallisation. ● Les fragmental breccia : ces brèches sont composées de clastes (anciennes brèches, roche ignée, minéral) dans une matrice composée de fragments fins de cette même roche (ex : 67975). Ces clastes sont souvent angulaires et de taille millimétrique. Des fragmental breccia ont été collectées sur les sites des missions Apollo 14, 16 et 17. ● Les glassy melt breccia : Ces brèches possèdent des clastes pris dans une matrice fondue qui n'a pas cristallisé et implique donc un refroidissement rapide de la roche (ex : 60095). ● Les impact melt breccia : ces roches sont similaires aux glassy melt breccia mais la matrice a refroidi plus lentement, formant une roche cristallisée (matrice et clasts). Le nombre de clastes dans ces roches varie de quelques-uns à plus de 50% de la roche (ex : 14169). Ce sont en majorité des clastes de plagioclases ; les pyroxènes, olivines ou fragments de roches étant minoritaires. La majorité des matériaux composant ces brèches d'impact ont été totalement fondus pendant leurs formations. ● Les clast-poor impact melt breccia : ces brèches présentent une texture similaire à celle des laves car elles sont peu riches en clastes. La présence d'éléments météoritiques dans ces brèches les différencie des laves. La plupart de ces brèches sont datées à 3.9 Ga, ce qui coïncide avec la période de la formation des grands bassins d'impacts. ● Les granulitic impact melt breccia : ces brèches sont des roches cristallines qui proviennent de la recristallisation d'anciennes brèches, soumises à de hautes températures (1000°C) sur de longues périodes de temps. ● Les dimict breccia : ce sont des brèches composées de deux lithologies distinctes, combinées en une roche (ex : 61015). Les deux lithologies étant mutuellement intrusives, la brèche formée se compose alors de veines de roches sombres et claires. La roche sombre est généralement de l'impact melt breccia et la roche claire de l'anorthosite. L'équivalent sur Terre est la pseudotachylite (Heiken et al., 1991). ● Les regolith breccia : ces brèches ont été lithifiées par choc, c'est-à-dire compactées et indurées suite au passage des ondes de choc dans le sol lunaire après un impact (Heiken et al., 1991). Elles sont ainsi formées en subsurface (ex : 10018) et reflètent les propriétés du sol à partir duquel elles sont formées (Fuller et Cisowski, 1987). Ces brèches ont une texture globale qui varie de friable à cohérente, selon leurs degrés de lithification (Heiken et al., 1991). Pour la suite de ces travaux de thèse, nous avons regroupé les lithologies des brèches en 3 sous-groupes seulement à partir des descriptions et des catégories décrites ci-dessus: les impact melt breccia, les dimict breccia et les regolith breccia. Les roches plutoniques sont
  32
  Evolution temporelle du champ magnétique lunaire Chapitre 1 : La Lune, notre satellite regroupées sous l'appellation de roches feldspathiques. Les roches fondues non bréchifiées sont appelées impact melt rock.
  1.3
  Evolution de la Lune
  1.3.1 Formation et différenciation de la Lune Formation Le système Terre-Lune se serait formé à 4421.5 ± 4.5 Ma (Connelly et al., 2016), soit 62 Ma après la formation du Système solaire (Touboul et al., 2007). L'hypothèse la plus probable serait que la Lune soit le résultat de la collision d'un impacteur de la taille de Mars (nommé Theia) avec la Terre primitive qui ne faisait que 70% de sa taille actuelle (Hartmann and Davis, 1975). Cette théorie de l'impact unique (Benz et al., 1986 ; Cameron, 1997 ; Palme, 2004) est attestée par des modèles récents de la structure interne lunaire (Matsuyama et al., 2016). Les débris de l'impacteur se seraient accrétés afin de former la proto-Lune qui serait composée à 80% du manteau de l'impacteur (Canup et al., 2001, 2013). Cette hypothèse n'explique cependant pas totalement la présence de certains éléments isotopiques terrestres retrouvés sur la Lune (Matsuyama et al., 2016). D'autres auteurs (Citron et al., 2014 ; Rufu et al., 2017) ont testé l'hypothèse d'une collision de la proto-Terre par de petits impacteurs successifs, grâce à des simulations d'impacts, arrachant successivement des morceaux du manteau primitif terrestre. Les débris, essentiellement d'origine terrestres, se seraient accrétés au fil des impacts, par gravité, pour former la proto-Lune. Différenciation Suite à l'accrétion de la proto-Lune, les éléments lourds auraient migré au centre de la Lune pour former le noyau tandis qu'un océan magmatique se serait formé entre 200 et 1000 km d'épaisseur (Kring et al., 2012). Cet océan s'est ensuite cristallisé par étapes afin de former la croûte d'anorthosites et le manteau. La présence des basaltes KREEP et des highlands en surface indique la présence d'au moins deux sources magmatiques lors de cette différenciation lunaire. Plusieurs hypothèses sont avancées pour expliquer la chronologie exacte de cette différenciation qui est encore débattue (voir Wieczorek et al., 2006 ; Kring et al., 2012 pour détails). Les éléments légers du manteau primitifs auraient formé en surface la croûte anorthositique primaire (couche appelée « FAN » pour ferroan anorthosites). Les roches plutoniques auraient cristallisé les unes après les autres, surplombant un manteau primitif (Figure 1.7a) (Kring et al., 2012). Les résidus de magma issus de la cristallisation de l'océan magmatique auraient ainsi donné les basaltes urKREEP qui formeraient une couche à la base de la croûte. Les données isotopiques du plomb proposent un événement magmatique majeur vers 4376 ± 18 Ma, âge moyen possible de la formation des urKREEP et des FAN (Snape et al., 2016). A partir de 3 Ga, le volcanisme lunaire et les impacts auraient contribué aux variations d'épaisseur de la croûte et aux différences de lithologies retrouvées entre la face cachée et la face visible. La couche des urKREEP serait alors sous forme de patchs à la base de la croûte (Figure 1.7b) (Kring et al., 2012). Walker (1983) propose comme autre hypothèse la formation de ce complexe croûte/manteau supérieur par la cristallisation successive de mélanges magmatiques plus ou moins fondus.
  Evolution temporelle du champ magnétique lunaire 33
  1 1
  Chapitre 1 : La Lune, notre satellite a) Lune à ~4.4 Ga
  b) Lune à ~3 Ga Face visible
  65 km
  Face cachée Croûte anorthosiCque (FeO ~4.5 wt.%)
  1738 km
  Manteau primiCf
  Manteau appauvri 40-50 km
  Croûte
  N.I
  Noyau
  Augm. Fe
  Augm. Fe
  90 km
  N.E
  Basaltes (FeO ~15-22 wt.%)
  Couche supposée urKREEP
  Manteau
  urKREEP résiduel
  40 km
  Bassin Pôle Sud Aitken
  Figure 1.7 : Interprétation possible de la différenciation de la Lune à 4.4 et 3 Ga. a) Structure interne FIGURE 3.22 vers Cross sections the Moon. (a) Depiction of lunar interior around après la cristallisation de l'océan magmatique 4.4 Ga.of Les séquences minéralogiques dans la 4.4 Ga, just after m ocean crystallization. A thin urKREEP layer lies between the crust and mantle. (b) Depiction of croûte et le manteau sont représentées. Le manteau primitif serait composé de matériaux fondus et interior around 3 Ga, near the end of mare basaltic volcanism. non Notice the asymmetry of the lunar c material,enunknown extent of urKREEP layer (adapted from McCallum, le noyau encore petit. Une couche enrichie KREEP primitif (urKREEP) marquerait la limite2001). croûtemanteau. b) Structure interne vers 3 Ga.Given La face visible et la face cachée ont des épaisseurs de croûte the high uncertainties that exist concerning the KREEPy material (origin, extent, différentes. Le noyau interne solide (N.I) se distingue duofnoyau liquidewould (N.E). Modifié d'après representative samples KREEPexterne and urKREEP provide invaluable insight regarding the ac of the lunar magma ocean theory and the bulk Moon, as it represents the last liquid to solidify. G Kring et al. (2012).
  truth data are also needed to provide more accurate global estimates of incompatible element abun that have only been derived so far from orbital data. In addition, heat producing elements existing KREEP material may have had a large effect on lunar volcanic processes. For such reasons, it recommended targetGa, areasaprès with high signatures important information ab La formation du noyau lunaire highly est estimée versto 4.1 la KREEP formation de tolagain croûte extent and composition of the urKREEP layer, and target deep impact basins that could potentially s anorthositique à 4.4 Ga, par la migration des éléments lourds non cristallisés vers le centre de pristine urKREEP.
  la Lune (Runcorn, 1996). FIGURE 3.22 Cross sections of the Moon. (a) Depiction of lunar interior around 4.4 Ga, just after magma 1.3.2 debetween la surface lunaire ocean crystallization. A thin Chronologie urKREEP layer lies the crust and mantle. (b) Depiction of lunar interior around 3 Ga, near the end of mare basaltic volcanism. Notice the asymmetry of the lunar crustal La chronologie surface2001). de la Lune est basée sur material, unknown extent of urKREEP layer (adaptedde fromlaMcCallum,
  la datation isotopique des échantillons lunaires issus des missions Apollo et Luna. L'analyse des cratères a également Given the high uncertainties that exist concerning the KREEPy material (origin, extent, etc.), permis de déterminer sa chronologie : étude de la stratigraphie des cratères (principe de representative samples of KREEP and urKREEP would provide invaluable insight regarding the accuracy 158 recoupement) ; étude de la distribution (taille et fréquence) des impacts selon l'unité of the lunar magma ocean theory and the bulk Moon, as it represents the last liquid to solidify. Ground truth data are also needed to provide more accurate global estimates of incompatible element abundances stratigraphique (datation relative) ; et étude du degré de dégradation des cratères. that have only been derived so far from orbital data. In addition, heat producing elements existing within LaaLune comporte grandes unitésFor stratigraphiques (du plus ancien au plus récent) : KREEP material may have had large effect on lunar 5volcanic processes. such reasons, it is thus highly recommendedPré-Nectarien, to target areas with Nectarien, high KREEP signatures to gain important information about the Imbrien, Erathosténien et Copernicien. Elles ont été définies à extent and composition of the urKREEP layer, and target deep impact basins that could potentially sample partir des âges de formation des grands bassins d'impacts. Dix-neuf grandes mers lunaires, pristine urKREEP. anciens bassins d'impacts remplis par les laves basaltiques, sont répertoriées (Table 1.2). La répartition des différents types de roches sur la surface lunaire et les unités stratigraphiques associées est présentée sur les deux cartes géologiques lunaires en Annexe 1.
  158
  34
  Evolution temporelle du champ magnétique lunaire Chapitre 1 : La Lune, notre satellite Système Copernicien
  Age Bassins d'impact (Ga) principaux
  Diamètres Volcanisme bassins (km)
  actuel Aucun 0.1
  1.1
  Aucun
  3.15 3.2 3.22 3.3 Schrödinger 3.41
  Imbrien
  Nectarien
  PréNectarien
  3.58 3.72 Orientale 3.74 3.75
  3.8 3.85 3.85 Imbrium 3.87 Serenitatis, Crisium Humorum Nectaris 3.92 Nectaris 3.92 Grimaldi Schiller-Zucchius Smythii, Nubium Fecunditatis, Tranquillitatis 3.7 Australe 4.3 Aitken, Procellarum 4.42
  Formations principales
  Tycho Tycho Flots de Lichtenberg Copernic Copernic Flots jeunes Imbrium Surveyor Oceanus Procellarum Eratosthène
  0.8 Eratosthéni en
  Cratères d'impact principaux
  Mare Crisium Sharp, Atlas Flots vieux Imbrium Humboldt Mare Fecunditatis Archimède Mare Tranquillitatis jeune Platon Sinus iridum Mare Nectaris Piccolomini Mare Serenitatis Arzachel, Cassini Mare Australe, Mare Tranquilitatis vieux Pétavius Maupertuis
  312
  900
  1160 920 / 740 825 340 340 740 / 690
  Helevius Formation
  Apennine Bench Formation Fra Mauro Formation Cayley Formation
  Bailly, Clavius Gauss début du volcanisme Longomontanus
  909 / 700 603 2500 / > 3000
  Ptolémée Hipparchus Maginus
  Descartes Formation
  Janssen Deslandres
  Formation de la Lune
  Tableau 1.2 : Unités stratigraphiques depuis la formation de la Lune avec chronologie des bassins d'impact majeurs, des cratères d'impact principaux et du volcanisme avec formation des mers. Adapté d'après Spudis (1993) ; Stöffler et Ryder (2001) ; Stöffler et al. (2006) et Hiesinger et al. (2011). Les grandes étapes de formation et d'évolution de la Lune peuvent être synthétisées selon l'âge d'occurrence (Figure 1.8) : formation et différenciation ; épisodes volcaniques ; champ de dynamo ; formation des grands bassin d'impacts.
  1.3.3 Discussion sur la stratigraphie Le faible nombre d'échantillons (en terme de quantité et de diversité) ne permet pas de dater avec précision chaque étage stratigraphique. La limite entre le pré-Nectarien et le Nectarien est généralement admise à 3.92 Ga avec la formation du bassin Nectaris. Mais d'autres études placent la limite à 4.1 Ga (Neukum et Ivanov, 1994) ou 4.2 Ga (Wilhelms et al., 1987). La limite entre le Nectarien et l'Imbrien inférieur est également discutée dans la
  Evolution temporelle du champ magnétique lunaire 35
  1 1
  Chapitre 1 : La Lune, notre satellite
  littérature. Elle transite entre 3.92 (Neukum et Ivanov, 1994) et 3.85 Ga (Wilhelms et al., 1987) et entre 3.85 et 3.77 Ga pour Stöffler et Ryder (2001). Puis la transition Erathosténien-Copernicien varie entre 1.5 Ga (Neukum et Ivanov, 1994), 1.1 Ga (Wilhelms et al., 1987) et 0.8 Ga (Stöffler et Ryder, 2001). Stöffler et al (2006) propose une nouvelle échelle de temps pour la stratigraphie lunaire, en incluant les nouvelles datations de certains cratères et des données de la littérature. Les âges de certains bassins d'impacts sont encore débattus et difficilement datables, comme le discute Hiesinger et al. (2011).
  Figure 1.8 (page suivante) : Grandes étapes de l'évolution de la Lune. Les âges associés à chaque étape sont indiqués par leur couleur. En gris, les différents étages de la stratigraphie lunaire ; en violet, période de la formation et différenciation lunaire ; en vert, la présence d'un champ de dynamo ; en rouge, la période de flux et d'éruptions volcaniques ; en bleu, la formation des bassins d'impact ; en orange, les âges des roches rapportées lors des missions Apollo. Certaines époques du champ de dynamo ne sont pas connues, et indiquées par des traits en pointillé. Le champ faible de dynamo est contrait par les données récentes de littérature (Cournède et al., 2012 ; Tikoo et al., 2012,2014,2017 ; Wang et al., 2017)
  36
  Evolution temporelle du champ magnétique lunaire Forma&on du système solaire
  4.568
  4.25
  4
  3.88
  3 3.56 3.29 3.9 3.6 3.08 3.8 3.3
  Flux de volcanisme lunaire
  Grands bassins d'impact
  ?
  Champ fort de dynamo
  Apollo 16
  Apollo 17
  Apollo 12
  3.2
  Apollo 15
  Imbrien
  Apollo 11
  Apollo 14
  Pré-Nectarien N
  4.42 4.3
  Forma&on & différencia&on Lune
  3.92 3.85
  2
  Erup&ons épisodiques
  ?
  Erathosténien
  1.5
  1.2
  1
  ?
  ?
  0
  0
  Copernicien
  Champ faible de dynamo
  1
  Temps (Ga)
  Temps (Ga)
  Chapitre 1 : La Lune, notre satellite 1
  Evolution temporelle du champ magnétique lunaire 37 1
  38
  Chapitre 1 : La Lune, notre satellite
  Evolution temporelle du champ magnétique lunaire Chapitre Deux Le champ magnétique lunaire
  Image schématique de l'intérieur de la Lune et de ses lignes de champ Copyright ©
  Evolution temporelle du champ magnétique lunaire 39 40
  Evolution temporelle du champ magnétique lunaire Chapitre 2 : Le champ magnétique lunaire
  2
  Chapitre 2 Le champ magnétique lunaire Ce chapitre rappelle tout d'abord des notions sur le champ magnétique dont le magnétique terrestre est présenté comme cas général. Dans une seconde partie, l'origine du champ magnétique lunaire est détaillée ainsi que les mécanismes qui pourraient avoir alimenté la dynamo lunaire. Puis une partie sur l'évolution du champ magnétique lunaire permettra de comprendre quelles connaissances et incertitudes nous avons sur ce champ magnétique.
  2.1 Généralités Le champ magnétique terrestre est généralement assimilé à un champ dipolaire (mais des composantes de champ non dipolaire s'additionnent à ce champ dipolaire) situé au centre de la planète. Le vecteur champ magnétique ! se décompose en tout point de l'espace en 3 composantes : Bx (orientée Nord-Sud), By (orientée Est-Ouest) et Bz (d'orientation verticale, positive vers le centre de la planète). Plus généralement, on décrit le champ magnétique en terme d'intensité B et de direction. La direction se divise en déclinaison D (angle entre le Nord géographique et la direction de la composante horizontale positive) et en inclinaison I (par rapport à l'horizontal, positive vers l'intérieur de la planète), comme l'illustre la Figure 2.1. N Bx
  Composante horizontale
  D By
  E
  I B
  Bz
  Z
  Figure 2.1 : Les composantes du champ magnétique. Sur Terre, le champ magnétique n'est pas stable dans le temps, il peut s'inverser. Actuellement, le champ magnétique est normal (inclinaison du champ magnétique négative dans l'hémisphère Sud et positive dans l'hémisphère Nord), la dernière inversion datant de
  Evolution temporelle du champ magnétique lunaire 41 2
  Chapitre 2 : Le champ magnétique lunaire
  772,5 ± 3,5 ka et correspond à la limite Matuyama-Brunhes (Simon et al., 2017). En paléomagnétisme plus spécifiquement, si l'échantillon analysé est orienté, il est possible de retrouver la direction et l'intensité du champ magnétique ambiant au moment où l'échantillon a enregistré ce champ. Dans le cas des roches lunaires, seule l'intensité du champ passé peut être retrouvée car le matériel lunaire (météorite ou roches Apollo) n'ont pu être orientés lors de leur échantillonnage. Cependant, des méthodes pour retrouver la paléohorizontale (à partir de la foliation magnétique de l'échantillon) et donc le paléopôle de l'échantillon ont été développées (Potter et al., 2011) et utilisées, notamment dans les études de Potter et al. (2011) ou de Cournède et al. (2012). La Terre possède un champ magnétique global et est le premier corps planétaire dont les composantes du champ magnétique ont pu être analysées en détail. Que ce soit à l'aide de satellites en orbite autour de la Terre et des observatoires au sol pour des mesures directes et en continue ; ou par des mesures indirectes sur du matériel géologique (coulées volcaniques, sédiments lacustres et marins) et archéologique (terres cuites) ; les évolutions temporelle, directionnelles et d'intensité du champ magnétique peuvent être retracées. Ses champs d'application sont très diverses (variété du matériel d'étude, du lieu géographique étudié et de la période temporelle analysée). L'archéomagnétisme est utilisé, par exemple, dans la datation de certains matériaux (grâce à des courbes de référence de variation du champ magnétique au cours du temps) et s'utilise dans la reconstruction de modèles de variation du champ magnétique (composante directionnelle et d'intensité) (Hervé et al., 2017). En paléomagnétisme, l'étude du champ magnétique permet de reconstruire le mouvement des structures géologiques (à l'échelle locale comme à l'échelle d'une plaque tectonique) ; d'étudier les variations du champ magnétique terrestre (excursions, inversions du champ) au cours des temps géologiques ; ou de dater des évènements géologiques (comme les cratères d'impacts ou des coulées volcaniques). En préliminaire de ce travail de thèse, j'ai par exemple mené une étude paléomagnétique pour dater la structure d'impact de Tunnunik, dans le grand Nord canadien (Lepaulard et al. 2019, en Annexe 2) L'origine du champ magnétique terrestre est connue, il est généré par une dynamo. Des études se concentrent sur l'évolution de ce champ magnétique afin de comprendre les variations de directions du champ (excursions) et l'âge du début de la dynamo terrestre. Biggin et al. (2015) a notamment montré de franches variations de l'intensité du champ sur de différentes périodes temporelles (par exemple, de grandes variations du moment dipolaire ont été notées sur de petites échelles de temps, de 2,5 à 1,3 Ma). Par ailleurs, les variations du Moment Dipolaire Virtuel terrestre témoignent d'une variation d'un facteur 6 sur ces derniers 100 Ma (eg., Biggin et al. 2015), ce qui implique des changements dans le cycle thermique du noyau. D'autres études se concentrent sur l'âge du début de la dynamo terrestre. Tarduno et al. (2010) montre la présence d'un champ magnétique entre 18 et 28 μT vers 3,4-3,45 Ga (en Afrique du Sud avec le craton Kaapvaal). Associés aux données de paléochamps de l'étude de Tarduno et al. (2007), ces résultats indiquent la présence d'une magnétosphère viable tôt dans l'histoire de la Terre et un suggèrent un début de fonctionnement de la dynamo terrestre dès 3,9 Ga.
  42
  Evolution temporelle du champ magnétique lunaire Chapitre 2 : Le champ magnétique lunaire
  2
  La compréhension de l'évolution du champ magnétique terrestre et de son mécanisme (dynamo) permet de faire une analogie pour les autres systèmes planétaires qui ont (comme Mercure, Jupiter ou Saturne) ou ont eu (comme la Lune ou Mars) un champ magnétique global. Dans le cas de la Lune, les mécanismes à l'origine de ce champ magnétique sont encore mal connus, ainsi que l'âge de son arrêt.
  2.2 Origine du champ magnétique lunaire Actuellement, la Lune ne possède plus de champ magnétique global, comme l'ont attesté, dès 1959, les données obtenues par le magnétomètre installé sur la sonde Luna 2. Cependant, les analyses paléomagnétiques préliminaires des échantillons Apollo 11 puis Apollo 12 et 15 ont montré que certains échantillons avaient enregistré des paléointensités de allant jusqu'à 100 μT, indiquant l'existence d'un ancien champ magnétique lunaire (Banerjee et al., 1974b ; Stephenson et al., 1974 ; Collinson et al., 1977). De plus, les mesures satellitaires en orbite autour de la Lune (Clémentine, Lunar Prospector, Kaguya) ont montré la présence d'un champ crustal résiduel en surface. Ce champ n'est pas homogène sur la surface. Les mécanismes à l'origine du champ magnétique lunaire et des anomalies magnétiques restes discutés : généré par une dynamo, par les successions d'impacts météoritiques ?
  2.2.1 Champ magnétique par dynamo Le principe de la dynamo repose sur la conversion d'une énergie cinétique en énergie magnétique et électrique par les processus d'induction. Sous des conditions de réchauffement par la radioactivité ou la différenciation chimique, le noyau liquide externe de la Lune peut passer dans un état turbulent et le fer liquide contenu dans le noyau peut entrer en mouvement (convection). Le mouvement du fer (énergie cinétique) va induire des courants électriques qui vont générer un champ magnétique. Pour que ce système soit autoentretenu et que le champ magnétique perdure, il faut qu'il y ait suffisamment d'énergie dans le noyau pour maintenir ces mouvements de convection. L'élément indispensable pour générer une dynamo est la présence d'une zone fluide, conductrice et en perpétuel mouvement non uniforme (Stevenson et al., 1983). Si le noyau liquide est entièrement cristallisé, les mouvements de convection et l'apport d'énergie magnétique pour l'entretien du champ magnétique s'effondrent. L'existence d'une dynamo lunaire nécessite donc la présence d'un noyau externe métallique liquide.
  2.2.2 Champ magnétique par impacts Il est également possible d'aimanter une roche par un impact (Cisowski et al., 1975), générant une aimantation de choc (Cf. Chapitre 3). En effet, certaines roches Apollo ayant enregistré de fortes intensités de champ magnétique (40-95 μT, Gattacceca et al., 2010a) présentent des indices de choc suite à un impact (Cisowski et al., 1975). L'étude de Gattacceca et al. (2010a) montre notamment que l'aimantation rémanente maximale pouvant être enregistrée par les roches lunaires lors d'un chocs de basse pression (< 10 GPa) n'excède pas 1/3 d'une aimantation acquise par refroidissement thermique sous un même champ ambiant. Cependant, d'autres roches Apollo ont aussi enregistré des intensités de champ
  Evolution temporelle du champ magnétique lunaire 43 2
  Chapitre 2 : Le champ magnétique lunaire
  magnétique du même ordre de grandeur mais sans traces d'impact, ce qui implique que les impacts seuls ne peuvent expliquer l'aimantation acquise dans toutes les roches. Hood et Artemieva (2008) ont montré qu'il y avait une augmentation transitoire du champ magnétique en surface après un impact, expliquant la présence d'anomalies magnétiques dans les bassins d'impact. En effet, la croûte lunaire n'est pas aimantée de façon homogène (Mitchell et al., 2008), des variations de champ magnétique ont été enregistrées sur la surface par les satellites (missions Lunar Prospector, Kaguya) et des anomalies magnétiques sont retrouvées dans certains bassins d'impact (Crisium) ou aux antipodes des grands bassins d'impact (Imbrium, Serenitatis, Crisium, Orientale) (Hood et al., 2001 ; Langlais et al., 2010). Hood et Huang (1991) ont montré qu'un grand impact pouvait générer un nuage de plasma qui serait capable d'amplifier localement le champ magnétique ambiant à l'antipode de l'impact. Les roches aux alentours de cet antipode pourraient alors s'aimanter par choc (SRM) (Gattacceca et al., 2010a) après la convergence des ondes sismiques depuis l'impact vers l'antipode (Hood et Huang, 1991 ; Wieczorek et al., 2001b).
  2.2.3 Champ magnétique crustal La croûte lunaire possède un champ rémanent comme l'ont montré les données satellitaires acquises par les magnétomètres (entre 10 et 45 km d'altitude) lors des missions Clémentine puis plus récemment, Lunar Prospector et Kaguya. Les données de Lunar Prospector (Figure 2.2) et de Kaguya montrent un champ magnétique total majoritairement autour de 2 nT (Tsunakawa et al., 2015). Certaines zones présentent des intensités de champs magnétiques anormalement fortes (jusqu'à 20 nT) ou anormalement faibles (< 0,1 nT). La répartition de ces anomalies apparait très hétérogène (Mitchell et al., 2008) : certaines se concentrent dans les bassins d'impact (comme Moscoviense, Mendel–Rydberg, Humboldtianum et Crisium, Hood et al., 2011), d'autres aux antipodes de ces bassins (comme les bassins Orientale et Imbrium, Hood et al., 2001) et d'autres sans liens avec ces bassins. Certains cratères présentent deux anomalies de champ en périphérie et non pas une centrale, comme Mare Crisium (Baek et al., 2017). Deux grandes zones de champ faibles ont été identifiés : une sur la face visible qui englobe le bassin Mare Imbrium et une au nord-est de la face cachée dans les highlands. La première anomalie peut être liée à la désaimantation partielle du bassin et des environs par l'onde de choc générée lors de l'impact Imbrium (Mitchell et al., 2008). Quelques anomalies fortes sont liées aux processus magmatiques, comme la portion nord-ouest du bassin Sud Aitken dont l'anomalie serait due à la présence de dikes magmatiques qui auraient refroidis en présence d'un champ magnétique global (Purucker et al., 2012). Cependant, la plupart des anomalies magnétiques n'ont pas de lien avec la topographie de surface (impacts) ou la géologie et sont répartis aléatoirement sur la surface. Leur origine est donc incertaine. Des études récentes tentent de retrouver les sources d'aimantation (profondeur, taille, intensité) des anomalies locales de la croûte, à partir de modèles d'inversion, afin de trouver leur origine (Carley et al., 2012 ; Wieczorek et al., 2017). Wieczorek et al. (2017) trouve que la source des aimantations crustales est située entre la surface et 25 km de profondeur. Bien que quelques aimantations crustales puissent être associées avec des dépôts d'impacts près de la surface (Hood, 1981), la majorité des aimantations est localisée à plus 10 km de profondeur. Ces aimantations ne peuvent être issues de magmatismes intrusifs dus à l'absence de volcanisme dans ces régions. Ces aimantations seraient issues de matériaux crustaux aimantés sous un champ magnétique
  44
  Evolution temporelle du champ magnétique lunaire Chapitre 2 : Le champ magnétique lunaire
  2
  global qui auraient refroidis moins de 100 Ma après la formation de la croûte (Arkani-Hamed et Boutin, 1017).
  Figure 2.2 : Modèle en harmoniques sphériques du champ magnétique total à 30 km de la surface, d'après les données Lunar Prospector. La carte obtenue par modélisation est couplée avec des données de topographie lunaire (généré par Y.Quesnel).
  2.3 Mécanismes à l'origine de la dynamo lunaire Le champ magnétique lunaire aurait été le plus probablement généré par une dynamo, de part sa durée dans le temps et avec d'aussi fortes intensités. Le manque de données directes de l'intérieur de la Lune et le manque de connaissances de l'évolution thermique de la Lune font que les mécanismes à l'origine et permettant le maintien de cette dynamo sont encore mal connus. Deux origines sont possibles (Table 2.1) pour la dynamo lunaire : une origine mécanique (par la précession ou les impacts) et une origine convective (convection thermique pure – sèche ou hydratée et convection thermochimique).
  2.3.1 Origine mécanique 2.3.1.1
  Précession
  La précession ne peut exister sans la présence d'un noyau liquide lunaire, qui a été montré par les données sismiques lors des missions Apollo (Wieczorek et al., 2006). La précession est le changement de l'axe de rotation de la Lune, sous l'influence de la Terre. Elle entraine alors un mouvement différentiel entre la base du manteau solide et le noyau liquide de la Lune. Ce mouvement conduit à un brassage mécanique (convection) en continu de ce noyau liquide qui permet de générer une dynamo. Les modèles de précession montrent qu'un champ magnétique peut être généré par la précession, en continu pendant plus de 1
  Evolution temporelle du champ magnétique lunaire 45 2
  Chapitre 2 : Le champ magnétique lunaire
  Ga, avec un champ de surface d'environ 1 μT (Dwyer et al., 2011). Dwyer et al. (2011) montre également par modélisations de dynamo que la précession lunaire a fonctionné sur la période 4,2 – 2,7 Ga, quand la distance Terre-Lune était inférieure ou égale à 48 fois le rayon de la Terre. Selon le modèle d'évolution orbitale terrestre choisi, cette distance est proposée atteinte à 1,8 Ga sous certaines conditions (e.g Dwyer et al., 2011). Weiss et al. (2014) propose que, sous certaines conditions, la précession pourrait perdurer jusqu'à 0,6 Ga mais les modèles ne l'ont pas encore montré. 2.3.1.2
  Impacts
  La dynamo lunaire peut aussi être générée par des impacts d'astéroïdes. S'ils sont de forte amplitude, l'onde de choc provoquera une distorsion tidale au niveau de la limite manteau inférieur – noyau liquide et donc générera des mouvements de matière dans le noyau liquide. Ces mouvements internes peuvent générer une dynamo lunaire temporaire, sur des périodes étalées entre 2000 et 8000 ans (Le Bars et al., 2011) et générer des champs de l'ordre quelques micro-teslas. Les grands bassins d'impact du pré-Nectarien et du Nectarien (décrits dans le Chapitre 1) auraient pu alimenter cette dynamo temporairement, comme le suggèrent les grandes anomalies magnétiques situées au centre des bassins (voir partie 2.2.3). Par exemple, un cratère de 700 km de diamètre pourrait fournir un champ de surface de l'ordre de 0,2 à 4 μT (Le Bars et al., 2011). Le dernier bassin d'impact suffisamment important pour générer une dynamo lunaire est survenu à 3,7 Ga, et marque la date de l'arrêt de la génération d'une dynamo par les impacts.
  2.3.2 Origine convective Il existe deux types de convection : convection thermique uniquement ou convection thermochimique, qui fait intervenir la cristallisation du noyau interne solide. Le champ magnétique produit par convection est relativement stable mais de faible valeur (quelques μT). 2.3.2.1
  Convection thermique
  Les mouvements de convection sont générés par des instabilités thermiques provenant d'une différence thermique entre la base et la surface du noyau liquide lunaire. Ce gradient thermique se concentre sous forme de flux de chaleur. Ce flux de chaleur provient du noyau liquide et en s'évacuant hors du noyau, se retrouve contrôlé par le comportement thermique du manteau inférieur ; le flux total sera assez fort ou non pour provoquer des mouvements de convection. On parle de convection thermique pure quand la cristallisation du noyau n'est pas prise en compte (Nimmo et al., 2007). L'intensité du champ magnétique et sa longévité dépendent directement de l'intensité du flux de chaleur et de l'épaisseur de la cellule de convection (Konrad and Spohn, 1997; Laneuville et al., 2013). Convection avec couche thermique Stegman et al. (2003) suggère qu'une couche dite thermique entre le manteau et le noyau a pu retarder le début de la convection, limitant la quantité de chaleur libérée par le noyau. Les modèles d'évolution thermique de la Lune proposent une convection du noyau suffisante pour générer une dynamo entre 4,2 et 3,5 Ga.
  46
  Evolution temporelle du champ magnétique lunaire Chapitre 2 : Le champ magnétique lunaire
  2
  Convection thermique pure Pour ce type de convection thermique pure, les modèles de l'évolution thermique proposent un refroidissement monotone de la limite noyau – manteau, induisant une persistance de la dynamo entre la période 4,3 et 4,1 Ga seulement (Konrad et Spohn., 1997 ; Laneuville et al., 2013). Convection thermique hydratée Un autre type de convection thermique est proposé par Evans et al. (2014), qui suggère un enrichissement en eau dans le manteau inférieur (40 ppm). Cet enrichissement aurait un rôle de catalyseur du flux de chaleur du noyau vers le manteau et réduirait sa viscosité. Les modélisations de ce type de convection donnent une dynamo plus longue, qui pourrait persister jusqu'à 2,5 Ga. 2.3.2.2
  Convection thermochimique
  La convection thermique peut être renforcée ou modifiée par une convection chimique, suite à la cristallisation du fer dans le noyau liquide. Le liquide résiduel est alors enrichi en éléments légers qui migrent vers la limite noyau- manteau inférieur. Cette cristallisation peut générer un champ de surface de 0,3 μT et dure plusieurs milliards d'années. La présence de 6-8 % en masse de soufre dans le noyau induirait la présence d'un noyau solide de 120-160 km et une persistance de dynamo jusqu'à 1 Ga (Laneuville et al., 2014). Origine de la dynamo Origine mécanique Précession Impacts * Origine convective thermique avec couche thermique thermique pure thermique hydratée thermo-chimique
  Période de fonctionnement (Ga)
  Intensités en surface (μT)
  4.2-2.7 4.2-3.7
  1 Quelques μT
  4.2-3.5
  Quelques μT
  4.3-4.1 4.2-2.5 4.2-1
  Quelques μT Quelques μT <1
  Table 2.1 : Récapitulatif des différents mécanismes pouvant être à l'origine d'une dynamo lunaire. *La dynamo par impacts, signalée par une étoile, fonctionne par intermittence et sur de très courtes périodes suite à un grand impact.
  Evolution temporelle du champ magnétique lunaire 47 2
  Chapitre 2 : Le champ magnétique lunaire
  2.4 Evolution du champ magnétique et incertitudes Les premiers résultats des données paléomagnétiques, d'âges compris entre 4,2 et 1 Ga, ont montré la présence d'un ancien champ magnétique lunaire (e.g., Runcorn, 1973 ; Strangway et al., 1973 ; Dyal et al., 1974 ; Stephenson et al., 1976 ; Banerjee et al., 1976a,1977 ; Daily et Dyal, 1979 ; Cisowski, 1983 ; Collinson, 1993 ; Fuller, 1998), qui a pu être généré par des impacts ou une dynamo. La dynamo lunaire est le mécanisme le plus probable à l'origine du champ magnétique, en terme de durée et d'intensité du champ. Cependant, les modèles utilisant les données paléomagnétiques contraignent encore mal l'évolution du champ magnétique lunaire. C'est l'acquisition de données paléomagnétiques qui va permettre d'enrichir les modèles de dynamo et donc de mieux contraindre la durée de ce champ magnétique et les mécanismes à l'origine de ce champ. Dès le retour des échantillons Apollo, de nombreuses études paléomagnétiques ont été réalisées afin de déterminer la paléointensité enregistrée par ces roches (Banerjee et al., 1974,1976b; Fuller, 1974 ; Stephenson et al., 1974,1977 ; Collinson et Stephenson, 1976 ; Sugiura et al., 1979 entre autres). Ces données ont été compilées (données en gris, Figure 2.3) dans les synthèses de Cisowski (1982) puis Fuller et Cisowski (1987) afin de mieux comprendre l'évolution du champ magnétique lunaire. Une nouvelle vague d'analyses (Lawrence et al., 2008 ; Garrick-Bethell et al., 2009; Cournède et al., 2012; Shea et al., 2012; Tikoo et al., 2012, 2014, 2017; Garrick-Bethell and Weiss, 2013; Suavet et al., 2013; Wang et al., 2017) de ces échantillons est survenue au début des années 2000 (données en bleu, Figure 2.3). Ce nouveau jeu de données est synthétisé dans Weiss et al. (2014) notamment. Au total, c'est une centaine de données de paléointensités (Figure 2.3, Table 2.2) qui ont été acquises depuis ces 40 dernières années, données étalées principalement dans la période entre 4 et 3 Ga. Deux grandes époques se distinguent (Fuller et Cisowski, 1987) : une époque avant 3,5 Ga appelée « époque de champ fort » (High Field Epoch, HFE) et une après 3,5 Ga, appelée « époque de champ faible » (Weak Field Epoch, WFE).
  48
  Evolution temporelle du champ magnétique lunaire Chapitre 2 : Le champ magnétique lunaire
  2
  Figure 2.3 : Compilation des données de paléointensités de la littérature. En gris, les données anciennes (avant 2000) issues de Fuller et Cisowski. (1987) ; en bleu les données récentes (après 2000) compilées à partir des données rassemblées dans Weiss et Tikoo. (2014) ; Tikoo et al. (2017), Wang et al. (2017). Le principal problème des données anciennes de paléointensités (en gris sur la Figure 2.3) est qu'elles ont été acquises par des méthodes parfois peu adaptées aux échantillons lunaires (chauffes en utilisant de la poudre de titane, Suguira et al., 1980 ; chauffes sous vide, Collinson et al., 1973; Dunn and Fuller, 1972; Fuller et al., 1979; Gose et al., 1973; Hoffman et al., 1979; Larson, 1978) ou en cours de développement (Gose et al., 1973). Les phénomènes de choc dans les roches ont commencé à être analysés dans la littérature mais restent mal compris (Cisowski et al., 1975). De plus, l'âge radiométrique des âges ne correspond pas forcément à l'âge de l'acquisition de l'aimantation ; âge qui est contrôlé par datation sur les échantillons récents (voir détails Chapitre 3). Ces données doivent être triées et utilisées avec précaution. C'est pourquoi elles ne seront pas utilisées dans la suite de ce travail.
  Evolution temporelle du champ magnétique lunaire 49 R ES E A RC H | R E V IE W
  2
  Chapitre 2 : Le champ magnétique lunaire
  EA H EEA |ARC R EH V IE|| WR R R ES E A RC H | RREES V IE WRC RES ES RC H REEV VIE IEW R ES E A RCavons H | R Edonc V IE W vu dans la partie précédente Nous
  les mécanismes pouvant générer une E S Echamp A R C H | magnétique. REVIEW dynamo et donc Run Chaque mécanisme pris individuellement ne peut EV VIE WES E A RC H | R E VIE W R ES E A RC H | R of anoth RE IER W o of Si another geologic associatio of expliquer la longue| durée du champ magnétique enregistré dans les roches lunaires. l'on of another geologic association wit ern farsi eli ern farside anomalies: They er ern farside anomalies: They lie on of another geologic ass regarde les données récentes du paléomagnétisme, les mécanismes à l'origine de la edge dynamo edge of t eT of the vast South Pole-A ed ern farside anomalies: edge of of the vast South Pole-Aitken of an ofofanother another geologic associatio another geologic associat geologic associatio lunaire peuvent être répartis selon leur âge (Figure 2.4) : the large th the largest known impact crat th edge of the vast South the largest known impact crater on
  ernldl ern farside anomalies: They ernfarside farside anomalies: They ern anomalies: They This the largest known impa This discovery has led to This discovery has led to two edgel edge of the vast South Pole-A edge of the vast South Pole edge of the vast South Pole-A Convection This discovery has these fo for these anomalies asfor alternat fo for these anomalies as alternatives t thermochimique the the largest known impact cra thelargest largest known impact cr the known impact crat for these anomalies as al plasma f p plasma fields hypothesis. Both pl plasma fields hypothesis. Both prop Th fieldshas hypothesis Thisplasma discovery has ledto t This discovery led This discovery has led to er than etr erfor than reflecting locally high er er than reflecting locally high Bpal eranomalies than reflecting local for these as Précession for these anomalies altern for these anomalies asasalternat alterna manifest manifestation of materials with loca m manifestation materials with m manifestation of materia bis plasma fields hypothesis. Both plasm plasma fieldsof hypothesis. Bot plasma fields hypothesis. Both that were in magnetized in thatthat were magnetized a dynamo f that were were magnetized in a dyna th th 1000 er high er th thanreflecting reflectinglocally locally hi ererthan than reflecting locally hig of these(71) proposals (71) of these proposals identifies th ofmanifestation these proposals (71) identif o of of these of materials wit manifestation materials w man manifestation ofof materials wit with the inferred with the the inferred locations oflocat <4.0 with inferred locations ow w with the that magnetized in thatwere were magnetized adyn dy that were magnetized ininaathat dyn dikes located within the dikes located within the SPA (Fig. 3). dikes located within the SPA (F di d dikes loc of these proposals (71) identi these proposals (71)identi iden great vertical(71) thickness of th ofofthese proposals great vertical thickness (30 km) o great vertical thickness (30 gr gkoo great ver enables them to produc with the inferred locations with the inferred locations with with thetoinferred locations 100 enables themthem produce the strong alies despite the weak NR enables to produce the en et dikes within the SPA (F enables dikeslocated located within the SPA dike dikes located within the SPA (F aliesalies despite theThe weak NRMNRM ofproposa typical despite the second weak ofal atkk great vertical thickness (30 alies desp great vertical thickness (30 grea great vertical thickness (30 TheThe second the proposal anomalies(72) are emp in th second proposal (72) enables them to produce the enables them to produce th The enab enablesejecta them to the produce thes impact mel the anomalies are and in expected alies despite the weak NRM of t the anomalies are in the exp th th alies despite the weak NRM o alies the anom alies despite the weak NRM of impactor (Fig. 3). The ejecta and impact melt derived fr 10 The second proposal (72) ejecta and impact melt deriv ej The second proposal (72 e proposal is that typica Th The second proposal (72) ejecta an impactor (Fig. 3). The key advan the are in the exp which far F impactor (Fig. are 3). The key a im theanomalies are inmore the ex im the the anomalies are in the exp impacto proposal isanomalies that typical chondriti 4 times that of m and 10 ejecta and impact melt deri proposal is that typical chon pr ejecta and impact melt de p eject andmore impact melt deri proposa whichejecta are far Fe-rich, have feldspathic rocks, respe impactor (Fig. 3). The key 4 are far more Fe-rich, impactor (Fig. 3). The key w imp w impactor (Fig. 3). The key which a times that of mare basalts andwhich 10 osition of relatively thin proposal is that typical chon 4a proposal is that typical ch times that of mare ba and 104 rocks, an prop proposal is that typical cho feldspathic respectively. Th 1 and 10 to thousands of meters which are far more Fe-rich which arerocks, far more Fe-ric feldspathic respectivel fe whic which are far more Fe-rich fe have fundamen osition of relatively thin layers (perh 4should that offeldspath mare ba and 1010 4 4times times that of mare and osition of relatively thin layers netic properties on the and times that of mare b and 10 ooss to thousands of meters thick) ofos osition feldspathic rocks, respectivel Fig. 6. Paleointensity measurements of the lunar magnetic field using modern methods andto estithat from more recen feldspathic rocks, respectiv thousands of meters thick felds to feldspathic rocks, respective should have as fundamentally enhanc to to osition ofofrelatively thin layers mated lifetimes of various lunar dynamos. Each point represents measurements of a single Apollo count for most of thous the oth osition relatively thin laye should have fundamentally en ositi sh osition of relatively thin layer netic properties on the surface. SPA sh should sample. Circles represent actual paleointensities, downward arrows represent upper limits on paleoalies. However, thusthick farh totothousands ofofmeters 0.1 6. Paleointensity thousands meters Fig. measurements of the lunar magnetic field using modern methods and estitothic th netic properties onindicating the surface. ne tofrom thousands of meters thick as that more recent impacts, c n intensity, and right arrows represent upper limits on age. Green and blue points were measured using the identified that netic pro should have fundamentally e should have fundamentally Fig. 6. Paleointensity measurements of the Each lunar magnetic field using modern methods and estimated lifetimesIRM of various lunar dynamos. point represents measurements of a single Apollo Fig. 6. Paleointensity Paleointensity measurements of thelimits lunar magnetic field using methods and estishou asshould that from more recent impa have fundamentally ec count for most ofestithe other isolated as Fig. 6. measurements of the lunar magnetic field using modern methods and estimethods, respectively, thelunar upper were derived from the ARMmodern method. afr (top ~30 cm) of the so Fig.and 6. Thellier-Thellier Paleointensity measurements ofand the magnetic field using modern methods and netic properties on the surface as that netic properties on the surfac sample. Circles of represent actual downward represent upper limits on paleomated lifetimes various lunar dynamos. Each point represents measurements ofbox a single Apollo mated lifetimes of various lunar dynamos. Each point represents measurements of aathus single Apollo netic Note the datum atpaleointensities, <7 Ma at the of extreme rightlunar of arrows the dynamos. figure.The shaded green encompasses the mean alies. However, no evidence netic properties on the surface count for most of the other isol co region isfar anomalously ir mated lifetimes various Each point represents measurements of single Apollo co Fig. 6. 6. Paleointensity Paleointensity measurements of the lunar magnetic field using modern methods and estimated lifetimes of various lunar dynamos. Each point represents measurements of as aassingle Apollo that from more recent imp count for Fig. measurements of the lunar magnetic field using modern methods and estithat from more recent im paleointensity value for the period 3.56 to 4.25 Ga (central green line) and its estimated 2 SD uncertainty intensity, and right arrows actual represent upper limits on age. Green and blue points were measured using the Fig. 6. Paleointensity measurements of the lunar magnetic field using modern methods and estiFig. 6. Paleointensity measurements of the lunar magnetic field using modern methods and estisample. Circles represent paleointensities, downward arrows represent upper limits on paleoBoth of these new pr sample. Circles represent actual paleointensities, downward arrows represent upper limits on paleoidentified indicating that at least the as th as that from more recent imp alies. However, thus far no evid al sample. Circles represent actual paleointensities, downward arrows represent upper limits on paleoa mated lifetimes of various lunar dynamos. Each point represents measurements of a single Apollo count for most of the other isois sample. Circles represent actual paleointensities, downward arrows represent upper limits on paleoalies. Ho lifetimes ofand various lunar dynamos. Each point represents measurements of amethod. single Apollo (upper lower lines) (see table S6). The paleointensity value for 76535 (leftmost green point) is measured count for most the other fields rather than IRMmated andand Thellier-Thellier methods, respectively, and the upper limits were from the ARM mated lifetimes of lunar dynamos. Each point represents measurements of anamo single Apollo mated lifetimes of various lunar dynamos. Each point represents measurements of aand single Apollo intensity, right arrows represent upper limits onvarious age. Green and bluederived points were measured using the (top ~30 cm) of the southern fars intensity, and right arrows represent upper limits on age. Green blue points were using the count for most ofof the other iso identified indicating that atcoun lea id intensity, and right arrows represent upper limits on age. Green and blue points were measured using the 0.01 sample. Circles represent actual paleointensities, downward arrows represent upper limits on paleoid alies. However, thus far no evi intensity, and right arrows represent upper limits on age. Green and blue points were measured using the currently not well constrained due to spurious demagnetization effects. Vertical dashed lines show sample. Circles represent actual paleointensities, downward arrows represent upper limits on paleoidentified the magnetic anomalie alies. However, thus far no e Note the datum at <7 Ma at the extreme right of the figure.The shaded green box encompasses the mean sample. Circles represent actual paleointensities, downward arrows represent upper limits on paleosample. Circles represent actual paleointensities, downward arrows represent upper limits on paleoregion is anomalously iron metal-ri IRM and Thellier-Thellier methods, respectively, and the upper limits were derived from the ARM method. alies IRM and Thellier-Thellier methods, respectively, and the upper limits were derived from the ARM method. alies. However, thus far no evi (top ~30 cm) of the southern (t(t IRM and Thellier-Thellier methods, respectively, and the upper limits were derived from the ARM method. intensity, and right arrows represent upper limits on age. Green and blue points were measured using the identified indicating that at lea estimated maximum lifetimes of various proposed lunar dynamo mechanisms: purely thermal convection a dynamo comes from IRM and Thellier-Thellier methods, respectively, and the upper limits were derived from the ARM method. intensity, and right arrows represent upper limits on age. Green and blue points were measured using the (top ~30 identified indicating that at lt paleointensity value for the period 3.56 to 4.25 Ga (central green line) and its estimated 2 SD uncertainty intensity, and right arrows represent upper limits on age. Green and blue points were measured using the Both of these new proposals req intensity, and right arrows represent upper limits on age. Green and blue points were measured using the iden Note the atin <7a Ma the extreme right of<7 the figure.The shaded green themethod. mean identified indicating that at lea Note the datum datum at <7 Ma atthe the extreme right ofbox theencompasses figure.The shaded green box encompasses the mean region isofanomalously iron me re Note the at Ma at the extreme right of the figure.The shaded green box encompasses the mean IRMdatum and Thellier-Thellier methods, respectively, and upper limits were derived from thegreen ARM dry at mantle (with and without an early thermal blanket surrounding the core); impact-driven mantle re many anomalies with (top ~30 cm) of the souther Note the datum at <7 Ma at the extreme right of the figure.The shaded box encompasses the mean IRM and Thellier-Thellier methods, respectively, and the upper limits were derived from the ARM method. (upper and lower lines) (see table S6). The paleointensity value fortoand 76535 (leftmost green point) is derived region is (topthe ~30 cm) ofthe the southe IRM and Thellier-Thellier methods, respectively, and the from upper limits were from ARM method. namo fields rather than plasma fiel IRM Thellier-Thellier methods, respectively, and the upper limits were derived the ARM method. (top paleointensity for period 3.56 to 4.25 Gafor (central green itsGa estimated 2green SD uncertainty (top ~30 cm) of souther paleointensity value for the period 3.56 4.25 Ga (central line) and its 2 SD uncertainty rotational changes, purely thermal convection in3.56 aline) wet precession; and thermochemical core Both of these new Nectarian (i.e., proposal ~3.85 to 3 Noteand thevalue datum atthe <7 Ma at the extreme right of the figure.The shaded green box encompasses the mean paleointensity value the period tomantle; 4.25 (central green line) and its estimated estimated 2 SD uncertainty is anomalously iron mm Note the datum at <7 <7 Ma Ma at the extreme right ofMa the figure.The shaded green box encompasses the mean paleointensity value for the period 3.56 to 4.25shaded Ga (central green line)green and its estimated 2region SD uncertainty currently not well constrained due to spurious demagnetization effects. Vertical dashed lines show region is anomalously iron Both o the magnetic anomalies. Yet more Note the datum at <7 at the extreme right of the figure.The shaded green box encompasses the mean Note the datum at at the extreme right of the figure.The green box encompasses the mean regio convection driven by core crystallization. The horizontal line shows the maximum lunar surface field as region is anomalously iron m (upper and lower lines) (see table S6). The paleointensity value for 76535 (leftmost point) is (upper and lower lines) (see table S6). The paleointensity value for 76535 (leftmost green point) is pre-Nectarian (i.e., mor namo fields rather than plasm na paleointensity value for (upper the period 3.56 to 4.25 Ga (see (central green line) andpaleointensity its estimated 2value SD uncertainty and lower lines) table S6). The for 76535 (leftmost green point) is Both of these new proposa n paleointensity value for the period 3.56 to 4.25lunar Ga (central green line) and its(central estimated 2SD SD uncertainty estimated lifetimes of various proposed dynamo mechanisms: purely thermal (upper and lower lines) (see table S6). The paleointensity value for 76535 (leftmost green point) is 0.001 Both these new propos aVertical dynamo from the recent id from Eq. 2.to The lifetimes of the precession and thermochemical dynamos are highly namo fie paleointensity value for the period 3.56 to 4.25 Ga green line) and itsuncertain. estimated 2comes SD paleointensity value for the period 3.56 to 4.25 Ga (central green line) and its estimated 2convection uncertainty (Fig. 3) (74–76). These Bo Both ofofuncertainty these new proposa currently notmaximum wellestimated constrained due spurious demagnetization effects. Vertical dashed lines show currently not well constrained due to spurious demagnetization effects. dashed lines show the magnetic anomalies. Yet m (upper and lower lines) (see table S6). The paleointensity value for 76535 (leftmost green point) is th namo fields rather than plasm currently not well constrained due to spurious demagnetization effects. Vertical dashed lines show th and lower lines) (see table S6). The paleointensity value for is 76535 (leftmost green point) is in a (upper dry mantle (with and without an early thermal blanket surrounding the core); impact-driven mantle Paleointensity data set is S6). from (14, 18, 50,(see 59–61, 63, 66) and also1.5 listed in table S6. namo fields rather than plas 4.5 4 3.5 3 2.5 2 1 0.5 0 currently not well constrained due to spurious demagnetization effects. Vertical dashed lines show of many anomalies within the inte tized any existing NRM (upper and lower lines) table S6). The paleointensity value for 76535 (leftmost green point) is (upper and lower lines) (see table The paleointensity value for 76535 (leftmost green point) is the mag nam namo fields rather than plasm estimated maximum lifetimes of various proposed lunar dynamo mechanisms: thermal convection estimated maximum lifetimes of various various proposed lunar dynamo mechanisms: thermal convection currently not well constrained due to spurious demagnetization effects.purely Vertical dashed lines show purely a dynamo comes from the rec a the magnetic anomalies. Yet estimated maximum lifetimes of proposed lunar dynamo mechanisms: purely thermal convection a currently not estimated well constrained duenot to well spurious demagnetization effects. Vertical dashed lines show rotational changes, purely thermal convection in constrained a wet mantle; precession; and thermochemical core sheets that should have the magnetic anomalies. Ye Nectarian (i.e., ~3.85 to 3.92 Ga)the bas lifetimes of various proposed lunar dynamo mechanisms: purely thermal convection Fig. 6.in Paleointensity measurements the lunar magnetic field using modern methods and es currently due tomechanisms: spurious demagnetization effects. Vertical dashed lines show currently well due to spurious demagnetization effects. Vertical dashed lines show a dynam Age (Ga) m magnetic anomalies. Yet a dry mantle not (with andconstrained without an early thermal blanket surrounding the core); impact-driven mantle inmaximum dry mantle (withof and without an early early thermal blanket surrounding the core); impact-driven mantle estimated maximum lifetimes of mantle various proposed lunar dynamo purely thermal convection ofathe many anomalies within the of dynamo comes from the rec in aa dry (with without an thermal blanket surrounding thepre-Nectarian core); impact-driven mantle cite Curie point tothe amb persisted in lunar asurrounding weakened state (surface field of toin be able to power avarious dynamo this and late in history or estimated maximum lifetimes of proposed lunar dynamo mechanisms: purely thermal convection convection driven by core crystallization. The horizontal line shows the maximum lunar surface field as a dynamo comes from (i.e., more than ~3.9 estimated maximum lifetimes of various proposed dynamo mechanisms: purely thermal convection a dry mantle (with and without an early thermal blanket the core); impact-driven mantle estimated maximum lifetimes ofconvection various proposed lunar dynamo mechanisms: purely thermalprecession; convection a dy of many a dynamo comes from the re rotational changes, purely thermal in a wet mantle; precession; and thermochemical core in a dry mantle (with and without an early thermal blanket surrounding the core); impact-driven mantle rotational changes, purely thermal convection in a wet mantle; and thermochemical core Nectarian (i.e., ~3.85 to 3.92 Ga of many anomalies within th N mated lifetimes of various lunar dynamos. Each point represents measurements ofanomalies a mantle single Apo peratures far more slo ~2 mT) until sometime after 3.3 Ga. (Figs. 4 and 6).without rotational changes, purely thermal convection in acore); wethighly mantle; precession; and thermochemical core N in aa: dry mantle (with and an early thermal blanket surrounding the impact-driven mantle estimated from Eq. 2. The lifetimes of the precession andblanket thermochemical dynamos are uncertain. of many within t (Fig. 3) (74–76). These impacts Figure 2.4 Bilan des divers mécanismes permettant lemantle; maintien de lacore); dynamo dans leand temps, associés in a dry mantle (with and without an early thermal blanket surrounding the core); impact-driven in dry mantle (with and without an early thermal surrounding the impact-driven mantle rotational changes, purely thermal convection inprecession; amaximum wet mantle; precession; core oflike m of many anomalies within th Nectaria rotational changes, thermal convection in acrystallization. wet and thermochemical core convection driven by corepurely crystallization. The horizontal line shows the lunar surface field asthermochemical convection driven by core core crystallization. The horizontal line shows the maximum lunar surface field as Nectarian (i.e., ~3.85 to 3.92 G These results confirm some Apollo-era concluthicknesses, over athan per pre-Nectarian (i.e., more pr convection driven by The horizontal line shows the maximum lunar surface field as Paleointensity data set is from (14, 18, 50, 59–61, 63, 66) and is also listed in table S6. rotational changes, purely thermal convection in a wet mantle; precession; and thermochemical core p tized any existing NRM, producing Nectarian (i.e., ~3.85 to 3.92 sample. Circles represent actual paleointensities, downward arrows represent upper limits on pale The global context of crustal magnetism rotational changes, purely thermal convection in a wet mantle; precession; and thermochemical core rotational changes, purely thermal convection in a wet mantle; precession; and thermochemical core convection driven by core crystallization. The horizontal line shows the maximum lunar surface field as Nect Nectarian (i.e., ~3.85 to 3.92 G aux estimated données récentes (dès 2009) de paléointensités. Les cercles : valeurs AF ; les losanges : valeurs convection driven by core crystallization. The horizontal line shows the maximum lunar surface field as pre-Nect any impact-generated sions and refute others. Modern paleomagnetic from Eq. 2. The lifetimes of the precession and thermochemical dynamos arethermochemical highly uncertain.dynamos estimated from Eq. 2.The The lifetimes ofshows the precession precession and are highly pre-Nectarian (i.e., more than (Fig. 3) should (74–76). These impact (F estimated from Eq. 2. lifetimes the and thermochemical dynamos are highly uncertain. uncertain. convection driven by core core crystallization. The horizontal lineof thehorizontal maximum lunar surface field as (F sheets that have cooled fro pre-Nectarian (i.e.,as more tha convection driven by core The line the maximum lunar surface field convection driven crystallization. The horizontal line shows the lunar surface field as central basin anomalie Coincident with these new paleointensity exmeasurements have now demonstrated that aalso pre-N estimated from Eq.50, 2. lifetimes ofcrystallization. the precession and thermochemical dynamos are highly uncertain. pre-Nectarian (i.e., more than estimated from Eq. 2. The lifetimes ofThe the precession and thermochemical dynamos areshows highly uncertain. (Fig. 3)tiz ( Paleointensity data setEq. isby from (14, 18, 59–61, 63, and is listed inmaximum table S6. (Fig. 3) (74–76). These impac Paleointensity data set is66) from (14, 18, 50, 59–61, 63, 66) and is also listed in table S6. tized any existing NRM, prod intensity, and right arrows represent upper limits on age. Green and blue points were measured using thermiques ; les rectangles : limites supérieures (voir explications Chapitre 3). La zone grisée Paleointensity data set is from (14, 18, 50, 59–61, 63, 66) and is also listed in table S6. estimated from 2. The lifetimes of the precession and thermochemical dynamos are highly uncertain. cite Curie point to ambient lunar persisted in a weakened state (surface field of to beestimated able to power a dynamo this late in history tit (Fig. 3) (74–76). These impa field like that expecte periments, there have also been important recore dynamo likely existed on the Moon at least estimated from Eq. 2. The lifetimes of the precession and thermochemical dynamos are highly uncertain. from Eq. 2. The lifetimes of the precession and thermochemical dynamos are highly uncertain. (Fig. (Fig. 3) (74–76). These impac PaleointensityPaleointensity data set is from (14,set 18, 50, 59–61, is also listed table S6. listed in table S6. sheets data is from (14,63, 18,66) 50,and 59–61, 63, 66) in and is also tized any existing NRM, prod tized any that should have coole sh Paleointensity set is from (14, 18, 50, 59–61, 63, 66)and and also listedafter in table S6. peratures more slowly (given ~2 mT) until 3.3aux Ga.the (Figs. 4 àand cent investigations of lunar crustal during the Nectarian an asdata far back inchamp time (14, as 4.25 Ga, in contradiction to 18, tized any existing NRM, pro sh la 6). l'époque de fort. Les pointillés gris correspondent dates de finfar de IRMcorrespond and Thellier-Thellier methods, respectively, the upper limits were derived from the ARM metho Paleointensity data set is from (14, 50, 59–61, 63, 66) and is also probables listed in magnetic table S6. Paleointensity data set is from 18, 50, 59–61, 63, 66) and isissometime also listed in table S6. tized tized any existing NRM, prod sheets that should have coole sheets th 4 cite Curie point to ambient persisted in a in weakened (surface field of thicknesses, to beThese able to power dynamo in history ci persisted in aa weakened state (surface field of tothis be able to power a dynamo dynamo this late lateanomalies. in historyAsstate discussed above, a key finding However, once again, thea hypothesis of able alate lateto origin (i.e., after 4.0 Ga). results confirm some Apollo-era concluover ashould period of shee >10 sheets that have coo cltl persisted in weakened state (surface field of to be power a this history sheets that should have coole −1 chaque mécanisme. Modifié d'après Cournède et al. (2012), Suavet et al. (2013), Weiss et Tikoo. cite Curie point tomthe ambient persisted in a the weakened state (surface field of theafter toand be able toOn power a dynamo this late in history The global context of crustal magnetism cite Curi persisted in a weakened state (surface field of to be able to power a dynamo this late in history Note the datum at <7 Ma at the extreme right of the figure.The shaded green box encompasses me peratures far more slowly (g ~2 mT) until sometime after 3.3 Ga. (Figs. 4 6). ) exceed sities (~1 A from Apollo-era measurements was disthe other hand, modern measurements have pe ~2 mT) until sometime 3.3 Ga. (Figs. 4 and 6). any impact-generated field. Ther sions and refute others.aModern paleomagnetic cite3.3 Curie point to ambient persisted in ahistory weakenedpersisted state (surface fieldof of after to be able to power dynamo this late in history history p ~2 mT) until Ga.far (Figs. andlate 6).power cite in asometime weakened state (surface field to be4this able to a dynamo this insometime cite Curie point to ambient persisted in aconcluweakened state (surface field to be able to power peratures more slowly (g ~2 mT) until 3.3 Ga. (Figs. 4etand 6). covery of intense magnetic anomalies, endogenous lun confirmed the existence ofconcluain high-field (mean of late perature ~2several mT)after until sometime after 3.3thicknesses, Ga. (Figs. 4a dynamo and 6). These results confirm some Apollo-era over a of period of These results confirm some Apollo-era th (2014), Tikoo al. (2017), Wang etresults al. (2017). central basinknown anomalies likely req Coincident with new paleointensity exmeasurements have now demonstrated that a 4.25 peratures farmore more slowly ~2 mT)(central until these sometime after 3.3 Ga. (Figs. 44value and 6). These confirm some Apollo-era conclupaleointensity for the period 3.56 to Ga green line) and its estimated 2 SD uncertain th pera ~2 mT) until sometime after 3.3 Ga. (Figs. 4 and 6). peratures far slowly ( ~2 mT) until sometime after 3.3 Ga. (Figs. and 6). These results confirm some Apollo-era concluthicknesses, over a period of including those on the southern farside, that flect the addition of Fe~77 mT from six samples) epoch lasting from at The global context of crustal magnetism The global context of crustal magnetism These results confirm some Apollo-era concluthicknes impact-generated field. sions refute others. Modern sions and refute others. Modern paleomagnetic Theimportant global context ofany crustal magnetism field like that expected aan periments, therecontext have alsoofbeen recoreand dynamo likely existed on thepaleomagnetic Moon at least These results confirm some Apollo-era conclu-recognized thicknesses, over afrom period The global crustal magnetism aoc sions and refute others. Modern paleomagnetic apparently require crustal materials with NRM It has been suggested least 3.85 to 3.56 Ga, as originally These results confirm some Apollo-era concluthick These results confirm some Apollo-era concluthicknesses, over a period any impact-generated field. sions and refute others. Modern paleomagnetic The global context of crustal magnetism (uppermeasurements and lower lines) (see table S6). The paleointensity value for 76535 (leftmost green point) The global context of crustal magnetism any imp sions and refute others. Modern paleomagnetic central basin anomalies Coincident with these new paleointensity exhave demonstrated that a now ce cent investigations of thea crustal magnetic during the Nectarian and expre-Necta as far back in time asnow 4.25 Ga, in contradiction towe Coincident with these new paleointensity measurements have now demonstrated that alunar any impact-generated field sions and refute others. Modern paleomagnetic The global context of crustal magnetism The context of crustal magnetism lack of anomalies anomalies in th larger than that of paleointensity nearly all known Apollo during thenow Apollo era. However, now see thatglobal c Coincident with these new paleointensity ex- likel measurements have demonstrated L'arrêt d'une dynamo est lathat plus probable mais l'âge exact de any sions and refute others. Modern paleomagnetic central basin likel any impact-generated field sions andprogressif refute others. Modern paleomagnetic Coincident with these new exmeasurements have demonstrated that a l'hypothèse field like that expected from periments, there have also been important recore dynamo likely existed on the Moon at least anomalies. As discussed above, a key finding However, once again, the inferred the hypothesis of a late origin (i.e., after 4.0 Ga). central Coincident with these new paleointensity exmeasurements have now demonstrated that a fie periments, there have also been important recore dynamo likely existed on the Moon at least central basin anomalies lik Coincident with these new paleointensity exmeasurements have now demonstrated that a samples (35). Global magnetic field mapsnew from sins (<1 nT at the surf the dynamocore subsequently declined precipitously currently not well constrained due to spurious demagnetization effects. Vertical dashed lines sho fib periments, there have also been important redynamo likely existed on the Moon at least cent Coincident with these paleointensity exmeasurements have now demonstrated that a field like thatanomalies expected like from there have also important recoreest dynamo likely existed on the Moon(Weiss atthat leasta et periments, central basin Coincident with these newbeen paleointensity exmeasurements havemal now demonstrated sonasarrêt encore modélisé al., 2014). De plus, les incertitudes liées à−1Nectarian ) exceed what ispreach sities (~1 Athe m from the Apollo-era measurements was important the disOn hand, modern measurements have cent investigations of the lunar crustal magnetic during and far the backother in time as 4.25 Ga, infar contradiction to field lik thethere Lunar Prospector spacecraft, which only becentral impact melt laye tocore <∼4 mT byas 3.2 Ga. In particular, modern analyperiments, there have also been important redynamo likely existed on the Moon at least cent investigations of the lunar crustal magnetic du back in time as 4.25 Ga, in contradiction to field like that expected fro periments, have also been recore dynamo likely existed on the Moon at least cent investigations ofpurely the lunar crustal magnetic d as core far in time asleast 4.25 inthe contradiction to investigations of the lunar crustal magnetic during the−3 Nectarian andindic preas fardynamo back inses time asexisted 4.25 Ga, in contradiction to Ga,cent field periments, there have also beenthermal important redynamo likely existed on Moon atdynamo least field like that expected from periments, there have also been important recore likely onback the Moon at −1 estimated maximum lifetimes of various proposed lunar mechanisms: convecti A lunar m ) may ~10 came available 6anomalies. years (8,of 29, 68), have now of samples 3.3 Gacontradiction younger that covery of investigations several intense anomalies, known endogenous materials the of4.25 athe high-field (mean anomalies. As discussed above, aago key finding However, again, the inf theconfirmed hypothesis late origin (i.e., after 4.0 l'intérieur de laofexistence Lune et àthe son évolution thermique ne permettent pas encore de prédire avec As discussed above, aaonce key finding H hypothesis ofGa). aoflate late origin (i.e., after 4.0 Ga).ofmagnetic cent investigations the lunar crustal magnetic during th far back in time asor 4.25 Ga, insuggest toAs cent the lunar crustal magnetic during the Nectarian and pr as back inaas time as Ga, in tocontradiction anomalies. As discussed above, key finding H hypothesis of a origin (i.e., after 4.0 Ga). anomalies. discussed above, a key finding However, once again, the inf thefar hypothesis of a late origin (i.e., after 4.0 Ga). cent investigations of the lunar crustal magnetic duri as far back in time as 4.25 Ga, in contradiction to cent investigations of the lunar crustal magnetic during the Nectarian and pre as far back in time as 4.25 Ga, in contradiction to −1 active by the confirmed the unique size and intensity of the addition or all epoch of thethe Apollo-era paleointensity analyincluding those on the southern farside, that flect the of−1)Fe-rich impacto mT from six most samples) lasting from atmodern exceed what sities (~1no A longer m from the4.0 Apollo-era measurements was the disOn~77 thethe other hand, modern measurements have si from the Apollo-era measurements was the disOn other hand, measurements have anomalies. As discussed above, a key finding However, once again, the i hypothesis ofand a late origin (i.e., after 4.0 Ga). anomalies. As discussed above, a key finding However the hypothesis of a late origin (i.e., after Ga). in aprécision dry mantle (with without an early thermal blanket surrounding the core); impact-driven man la fin de la dynamo lunaire. De plus, l'absence de données entre 3 et 1,5 Ga rend la ) exceed what sities (~1 A m from the Apollo-era measurements was the disOn the other hand, modern measurements have si from the was the dis-Ga). On thehypothesis other hand, have anomalies. As discussed above, a(i.e., key finding How the of amodern lateapparently origin (i.e.,require after 4.0 Ga). anomalies. As discussed above, a Apollo-era key finding However, once again, the in the hypothesis of a as late origin (i.e., after 4.0 Ga). southern farside anomalies and theirmeasurements spatial after ~3.85 H ses Ga, from this late epoch, which ranged up measurements to apcrustal materials with NRM Itknown has been suggested (29) that leastOn 3.85 to 3.56 recognized covery of the several intense magnetic anomalies, endogenous lunar mat confirmed the existence of aoriginally high-field (mean ofupper wha sities (~1 Aconsistent m−1−1) exceed from Apollo-era measurements was theintense disthe other hand, modern measurements have covery of several magnetic anomalies, kn confirmed the existence of acovery high-field (mean ofwith sities (~1 from the Apollo-era measurements was the disOn the other hand, modern measurements have of several intense magnetic anomalies, known endogenous lunar ma confirmed the existence of a high-field (mean of correlation antipodes of four, or perhaps not with th parent values of 20 mT, are only limits on sitie from the Apollo-era measurements was the disOn the other hand, modern measurements have covery of several intense magnetic anomalies, k confirmed the existence of a high-field (mean of ) exceed what sities (~1 A m from the Apollo-era measurements was the disOn the other hand, modern measurements have compréhension de la transition HFE-1.5 Gaoflarger difficile. rotational changes, purely thermal convection inalasting athose wet mantle; precession; and thermochemical co lack ofknown anomalies in the centers of than that of nearly all known Apollo the Apollo era. However, we now see that including on the southern farside, that flect the addition ofofthat Fe-rich im ~77during mTconfirmed from samples) epoch lasting from at covery of several intense magnetic anomalies, endogenous lunarpa m the existence of amT high-field (mean including those on the southern farside, fle ~77 from six samples) epoch from at covery several intense magnetic anomalies, known confirmed the existence of a high-field (mean of dynamo from even five (69), the eight basins. the lunar field therefore do not require those on southern farside, that flect thecore addition Fe-rich im ~77 mT six from six samples) epoch lasting from at covery ofyoungest several intense magnetic anomalies, know confirmed the existence ofepoch aincluding high-field (mean oftheofof covery ofGlobal several intense magnetic anomalies, known endogenous lunar ma confirmed the existence of aand high-field (mean ofsamples including those on theHowsouthern farside, that fle ~77 mT from six samples) lasting from at (35). magnetic field maps from sins (<1 nT at the surface, sugges the dynamo subsequently declined precipitously including those on the southern farside, that flect the addition offlect Fe-rich ~77 mT from six samples) epoch lasting from atas apparently require crustal materials with NRM It has been suggested (29) least 3.85 to 3.56 Ga, as originally recognized apparently require crustal materials with NRM least 3.85 to 3.56 Ga, originally recognized ever, the acquisition of the first high-resolution 3.56 to 3.85 Ga (Imbria dynamo at this time. Finally, a recent analysis convection driven by core crystallization. The horizontal line shows the maximum lunar surface field including those on the southern farside, that the ~77 mT from six samples) epoch lasting from at apparently require crustal materials with NRM It has been suggested (29) least 3.85 to 3.56 Ga, as originally recognized those onthat the southern that flect ~77 mT from sixfrom samples) epoch lasting fromspacecraft, including those onatthe including southern farside, flect thefarside, addition of Fe-rich im ~77mT mTbyfrom six In samples) epoch lasting apparently require crustal materials with NRM least 3.85 to 3.56 Ga,atas originally recognized the Lunar Prospector which only becentral impact melt layer has an N to <∼4 3.2toof Ga. particular, modern analyglobal topography data in 1993 by the Clemenbasalts above). In a potentially very young lunar sample (66) apparently require crustal materials with NRM It has been(see suggested (29 least 3.85 3.56 Ga,However, as originally recognized lack of anomalies in the larger than that of nearly all known Apollo during the Apollo era. However, we now see that laf larger than that of nearly all known Apollo during the Apollo era. However, we now see that apparently require crustal with NRM Itcente has least 3.85 3.56 Ga, as originally recognized of anomalies in the cent larger that of nearly all known Apollo during the Apollo era. we now see that −3lack −1 apparently require crustal materials with NRM It least 3.85 to 3.56 Ga, as originally recognized apparently require crustal materials with NRM Itm has been suggested (29) least 3.85 to 3.56 Ga, asto originally recognized la larger than that ofmaterials nearly all known Apollo during the Apollo era. However, wethan now see that estimated from Eq. 2. The lifetimes of the precession and thermochemical dynamos are highly uncerta A ) may indicate that the ~10 came available 6 years ago (8, 29, 68), have now ses of samples 3.3 Ga or younger suggest that tine laser altimeter (70) led to the identification bital data sets are cons hints that the dynamo may have nevertheless lack of anomalies in theofcen larger than that of nearly allfield known Apollo during the Apollo era.declined However, we now see thatwe samples (35). Global magnetic field maps from sins (<1 nT the surface, su the dynamo subsequently precipitously samples (35). Global magnetic field maps si the dynamo subsequently declined precipitously samples (35). Global maps from sins (<1 nTat atApollo thefrom surface, su the dynamo subsequently declined lack a larger than of nearly alllonger known Apollo during Apollo era.precipitously However, now see that larger than that of nearly all known during the Apollo we now see that lack of anomalies in theoflack cent larger than that of magnetic nearly allthat known Apollo during the Apollo era.the However, we now seeera. thatHowever, samples (35). Global magnetic field maps from si the dynamo subsequently declined precipitously no active by the time the confirmed the unique size and intensity of the most or all of the Apollo-era paleointensity analysamples (35). Global magnetic field maps from sins (<1 nT at thelayer surface, the dynamo subsequently declined precipitously theprecipitously Lunar Prospector spacecraft, which only becentral impact melt has <∼4 bydata 3.2by Ga. InGa. particular, modern analythe Lunar Prospector spacecraft, which only beces to <∼4 mT byprecipitously 3.2 Ga. In 59–61, particular, modern analyPaleointensity set isIn from (14, 18, 50, 63, 66) and issamples also listed in table S6. the Lunar Prospector spacecraft, which only becentral impact melt layer has tomT <∼4 mT 3.2 particular, modern analysamples (35). Global magnetic field maps from sins (<1 the dynamo subsequently declined 50 to ses Evolution temporelle du champ magnétique lunaire (35). Global magnetic field maps from sins the dynamo subsequently declined precipitously samples (35). Global magnetic field maps from sins (<1 nT at the surface, the dynamo subsequently declined the Lunar Prospector spacecraft, which only bec to <∼4 mT by 3.2 Ga. In particular, modern analysouthern farside anomalies and their spatial (i.e., after ~3.85 Ga). However, thi from this late epoch, which ranged up to ap−3 −1−1 the Lunar Prospector spacecraft, which only be- ago central impact melt layertha ha <∼4 mT by 3.2 Ga. Inyounger particular, modern analy1246753-8 5 DECEMBER 2014 •Ga VOLor 346 ISSUE 6214 s m ) )may indicate ~10 came 6 years ago (8, 29, 68), have now ses ofto samples 3.3 Ga orGa suggest that ~1 came available 6 years (8,−3A 29, 68), have now ses of samples 3.3 younger suggest that A m may indicate tha ~10 came available 6the years ago (8, 29, 68), have ses of samples 3.3 or suggest that Lunar Prospector spacecraft, which only central to <∼4 mTses by 3.2 Ga. In particular, modern analythe Lunar Prospector spacecraft, which only betoyounger <∼4 mTmodern by 3.2 Ga. Inor particular, modern analytheavailable Lunar Prospector spacecraft, only becentral impact melt layercent has to <∼4 mT of by 3.2 Ga. In particular, analy−1becorrelation with antipodes of four, orwhich perhaps not consistent with the strong ev parent values 20 mT, are only upper limits ~ti came available 6 now years ago (8,−3 29, 68), have now of samples 3.3on Ga younger suggest that A m ) may indicate ~10 came available 6 years ago (8, 29, 68), have now ses of samples 3.3 Ga or younger suggest that −3 − −3 −1 no longer active by the time o confirmed the unique size and intensity of the most or all of the Apollo-era paleointensity analyno confirmed the size and ofindicate the most or all of the Apollo-era paleointensity analyno longer active by confirmed unique size and intensity of the most all ofand the3.3 Apollo-era paleointensity analy~10 A ~10time came available 6unique years ago (8, 29,have 68), have now of3.3 samples 3.3 Ga even or suggest younger suggest that available 668), years ago (8, 29, 68), now ses of Ga samples Ga orApollo-era younger that Aintensity mfrom ) may th ~10 came available 6came years ago (8, 29, have now ses oforfield samples orses younger suggest core dynamo paleomagnetic five (69), ofthe the eight youngest basins. Howthe most lunar therefore do not require a that n confirmed the unique size and intensity ofbythe most or all of the paleointensity analyno longer active the tim confirmed the unique size and intensity of the or all of the Apollo-era paleointensity analysouthern farside anomalies and their spatial (i.e., after ~3.85 Ga). Howeve ses from this late epoch, which ranged up to apsouthern farside anomalies and their spatial (i.e., after ~3.85 Ga). Howeve ses from this late epoch, which ranged up to apsouthern farside anomalies and their spatial (i. ses from this late epoch, which ranged up to appersisted in weakened state (surface field to be able tofrom power a Apollo-era dynamo this late inap-history no lo confirmed the unique sizeintensity and intensity theno most or all analysis oflate theepoch, Apollo-era paleointensity analyno longer active by the time confirmed unique size and intensity of anomalies the most or this all oftime. the paleointensity analylonge confirmed thea unique size and of theofspatial most or all Apollo-era paleointensity analy-the ever, theranged acquisition of the first high-resolution 3.56 to 3.85 Ga (Imbrian-aged) Apo dynamo at Finally, athe recent southern farside and their (i sesof from this which up to apsouthern farside anomalies and their spatial (i.e., after ~3.85 Ga). Howe ses this late epoch, which ranged up tomT, correlation with antipodes of four, or perhaps not consistent with the stro parent values of mT,only are only upper limits on correlation with antipodes ofby four, or perhaps not consistent with the stron parent values of 20very mT,20 are upper limits on correlation with antipodes of four, or perhaps no parent values of 20 are only upper limits on southern farside anomalies and their spatial (i.e., ses from this late epoch, which ranged up to apsouthern farside anomalies and their spatial (i.e., after ~3.85 Ga). Howev ses from this late epoch, which ranged up to apsouthern farside anomalies and their spatial (i.e., afte ses from this late epoch, which ranged up to apglobal topography data in 1993 the Clemenbasalts (see above). In fact, the sam of a potentially young lunar sample (66) correlation with antipodes of four, or perhaps n parent values of 20 mT, are only upper limits on ~2 mT) until sometime after 3.3 Ga. (Figs. the 4 and 6). correlation with antipodes offive four, orof perhaps notdynamo consistent with the str parent values oftherefore 20 mT, aredolunar only upper limits on core dynamo from paleoma of(70) the youngest basins. Howthethat lunar field therefore do not require atine core from paleomag even five (69), of the eight youngest basins. Howlunar field and require a therefore co even (69), the eight youngest basins. Howthe field and dofive not(69), require aeight with antipodes of or four, or perhaps not parent values of 20 mT, are even only upper limits on correlation with antipodes four, or perhaps not consistent with theifcons stro parent values ofand 20 mT, are only upper limits on laser altimeter led tocorrelation theofwith identification bital data sets are consistent the hints the dynamo may have nevertheless correlation antipodes of four, perhaps not parent values ofnot 20 mT, are only upper limits on Impacts
  Convection thermique pure avec couverture thermique
  Précession Convection thermique hydratée
  3.7 Ga
  4.2 Ga
  1 Ga
  1.8 Ga
  2.5 Ga
  2.7 Ga
  3.5 Ga
  Paleointensities (μT)
  Convection thermique pure Chapitre 2 : Le champ magnétique lunaire
  2
  Sample
  Age (Ga)
  Dating method
  Dating references
  Paleointensity (μT)
  Method
  Measures references
  Basalt 10003 10017 10017+ 10020 10020+ 10022 10047 10049 10049+ 10050 10050 10057 10057 10069 10069 10072 12002 12002+ 12017 12018 12022 12022 14053 14053+ 14072 15016 15058 15495 15499 15535 15597 70017 70017+ 70035 70135 70215 70215+ 71055 71505+ 71567+
  3,91 3,037 3,037 3,77 3,77 3,59 3,74 3,56 3,56 3,75 3,75 3,63 3,63 3,68 3,68 3,52 3,21 3,21 0,007 0,65 3,11 3,11 3,95 3,95 4,04 3,29 3,36 3,36 3,34 3,3 3,35 3,8 3,8 3,7 3,75 3,84 3,84 3,64 3,7 3,75
  Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Rb-Sr Rb-Sr Rb-Sr Rb-Sr Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar† Ar-Ar† Ar-Ar Ar-Ar Ar-Ar Rb-Sr Ar-Ar Ar-Ar Rb-Sr Ar-Ar† Ar-Ar†
  [1] [2] [2] [3] [3] [4] [5] [2] [2] [3];[5] [3];[5] [6] [6] [6] [6] [1] [1] [1] [8] [7] [1] [1] [9] [9] [1] [10] [11] [12] [11] [13] [13] [15] [15] [1] [16] [1] [1] [17] [13] [13]
  6,8 19,3 71,0 58,3 66,0 1,8 18,4 7,0 77,0 28,2 38,0 6,8 14,0 5,0 3,6 50,0 4,5 50,0 7,0 7,4 4,8 9,2 34,4 20,0 44,9 15,3 4,9 2,2 35,3 7,6 13,2 50,0 42,0 37,6 92,0 2,0 20,0 43,0 95,0 111,0
  sIRM sIRM sIRM sIRM sIRM thellier-thellier sIRM shaw sIRM sIRM ARM sIRM ARM thellier-thellier sIRM ARM thellier-thellier sIRM ARM sIRM thellier-thellier sIRM sIRM sIRM sIRM sIRM ARM ARM sIRM ARM sIRM thellier-thellier sIRM sIRM sIRM thellier-thellier sIRM single pTRM sIRM sIRM
  [24] [24] [2] [24] [32] [33] [24] [34] [2] [24] [35] [24] [35] [33] [33] [36] [37] [38] [8] [24] [40] [40] [24] [38] [24] [24] [42] [42] [24] [42] [24] [35] [38] [24] [24] [35] [38] [43] [38] [38]
  Evolution temporelle du champ magnétique lunaire 51 2
  Chapitre 2 : Le champ magnétique lunaire
  3,85 3,85 3,79 3,7 3,8
  Rb-Sr Rb-Sr Ar-Ar Ar-Ar† Ar-Ar
  [18] [18] [19] [13] [20]
  3,3 28,0 14,0 68,5 5,3
  single pTRM sIRM thellier-thellier sIRM sIRM
  [43] [38] [44] [24] [24]
  Dimict breccia 67915 3,9 68815 3,5 72275 4,01
  Ar-Ar Ar-Ar Rb-Sr
  [1] [21] [1]
  4,7 40,6 19,0
  sIRM sIRM ARM
  [1] [24] [45]
  Feldspatic rock 60015 60015 60015 76535+
  3,5 3,5 3,5 4,25
  Ar-Ar Ar-Ar Ar-Ar Ar-Ar
  [1] [1] [1] [1]
  15,3 33,0 5,0 40,0
  sIRM ARM thellier-thellier sIRM
  [24] [46] [47] [48]
  Impact melt breccia 14066 3,93 14305 3,92 14306 3,9 15455 4,42 60315 3,78 61016 4,1 62235 3,79 62235 3,79 62295 3,86 62295 3,86 65015 3,98 68416 3,85 68416 3,85 72215 3,87 72255 3,85 73235 3,97 77017 3,91 77017 3,91 77115 3,85 77135 3,9 77135 3,9
  Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar Ar-Ar
  [22] [23] [24] [22] [25] [1] [25] [25] [25] [25] [10] [1] [1] [13] [1] [1] [15] [15] [1] [26] [26]
  4,7 1,7 0,9 18,1 40,0 3,3 86,2 132,0 7,5 50,0 45,4 120,0 51,3 41,0 35,0 1,1 4,1 81,0 9,7 31,7 16,0
  sIRM sIRM sIRM sIRM single pTRM sIRM sIRM thellier-thellier sIRM single pTRM sIRM ARM sIRM shaw ARM thellier-thellier sIRM single pTRM sIRM sIRM single pTRM
  [24] [24] [24] [24] [49] [24] [24] [35] [24] [49] [24] [35] [35] [50] [45] [51] [24] [43] [24] [24] [43]
  Impact melt rock 68415 3,85 68415 3,85
  Ar-Ar Ar-Ar
  [27] [27]
  54,2 5,0
  sIRM thellier-thellier
  [24] [46]
  74275 74275+ 75035 78505 79155
  52
  Evolution temporelle du champ magnétique lunaire Chapitre 2 : Le champ magnétique lunaire
  Regolith breccia 14318 3,88 15015 0,5 15015+ 0,5 15465+ 1,19 15498 1,32 15498 1,32 15498+ 1,32 60255 1,7 70019 0,175
  Ar-Ar‡ Ar-Ar Ar-Ar Ar-Ar Ar-Ar‡ Ar-Ar‡ Ar-Ar‡ Ar-Ar‡ U-Pb
  [28] [29] [29] [30] [28] [28] [28] [28] [31]
  0,7 7,0 0,4 0,01 6,7 2,2 5,0 0,5 1,2
  sIRM sIRM thellier-thellier thellier-thellier sIRM thellier-thellier thellier-thellier thellier-thellier thellier-thellier
  2
  [24] [24] [30] [30] [52] [52] [39] [53] [54]
  Table 2.2 : Récapitulatif des données de paléointensités de la littérature et leurs âges respectifs. L'échantillon 14306 a un âge qui correspond à la formation du bassin Serenitatis (Cisowski et al., 1983). Pour les âges notés « † », j'ai utilisé des âges moyens, caractéristiques de la lithologie de la roche et de la mission Apollo associée (Stöffler et al., 2006). Les âges notés « ‡ » proviennent d'âges « trapped argon » (ArTr) recalculés par Fagan et al. (2014). Les échantillons notés « + » sont les données récentes de la littérature (après 2008). [1] Turner (1977)
  [19] Turner and Cadogen (1974)
  [37] Hesley et al. (1972)
  [2] Suavet et al. (2013)
  [20] Kirsten and Horn (1974)
  [38] Cournède et al. (2012)
  [3] Geiss et al. (1977)
  [21] Schaeffer et al. (1977)
  [39] Tikoo et al. (2017)
  [4] Alexander et al. (1972)
  [22] Alexander and Davis (1974) [40] Helsley et al. (1971)
  [5] Guggisberg et al. (1979)
  [23] Eugster et al. (1984)
  [41] Tikoo et al. (2014b)
  [6] Papanastassiou et al. (1970)
  [24] Cisowski et al. (1983)
  [42] Banerjee and Mellema (1974)
  [7] Stettler et al. (1973)
  [25] Norman et al. (2006)
  [43] Brecher et al. (1974)
  [8] Tikoo et al. (2012)
  [26] Stettler, 1974
  [44] Sugiura et al. (1978)
  [9] Turneret al. (1971)
  [27] Huneke et al. (1973)
  [45] Banerjee and Swits (1975)
  [10] Kirsten et al. (1973)
  [28] Fagan et al. (2014)
  [46] Stephenson and Collinson (1974)
  [11] Hussain et al. (1974)
  [29] Meyer (2012)
  [47] Lawrence et al. (2008)
  [12] Papanastassiou et al. (1973)
  [30] Wang et al. (2017)
  [48] Garrick-Bethell et al. (2017)
  [13] Stöffler et al. (2006)
  [31] Nunes et al. (1975)
  [49] Brecher et al. (1973)
  [14] Kirsten et al. (1972)
  [32] Shea et al. (2012)
  [50] Banerjee and Mellema (1976)
  [15] Phinney et al. (1975)
  [33] Helsley et al. (1970)
  [51] Watson et al. (1974)
  [16] Nyquist et al. (1975)
  [34] Fuller et al. (1979)
  [52] Gose et al. (1973)
  [17] Tera et al. (1974)
  [35] Stephenson et al. (1974)
  [53] Sugiura and Strangway 1980
  [18] Murthy and Coscio (1977)
  [36] Chowdary et al. (1987)
  [54] Sugiura et al. (1979)
  Evolution temporelle du champ magnétique lunaire 53 2
  Chapitre 2 : Le champ magnétique lunaire
  2.5 Conclusion Ce chapitre de thèse a permis de voir les différents processus qui peuvent être à l'origine du champ magnétique lunaire passé, la dynamo étant le plus probable. Les mécanismes potentiellement à l'origine de cette dynamo ont été décrits. Les enjeux de ce travail de thèse sont donc d'acquérir de nouvelles données de paléointensités afin de mieux comprendre l'évolution et la durée du champ magnétique lunaire et les mécanismes associés.
  54
  Evolution temporelle du champ magnétique lunaire Chapitre Trois Matériel et méthode en magnétisme
  Echantillons lunaires analysés au Johnson Space Center et au laboratoire (CEREGE). Sont représentés, dans l'ordre, un regolith breccia (10018), un basalt (12005), un second basalt (10057) dans un double emballage de téflon et une dimict breccia (61015). Copyright © Camille Lepaulard
  Evolution temporelle du champ magnétique lunaire 55 56
  Evolution temporelle du champ magnétique lunaire Chapitre 3 : Matériel et méthodes en magnétisme
  3
  Chapitre 3 Matériel d'étude et méthodes en magnétisme Ce chapitre présente les différents échantillons lunaires analysés lors de ce travail de thèse. Il introduit également les notions de base du magnétisme des roches et notamment de l'aimantation naturelle des roches lunaires. Les minéraux magnétiques présents dans les roches lunaires sont ensuite présentés, ainsi que les méthodes de détermination de la paléointensité.
  3.1 Échantillons lunaires Au cours de ce travail de thèse, deux grandes études ont été menées. La première a consisté en l'analyse de 161 roches Apollo au Johnson Space Center (Houston, USA). Elle est décrite dans le Chapitre 4. La seconde a consisté en l'analyse de vingt-sept roches lunaires au laboratoire de magnétisme des roches du CEREGE. Elle est décrite dans le Chapitre 5.
  3.1.1 Détail des échantillons analysés au Johnson Space Center Cent soixante-et-une roches Apollo ont été analysées dans le Lunar Sample Laboratory Facility, lieu de stockage des échantillons Apollo, au Johnson Space Center à Houston en 2016. Depuis leur retour sur Terre, seulement 5% des échantillons Apollo ont été mesurés en laboratoire. Cette étude a eu pour but d'acquérir de nouvelles données sur les propriétés magnétiques afin de regarder le potentiel paléomagnétique de roches dont les masses étaient supérieures à 50 g et de lithologie bréchique. Les échantillons étaient rangés dans des boites en mumétal dans des étagères (Figure 3.1.a), sous un mélange d'azote afin de ne pas les altérer. Chaque échantillon était lui-même dans un double emballage de téflon, mis également sous azote afin d'éviter leur contamination avec l'atmosphère terrestre (Figure 3.1.d-e). Nous avons mesuré l'aimantation rémanente naturelle (NRM) et la susceptibilité magnétique de 123 et 154 échantillons respectivement, à l'aide d'un magnétomètre portatif et d'un susceptibilimètre portatif (Figure 3.1.b) développés spécialement au CEREGE. Avant chaque mesure, les échantillons ont été placés dans des cubes en plexiglasse (Figure 3.1.c) afin de mesurer les propriétés des roches sous trois directions perpendiculaires (détails Chapitre 4). Le centrage de chaque échantillon dans les cubes a été permis par des traits perpendiculaires tracés sur chaque cube et de l'emballage plastique a été utilisé pour caler les échantillons dans les cubes. Le détail de ces instruments est décrit dans le Chapitre 4.
  Evolution temporelle du champ magnétique lunaire 57 3
  Chapitre 3 : Matériel et méthodes en magnétisme
  a) Etagère de stockage des échantillons
  c) Echantillon 71035, avant analyse
  b) Magnétomètre et susceptibilimètre
  d) Echantillon 14169, sous téflon
  e) Echantillon 61015, sous téflon
  Figure 3.1 : Présentation des conditions de mesures au JSC. a) Présentation du meuble de stockage des échantillons Apollo, sous azote ; b) Magnétomètre portatif (gauche) et susceptibilimètre portatif (droite). L'ordinateur de contrôle est situé entre les deux appareils, eux même éloignés des zones de stockage des échantillons ; c) Echantillon centré dans un cube en plexiglasse avant analyse au magnétomètre et au susceptibilimètre ; d-e) Echantillons 14169 et 61015, conditionnés dans un double emballage de téflon et sous azote.
  Les lithologies analysées sont reportées dans la Figure 3.2. Ces roches étaient principalement des impact melt breccia ou des regolith breccia. La masse des roches analysées était comprise entre 21 et 4688 g avec une majorité entre 100 et 500 g (Figure 3.2).
  58
  Evolution temporelle du champ magnétique lunaire Chapitre 3 : Matériel et méthodes en magnétisme
  3
  Feldspathic rocks
  80
  Basalt Impact Melt Rock Dimict breccia Regolith breccia
  Nombre d'échantillons
  60
  IMB
  40
  20
  0
  0
  50
  500
  100
  1000
  2000
  5000
  Masse (g)
  Figure 3.2 : Répartition des masses des roches Apollo analysées selon leur lithologie.
  3.1.2 Description des échantillons analysés en laboratoire Vingt-cinq roches Apollo et deux météorites lunaires ont été analysées en laboratoire afin de déterminer des valeurs de paléointensités (détails Chapitre 5). Sur ces vingt-sept échantillons, neuf (indiqués par un astérisque dans la liste) proviennent de la pré-sélection basée sur les mesures préliminaires (détaillées dans la partie précédente et le Chapitre 4) au Johnson Space Center (JSC), Houston (Lepaulard et al., submitted – Chapitre 4). Les autres échantillons ont été prêtés par différents laboratoires de recherche français (IPG Paris, LMV Clermont-Ferrand, CRPG Nancy) pour des mesures magnétiques avant leurs analyses géochimiques destructives. Ces échantillons, de masse comprise entre 1 et 42 gr, sont de lithologies différentes (voir Chapitre 1, les classifications des roches lunaires) : - 16 basaltes (échantillons Apollo 10003, 10017, 12005*, 12012, 12016, 12018, 12021*, 14078, 15529*, 15555, 70017, 70135, 74255, 74275 et 75075; la météorite lunaire LAP02205) - 4 brèches de régolithe (échantillons Apollo 10018*, 15505*, 61195* et la météorite NWA10782) - 1 impact melt breccia (échantillon Apollo 14169*) - 1 dimict breccia (échantillon Apollo 61015*) - 1 impact melt rock (échantillon Apollo 65055*) - 4 roches feldspathiques (échantillons Apollo 60015, 60215, 62255 et 65315)
  Evolution temporelle du champ magnétique lunaire 59 Chapitre 3 : Matériel et méthodes en magnétisme
  3
  Les planches photographiques de tous les échantillons sont visibles dans l'Annexe 3.
  3.1.3 Datation des échantillons L'acquisition de nouvelles données de paléointensités permet de retracer l'évolution du champ magnétique lunaire en fonction du temps si on connaît l'âge de l'enregistrement du champ magnétique par la roche. L'aimantation est portée par les minéraux ferromagnétiques. Les minéraux porteurs de l'aimantation sont essentiellement la kamacite mais aussi la taénite ou le fer métallique. Leurs températures classiques de blocage sont de 780°C, 600°C et 765°C respectivement (Fuller, 1974 ; Wasilewski, 1982 ; Garrick-Bethell et al., 2010). Ces températures de blocage sont souvent inférieures aux températures de remise à zéro des chronomètres utilisés pour les datations (Ar/Ar, etc). La durée de l'événement de surface (impacts, mise en contact de la roche avec une nappe de roche fondue) influe également sur la température nécessaire pour la remise à zéro du chronomètre. En général, cette température atteint 600-700°C. Si l'âge radiométrique est remis à zéro, l'âge de l'aimantation sera généralement aussi remis à zéro (e.g McDougall et al., 1988), mais l'inverse n'est pas forcément vrai. La plupart des roches Apollo ont été datées lors de leur caractérisation initiale dans les années 70 (voir synthèse dans Turner et al., 1977). La majorité de ces âges sont obtenus par K/Ar ou 40Ar/39Ar, méthodes les plus couramment utilisées dans la datation de roches lunaires. Des âges moyens par lithologie et mission Apollo ont été obtenus à partir de la compilation d'âges réunis dans Stöffler et al. (2006). Fagan (2014) apporte également de nouveaux âges pour des brèches de régolithes, par « ArTr » (pour trapped argon), qui correspondent à des âges de fermeture du système à partir du rapport d'isotopes piégés 40Ar/36Ar. L'avantage des datations utilisant l'argon est que ce dernier a tendance à diffuser hors des minéraux facilement et le système peut se remettre à zéro plus facilement qu'avec un autre élément (notamment lors d'un impact de météorite). D'autres méthodes de datation ont été employées dans la littérature et témoignent plutôt de l'âge de cristallisation de la roche : Nd/Sm, U/Pb et Rb/Sr (e.g discussion dans Turner et al., 1977). Elles peuvent aussi être considérées comme moins robustes car moins précises en terme d'âge obtenu (pas d'âge plateau contrairement aux datations 40Ar/39Ar modernes par stepheating) et en terme de technique de mesure (datations anciennes, avec éventuellement l'utilisation de protocoles obsolètes ou d'instruments moins précis que les instruments modernes). Pour les brèches, qui constituent une proportion importante des échantillons lunaires et qui permettent d'accéder à des périodes de l'histoire magnétique de la Lune qui sont encore peu connus (transition entre période de champ fort et de champ faible, arrêt de la dynamo), les âges sont souvent acquis sur des zircons ou des clastes qui indiquent malheureusement l'âge de formation de ces éléments, mais qui ne témoignent pas forcément de l'âge de l'acquisition de l'aimantation par la roche (Jessberger et al., 1974). Récemment, la datation par sonde laser (Mercer et al., 2015), avec ses analyses spatialement résolues, a montrée sa capacité à mieux caractériser la chronologie complexe de ces roches, et de différencier les divers événements que la roche a subi, en particulier dans les IMB, qui peuvent contenir de nombreuses veines de roche fondues. C'est l'âge de la partie fondue de la roche qui nous renseigne sur l'âge de l'acquisition l'aimantation. Cinq échantillons ont été daté dans le cadre de ce travail de thèse après leur analyse 40
  39
  paléomagnétique : deux échantillons (basaltes 12005 et 15529) par la technique Ar / Ar et trois échantillons (regolith breccia 10018, IMB 14169 et regolith breccia 15505) par la technique 40
  39
  Ar / Ar mesurée par sonde laser. La datation par la technique par sonde laser a nécessité la
  60
  Evolution temporelle du champ magnétique lunaire Chapitre 3 : Matériel et méthodes en magnétisme
  3
  réalisation de cartes des échantillons, au microscope électronique à balayage (MEB) afin d'identifier les zones riches en potassium pour les analyses. Ces trois cartes sont visibles dans l'Annexe 4. L'échantillon 12005 ne donne pas de plateau bien défini et les paliers hautes températures indiquent une domination du 36Ar par une source cosmogénique. De ce fait, seul un âge minimal de l'échantillon a pu être estimé, à 2,8 Ga. Les deux sous-échantillons de 15529 donnent un âge de plateau bien défini à haute température, de moyenne 3,15 ± 0,007 Ga. (Les résultats des datations obtenues sont détaillés dans le Chapitre 5). Les échantillons 10018 et 14169 donnent des âges moyens d'assemblage de la brèche à 1,53 ± 0,09 Ga et 3,88 ± 0,16 Ga respectivement. L'échantillon 15505 semble donner un âge moyen d'assemblage à 0,47 ± 0,13 Ga. Ces résultats sont en cours d'analyse et l'âge du 15505 est sujet à s'affiner.
  3.1.4 Préparation des échantillons en laboratoire Sur les vingt-sept échantillons analysés en laboratoire, neuf échantillons, de 2 g chacun environ, ont été collés avec de la cire à basse température de fusion (40°C) ou de la colle cyanoacrylate sur un disque en aluminium, puis découpés à la scie à fil diamanté (diamètre de fil 220 μm). Des cubes d'environ 3x3x3 mm ont été obtenus pour des analyses en laboratoire. La dimension des cubes est adaptée de telle sorte que la NRM de l'échantillon -9
  2
  soit supérieure à 1.10 Am pour être d'au moins deux ordres de grandeur supérieurs au bruit de fond du magnétomètre à SQUID utilisé pour les mesures. Tous les morceaux sont orientés mutuellement d'après l'orientation du bloc de 2 g avant découpe, comme illustré sur la Figure 3.3. Bulk sample 10018 (2.3 g)
  B A
  C
  Sample 10018-B
  Sample 10018-B1 (140 mg)
  D
  a
  1
  b
  2 3 mm
  3 4 N Down
  3 mm
  N
  N
  E
  E
  E Down
  Down
  Figure 3.3 : Exemple de découpe de la brèche de régolithe 10018. Tous les sous-échantillons sont mutuellement orientés. L'échantillon est découpé en tranches avec une scie à fil diamanté (gauche). La tranche B, dont l'orientation est gardée, est découpée en 4 morceaux (milieu). Puis le sous-échantillon 1 est à nouveau découpé en 2. Chaque sous-échantillon a été collé sur un disque en quartz (Figure 3.4.a) pour les mesures en champ alternatif ou placé dans des cylindres en quartz et bloqué avec de la laine de verre pour les mesures thermiques (Figure 3.4.b). Les moments des disques et cylindres ont été mesurés et se situent autour de 5.10 aux NRM mesurés des échantillons.
  -11
  2
  Am et 1.10
  -11
  2
  Am respectivement, soit très inférieurs
  Evolution temporelle du champ magnétique lunaire 61 3
  Chapitre 3 : Matériel et méthodes en magnétisme
  a) 12021-C (220 mg)
  b) 12021-A1 (287 mg)
  1 cm
  0.5 cm
  Figure 3.4 : Préparation d'un échantillon pour analyse. a) Echantillon 12021 sur un disque de quartz pour analyse AF; b) Echantillon 12021 dans un cylindre en quartz pour analyse thermique. Ainsi, chaque échantillon possède au moins 2 sous-échantillons qui ont été analysés par désaimantation progressive en champ alternatif et/ou par désaimantation progressive lors de chauffes successives à des températures croissantes (détails dans la suite de ce Chapitre).
  3.2 Magnétisme des roches lunaires 3.2.1 Propriétés magnétiques de l'atome à la roche L'aimantation d'une roche tire son origine du mouvement des électrons. A l'échelle atomique, la combinaison du mouvement des électrons autour des noyaux atomiques et du moment magnétique du spin d'un électron provoque des courants microscopiques dans la matière. Ces courants sont définis comme le moment magnétique (en Am2), et proviennent du non-appariement des spins des électrons. La somme des moments magnétiques constitue -1
  l'aimantation volumique (normalisée au volume, en Am ) ou massique (normalisée à la masse, en Am2kg-1) (Dunlop et Özdemir, 1997). Dans de très petites particules (<40 nm), les spins des électrons sont essentiellement alignés de façon parallèle et génèrent une aimantation uniforme. Ces grains sont appelés mono-domaines (single domain, SD) (Figure 3.5.a). Dans de plus grandes particules (>200 nm), l'énergie produite par les électrons est trop grande pour permettre une aimantation uniforme de la particule qui formera des parois de Bloch pour compartimenter l'aimantation des spins dans différents domaines (Figure 3.5.b). La particule est dite poly-domaines (multidomain, MD). Les spins sont alors alignés de façon parallèle. Les particules au comportement entre le SD et le MD sont des pseudo-monodomaines (pseudo-single domain, PSD) (Day et al., 1977). Si les domaines ne sont pas répartis de façon parallèle dans la particule, le champ magnétique résultant est alors nul (Figure 3.5.c), on parle de domaines de fermeture.
  62
  Evolution temporelle du champ magnétique lunaire Chapitre 3 : Matériel et méthodes en magnétisme a)
  b)
  3
  c)
  Figure 3.5 : Répartition des domaines dans une particule avec le champ généré en bleu et le spin en rouge. a) particule mono-domaine, b) particule multi-domaines, c) particule contenant deux domaines et deux domaines fermés, le champ résultant étant nul. Sous l'effet d'un champ inducteur, un corps peut acquérir une aimantation induite qui s'annulera en cas d'arrêt de ce champ. Cette capacité s'appelle la susceptibilité magnétique. Sa mesure permet de montrer le type de comportement magnétique que possèdent les minéraux à l'échelle cristalline : le ferromagnétisme, le diamagnétisme et le paramagnétisme. En dessous de leur température de Curie, certains minéraux ont une aimantation rémanente issue de l'alignement des spins non appareillés. Ces minéraux ont un comportement ferromagnétique sensu lato. Au-delà de sa température de Curie, un minéral ferromagnétique sensu stricto devient paramagnétique. Un minéral paramagnétique ne possède pas d'aimantation spontanée mais sous l'application d'un champ extérieur, les spins vont s'aligner selon la direction de ce champ. L'intensité de l'aimantation va diminuer avec la température et si le champ disparaît, les spins reprennent leur orientation aléatoire, il n'y a pas de rémanence. Dans le cas de diamagnétisme, les grains vont générer une aimantation faible sous la présence d'un champ externe, dans le sens inverse au champ, qui va disparaître avec l'arrêt du champ. Il n'y a pas de phénomène de rémanence. Le ferromagnétisme se divise en trois comportements principaux selon l'orientation des spins : ferro- sensu stricto, ferri- et antiferro-magnétisme (Dunlop et Özdemir, 1997) selon l'alignements des spins des électrons et leur réaction face à un champ magnétique extérieur (Figure 3.6). Des spins alignés, de façon parallèles et dans le même sens, donnent le ferromagnétisme sensu stricto. Un alignement antiparallèle des spins donne l'antiferromagnétisme. Parfois, l'alignement des spins n'est pas parfait en position antiparallèle et donne un antiferromagnétisme incliné dont le moment résultant est faible. C'est le cas par exemple de l'hématite. Enfin, quand les spins sont alignés de façon antiparallèle mais avec des différences de valence au sein de l'atome (et donc de moment), on parle de ferrimagnétisme. Sous l'influence de la température (en dessous de la température de Curie du grain), des grains ferro- ou ferri- magnétiques peuvent devenir superparamagnétiques (SP). Dans ce cas, sous l'effet d'un champ extérieur, les grains acquièrent une aimantation.
  Evolution temporelle du champ magnétique lunaire 63 Chapitre 3 : Matériel et méthodes en magnétisme
  3
  a)
  b)
  c)
  d)
  Figure 3.6 : Différents types d'alignement des spins dans le ferromagnétisme sensu largo. a) ferromagnétisme sensu stricto, b) antiferromagnétisme, c) antiferromagnétisme incliné, d) ferrimagnétisme.
  3.2.2 Minéraux magnétiques lunaires 3.2.2.1
  Description Les minéraux magnétiques lunaires sont différents des minéraux terrestres habituellement trouvés. Les minéraux magnétiques lunaires les plus communs sont essentiellement métalliques, composés de fer, nickel, cobalt. Rochette et al., (2010) précise notamment la nature exacte et la teneur des minéraux des roches lunaires. Le fer est souvent présent sous forme d'alliage dans les roches ou associé à des sulfures et rarement sous forme pure. Le fer (métallique Fe0 ou contenu dans les minéraux) peut avoir diverses origines : origine magmatique, provenir des météorites, provenir de la décomposition de la troilite suite à des impacts (fer secondaire) ou encore provenir du verre d'impact (trouvé sous forme de grains sphériques ultrafins). Le fer est souvent associé avec une petite proportion de nickel, donnant une grande variété d'alliages Fe-Ni comme la kamacite ou la taénite majoritairement, l'awaruite secondement. Ces alliages vont dépendre de la phase du fer, de sa vitesse de refroidissement et de sa teneur en nickel. Alliages fer-nickel : La kamacite est issue de l'alliage α-(Fe-Ni), avec plus de 90% de Fe. C'est l'un des minéraux, avec la troilite, les plus trouvés dans les roches lunaires. La taénite est l'alliage γ(Fe-Ni) avec une proportion de Ni entre 20 et 65% et est trouvé dans les météorites lunaires. L'awaruite peut être trouvée associée à la kamacite et se compose de Ni2Fe à Ni3Fe. Autres alliages du fer : Associé au soufre, le fer donne Fe-S et forme de la troilite. Associé au carbone, le fer donne α'-(Fe-C) et forme de la martensite, trouvé dans les météorites lunaires. Le fer peut s'associer au chrome : FeCr2O4 pour former de la chromite. Associé au titane, le fer donne FeTiO3 et forme de l'ilménite. 3.2.2.2
  Identification des grains magnétiques
  L'observation en lumière réfléchie des neuf lames minces correspondant aux neufs échantillons Apollo sélectionnés du Lunar Sample Laboratory Facility (LSLF) permet d'identifier les minéraux magnétiques principaux contenus dans les échantillons (Figure 3.7). Le niveau de choc a également été déterminé par observation en lumière transmise (polarisée
  64
  Evolution temporelle du champ magnétique lunaire Chapitre 3 : Matériel et méthodes en magnétisme
  3
  analysée), afin d'estimer la pression de choc maximale que la roche a pu subir après sa cristallisation. Les minéraux magnétiques observés en lame mince sont du fer métallique inclus ou non dans des grains de troilite (minéral trouvé dans toutes les lames, en proportion variable). De l'ilménite est vue dans certaines lames (61195, 65055). a) 10018 (regolith breccia)– Lumière naturelle
  b) 10018 – Lumière polarisée analysée
  c) 14169 (IMB)– Lumière naturelle
  d) 15529 (basalt)– Lumière naturelle
  Figure 3.7 : Observation de lames minces de certains échantillons Apollo au microscope optique. a) Apollo 10018, vue en lumière naturelle. Le point bleu montre une sphérule de verre, b) Apollo 10018, vue en lumière polarisée analysée, c) Apollo 14169, vue en lumière naturelle, d) Apollo 15529, vue en lumière naturelle. Les flèches rouges pointent quelques minéraux magnétiques (fer, troilite supposés). 3.2.2.3
  Propriétés magnétiques des roches lunaires
  Le ferromagnétisme s.s. dans les roches lunaires est dominé par le fer métallique présent sous forme d'alliages avec du cobalt ou du nickel (Fuller et al., 1974). La kamacite est le minéral principal porteur de l'aimantation sur la Lune. Il est relativement stable face aux perturbations thermiques lunaires et face aux perturbations lors du stockage sur la Terre (Garrick-Bethell et Weiss, 2010). Les autres minéraux ferromagnétiques sont souvent associés au fer métallique (Fe α) dans les brèches des highlands ou les régolithes (Rochette et al., 2010). La martensite et la taénite sont également des minéraux ferromagnétique. La chromite et l'ilménite ont un comportement paramagnétique selon la température d'observation. La troilite, qui fait partie du groupe des pyrrhotites (comme l'hématite sur la Terre), est présent en abondance dans les roches lunaires et possède un comportement antiferromagnétique (Rochette et al., 2010).
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  Les basaltes lunaires possèdent en général des grains de fer métallique polydomaines tandis que le sol lunaire possède principalement des mélanges de grains monodomaines et superparamagnétiques (Fuller et al., 1987). Les brèches d'impact ont plutôt des grains poly-domaine tandis que les brèches de régolithe ont des grains superparamagnétiques (Fuller et al., 1987).
  3.2.3 Etude du magnétisme des roches lunaires La susceptibilité magnétique des échantillons a été mesurée à l'aide d'un instrument MFK1 (champ de 200 Am-1, fréquence 976 Hz, sensibilité de 5.10-13 m3) à température ambiante. Toutes les mesures de rémanence réalisées au laboratoire de magnétisme du CEREGE ont été mesurées par un magnétomètre à SQUID (2G Enterprises, model 755R, bruit -11
  2
  de mesure 10 Am ). L'aimantation naturelle des roches a été mesurée et désaimantée par paliers par champ alternatif, jusqu'à 200 mT ou par paliers lors de chauffes successives à des températures croissantes (600°C). Chaque palier de désaimantation par champ alternatif, suivi de sa mesure, a été répété trois fois. Les mesures d'aimantations résultantes ont été moyennées afin de réduire les effets parasites pouvant survenir lors des désaimantations par champ alternatif. Chaque palier de température a été mesuré une fois, n'étant pas influencé par les effets du champ alternatif. La structure en domaines des grains magnétiques a été caractérisée par la mesure de cycles d'hystérésis (Figure 3.8) grâce à un magnétomètre à échantillon vibrant (sensibilité ~10 nAm2) et des électroaimants permettant d'appliquer des champs jusqu'à 1 T. Le champ nécessaire pour réduire l'aimantation rémanente à saturation (Mrs) de l'échantillon à zéro est le champ coercitif rémanent (Bcr). M : moment (Am2) 3.10-4 Ms 2.10-4
  1.10-4
  -1
  -0.8
  -0.6
  -0.4
  -0.2
  Mrs Bcr
  0.2
  0.4
  0.6
  0.8
  1
  Bc
  -1.10-4
  -2.10-4
  -3.10-4
  66
  Paramètres du 10018-A2a Bc = 6 mT Bcr = 42 mT Ms = 1.73 Am2kg-1 Mrs = 0.15 Am2kg-1
  Evolution temporelle du champ magnétique lunaire
  B : champ (T) Chapitre 3 : Matériel et méthodes en magnétisme
  3
  Figure 3.8 : Cycle d'hystérésis de l'échantillon 10018 après correction de la pente sur la plage -1/1 T. La courbe de rémanence, en bleue, permet d'obtenir la valeur du Bcr.
  Le rapport de Mrs sur Ms en fonction du rapport Bcr sur Bc permet de placer les points sur diagramme Mrs/Ms vs Bcr/Bc, classiquement appelé diagramme de Day (Day et al., 1977) et d'estimer, selon les valeurs des deux rapports, le domaine des grains de l'échantillon. Un diagramme de Day pour les roches lunaires est présenté dans le Chapitre 5.
  3.3 Aimantation rémanente naturelle des roches Les minéraux ferromagnétiques ont la capacité d'acquérir une rémanence, c'est-à-dire qu'ils peuvent conserver une aimantation qui a été acquise dans un champ après l'arrêt de ce dernier. On parle alors d'aimantation rémanente naturelle (Natural Remanent Magnetization, NRM). Selon son mécanisme d'acquisition, la NRM aura des propriétés différentes.
  3.3.1 Aimantation thermorémanente L'aimantation thermorémanente (ThermoRemanent Magnetization, TRM) est acquise par refroidissement d'une roche volcanique depuis sa température de Curie en présence d'un champ magnétique. Cette aimantation peut être partielle (Partial ThermoRemanent Magnetization, pTRM) si la roche refroidit à partir d'une température inférieure à sa température de Curie (Thellier et Thellier, 1959).
  3.3.2 Aimantation rémanente de choc Description Le second type d'aimantation qui peut être enregistrée par les roches lunaires est l'aimantation rémanente de choc (Shock Remanent Magnetization, SRM) lors du passage d'une onde de choc en présence d'un champ magnétique. Plusieurs processus ont été mis en évidence : (i) l'aimantation primaire pré-choc peut être effacée partiellement ou complètement par désaimantation en présence d'un champ faible ou nul (Gattacceca et al., 2010a) ; (ii) une SRM peut être acquise par choc en présence d'un champ ambiant (Gattacceca et al., 2008) et (iii) le choc peut modifier les propriétés magnétiques de la roche (sa susceptibilité magnétique par exemple ; Gattacceca et al., 2005 ; 2008). Tout comme une TRM, la SRM est parallèle au champ ambiant et son intensité proportionnelle à ce champ au moment de l'impact (Gattacceca et al., 2010b). L'étude de Tikoo et al. (2015) montre notamment que les SRM sont difficiles à préserver dans les échantillons après un impact et sont donc peu observées dans les échantillons lunaires. En laboratoire Comme attendu pour un processus isotherme, la SRM se désaimante facilement par champ alternatif, mais ses températures de déblocage peuvent aller jusqu'à la température de Curie des minéraux (Tikoo et al. 2015). La pression générée par le choc peut être identifiée et estimée à partir notamment de l'analyse pétrographique de lames minces des échantillons : minéraux fracturées, veines et poches de matériel fondu, déformation du
  Evolution temporelle du champ magnétique lunaire 67 3
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  réseau cristallin de l'olivine sont autant d'indicateurs de choc (Stöeffler et al., 1991). La plupart des roches ayant été aimantés par choc ont subit des chocs de faible pression (< 10 GPa). Ainsi, une SRM de faible pression (< 10 GPa) peut atteindre, dans certaines conditions, jusqu'à un tiers de la TRM acquise à champ équivalent dans certaines roches riches en Fe-Ni (Gattacceca et al., 2010a).
  3.3.3 Aimantation rémanente visqueuse Description L'aimantation rémanente visqueuse (Viscous Remanent Magnetization, VRM) est un processus activé thermiquement qui consiste en l'acquisition au cours du temps et à température constante d'une aimantation rémanente (dont l'intensité est proportionnelle à log t). Par exemple, elle peut être acquise par les roches lunaires si elles restent en présence d'un champ magnétique faible (que ce soit le champ ambiant sur la surface lunaire ou sur Terre depuis leur retour). Quand le champ magnétique est nul, l'aimantation visqueuse acquise décroit avec le temps. Ce type d'aimantation est généralement secondaire et se superpose à la première aimantation (TRM ou SRM dans le cas lunaire) enregistrée par la roche. En laboratoire Comme cette aimantation croît en fonction du logarithme du temps, il est possible de déterminer en laboratoire dans quelle proportion la VRM contribue à la NRM. Pour cela, une VRM artificielle est acquise en fonction du temps dans un champ de direction connue et d'intensité de l'ordre du champ terrestre (dizaines de μT). Le taux de décroissance en fonction du temps de la VRM est ensuite mesuré en plaçant l'échantillon en champ magnétique nul (Enkin and Dunlop, 1980). Cette aimantation visqueuse acquise depuis des années (sur Terre) voire des millions d'années (sur la Lune) est facilement effacée par des champs alternatifs faibles ou des chauffes à relativement basse température (voir Chapitre 5). Nos expériences d'acquisition (pendant 2 mois) et de décroissance de la VRM sur 9 échantillons sont présentées dans le Chapitre 5).
  3.3.4 Aimantation rémanente chimique Les grains ferromagnétiques qui cristallisent à basse température peuvent acquérir une aimantation rémanente chimique (Chemical Remanent Magnetization, CRM) plutôt qu'une TRM. Cette aimantation peut également s'acquérir par l'altération d'un minéral préexistant en un minéral ferromagnétique (Dunlop et Özdemir, 1997).
  3.3.5 Aimantation gyro-rémanente Description Lors de la désaimantation par champ alternatif d'un échantillon, le moment magnétique des grains va se retourner et changer de direction. Cette rotation du moment peut prendre une forme gyroscopique et entrainer l'acquisition d'une aimantation gyrorémanente (Gyro-Remanent Magnetization, GRM) par l'échantillon, de direction perpendiculaire au champ AF appliqué et perpendiculaire à l'alignement préférentiel des grains de l'échantillon. Cette aimantation non désirée peut s'ajouter à la NRM et rendre sa désaimantation instable.
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  Evolution temporelle du champ magnétique lunaire Chapitre 3 : Matériel et méthodes en magnétisme
  3
  En laboratoire Des études ont permis de mettre au point un protocole pour corriger ce phénomène lors de la désaimantation de la NRM par champ alternatif (Stephenson et al., 1993 ; Finn and Coe, 2016).
  3.3.6 Aimantation rémanente isotherme L'aimantation rémanente isotherme (Isothermal Remanent Magnetization, IRM) est acquise par l'échantillon lorsqu'il est soumis à un champ fort (> mT). Naturellement, elle peut être acquise suite à un foudroiement de la roche. Elle peut être donnée artificiellement, volontairement ou involontairement, si l'échantillon est placé près d'une source générant un champ très fort (bobine, aimant) à côté de l'échantillon. En laboratoire, l'IRM est volontairement donnée en exposant l'échantillon à une bobine (pulse magnetizer) afin de regarder notamment la composition et les domaines portés par les minéraux magnétiques d'une roche. Lors de leur découverte par les « chasseurs de météorites », elle peut être donnée involontairement sur certaines météorites lunaires. En effet, ces derniers utilisent des aimants pour les distinguer des autres roches.
  3.4 Détermination des paléointensités 3.4.1 Evaluation de la qualité des échantillons Dans tous les cas, il faut que la composante primaire soit une TRM. Il faut également que cette composante soit stable thermiquement. Généralement, cette composante primaire tend vers l'origine lors de la désaimantation. Cette composante stable est appelée composante caractéristique (ChRM). Les désaimantations des échantillons lunaires et leurs différentes composantes sont détaillées dans le Chapitre 5 et rassemblées dans l'Annexe 5. Les roches lunaires sont sujettes à l'enregistrement d'aimantations secondaires de type IRM ou VRM (lors de leur retour ou de leur stockage sur Terre qui se fait malheureusement dans le champ magnétique terrestre). Ces composantes secondaires peuvent oblitérer plus ou moins totalement l'aimantation primaire, rendant sa caractérisation difficile pour la détermination de la paléointensité (Dunlop et Özdemir, 1997).
  3.4.2 Protocoles R et REM' La paléointensité peut également être obtenue par désaimantation de la NRM de l'échantillon par champs alternatifs et normalisation par une aimantation de laboratoire : ARM ou IRM. Cette ARM (ou IRM) est également désaimantée en champ alternatif aux mêmes paliers que ceux utilisés pour la désaimantation de la NRM. Puis la NRM désaimantée est comparée à l'ARM (ou IRM) aux mêmes paliers afin de calculer une paléointensité. C'est une méthode alternative aux méthodes thermiques dérivées de la méthode de Thellier et Thellier (1959), et utilisée pour les échantillons dont la minéralogie magnétique est instable lors des chauffes ou qui ne peuvent tout simplement pas être chauffé pour des raisons de curation. De plus, c'est une méthode non destructive, qui peut se révéler intéressante pour l'étude de certains types d'échantillons (par exemple, pour les échantillons qui doivent être ensuite détruits pour des analyses chimiques).
  Evolution temporelle du champ magnétique lunaire 69 3
  Chapitre 3 : Matériel et méthodes en magnétisme
  La première méthode utilisée (notée R) est la méthode de normalisation de la NRM par une ARM, développée notamment par Stephenson and Collinson (1974) ; Stephenson et al. (1977) puis reprise par Kohout et al. (2008) et Tikoo et al. (2012, 2014). Cette méthode a été appliquée sur 11 échantillons dont les résultats sont décrits dans le Chapitre 5. ∆!"# !
  PARM =
  .
  ∆!"# !!
  (1)
  où ∆NRM est la composante stable préalablement définie (appelée ChRM) de haute coercivité (HC) et ∆ARM la composante établie sur le même intervalle ; b est le champ alternatif utilisé lors de l'acquisition de l'ARM ; f' est une constante de calibration, dont la valeur dépend du minéral magnétique porteur de l'aimantation (Stephenson and Collison, 1974). Dans cette étude, nous utilisons la valeur de 1,34, qui correspond à la valeur moyenne pour les roches lunaires (Stephenson et Collinson, 1974). La seconde méthode, connue sous le nom de REM', est basée sur la normalisation de la NRM par une SIRM précédemment acquise, développée par Cisowski et al. (1983) et adaptée par Gattacceca et Rochette (2004) et Muxworthy and Heslop (2011). Cette méthode a été appliquée sur 10 échantillons dont les résultats sont décrits dans le Chapitre 5.
  PIRM =
  ∆!"# ∆!"#
  .!
  (2)
  où ∆NRM est la composante stable préalablement définie (appelée ChRM) et ∆IRM la composante établie sur le même intervalle; a est la constante de calibration estimée à 3000 μT (Gattacceca and Rochette, 2004). Cette constante de calibration est une valeur moyenne issue de calculs sur différents types d'échantillons (Apollo, échantillons synthétiques lunaires, roches terrestres) et de minéraux (Supplementary material, Weiss et Tikoo, 2014). L'incertitude associée au résultat de la méthode ARM est d'environ un facteur 2 (Supplementary material, Weiss et Tikoo, 2014). L'incertitude associée au résultat de la méthode REM' varie d'un facteur 2 (Gattacceca and Rochette, 2004) à un facteur de presque 0.5 (Muxworthy et al., 2011). Les barres d'erreurs (b.e) des résultats R et REM' ont été obtenues à partir de la régression linéaire entre la courbe de désaimantation de la NRM en fonction de celle de l'ARM (R) ou de l'IRM (REM') :
  b.e
  (ARM)
  b.e
  (IRM)
  =
  !"#$" . ! ƒ"
  (3)
  et
  = pente. !
  (4)
  Ces résultats sont dans le Chapitre 5.
  3.4.3 Protocole de Thellier-Thellier La méthode de détermination de paléointensité la plus robuste est le protocole de double chauffe développé par Thellier et Thellier (1959) et ses méthodes dérivées. Cette méthode consiste en la chauffe successive de l'échantillon dans un champ d'intensité connue
  70
  Evolution temporelle du champ magnétique lunaire Chapitre 3 : Matériel et méthodes en magnétisme
  3
  à des températures croissantes (notées T1 à T4 sur la Figure 3.9) afin de désaimanter, par paliers, son aimantation naturelle NRM et d'acquérir en parallèle une aimantation thermorémanente partielle (pTRM). Ainsi, on acquiert artificiellement une TRM, qui est proportionnelle au champ appliqué au laboratoire. La comparaison avec les intensités de TRM et de NRM donne la paléointensité. La paléointensité du champ ancien Banc (μT) que l'on cherche est ainsi calculé :
  Banc =
  ∆!"# ∆!"#
  . Blab
  (5)
  où ∆NRM est l'intensité de la NRM désaimantée dans l'intervalle de température où est définie la ChRM ; ∆TRM est l'intensité de la TRM acquise dans le même intervalle; Blab est le champ appliqué en laboratoire (μT) qui doit être le plus proche de celui qui a été enregistré par l'échantillon. Cette méthode repose sur trois lois : - la loi d'additivité : la somme des pTRM acquises, sous un même champ, entre T1 et T2 et entre T2 et T3 est égale à la pTRM acquise entre T1 et T3. - la loi d'indépendance : la pTRM acquise par refroidissement entre deux paliers de température (entre T1 et T2) est indépendante de celle acquise à deux autres paliers de température (entre T2 et T3).
  - la loi de réciprocité : l'aimantation acquise par refroidissement à partir d'une température donnée est entièrement remplacée par l'aimantation acquise lors de la seconde chauffe au même palier de température. La méthode que j'ai utilisée dans ma thèse est la méthode IZZI (in field / zero field / zero field/ in field, détaillée par Yu et Tauxe, 2004). Pour retrouver la pTRM (MpTRM) acquise à chaque palier de température, l'échantillon est chauffé deux fois au même palier. Il y a ainsi deux étapes de chauffe : une étape de chauffe sous champ nul et une étape de chauffe sous champ connu (en μT) à un même palier de température: M =M (6) et M =M +M (7) M est ainsi calculé par différence vectorielle entre les deux chauffes. Ces deux chauffes sont répétées pour chaque palier de température, qui est de plus en plus élevée, jusqu'à atteindre la température de déblocage maximum de la NRM de l'échantillon. 1
  NRM
  2
  NRM
  pTRM
  pTRM
  Evolution temporelle du champ magnétique lunaire 71 3
  Chapitre 3 : Matériel et méthodes en magnétisme IZZI ZI
  ZI
  IZ
  IZ
  T3 T2
  Blocking temperature
  T4
  T1 Zero field
  In field
  In field
  NRM remaining
  Zero field
  Zero field
  pTRM check
  In field
  In field
  Zero field
  pTRM gained
  NRM lost
  Figure 3.9 : Description du protocole de désaimantation thermique IZZI pour l'estimation de paléointensités. Modifié d'après Ben-Yosef et al. (2008). Pour les roches dont les minéraux magnétiques sont des alliages de fer métallique (fer, kamacite, taenite) comme les roches lunaires, l'oxydation des porteurs magnétiques lors des chauffes est un problème sérieux pour la détermination des paléointensités. Suavet et al. (2014) a ainsi développé un système de chauffe sous fugacité d'oxygène contrôlé, basé sur les travaux théoriques de Prunier et Hewitt (1981). Un mélange de CO2 et de H2 est introduit lors de la chauffe. La proportion de ces deux gaz permet de contrôler la fugacité d'oxygène et d'éviter les phénomènes d'oxydation ou de réduction si l'on se place dans des conditions voisines de la fugacité d'oxygène lors de la formation de la roche étudiée. Nous avons ainsi développé ce système de chauffe sous oxygène contrôlé (Figure 3.10) au laboratoire de magnétisme du CEREGE, en adaptant un système décrit dans Suavet et al. (2014). Le dispositif a été installé dans la chambre amagnétique tandis que les deux bouteilles de gaz (H et CO ) ont été installées à l'extérieur la chambre amagnétique. 2
  2
  Figure 3.10 : Système de chauffe sous oxygène contrôlé installé sur le four de paléointensité (à gauche et en haut à droite). Les boîtiers de contrôle du four et du champ appliqués sont à gauche et en bas à droite. Les boîtiers de contrôle des débits de gaz injectés dans le four sont en haut à droite. J'ai d'abord réalisé des tests de chauffe sous atmosphère contrôlée en utilisant du fer pur (Figure 3.11) afin de tester l'oxydation ou non du fer en fonction de la fugacité d'oxygène.
  72
  Evolution temporelle du champ magnétique lunaire Geochemistry, Geophysics, Geosystems
  10.1002/2013GC005215 Chapitre 3 : Matériel et méthodes en magnétisme 3 T [oC]
  800 30 20 10 5
  -20
  log(fO2) [log(atm)]
  -25
  700
  600 570
  500
  400
  300
  WM
  1 0.5 0.2 %CO2
  9 8
  IW
  0 IM -1 -2 -3
  -30
  7 5 6 et 10 4
  -35
  3 et 11
  2 10 5
  -40 1 et 12
  -45
  9
  10
  11
  12
  13
  10,000 / T [K]
  14
  15
  16
  1 0.5 0.2
  17
  Figure 3.11of: the Calibration fluxcontrolled-atmosphere de gaz utilisé dans la désaimantation thermique sousis atmosphère Figure 2. Calibration gas mixturesdu in our thermal demagnetization apparatus. Shown the inferred oxygen function of inverse temperature, 1/T. Black thick lines labeled are iron-w€ ustite and iron-magnetite (IM) solid fugacity,contrôlée. log(fO2), as aLa fugacité d'oxygène est montrée en fonction de ''0'' l'inverse de la (IW) température. La droite buffer curves from Eugster and Wones [1962] (based on measurements for IW, and theoretical calculations for IM), which intersect at noire « 0 » correspond à la courbe iron-wüstite (IW) et iron-magnetite (IM) d'après Eugster et Wones ustite-magnetite (WM) curves. Additional black solid lines represent "570! C. Above this temperature, the IM curve splits into IW10 and a w€ Les(atm) autres droites noires représentent lesrepresent courbescalculated en -1, -2 et -3 log Les droites 21, 22,(1962). and 23 log below the IW/IM buffer. Red solid lines fugacities from(atmosphère). Prunier and Hewitt [1981] for H2-CO2 rouges montrent les fugacités calculées d'après et Hewitt (1981) pour les mélanges de gaz H2-area gas mixtures of different compositions (in vol % CO2), and dashedPrunier lines are low-temperature linear extrapolations. The dark shaded encompasses in which precipitate [Prunier and Hewitt, representde ourSuavet experiments for CO2 the (enconditions CO2 vol%). Lesgraphite pointscanbleus, violets et verts sont1981]. issusGreen des circles expériences et al. which we observed iron was stable, blue circles represent experiments for which we observed that iron was oxidized to w€ ustite, and pur(2014), le violet étant pour l'oxydation du fer en magnétite ; le vert montrant le fer restant stable et le ple circles represent experiments for which we observed that iron was oxidized to magnetite. The fugacity for each of these experiments bleu l'oxydation fer enH2wüstite. Nos expériences -CO2 gas mixture composition. sont numérotées de 1 à 12 et signalées par des was calculated based on the du controlled étoiles vertes et violettes en respectant le code de couleur utilisé par Suavet et al. (2014). Modifié d'après Suavet al.oven (2014). field measured insideetthe is typically < 20 nT. Samples are placed on a removable ceramic holder attached
  to a 6 mm outer diameter gas inlet quartz tube with ceramic paste. A platinum wire acting as a catalyst [BurCes Ulmer, tests ont validé le fonctionnement de ce système queinlet. j'ai Once utiliséthe pour obtenir les résultats khard and 1995] is wrapped around the opening of the gas peak temperature is reached ! dans le Chapitre 5. Pour les the chauffes avec desisroches utilisé une with fugacité in the décrits oven, the temperature gradient along sample holder <5 C. lunaires, The quartzj'ai tube is sealed a silicone d'oxygène à IM/IW-1 (Sato et al., 1973) en introduisant les quantités de gaz détaillées dans stopper. The gas mixture composition is controlled from outside the shielded room by adjusting the flowlarates 26 Table pour chaque palier de température à partir de 300°C. of H2 and CO3.1, m3/s for CO2 and 2 with two rotameters. The range of possible flow rates is 0.01–2.28 3 10 26 3 0.58–5.88 3 10 m /s for H2. CO detectors are used to monitor potential H2 leaks, but these are not expected to be problematic because of the small H2 flow rates (the total flow is "6 3 1026 m3/s during filling).
  3. Calibration To calibrate and test our system, we heated iron metal fragments (guaranteed 99.99% Fe by the producer) in gas mixtures of different compositions to temperatures between 300 and 800! C (Figure 2). At temperatures below "570! C, iron oxidizes to form magnetite when the atmosphere is too oxidizing. Above "570! C, Evolution temporelle du champ magnétique lunaire 73 €stite, which then further iron oxidizes to form wu oxidizes to magnetite. The fO 2 conditions at which these reactions occur are given by the equations of Eugster and Wones [1962], which define the IW, IM, and €site-magnetite (WM) buffers. We calculated the proportions of H2 and CO2 required to reach fO2 levels wu above and below the IW and IM buffers at different temperatures based on the tables of Prunier and Hewitt 3
  Chapitre 3 : Matériel et méthodes en magnétisme Température (°C)
  Débit H2 (L/m)
  300 330 360 400 450 480 500 520 540 560 580 610 630
  0.6 0.6 0.6 0.5 0.5 0.5 0.3 0.3 0.2 0.2 0.2 0.2 0.2
  CO2 (%) 0.2 0.35 0.5 1 2 2.5 3 3.5 4 4 4.5 5 7.5
  Débit CO2 (L/m) 0.0012 0.0021 0.003 0.005 0.01 0.0125 0.009 0.01 0.008 0.008 0.009 0.01 0.015
  Tableau 3.1 : Mélanges de gaz utilisés pour les expériences de désaimantation de roches lunaires.
  3.4.4 Cas des échantillons sans composante stable Certains échantillons ne possèdent pas de composante d'aimantation stable et ne sont donc pas propices à donner une valeur de paléointensité. Il est cependant parfois possible d'obtenir une limite supérieure de paléointensité pour ces échantillons (Tikoo et al. 2012). Cette méthode repose sur la normalisation de deux composantes HC (∆ARM1 et ∆ARM2) obtenues après une acquisition d'ARM dans deux différents champs DC puis désaimantées. La première (ARM1) est acquise à fort champ DC (100 ou 200 μT), noté C ; et la seconde (ARM2) est acquise dans un champ DC plus faible (allant de 100 à 5 μT), noté L. Pour cela, on procède de la même façon que pour la détermination de la paléointensité par la méthode ARM sauf que l'on normalise une ARM par une autre ARM, le but étant de retrouver la valeur du champ DC appliqué lors de l'acquisition de l'ARM : ∆!"#!
  I = ∆!"#! . C
  (8)
  Pour savoir si le champ retrouvé I (μT) est valide, deux paramètres de fidélité sont calculés : la différence D (%) et l'erreur E (%), décrits par Tikoo et al. (2012) : D = | L – I| / L. 100
  (9)
  Et E = W / L. 100 (10) où W est l'intervalle de confiance à 95% du champ retrouvé par rapport au champ appliqué. Si les deux paramètres E et D sont inférieurs à 100%, alors on considère que l'échantillon est capable d'enregistrer ce champ L et l'absence de composante stable de la NRM indique alors que le paléochamp était inférieur à L. En répétant ces expériences avec des champs L de plus en plus faibles (de 50 à 5 μT), on peut ainsi déterminer une limite supérieure pour la paléointensité.
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  Evolution temporelle du champ magnétique lunaire Chapitre 3 : Matériel et méthodes en magnétisme
  3
  Une fois le champ L minimum obtenu, on le convertit en champ équivalent pour une TRM (U ) (Supplementary material, Tikoo et al., 2014) : L
  U = L/f' (11) Avec f' la constante de calibration de l'ARM décrit en formule (1). L
  Il faut cependant remarquer que cette méthode repose sur des critères de sélection subjectifs (E et D < 100%) et les limites supérieures (U ) ainsi obtenues doivent être interprétées avec précaution. Il faut aussi garder à l'esprit qu'il ne s'agit pas d'une propriété intrinsèque de la roche mais plutôt d'une limitation des méthodes expérimentales, en général les aimantations parasites introduites par les désaimantation par champs alternatifs dans les grains de Fe,Ni poly-domaines. Cette méthode a été appliquée sur vingt-sept échantillons Apollo (représentant vingt roches Apollo), dont les résultats sont décrits dans le Chapitre 5. L
  Evolution temporelle du champ magnétique lunaire 75 3
  76
  Chapitre 3 : Matériel et méthodes en magnétisme
  Evolution temporelle du champ magnétique lunaire Chapitre Quatre A survey of the Natural Remanent Magnetisation and the magnetic susceptibility of Apollo whole rocks
  Magnétomètre portatif, développé au CEREGE, salle de Stockage NASA (Houston, USA) Copyright © Camille Lepaulard
  Evolution temporelle du champ magnétique lunaire 77 78
  Evolution temporelle du champ magnétique lunaire Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  4
  Chapitre 4 A survey of the Natural Remanent Magnetisation and the magnetic susceptibility of Apollo whole rocks Les propriétés magnétiques de seulement 5% des roches lunaires (< 70 sur les 1402 roches disponibles) ont été étudiées en laboratoire depuis leur échantillonnage lors des missions Apollo. De plus, la plupart de ces études se concentrent sur des échantillons de quelques centaines de mg, peu représentatifs d'une roche compte tenu de son hétérogénéité lithologique (notamment les roches d'impact). Notre étude vise donc, dans un premier temps, à compléter cette base de données par des mesures d'aimantation rémanente naturelle (NRM) et de susceptibilité magnétique sur des échantillons de masse plus conséquente de la collection Apollo. Pour cela, deux appareils portatifs (un magnétomètre et un susceptibilimètre) ont été développés au CEREGE afin de réaliser ces mesures dans la pièce de stockage des échantillons. Le magnétomètre est décrit dans l'article de Uehara et al. (2017) (Annexe 3). Les résultats issus des mesures au magnétomètre et au susceptibilimètre, ainsi que le susceptibilimètre, sont décrits dans l'article de Lepaulard et al. (en cours d'expertise pour Physics of the Earth and Planetary Interiors) à la fin de ce Chapitre. Nous avons ainsi étudié cent-soixante et une roches Apollo avec une masse moyenne de 500 g. L'analyse des données de susceptibilité magnétique obtenues corrobore les résultats de la littérature en affirmant que les processus d'impact sont le principal facteur de contrôle de la quantité de minéraux ferromagnétiques dans les roches lunaires, avec un enrichissement magnétique dû à l'incorporation de métal météoritique. De plus, l'apport de nouvelles mesures de NRM alimente la base de données utilisée pour la modélisation et l'interprétation des anomalies de champ magnétique lunaire. Dans un second temps, nous avons comparé les données de susceptibilité et de NRM obtenues à celles de la littérature. D'une manière générale, les données de susceptibilité et de NRM de la littérature présentent, pour une même roche, une plus grande dispersion par rapport à nos propres données de susceptibilité et de NRM obtenues sur des roches plus grosses. Cette dispersion témoigne d'une hétérogénéité à petite échelle du contenu métallique des échantillons. Enfin, nous avons normalisé les valeurs de NRM par les valeurs de susceptibilité magnétique pour chaque roche (rapport noté ß), dans le but d'obtenir un proxy de l'intensité du paléochamp enregistré par une roche. La comparaison des valeurs de ß avec les valeurs de paléointensité de la littérature montre une corrélation des valeurs à partir de ß = 4 (voir détails dans l'article). L'évolution de cette donnée en fonction du temps (Figure 4.1 cidessous) montre les deux époques de fort champ magnétique (4,25 – 3,5 Ga) et de champ faible (après 3 Ga) définies dans la littérature. On note cependant un certain nombre d'échantillons avec des ß faibles dans l'époque de champ fort, pouvant correspondre à des échantillons de faible paléointensité. Cela suggère que le champ moyen de cette époque a pu être surestimé dans la littérature. Ces échantillons peuvent aussi avoir eu une NRM originelle partiellement ou totalement effacée par un autre processus, comme le choc par exemple, qui aurait diminué la valeur de ß obtenue. Une contamination tardive par un
  Evolution temporelle du champ magnétique lunaire 79 4
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  régolithe riche en métal ou une NRM acquise lors d'une bréchification froide est aussi possible. Ce jeu de données nous a permis la sélection de neuf échantillons (signalés par une étoile blanche) spécifiques pour des analyses détaillées en laboratoire (voir Chapitre 3). Une nouvelle demande d'échantillons a été faite suite à cette étude, signalés par des triangles Figure 8 sur la Figure 4.1, afin de vérifier si les hautes valeurs de ß obtenues correspondent à blancs des hautes valeurs d'intensité.
  30 Regolith breccia Impact melt breccia 25
  Dimict breccia
  200
  Impact melt rock
  ß (A.m-1)
  Feldspathic rock
  15
  100
  10
  50
  Paleointensity equivalence (μT)
  Basalt
  20
  20 10
  5
  Not defined
  0 4
  3
  2
  1
  0
  Time before present (Ga)
  Figure 4.1 : ß (avec les barres d'erreur) en fonction de l'âge des roches étudiées. Le code couleur réfère à la lithologie. Le trait gris correspond à une médiane glissante calculée sur quinze données successives de valeurs de ß. L'axe de droite indique les paléointensités approximatives pour ß>4 (ligne en pointillée). Les échantillons (neuf) demandés pour des analyses en laboratoire dans cette thèse ont une étoile dans le centre du cercle ; et ceux (quatre) avec un triangle sont ceux demandés pour des futures analyses après cette thèse. Figure extraite de l'article Lepaulard et al. (2018 – under review).
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  Evolution temporelle du champ magnétique lunaire Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
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  Article en révision à PEPI
  A survey of the natural remanent magnetization and magnetic susceptibility of Apollo whole rocks Camille Lepaulard1, Jérôme Gattacceca1, Minoru Uehara1, Pierre Rochette1, Yoann Quesnel1, Robert J. Macke, S.J.2, Walter Kiefer3 1
  Aix Marseille University, CNRS, IRD, Coll. France, INRA, CEREGE, Aix-en-Provence,
  France 2
  Vatican Observatory, Vatican City-State
  3
  Lunar and Planetary Institute, Houston, Texas, USA
  Key words Apollo samples; remanence; paleomagnetism; magnetic susceptibility Abstract Magnetic properties of lunar rocks may help understand the evolution of the past magnetic field of the Moon. However, these properties are often measured on small subsamples, which may not be representative of the bulk lunar rocks. In this study, a rough magnetic characterization of a large fraction of the Apollo collection was performed using non-destructive methods to determine the paleomagnetic signal, with a focus on large samples (mean mass 500 g). The natural remanent magnetization of 123 Apollo samples and the magnetic susceptibility of 154 Apollo samples were measured in the Apollo storage vault at the Lunar Sample Laboratory Facility in Houston. Magnetic susceptibility is strongly controlled by lithology and is clearly enhanced by impact processing and associated incorporation of meteoritic metal. The natural remanent magnetization is lower by a factor of ~3 than that measured by other researchers on small laboratory samples, suggesting magnetic contamination during small-sample preparation or handling, or directional heterogeneity in remanence within large samples. The laboratory measurements were conducted on subsamples of the same sample measured in this study. The ratio of natural remanence magnetization to magnetic susceptibility may be used as a selection tool for further paleomagnetic study. It is a crude paleointensity proxy and its overall temporal evolution is Evolution temporelle du champ magnétique lunaire 81 4
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  coherent with the existence of a high magnetic field from ~4 Ga to 3.5 Ga and a subsequent weaker field epoch as previously defined in the literature. Our data also raise the possibility that the intensity of the paleomagnetic field during the high field epoch may have been more variable and lower on average than previously suggested. 1. Introduction The existence of past or present magnetic fields on the Moon was a matter of debate before lunar exploration started, as it was one of the key factors in distinguishing between a "hot Moon" versus a "cold Moon" model. Early orbital magnetic measurements by Luna 10 (Dolginov et al., 1966) and Explorer 35 (Sonett et al., 1967) satellites were only able to determine upper limits of the magnetic fields in the vicinity of the Moon, indicating that any lunar fields would be much weaker than that of the Earth, and suggesting a non-magnetic body. However, the Apollo orbital and surface magnetometers revealed patches of significant field due to crustal remanence. The paleomagnetic study of Apollo samples suggested the record of past strong (i.e., Earth like) magnetic fields at the surface of the Moon (e.g., Fuller et al., 1987), besides the particular difficulties linked with retrieving reliable paleointensities in metal-bearing rocks (e.g. Lawrence et al. 2008). The analysis of Apollo samples establishes constraints on the temporal evolution of the lunar magnetic field. This evolution reveals two main magnetic epochs: a high-field epoch with paleointensities of several tens of μT between about 4.25 and 3.56 Ga (Garrick-Bethell et al., 2009, 2017; Cournède et al., 2012; Shea et al., 2012; Tikoo et al., 2012a; Suavet et al., 2013); followed by a weak-field epoch with paleointensities below 10 μT (Tikoo et al., 2014) and down to < 4 μT after 3.19 Ga (Tikoo et al., 2014, 2017; Weiss et al., 2014; Buz et al., 2015). In part because the age distribution of Mare basalts peaks between 3.8 and 3.3 Ga (Hiesinger et al., 2011), the high-field epoch is rather well constrained, with around 60 studied samples (Cisowski et al., 1983; Weiss et al., 2014). However, paleointensities up to 100 μT are at odds with dynamo models that predict much lower surface fields (Dwyer et al., 2011; Le Bars et al., 2011; Laneuville et al., 2014; Evans et al. 2017). Also, some samples may indicate paleointensities < 10 μT during this high-field epoch (Fuller et Cisowski, 1987), raising the possibility of a highly variable or intermittent dynamo (Evans et al., 2017). Further data from this high-field epoch is thus needed to strengthen the robustness of the high paleofields claims, and test the possibility of an intermittent or highly variable dynamo. 82
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  The weak-field epoch is defined as the period of decline of the field intensity, down to the present day situation with no internally generated field (Fuller and Cisowski, 1987; Tikoo et al., 2014). This epoch is constrained by 30 paleointensity estimates (Cisowski et al., 1983; Weiss et al., 2014; Tikoo et al., 2017; Wang et al., 2017). Recent analyses of young samples reveal surface field intensities from 5 ± 2 μT (~2.5-1 Ga, Tikoo et al., 2017) to <1 μT (~1 Ga, Wang et al., 2017). These results could be explained by the persistence of a weak dynamo powered by core crystallization and mantle precession from 2.5 to 1 Ga (Tikoo et al., 2017). Numerical modeling studies show that core convection could indeed generate surface fields of 2 μT until up to 200 Ma (Evans et al., 2017). The field evolution over the weak-field epoch is rather poorly defined, especially between 3 and 2 Ga, a time interval where both basalts and regolith breccias are rare (Hiesinger et al., 2011; Fagan et al., 2014). So far, fewer than 70 small (usually well below 1 g) samples of Apollo rocks have been studied for paleomagnetism, compared to a total of 1402 rock samples available in the Apollo collection (this latter number is a personal communication by Ryan Zeigler). The aim of this study is to obtain a rough estimate of the paleomagnetic potential of a larger fraction of the Apollo rock collection using non-destructive methods. We measured the natural remanent magnetization (NRM) and the magnetic susceptibility (χ) of whole Apollo samples. This data increase the database of lunar rocks magnetic properties. The measurement of these two properties on the same sample also allows a qualitative assessment of the paleomagnetic potential of these samples. The NRM intensity is a crude indication of whether a rock can be targeted for paleomagnetic study. χ is a proxy for the metal content. The ratio between NRM and χ can be viewed as a crude proxy for the intensity of the paleofield recorded by a rock, enabling the selection of samples for detailed laboratory demagnetization follow-up studies that address specific questions (timing of the dynamo, maximum field recognized in the weak-field epoch, etc). A high NRM/χ may also be an indication of suitable paleomagnetic intrinsic properties for laboratory follow-up studies. Finally, these analyses increase the database of lunar rock magnetic properties for the purposes of modeling the lunar crustal magnetic field (e.g., Oliveira et al. 2017).
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  2. Samples and methods 2.1 Samples We studied 161 Apollo samples with a focus on large samples (mean mass 497 g, median mass 232 g, mass range 21 to 4688 g, see Table S1). This is much larger than the typical size of specimens used in most previous studies, which were well below 1g (see section S2 in Supplementary material). Of the 161 samples in this study, only 28 had been studied previously for remanent magnetization, and only 37 for magnetic properties. We measured the natural remanent magnetization of 123 samples, the magnetic susceptibility of 154 samples, and 119 samples for which both properties were measured. All specimens in this survey were single coherent bulk samples. We studied basalts, anorthosites, norites, and different types of breccias, spanning the Apollo 11, 12, 14, 15, 16 and 17 landing sites. In the following, a simplified lithological classification of lunar rocks is used (Heiken and al., 1991). Breccias are classified into three categories: impact melt breccia (IMB) with clasts set in a melt matrix, dimict breccia (DB) with two distinct lithologies (usually anorthosite and melt rock) and regolith breccia that represent lithified regolith. The other lithologies are impact melt rocks (IMR), basalts, and feldspathic rocks from the lunar highlands (norites, troctolites, anorthosites). 2.2 Methods All measurements were performed directly in the Apollo storage vault at the Lunar Sample Laboratory Facility (LSLF) at Johnson Space Center (JSC), NASA, Houston. A specific susceptibility meter and a spinner magnetometer were used to be assembled in the LSLF and which satisfy curatorial constraints. The samples were left in their original packaging for this study. Sample packaging varied from sample to sample. Samples kept in steel containers could not be studied. We measured those stored in Teflon bags, aluminum containers, or a combination of both. Prior to measurements, the samples were placed at the center of a hollow cubic sample holder made of acrylic plastic. They were maintained in place using padding material made of acrylic and Teflon. The magnetometer and associated data processing methods are described in Uehara et al. (2017). The errors associated with the NRM measurements provided in section S1, Table S1, are based on the departure of the measured signal from that of the signal from an ideal dipole (Uehara et al., 2017). We validated only samples that fit a dipole source to better than 84
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  4
  20% (see discussion in Uehara et al., 2017). We did not obtain a satisfactory measurement for 24 samples, either because the magnetization was too low or because the measured signal departed too much from the signal generated by a centered dipole (samples noted with † in the Table S1). These 24 samples range in mass from 24.5 to 1444 g and do not correlate with lithology or size. We did not measure NRMs for 14 samples that consisted of multiple small pieces in a single container (samples noted - in Table S1 in Supplementary material). The susceptibility meter is described in section S3 in Supplementary material. It consists of an acrylic cylindrical body with two inducing coils and a Hall probe to measure the sum of the alternating inducing and induced fields (Figures S2 and S3 in Supplementary materials). The peak inducing field was 30 μTp-p, with absolutely no effect on the paleomagnetic signal, and its frequency was at 5 Hz. This low frequency limits eddy currents in metal and thus enables measurements of samples wrapped in aluminum foils or encapsulated in aluminum containers. This susceptibility meter can accommodate samples as large as 8000 cm3, corresponding to 20 x 20 x 20 cm sample capsule. Measurements are based on the assumption that samples are perfectly centered in the instrument. The method of calculation of the uncertainty on our magnetic susceptibility results is detailed in section S3 in Supplementary material. To limit errors from sample geometric offset and/or anisotropic sample shape, we measured the susceptibility in two to three orthogonal positions and used the average result. Magnetic anisotropy of lunar rocks is at most a few percent (e.g., Rochette et al., 2010; Cournède et al. 2012) and is neglected in this study. The magnetic susceptibility for 41 samples was also measured using a commercial susceptibility meter MS2 from Bartington equipped with a MS2C coil. This coil, with an internal diameter of 12 cm, is designed for measurement of cores and was calibrated for discrete samples against a KLY2 large volume kappabridge. The calibration and procedure is provided in section S4 in Supplementary material. Like during the calibration procedure, the measurements were performed along three orthogonal positions and averaged, not so much to account for the weak magnetic anisotropy of lunar rocks, but to account for the effect of sample anisometry on the measurements. For samples measured with both instruments (MS2 and our susceptibility meter), the results compare well, with a coefficient of correlation R2 = 0.98 and a slope of 1.03 (see section S5 in Supplementary material).
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  3 Results and discussion 3.1. Magnetic properties and lithology The NRM intensity spreads over two orders of magnitude (S1, Table S1) and shows a strong lithological dependence, in ascending order of NRM: feldspathic rocks, basalts, breccias and regolith breccia (Figure 1a, Table 1). This corroborates previous results, although on a much larger dataset. Magnetic susceptibility is also lithology-dependent, with impact-processed rocks being more magnetic (i.e., higher NRM and χ) (Figure 1b). This confirms that lunar impact processes are the main factors controlling the amount of ferromagnetic minerals in lunar rocks, with a magnetic enhancement due to incorporation of meteoritic metal as well as formation of nanoparticules of metal due to vapor reduction and condensation (e.g., Rochette et al., 2010). Figure 1 a)
  Feldspathic rocks (2,7) Basalt (29,39) Impact melt rocks (5,5)
  b) 60
  Dimict breccia (4,5) Regolith breccia (44,48)
  50
  Number of samples
  Impact melt breccia (39,49) 40
  40
  30
  30
  20
  20
  10
  10
  0
  -6 0 3E-063 101E-05 10-5 3E-053 10-51E-0410-4 3E-043 10-4
  0
  0 3E-07 3 10-71E-06 10-6 3E-063 10-61E-05 10-5 3E-05 3 10-51E-04 10-4
  NRM (Am2kg-1)
  Magnetic susceptibility (m3kg-1)
  Figure 1: a) Distribution of the NRM intensities of the large Apollo samples measured in this study, grouped by lithology. b) Distribution of the magnetic susceptibility of the large Apollo samples measured in this study, grouped by lithology. Numbers of analyzed samples are indicated for each lithology (number for NRM, number for susceptibility). Note the non-linear scales for both NRM and susceptibility.
  86
  Evolution temporelle du champ magnétique lunaire 199.79 44.55 80.50 13.50 37.24 6.05
  Literature data Regolith breccia Impact melt breccia Dimict breccia Impact melt rocks Basalts Feldspathic rocks
  328.06 46.96 7.50 13.14 70.86 8.90
  63.06 25.67 41.99 31.00 43.36 2.92
  69.50 23.20 80.50 9.00 15.85 1.04
  55.91 9.24 11.09 19.07 8.20 4.75
  NRM median (10-6Am2kg-1)
  31.00 1.80 73.00 2.00 0.54 0.17
  4.83 1.30 2.71 2.45 1.25 1.83
  NRM minimum (10-6Am2kg-1) N/No
  1000.0 147.0 88.0 34.0 500.0 30.2 Total
  7 24 2 4 61 11 109
  225.6 44 / 50 85.6 39 / 52 104.9 4/5 74.1 5/6 243.4 29 / 40 7.7 2/8 Total 123 / 161
  NRM maximum (10-6Am2kg-1)
  23.72 7.22 6.02 1.59 2.04
  19.11 4.11 6.10 6.07 1.81 1.91
  18.58 11.63 4.40 1.60 5.02
  16.65 2.78 3.81 4.78 1.48 2.35
  16.46 3.13 4.30 1.10 0.041
  13.89 3.69 1.57 4.32 1.41 1.15
  4.17 0.28 0.72 0.41 0.0016
  2.65 0.15 2.03 1.16 0.64 0.020
  N/No
  75.9 45.7 12.3 9.8 17.8 Total
  14 24 0 8 48 11 105
  73.8 44 / 50 11.5 49 / 52 10.6 5/5 13.1 5/6 9.8 39 / 40 7.8 8/8 Total 154 / 161
  χ S.D χ median χ mean (10-6m3kg (10-6m3kg- χ minimum χ maximum (10-6m3kg-1) (10-6m3kg-1) (10-6m3kg-1) -1) 1)
  Table 1: NRM and magnetic susceptibility sorted by lithology. N: number of reliable measurements (see text); No: number of samples studied in this work; S.D: standard deviation.
  75.20 20.72 32.44 33.54 18.87 4.75
  NRM S.D NRM mean (10-6Am2kg (10-6Am2kg-1) -1)
  Regolith breccia Impact melt breccia Dimict breccia Impact melt rocks Basalts Feldspathic rocks
  Lithology
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  3.2.Comparison with other measurements The NRM and the magnetic susceptibility values averaged by lithology are given in Table 1, both for this study and for literature data. The literature dataset consists of 109 NRM estimates (from 61 different Apollo rocks), and 105 susceptibility estimates (from 73 different Apollo rocks). A comparison of both datasets shows that the NRM intensities are larger by an average factor of ~2.7 in small sample studies (Figure 2a). This discrepancy is evidence of the NRM directional heterogeneity within some lunar rocks and/or of possible magnetic contamination of small samples during preparation and handling. The two datasets provide similar values for the susceptibility (R2= 0.86) with a slope of 0.9 (Figure 2b). Standard deviations in the literature data are generally higher than in ours, except for regolith breccias, indicating small-scale heterogeneities in metal content. Figure 2 a)
  b) 25
  Susceptibility, literature data (10-6 m3kg-1)
  NRM, literature data (10-5 Am2kg-1)
  (44,14)
  (44,7)
  20
  15
  10 (4,2)
  (39,24)
  5 (29,61)
  20
  15
  10 (49,24) Regolith breccia
  (5,8)
  5
  Impact melt breccia Impact melt rocks
  (8,11)
  Basalt
  (2,11)
  0 0
  (5,4)
  (39,48) 5
  10
  Feldspathic rocks
  0 0
  5
  NRM, this study (10-5 Am2kg-1)
  10
  15
  20
  Susceptibility, this study (10-6 m3kg-1)
  Figure 2: a) NRM average values of small samples (literature data, with standard deviations) versus NRM average values of large samples (this study, with standard deviations) by lithology. b) Magnetic susceptibility average values of small samples (literature data, with standard deviations) versus magnetic susceptibility average values of large samples (this study, with standard deviations) by lithology. In both figures numbers in parentheses refer to the number of samples in our dataset and the literature dataset, respectively, and the light broken line is a 1:1 line.
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  25 Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  4
  For the subset of samples that were previously studied in the literature (section S6, Table S2 in Supplementary material), we compare our NRM measurements on large samples with the laboratory measurements performed on small subsamples of the same rocks (Figure 3). The results of our bulk sample study do not correlate to those of the small subsamples (R2= 0.08). It is noteworthy that when several measurements are available in the literature, there is a large scatter between measurements (with a relative variation from 6% to 163%, 56% in average). This scatter is not due to analytical uncertainties that are small compared to magnetic moments, both in this study and in previous studies. This is rather further evidence of small-scale heterogeneity of NRM, attributed to lithological heterogeneities (illustrated in section S2), with heterogeneous distribution of metallic iron, especially in breccias. It can indicate too the presence, in the same rock, of lithologies that have recorded contrasting paleofields. Overall, the NRM is higher for small samples than large samples (see also Table 1), indicating possible magnetic contamination during small samples preparation or handling, or heterogeneous directions of NRM within a large sample. 1.E-03 10-3
  10-4 1.E-04
  NRM, literature data (Am2.kg-1)
  Figure 3
  10-5 1.E-05
  -6
  10 1.E-06
  Regolith breccia basalt Feldspathic rocks Dimict breccia Impact melt breccia
  10-7
  1.E-07 10-7 1.E-07
  -6
  10 1.E-06
  -5
  10 1.E-05
  -4
  10 1.E-04
  -3
  10 1.E-03
  NRM, this study (Am2.kg-1)
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  Figure 3: NRM intensities measured on small samples (literature data) versus NRM values of large intensities measured on large samples of the same rocks (this study). Box symbols connected by vertical lines are for rocks with several small sub-samples in the literature dataset. The error bars are smaller than the symbol size. The light broken line is a 1:1 line.
  Our susceptibility data for the large samples and the small subsamples in the literature data (Figure 4) are also not correlated (R2 = 0.03); even the most correlated lithology, which is the impact melt breccia, shows only R2 of 0.175. Feldspathic rocks show the largest discrepancy, our values being up to of 2 orders of magnitude larger than literature data. It is noteworthy that there is a large scatter in literature data (S5, in Table S2) for multiple measurements of subsamples from the same rock, especially for impact melt breccia and impact melt rocks with coefficient of variation of 43% and 59% respectively. Again, smallscale lithological heterogeneities can account for these variations, especially in smaller samples. For feldspathic rocks, the larger whole rock susceptibility may be due to the fact that these rocks often have a coating of melted or regolithic material and that this coating, enriched in metal, was avoided in the subsampling. As an example, samples 65315 and 60215 had diamagnetic χ values of −1.4 × 10−9 m3kg−1 and -5.4 × 10−9 m3kg−1, respectively, in the small sample study of Cournède et al. (2012) and 20 × 10−9 m3kg−1 and 228 × 10−9 m3kg−1, respectively, in this study.
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  igure 2 10-4 1E-04 Regolith breccia IMB Basalt 1E-05 10-5
  Feldspathic rocks
  Susceptibility, literature data (m3.kg-1)
  Impact Melt Rock
  -6
  10 1E-06
  10-7 1E-07
  10-8 1E-08
  -9
  10 1E-09
  10-9 1.E-09
  10-8 1.E-08
  10-7 1.E-07
  10-6 1.E-06
  10-5 1.E-05
  10-4 1.E-04
  Susceptibility, this study (m3.kg-1) Figure 4: Magnetic susceptibility of small samples (literature data) versus magnetic susceptibility values of large samples (this study, with error bars). Box symbols connected by vertical lines are for rocks with several sub-samples in the literature dataset. The light broken line is a 1:1 line.
  In Figure 5, we compare our susceptibility results with measurements performed at JSC on another set of large samples (mean mass 19.8 g) from the returned sample collection (Macke et al., 2014). These measurements were performed using a SM30 instrument calibrated following Gattacceca et al. (2004) and are provided in section S7, Table S3. The coefficient of correlation of 0.70, together with a slope of 0.998 between the two independent datasets confirm that the scatter observed between our measurements and the small sample laboratory measurements is due to the intrinsic properties of lunar rocks (mostly small scale heterogeneities) rather than instrumental issues. Still returned samples yield on average lower Evolution temporelle du champ magnétique lunaire 91 4
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  values than the whole rocks, again suggesting the possible effect of regolith contamination of the surface of whole rocks. It is also possible that measurements using the SM30 contact probe are underestimated because they are sensitive to the surface roughness of the samples and were calibrated for smooth pebble shaped samples (Gattacceca et al., 2004). 20 Magnetic suscpetibility (SM30 measurements, 10-6m3kg-1)
  Figure 5
  15
  y = 0.998x R2 = 0.703
  10
  5
  0 0
  5 10 15 -6 3 Magnetic susceptibility, this study (10 m kg-1)
  20
  Figure 5: Magnetic susceptibility data measured with our custom made susceptibility meter (average sample mass 1130 g for this subset of samples) versus magnetic susceptibility measured using a SM30 susceptibility meter on different samples (with average mass 19 g) of the same Apollo rocks. The dotted line is a linear fit between the two datasets.
  3.3. Paleomagnetic potential of this dataset The ratio of NRM to magnetic susceptibility, noted ß hereafter, may convey information about lunar paleomagnetism. Indeed, high ß values may point to more single domain like behavior (for a given paleofield) or to higher paleofields (for a given magnetic mineralogy). 92
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  4
  The paleointensity of 23 selected samples measured in this study have already been determined in previous laboratory studies (part S8, Table S4). ß appears broadly correlated with the paleointensity (Figure 6). However, a trend line does not intercept at the origin (B,ß)=(0,0). Indeed probably because of secondary magnetization components, hardly any samples have ß below 1. Qualitatively, we estimate that ß values below 4 have no real paleointensity significance and may correspond to null or weak paleointensity. Selecting samples with ß ≥4, and forcing the x-intercept of the linear fitting to ß=4, we have a relation B = 9.7 * (ß-4) (R2=0.63) with B in μT and ß in A/m. This relation indicates that the overall ß dataset may be informative about the overall evolution of lunar paleointensities and allows using ß as a crude paleointensity proxy. 120
  B=9.7*(ß-4)
  100
  Paleointensity, literature data (μT)
  Figure 6
  80
  60
  40
  20
  0 0
  2
  4
  6
  8
  10
  ß
  12
  14
  16
  18
  (Am-1)
  Figure 6: Paleointensity estimates (literature data, dark circles: paleointensity based on AF methods, grey circles: paleointensity based on thermal methods) as a function of ß (with error bars). When several estimates are available the average paleointensity is used. When both AF- and thermal-based results are available, the AF-based result is used, except for a recent result on 15015 that was obtained using oxygen fugacity control (Wang et al,. 2017). The single pTRM result of Brechet 1974 for 74275 was not used. The shaded area corresponds to ß <4 (see section 3.3). Sample 14305 was not included
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  in this plot because of its peculiar magnetic mineralogy
  (NRM being carried by cohenite; e.g
  Cournède et al., 2012) that sets it apart from all other Apollo samples.
  However, there are various reasons why ß may not be a good proxy for the paleointensity. First, the minerals that carry the magnetic susceptibility signal are not necessarily the same ones that carry the NRM. This may introduce a mineralogical bias in the interpretation. Unfortunately, the formation ages of the various lithologies are clustered in discrete and mostly non overlapping periods so that this lithological bias is difficult to circumvent. Second, it would be better to normalize NRM by ferromagnetic susceptibility only, since the paramagnetic minerals contribute to the susceptibility but not to the remanence. As we do not know the paramagnetic susceptibility of most Apollo rocks, however, we chose not to apply a blanket correction to the susceptibility that may introduce an additional source of uncertainty. Third, we measured the total NRM of the samples without any magnetic cleaning. Therefore these rocks may be affected by secondary magnetizations (i.e., not acquired on the Moon) like viscous remanent magnetization (VRM) acquired during the ~50 years stay of the Apollo rocks in the geomagnetic field, but also by all magnetic contaminations that may have occurred at differents stages of the Apollo samples' history (during sampling on the Moon, in the return spacecraft, during cutting and processing at JSC). Regarding VRM, it is noteworthy that the studied samples were kept in a mumetal container in ambient field < 200 nT for at least 24 hours before their NRM was measured, allowing for relaxation of about 50% of a 50 year VRM assuming a similar acquisition and decay rate for VRM. As an example we measured in the laboratory (CEREGE) the NRM and the magnetic susceptibility of a small piece of sample 14169 (73 mg). The NRM (6.45 x 10-5 Am2kg-1) was measured using a SQUID cryogenic magnetometer and the susceptibility (2.34 x 10-6 m3kg-1) using a MFK1 instrument. The NRM was then stepwise demagnetized using alternating fields (AF). This particular sample has a large secondary component (possibly a VRM), removed between 2 mT and 6 mT AF (Figure 7a). As a consequence ß decreases rapidly during AF demagnetization and reaches much lower values than those obtained on the whole rock at LSLF (Figure 7b). This ß value of 16.5, measured on a 72.4 g sample, is clearly dominated by the secondary component of the sample. It is therefore hardly related to the high coercivity component of magnetization which would relate to the lunar surface field during the formation of this rock on the Moon. This effect can enhance ß for some samples with no relation to the paleofield intensity. Finally the possibility that the large block magnetic 94
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  4
  susceptibility is biased toward higher value by surface contamination and coating by regolith material may add another bias since this contamination will increase the magnetic susceptibility but not necessarily the bulk NRM. Figure 7 a)
  b)
  Sample Sample 14169 14169B-2a N Up Sample 14169, 73 mg 73 mg
  30 25
  W
  ß (A.m-1)
  20
  E
  6 mT
  15 10
  3 mT
  5 0
  0
  10
  20
  30
  40
  50
  60
  70
  80
  90
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  Alternating field (mT)
  0NRM mT Scale markScale = S Down : 1e-9 Am 10-9 Am2
  2
  Figure 7: Laboratory NRM measurements for Apollo rock 14169. a) Orthogonal projection plot for a 73 mg sub-sample. b) ß (A.m-1) as a function of alternating field. The dashed line is the ß value for the 72.4 g block measured at JSC.
  A plot of ß as a function of time reveals different behaviors over the high-field epoch (between 4.25 and 3.56 Ga in literature) and weak-field epoch (after ~3.5 Ga) discussed above (Figure 8). There is more scatter in the high-field epoch with 7/40 samples with high ß value (ß > 10 A/m, including samples 14053 and 14305 with ß > 20 A/m) between 4 and 3 Ga (Figure 8). However, 23 samples, accounting for 64% of samples in this period, have low ß values (ß < 5 A/m). A majority of samples have ß < 5 A/m in the weak-field epoch (18/48 samples higher than 5 A/m) and only 1 sample has ß > 20 A/m (sample 12010 with ß = 25.2 A/m). A median value computed on a sliding window of 15 ß values (grey line in Figure 8) agrees qualitatively with the existence of the high-field and weak-field epochs. It is noteworthy that a significant fraction of samples from the high-field epoch have low ß values (23/40 with ß < 5 A/m). They may correspond to samples with low paleointensities, Evolution temporelle du champ magnétique lunaire 95 4
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  suggesting that the average field in the high field epoch may have been overestimated in previous studies because of the focus on highly magnetized samples. Alternatively they could correspond to samples whose original NRM has been partially or totally erased by a later process. Shock in particular is able to remagnetize rocks and in view of the low efficiency of shock magnetization compared to TRM (e.g., Tikoo et al., 2015), the result of shock remagnetization would be a drastic decrease of ß regardless of the intensity of the lunar surface field at the time of impact. Other explanation for low ß could be post NRM acquisition "cold" brecciation (without resetting of radiometric ages but clast-scale randomization of the NRM) and recent contamination by metal rich regolith. With all these caveats in mind, the dataset presented in Figure 8 allows selecting
  Figure 8
  samples for laboratory studies addressing specific questions about the lunar surface field evolution. 30 Regolith breccia Impact melt breccia 25
  Dimict breccia
  200
  Impact melt rock
  ß (A.m-1)
  Feldspathic rock
  15
  100
  10
  50
  Paleointensity equivalence (μT)
  Basalt
  20
  20 10
  5
  Not defined
  0 4
  3
  2
  1
  0
  Time before present (Ga)
  Figure 8: ß (with error bars) as a function of the age of studied rocks. Color-coding refers to lithology. The grey line corresponds to a sliding median, calculated on 15 successive ß values. The right axis indicates the approximate paleointensity for ß > 4 (broken line, see section 3.3 and fig. 6).
  96
  Evolution temporelle du champ magnétique lunaire Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  4
  4. Conclusions Using two portable instruments specifically designed for this project, we obtained magnetic susceptibility and NRM measurements on 154 and 123 Apollo whole samples, respectively. This dataset represents a significant increase in number (+52% for NRM and +64% for susceptibility) and in mass (80 kg against 20.5 g for the literature dataset) of the existing database of lunar magnetic properties. Our lithology-averaged NRM values may be more representative of the large-scale magnetic parameters than the values measured on small samples usually well below 1 gram that appears overestimated by an average factor of ~2.7. The NRM values obtained in this work are therefore more suitable to model and interpret the lunar magnetic field anomalies. Conversely, our magnetic susceptibility estimates may be overestimated for some weakly magnetic lithologies because of coating by glass or regolith material on bulk samples. Overall, magnetic susceptibility is strongly controlled by lithology and is clearly enhanced by impact processing and associated incorporation of meteoritic metal. The ratio of natural remanence and magnetic susceptibility may be used as a crude paleointensity proxy. Its overall evolution with time is coherent with the existence of the high field and weak field epochs defined in the literature. However our data raise the possibility that the paleointensity during the high field epoch may have been more variable and lower on average than previously suggested. Although this ratio cannot provide robust paleointensities, our dataset represents a valid tool to select samples of interest for detailed paleomagnetic laboratory study. Acknowledgements This work was funded by the Agence Nationale de la Recherche (project Maglune, ANR-14CE33-0012) and the Programme National de Planétologie (INSU/CNES).
  Evolution temporelle du champ magnétique lunaire 97 4
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
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  Rochette, P., Gattacceca, J., Ivanov, A.V., Nazarov, M.A. and Bezaeva, N.S., 2010. Magnetic properties of lunar materials: Meteorites, Luna and Apollo returned samples. Earth and Planetary Science Letters 292 (383-391), doi:10.1016/j.epsl.2010.02.007. Shea, E.K., Weiss, B.P., Cassata, W.S., Shuster, D.L., Tikoo, S.M., Gattacceca, J., Grove, T.L., and Fuller, M.D., 2012. A long-lived lunar core dynamo. Science 335 (453), doi: 10.1126/science.1215359. Sonett, C.P., Colburn, D.S., Currie, R. G. and Mihalov. J.D., 1967. The geomagnetic tail, topology, reconnection and interaction with the Moon. Physics of the Magnetosphere (461484). Steiger, R.H. and Jäger, E., 1977. Subcommission of geochronology : Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters 36 (359-362). Suavet, C., Weiss, B.P., Cassata, W.S., Shuster, D.L., Gattacceca, J., Chan, L., GarrickBethell, I., Head, J.W., Grove, T.L., and Fuller, M.D., 2013. Persistence and origin of the lunar core dynamo. Earth, Atmospheric, and Planetary Sciences 110 (8453-8458). Tikoo, S.M., Weiss, B.P., Buz, J., Lima, E.A., Shea, E.K., Melo, G., and Grove, T.L., 2012a. Magnetic fidelity of lunar samples and implications for an ancient core dynamo. Earth and Planetary Science Letters 337-338 (93-103). Tikoo, S.M., Weiss, B.P., Cassata, W.S., Shuster, D.L., Gattacceca, J., Lima, E.A., Suavet, C., Nimmo, F., and Fuller, M.D., 2014. Decline of the lunar core dynamo. Earth and Planetary Science Letters 404 (89-97). Tikoo, S.M., Gattacceca, J., Swanson-Hysell, N.L., Weiss, B.P., Suavet, C. and Cournède, C., 2015. Preservation and detectability of shock-induced magnetization. Journal of Geophysical Research: Planets 120 (1461-1475), doi:10.1002/2015JE004840. Tikoo, S.M., Weiss, B.P., Shuster, D.L., Suavet, C., Wang, H., and Grove, T.L., 2017. A twobillion-year history for the lunar dynamo. Science Advances 3, e1700207. Uehara, M., Gattacceca, J., Quesnel, Y., Lepaulard, C., Lima, E.A., Manfredi, M., and Rochette, P., 2017. A spinner magnetometer for large Apollo lunar samples. Review of Scientific Instruments 88 (104502). Wang, H., Mighani, S., Weiss, B.P., Shuster, D.L., and Hodges, K.V., 2017. Lifetime of the lunar dynamo constrained by young Apollo returned breccias 15015 and 15465. Lunar and Planetary Science XLVIII. Weiss, B.P., and Tikoo, S.M., 2014. The lunar dynamo. Science 346, 1246753, doi: 10.1126/science.1246753.
  Evolution temporelle du champ magnétique lunaire 99 100
  -
  1.19
  0.10
  0.53
  1.21
  1.36
  1.45
  -
  1.76
  0.79
  15015.0
  15028.0
  15059.0
  15086.0
  15265.0
  15266.0
  15285.0
  15287.0
  15295.0
  2.75
  14307.19
  14315.0
  3.79
  14301.0
  -
  -
  14309.0
  -
  0.84
  12034.9
  14055.0
  1.93
  12010.12
  14045.0
  1.42
  10068.5
  -
  -
  10065.7
  14042.0
  1.80
  10064.6
  -
  1.43
  10061.18
  14041.0
  1.76
  10060.6
  -
  1.88
  10046.193
  12073.93
  1.09
  10019.1
  1.56
  10018.0
  breccia
  Age (Ga)
  Regolith
  Sample
  Evolution temporelle du champ magnétique lunaire
  0.43
  0.43
  -
  0.43
  0.04
  0.02
  0.43
  0.32
  -
  -
  -
  0.01
  0.30
  -
  -
  -
  -
  -
  0.43
  0.43
  0.03
  -
  0.43
  0.01
  0.03
  0.43
  0.43
  0.43
  Error (Ga)
  68.2
  37.3
  220.7
  252.3
  180.0
  33.1
  487.7
  56.6
  2870.0
  84.4
  39.1
  21.6
  723.6
  85.1
  56.0
  89.7
  158.5
  65.7
  51.5
  263.1
  96.4
  145.1
  38.0
  81.0
  478.6
  120.2
  167.0
  196.4
  57.18
  50.94
  15.29
  39.64
  42.78
  7.25
  65.61
  -
  26.83
  17.77
  12.53
  38.43
  -
  54.63
  53.57
  83.65
  137.26
  47.92
  6.41
  121.63
  124.48
  103.38
  78.95
  209.88
  172.37
  224.66
  157.15
  178.21
  1.72
  1.53
  0.46
  1.19
  1.28
  0.22
  1.97
  -
  0.80
  0.53
  0.38
  1.15
  -
  1.64
  1.61
  2.51
  4.12
  1.44
  0.19
  3.65
  3.73
  3.10
  2.37
  6.30
  5.17
  6.74
  4.71
  5.35
  9.6
  5.4
  6.0
  5.5
  3.0
  13.3
  7.0
  -
  11.7
  8.7
  11.1
  7.2
  -
  8.0
  6.0
  6.0
  6.0
  7.0
  13.8
  4.0
  7.8
  6.7
  6.7
  4.3
  9.0
  6.0
  6.0
  4.0
  15.34
  10.36
  11.40
  12.45
  15.28
  9.93
  23.36
  3.37
  6.34
  2.65
  4.30
  16.86
  3.26
  28.20
  34.05
  25.70
  28.77
  17.38
  4.30
  4.83
  52.85
  33.37
  28.25
  70.63
  73.84
  34.67
  23.87
  45.46
  2.96
  2.03
  0.90
  2.37
  2.90
  1.92
  4.44
  0.77
  1.20
  0.52
  0.92
  3.33
  0.62
  2.29
  0.36
  2.17
  0.31
  0.19
  0.86
  0.92
  10.03
  6.34
  5.37
  13.42
  1.11
  0.37
  0.26
  8.64
  Mass NRM Error NRM Erreur fit χ Error χ (g) (10-6Am2kg-1) (10-6Am2kg-1) NRM (%) (10-6m3kg-1) (10-6m3kg-1)
  S1: Results of magnetic measurements on whole rocks at JSC, Houston
  3.73
  4.92
  1.34
  3.18
  2.80
  0.73
  2.81
  -
  4.23
  6.71
  2.91
  2.28
  -
  1.94
  1.57
  3.26
  4.77
  2.76
  1.49
  25.19
  2.36
  3.10
  2.79
  2.97
  2.33
  6.48
  6.58
  3.92
  NRM/χ
  0.73
  0.98
  0.11
  0.61
  0.54
  0.14
  0.54
  -
  0.81
  1.33
  0.63
  0.45
  -
  0.17
  0.05
  0.29
  0.15
  0.09
  0.30
  4.85
  0.45
  0.60
  0.54
  0.57
  0.08
  0.21
  0.21
  0.75
  error
  [16]
  [16]
  -
  [16]
  [16]
  [16]
  [16]
  [16]
  [17]
  -
  -
  [16]
  [16]
  -
  -
  -
  -
  -
  [16]
  [16]
  [16]
  -
  [16]
  [16]
  [16]
  [16]
  [16]
  [16]
  Ref age
  Susceptibilimeter Susceptibilimeter Susceptibilimeter MS2C
  ArTr ArTr ArTr -
  Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter
  ArTr ArTr Ar-Ar* ArTr ArTr ArTr ArTr ArTr ArTr ArTr
  2
  MS2C Susceptibilimeter
  -
  MS2C
  Susceptibilimeter
  -
  Susceptibilimeter
  Susceptibilimeter
  ArTr
  -
  Susceptibilimeter
  ArTr
  -
  MS2C Susceptibilimeter
  MS2C
  ArTr ArTr
  Susceptibilimeter
  ArTr ArTr
  χ measuring device used
  Dating method
  4 Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  Supplementary files
  S1: Results of magnetic measurements on whole rocks at JSC, Houston 1.70
  3.77
  3.41
  2.42
  -
  60255.45
  61135.1
  61195.3
  61295.11
  65095.21
  3.83
  3.83
  14305.18
  14305.27
  -
  14264.0
  3.95
  3.85
  14169.0
  14304.134
  4.22
  14082.0
  3.82
  3.80
  14066.0
  14303.288
  3.63
  14051.0
  breccia
  -
  79175.0
  3.94
  1.82
  79135.0
  12013.11
  1.82
  79115.0
  Impact melt
  1.66
  79035.45
  -
  3.35
  60019.4
  70295.0
  3.74
  60016.196
  -
  1.11
  15558.22
  70175.0
  0.10
  15505.0
  3.56
  1.32
  15498.141
  3.65
  2.35
  15459.0
  66035.0
  0.38
  15299.0
  65715.0
  0.28
  15298.0
  0.03
  0.03
  0.04
  0.04
  -
  0.03
  0.01
  0.03
  0.05
  0.07
  -
  0.05
  0.05
  0.03
  -
  -
  0.43
  0.43
  -
  0.43
  0.43
  0.43
  0.43
  0.43
  0.43
  0.43
  0.01
  0.43
  0.43
  0.33
  0.43
  926.3
  332.3
  363.0
  462.9
  176.5
  114.8
  72.4
  29.3
  364.4
  185.2
  60.0
  547.2
  1433.9
  180.4
  1481.3
  317.5
  310.3
  173.2
  26.9 -
  240.1
  71.6
  345.4
  94.5
  672.9
  386.3
  231.9
  944.2
  842.3
  227.8
  2122.3
  1133.6
  10.38
  85.40
  11.88
  -
  -
  85.64
  -
  -
  7.56
  -
  7.49
  34.87
  77.61
  151.89
  40.94
  4.83
  178.98
  32.90
  -
  81.07
  -
  10.55
  11.91
  12.94
  69.11
  225.57
  13.83
  61.25
  83.80
  80.97
  -
  0.31
  2.56
  0.36
  -
  -
  2.57
  -
  -
  0.23
  -
  0.22
  1.05
  2.33
  4.56
  1.23
  0.15
  5.37
  0.99
  -
  2.43
  -
  0.32
  0.36
  0.39
  2.07
  6.77
  0.41
  1.84
  2.51
  2.43
  14.0
  8.9
  12.1
  -
  -
  4.1
  -
  -
  9.6
  -
  7.9
  6.6
  3.8
  4.0
  8.0
  12.8
  5.8
  -
  5.1
  -
  7.3
  -
  8.2
  4.7
  7.8
  9.0
  8.5
  12.0
  10.3
  3.5
  8.0
  -
  3.64
  4.11
  3.89
  2.89
  5.19
  2.42
  9.44
  5.36
  0.97
  7.38
  20.76
  26.83
  40.80
  22.04
  2.85
  9.81
  5.38
  10.03
  6.96
  6.40
  3.77
  8.61
  5.39
  5.83
  12.50
  25.24
  -
  7.29
  15.57
  36.20
  _
  0.69
  0.78
  0.04
  0.03
  1.00
  0.60
  1.80
  1.03
  0.01
  1.40
  3.95
  5.10
  0.36
  4.19
  0.54
  1.66
  1.34
  1.91
  0.07
  1.22
  0.72
  1.64
  1.02
  1.11
  0.19
  4.80
  -
  1.39
  2.96
  0.49
  -
  23.47
  2.89
  -
  -
  16.50
  -
  -
  1.41
  -
  1.02
  1.68
  2.89
  3.72
  1.86
  1.70
  18.25
  -
  3.28
  -
  12.67
  -
  1.23
  2.21
  2.22
  5.53
  8.94
  -
  8.40
  5.38
  2.24
  -
  4.52
  0.56
  -
  -
  3.20
  -
  -
  0.27
  -
  0.20
  0.32
  0.56
  0.12
  0.36
  0.33
  3.13
  -
  0.63
  -
  2.45
  -
  0.24
  0.43
  0.43
  0.19
  1.72
  -
  1.62
  1.04
  0.07
  [22]
  [22]
  [18]
  [18]
  -
  [21]
  [21]
  [20]
  [19]
  [18]
  -
  [16]
  [16]
  [16]
  -
  -
  [16]
  [16]
  -
  [16]
  [16]
  [16]
  [16]
  [16]
  [16]
  [16]
  [16]
  [16]
  [16]
  [16]
  [16]
  Susceptibilimeter -
  Ar-Ar * Ar-Ar *
  3
  MS2C Susceptibilimeter
  MS2C Ar-Ar *
  Susceptibilimeter Ar-Ar *
  Susceptibilimeter ◊
  Susceptibilimeter
  Susceptibilimeter
  Susceptibilimeter -
  U-Pb
  Susceptibilimeter ArTr
  Ar-Ar *
  Susceptibilimeter ArTr
  MS2C
  Susceptibilimeter ArTr
  Susceptibilimeter
  Susceptibilimeter -
  Ar-Ar *
  Susceptibilimeter -
  Ar-Ar *
  Susceptibilimeter
  Susceptibilimeter
  ArTr
  ArTr
  Susceptibilimeter
  ArTr
  ArTr
  Susceptibilimeter
  ArTr
  MS2C
  Susceptibilimeter
  ArTr
  Susceptibilimeter
  Susceptibilimeter
  ArTr
  -
  Susceptibilimeter
  ArTr
  ArTr
  Susceptibilimeter
  Susceptibilimeter
  ArTr ArTr
  Susceptibilimeter
  ArTr ArTr
  Susceptibilimeter
  ArTr
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks 4
  Evolution temporelle du champ magnétique lunaire 101 102
  -
  14319.0
  Evolution temporelle du champ magnétique lunaire
  -
  3.87
  0.46
  4.20
  67235.0
  67435.7
  67627.0
  67955.1
  3.87
  3.90
  4.01
  3.89
  3.89
  3.83
  3.86
  3.86
  72215.14
  72255.23
  72275.102
  72395.7
  72535.0
  73215.2
  73235.9
  73275.1
  -
  3.86
  67016.1
  68115.1
  3.81
  66055.26
  -
  3.83
  65015.25
  67975.22
  3.79
  62235.11
  -
  -
  60018.25
  60115.1
  -
  60017.52
  -
  3.83
  15455.37
  60095.0
  1.28
  15405.0
  3.99
  -
  15206.0
  60035.0
  -
  15205.0
  3.84
  -
  14314.0
  14321.37
  3.60
  14311.0
  0.04
  0.04
  0.04
  0.02
  0.02
  0.04
  0.03
  0.02
  -
  -
  0.07
  0.03
  0.01
  -
  0.10
  0.02
  0.04
  0.03
  -
  -
  0.10
  -
  -
  0.04
  0.04
  -
  -
  0.05
  -
  -
  0.15
  250.2
  500.8
  365.5
  140.4
  251.9
  1253.7
  120.5
  172.1
  759.7
  220.4
  54.7
  42.2
  179.1
  886.8
  2494.0
  673.0
  1322.3
  181.2
  83.3
  39.9
  677.4
  692.3
  1229.6
  199.2
  201.3
  55.9
  136.2
  761.2
  156.6
  96.0
  2421.4
  2.19
  5.20
  4.79
  -
  2.56
  7.94
  59.03
  11.62
  8.72
  9.28
  6.81
  3.98
  9.24
  55.83
  20.30
  1.56
  35.66
  73.74
  71.74
  17.10
  11.27
  3.84
  72.22
  3.42
  -
  -
  -
  2.72
  2.89
  24.43
  13.29
  0.16
  0.14
  -
  0.077
  0.24
  1.77
  0.35
  0.26
  0.28
  0.20
  0.12
  0.28
  1.68
  0.61
  0.047
  1.07
  2.21
  2.15
  0.51
  0.34
  0.12
  2.17
  0.102
  -
  -
  -
  0.08
  0.09
  0.73
  0.40
  0.066
  8.4
  10.5
  -
  14.1
  8.3
  6.8
  6.9
  10.1
  15.0
  5.4
  15.0
  12.8
  4.9
  12.5
  15.6
  4.7
  11.0
  3.8
  6.0
  7.0
  13.8
  3.8
  8.7
  -
  -
  -
  15.2
  10.4
  11.0
  11.0
  12.1
  4.73
  1.06
  0.97
  3.69
  3.70
  6.31
  4.53
  2.89
  10.20
  3.22
  -
  5.97
  11.55
  10.66
  0.66
  6.73
  9.29
  5.56
  4.88
  8.92
  2.79
  6.87
  0.15
  1.14
  1.05
  1.11
  1.12
  3.80
  3.06
  2.94
  1.72
  0.90
  0.21
  0.20
  0.71
  0.71
  1.20
  0.89
  0.57
  0.30
  0.61
  -
  1.16
  2.20
  2.03
  0.13
  1.28
  0.41
  1.06
  0.05
  0.10
  0.01
  1.30
  0.03
  0.26
  0.30
  0.82
  0.21
  0.72
  0.03
  0.03
  0.33
  1.10
  4.52
  -
  0.69
  2.15
  9.36
  2.57
  3.01
  0.91
  2.12
  -
  1.55
  4.84
  1.90
  2.36
  5.30
  7.94
  12.91
  3.50
  1.26
  1.38
  10.52
  23.24
  -
  -
  -
  2.42
  0.76
  7.99
  7.16
  1.27
  0.21
  0.90
  -
  0.14
  0.42
  1.80
  0.51
  0.60
  0.04
  0.41
  -
  0.31
  0.93
  0.37
  0.45
  1.02
  0.42
  2.49
  0.11
  0.04
  0.04
  2.02
  4.91
  -
  -
  -
  0.47
  0.15
  0.25
  0.23
  0.24
  [18]
  [18]
  [32]
  [21]
  [21]
  [18]
  [18]
  [31]
  -
  -
  [30]
  [20]
  [29]
  -
  [28]
  [27]
  [18]
  [26]
  -
  -
  [25]
  -
  -
  [24]
  [23]
  -
  -
  [18]
  -
  -
  [19]
  Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter Susceptibilimeter
  Ar-Ar * Rb-Sr Ar-Ar * Ar-Ar * Ar-Ar * Ar-Ar * Ar-Ar *
  4
  Susceptibilimeter
  Ar-Ar *
  Susceptibilimeter -
  Susceptibilimeter Ar-Ar *
  Susceptibilimeter
  Susceptibilimeter Ar-Ar *
  -
  Susceptibilimeter -
  Nd-Sm
  Susceptibilimeter
  MS2C
  -
  Ar-Ar *
  Susceptibilimeter
  Ar-Ar *
  Susceptibilimeter
  Susceptibilimeter
  -
  Ar-Ar *
  Susceptibilimeter
  -
  Ar-Ar *
  Susceptibilimeter
  Ar-Ar *
  MS2C
  Susceptibilimeter
  Ar-Ar *
  Susceptibilimeter
  Susceptibilimeter
  -
  Ar-Ar *
  Susceptibilimeter
  -
  -
  MS2C Susceptibilimeter
  Ar-Ar *
  MS2C
  -
  Susceptibilimeter
  Ar-Ar *
  4 Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks -
  3.79
  3.75
  77035.23
  77115.15
  77135.86
  -
  65035.0
  3.04
  3.63
  3.45
  3.01
  3.17
  3.03
  3.11
  3.17
  0.65
  3.06
  10017.15
  10057.19
  10072.16
  12004.1
  12005.0
  12008.2
  12009.17
  12016.0
  12018.15
  12020.22
  3.76
  68415.3
  3.82
  3.71
  66095.13
  10003.25
  3.87
  65055.4
  Basalt
  3.78
  60315.46
  rock
  3.79
  14310.35
  Impact melt
  3.90
  3.89
  64535.0
  67915.11
  -
  64475.1
  breccia
  3.81
  61015.0
  Dimict
  3.78
  3.74
  77017.18
  79215.2
  3.85
  76215.0
  -
  3.88
  76055.0
  77515.0
  -
  3.84
  76035.0
  76015.18
  0.06
  -
  0.02
  0.07
  0.07
  0.02
  0.11
  0.08
  0.02
  0.01
  0.03
  0.04
  0.05
  0.02
  0.03
  0.06
  0.05
  -
  0.02
  -
  0.04
  0.04
  -
  0.03
  0.05
  -
  0.02
  0.04
  0.04
  -
  0.04
  195.0
  308.3
  1444.1
  280.5
  24.5
  400.4
  422.0
  38.1
  132.4
  173.3
  111.6
  110.1
  459.4
  195.8
  526.6
  1438.1
  396.2
  210.4
  228.6
  543.3
  953.5
  295.4
  305.5
  230.1
  51.3
  3002.9
  67.6
  258.7
  4688.2
  283.0
  1307.2
  8.21
  4.87
  -
  1.25
  -
  21.48
  5.21
  -
  -
  8.66
  15.23
  19.07
  67.48
  2.45
  74.06
  4.66
  -
  2.71
  8.75
  13.44
  104.88
  -
  2.70
  -
  -
  1.30
  16.28
  2.40
  36.26
  2.30
  4.67
  0.25
  0.15
  -
  0.037
  -
  0.64
  0.16
  -
  -
  0.26
  0.46
  0.57
  2.02
  0.074
  2.22
  0.14
  -
  0.081
  0.26
  0.40
  3.15
  -
  0.081
  -
  -
  0.039
  0.49
  0.072
  1.09
  0.07
  0.14
  7.7
  5.1
  -
  12.7
  -
  8.7
  10.6
  -
  -
  4.7
  3.4
  3.8
  9.8
  14.0
  5.5
  6.2
  -
  10.0
  16.7
  12.2
  6.7
  -
  14.0
  -
  -
  9.9
  4.1
  6.7
  5.6
  13.5
  15.1
  1.56
  2.71
  0.68
  0.87
  2.77
  2.48
  2.99
  3.15
  1.96
  1.47
  2.57
  1.88
  10.15
  4.32
  13.15
  1.57
  2.03
  4.79
  2.44
  10.63
  10.60
  0.72
  3.27
  3.53
  3.29
  3.77
  3.49
  4.28
  4.69
  4.52
  3.23
  0.32
  0.52
  0.13
  0.18
  0.85
  0.47
  0.57
  0.81
  0.40
  0.29
  0.53
  0.02
  1.93
  0.82
  2.50
  0.30
  0.39
  0.91
  0.47
  2.02
  2.01
  0.15
  0.04
  0.67
  0.74
  0.72
  0.71
  0.82
  0.89
  0.86
  0.61
  5.24
  1.80
  -
  1.43
  -
  8.67
  1.74
  -
  -
  5.87
  5.94
  16.48
  6.65
  0.57
  5.63
  2.97
  -
  0.57
  3.58
  1.26
  9.89
  -
  0.83
  -
  -
  0.34
  4.66
  0.56
  7.74
  0.51
  1.45
  1.08
  0.35
  -
  0.30
  -
  1.67
  0.34
  -
  -
  1.18
  1.23
  0.52
  1.28
  0.11
  1.08
  0.57
  -
  0.11
  0.69
  0.24
  1.90
  -
  0.03
  -
  -
  0.07
  0.96
  0.11
  1.49
  0.10
  0.28
  [18]
  [35]
  [21]
  [35]
  [35]
  [21]
  [37]
  [18]
  [18]
  [18]
  [18]
  [36]
  [18]
  [32]
  [26]
  [35]
  [18]
  -
  [32]
  -
  [26]
  [25]
  -
  [34]
  [18]
  -
  [27]
  [33]
  [18]
  -
  [33]
  Susceptibilimeter Susceptibilimeter Susceptibilimeter
  Ar-Ar * Ar-Ar *
  5
  Susceptibilimeter
  Susceptibilimeter
  Ar-Ar *
  ◊
  Susceptibilimeter
  Ar-Ar *
  Susceptibilimeter
  Susceptibilimeter
  Ar-Ar *
  Ar-Ar *
  Susceptibilimeter
  Ar-Ar *
  Susceptibilimeter
  Susceptibilimeter
  Ar-Ar *
  Ar-Ar *
  Susceptibilimeter
  -
  Susceptibilimeter
  Susceptibilimeter
  Ar-Ar *
  ◊
  Susceptibilimeter
  -
  Susceptibilimeter
  Susceptibilimeter
  Ar-Ar *
  Rb-Sr
  Susceptibilimeter
  Ar-Ar *
  Susceptibilimeter
  Susceptibilimeter
  -
  Ar-Ar *
  Susceptibilimeter
  Ar-Ar *
  Susceptibilimeter
  Susceptibilimeter
  Ar-Ar *
  Rb-Sr
  Susceptibilimeter
  -
  Rb-Sr
  Susceptibilimeter
  Ar-Ar *
  MS2C
  Susceptibilimeter
  Ar-Ar *
  Susceptibilimeter
  Susceptibilimeter
  Ar-Ar *
  Ar-Ar *
  Susceptibilimeter
  -
  Ar-Ar *
  Susceptibilimeter
  Ar-Ar *
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks 4
  Evolution temporelle du champ magnétique lunaire 103 104
  Evolution temporelle du champ magnétique lunaire
  3.21
  3.21
  3.21
  3.33
  3.33
  15555.49
  15555.1039
  15555.913
  15556.0
  15556.215
  60015.1
  3.43
  3.95
  3.74
  75075.7
  15418.0
  3.85
  74275.2
  rocks
  3.75
  71567.0
  Feldspathic
  -
  71035.0
  3.75
  3.21
  15555.53
  70215.261
  3.30
  15529.0
  3.66
  -
  15476.0
  70035.0
  3.28
  15076.0
  -
  3.38
  15075.5
  3.72
  3.31
  15016.0
  70017.8
  3.85
  14053.0
  15597.0
  3.18
  -
  12062.0
  3.18
  2.92
  12052.2
  12065.17
  3.21
  12051.3
  12064.11
  3.05
  12038.7
  3.12
  3.05
  12022.15
  12063.29
  3.30
  12021.34
  0.05
  0.06
  0.04
  0.08
  -
  -
  0.04
  0.07
  0.12
  -
  0.10
  0.10
  0.06
  0.06
  0.06
  0.06
  -
  -
  0.04
  0.20
  0.08
  0.05
  0.05
  0.09
  0.05
  -
  0.18
  0.05
  0.05
  0.04
  0.10
  1861.4
  480.1
  192.6
  649.9
  107.3
  141.8
  1882.0
  1539.1
  1424.0
  107.3 -
  354.0
  619.3
  309.9
  66.8
  147.4
  201.4
  1451.9
  206.9
  239.5
  171.5
  313.1
  49.3
  1134.6
  740.2
  1172.2
  720.1
  1101.3
  753.8
  327.5
  1135.6
  1284.0
  1.83
  -
  35.31
  1.85
  22.37
  31.73
  3.35
  12.99
  19.66
  2.82
  5.69
  -
  -
  -
  -
  9.64
  3.38
  -
  4.55
  -
  243.36
  8.20
  4.66
  6.57
  10.69
  13.62
  5.84
  2.75
  6.16
  27.26
  -
  0.055
  -
  1.06
  0.055
  0.67
  0.95
  0.100
  0.39
  0.59
  0.085
  0.17
  -
  -
  -
  -
  0.29
  0.10
  -
  0.14
  -
  7.30
  0.25
  0.14
  0.20
  0.32
  0.41
  0.18
  0.082
  0.18
  0.82
  17.1
  -
  7.6
  8.4
  6.5
  6.8
  19.0
  9.5
  9.5
  -
  15.1
  9.0
  -
  -
  -
  -
  10.3
  13.8
  -
  12.3
  -
  14.0
  6.7
  12.0
  11.7
  8.0
  4.3
  17.9
  3.8
  9.8
  10.3
  0.50
  0.40
  3.26
  2.04
  2.17
  1.15
  0.86
  2.33
  2.09
  1.26
  1.25
  -
  1.17
  1.49
  1.46
  1.52
  1.27
  1.18
  1.49
  1.41
  1.20
  9.80
  0.74
  1.10
  1.41
  1.52
  1.65
  0.64
  1.66
  1.08
  1.72
  0.10
  0.08
  0.63
  0.39
  0.02
  0.27
  0.16
  0.44
  0.40
  0.27
  0.24
  -
  0.23
  0.02
  0.02
  0.02
  0.24
  0.23
  0.30
  0.28
  0.23
  0.10
  0.14
  0.01
  0.27
  0.29
  0.31
  0.12
  0.32
  0.21
  0.33
  3.64
  -
  10.83
  0.91
  16.29
  27.63
  3.88
  5.57
  9.42
  -
  3.58
  -
  -
  -
  -
  -
  7.61
  2.87
  -
  3.24
  -
  24.83
  11.12
  4.25
  4.66
  7.03
  8.23
  9.10
  1.65
  5.71
  15.89
  0.71
  -
  2.10
  0.18
  0.52
  6.49
  0.75
  1.07
  1.81
  -
  0.44
  -
  -
  -
  -
  -
  1.47
  0.58
  -
  0.66
  -
  0.79
  2.15
  0.14
  0.90
  1.36
  1.58
  1.76
  0.32
  1.10
  3.06
  [18]
  [35]
  [46]
  [45]
  [44]
  -
  [18]
  [35]
  [42]
  -
  [43]
  [43]
  [18]
  [18]
  [18]
  [18]
  [21]
  -
  [35]
  [42]
  [41]
  [39]
  [18]
  [40]
  [18]
  -
  [37]
  [39]
  [18]
  [18]
  [38]
  Susceptibilimeter Susceptibilimeter Susceptibilimeter
  Ar-Ar *
  Susceptibilimeter Susceptibilimeter Susceptibilimeter
  Ar-Ar * Ar-Ar * Ar-Ar *
  6
  MS2C Susceptibilimeter
  Susceptibilimeter Rb-Sr
  Susceptibilimeter Rb-Sr
  Susceptibilimeter Ar-Ar *
  Ar-Ar *
  Susceptibilimeter
  Susceptibilimeter
  Ar-Ar *
  Ar-Ar *
  MS2C
  Ar-Ar *
  Susceptibilimeter
  MS2C
  Ar-Ar *
  -
  MS2C
  ◊
  Ar-Ar *
  Susceptibilimeter
  -
  -
  Susceptibilimeter
  Ar-Ar *
  Susceptibilimeter
  Susceptibilimeter
  Ar-Ar *
  Ar-Ar *
  Susceptibilimeter
  Ar-Ar *
  Ar-Ar *
  MS2C Susceptibilimeter
  Ar-Ar *
  MS2C
  Susceptibilimeter
  Rb-Sr
  Susceptibilimeter
  Susceptibilimeter
  Ar-Ar *
  Rb-Sr
  Susceptibilimeter
  Ar-Ar *
  Ar-Ar *
  Susceptibilimeter
  Rb-Sr Ar-Ar *
  4 Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks -
  3.95
  3.95
  3.93
  65315.46
  67075.39
  77215.18
  78235.0
  0.21
  0.03
  0.05
  -
  -
  0.06
  44.3
  83.5
  66.6
  136.5
  213.7
  157.1
  7.67
  -
  -
  -
  -
  -
  0.23
  -
  -
  -
  -
  -
  12.6
  -
  -
  -
  -
  -
  7.78
  0.99
  2.01
  0.02
  2.28
  1.66
  1.63
  0.01
  0.42
  0.00
  0.44
  0.33
  0.99
  -
  -
  -
  -
  -
  0.21
  -
  -
  -
  -
  -
  [47]
  [18]
  [18]
  -
  -
  [18]
  SM30‡ Susceptibilimeter MS2C Susceptibilimeter
  Ar-Ar * Ar-Ar * U-Pb
  Susceptibilimeter
  -
  Susceptibilimeter
  Ar-Ar *
  Table S1: Compilation of the data obtained after measurements at NASA. Results with no measurement have the symbol "-". Samples whose NRM measurement was unsuccessful (see text) are noted "†". Some magnetic susceptibility results were only obtained by measurements with the SM30 and are noted "‡" (Macke et al., 2014). Ref age is the reference for the radiometric age. All ages noted "*" were recalculated with the calibration constant from Steiger and Jäger (1977). ArTr indicate model closure ages determined from the trapped 40Ar-36Ar ratio (Fagan et al., 2014). For samples noted "◊" we used a characteristic average age for the corresponding lithology and Apollo mission (Stöffler et al., 2006).
  -
  4.09
  60215.8
  60025.241
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks 4
  Evolution temporelle du champ magnétique lunaire 105 4
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  S2. Typical samples discussed in this work 2 cm
  0.5 cm
  b )
  a)
  Figure S1: a) Typical whole rock sample (dimict breccia 61015, 953.5 g sample) measured at JSC (Houston); b) Typical laboratory paleomagnetism sample (sub-sample of 61015, 87 mg, studied at CEREGE, France). S3. Large block susceptibility meter This instrument was designed at CEREGE by Minoru Uehara S3.1.Theory of operation We measure the magnetic susceptibility by detection of the induced magnetization of the sample installed at the center of Helmholtz coils by means of application of low-frequency magnetic field (5 Hz, 30 μT, sinusoidal). The low-frequency of magnetic field variation minimizes the absorption of the magnetic field by aluminum foils occasionally covering the samples. The samples are enclosed in cubic capsules and put on the horizontal sample stage to ensure the positioning. The components are made of acrylic resin (PMMA) and installed on the aluminum table (see Figure S2). A 1-axis fluxgate magnetic sensor mounted at 12 cm from the center of the coils measures the induced magnetization. The sensor, sample capsule, and coils are co-axial to each other. The magnetic fields and coil currents are simultaneously measured by 50 kSPS (samples-par-second) 24-bit simultaneous A/D converter (see Figure S3).
  106
  Evolution temporelle du champ magnétique lunaire Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  4
  Figure S2: The illustration of the magnetic susceptibility meter that consists of a pair of Helmholtz coils, a single axis flux-gate sensor (red arrow, co-axial to the coils) mounted in a plastic rod, and the sample cube set at the center of the coils. The coil support tube, tube holders, sample stage plate, and sample cube are in plastics (PMMA) to avoid eddy currents. The susceptibility meter is mounted on an aluminum table (5 mm thick) to keep distance (14 cm) from the stainless steel table used in the LSLF at JSC. The calculated distribution of magnetic fields by means of FEMM software (Meeker D.C., 2013, Finite Element Method Magnetics, Version 4.2, http://www.femm.info), normalized by the field intensity at the center of the coils, shows that the variations within the sample cube is less than +/- 4%. We normalize the measured magnetic field signals by the calculated applied field at the coil center by means of the coil current signals to reduce the noises of the applied field. The symbol R indicates this normalized value hereafter. The measurement protocol is as follows: (1) measure the background signal without sample on the stage (Bair1), (2) measure the sample on the stage (Bsample), (3) measure the background signal (Bair2), (4) compare Bair1 and Bair2 and retry the entire measurement if jumping of the background is detected, and (5) save the measured waveforms. The duration of the measurement is 5 seconds. The amplitude of the normalized signals is calculated by FFT for every 1 second, making 5 points for each measurement (R = {R1,  R5}, Fig. S4). This very narrow FFT filter eliminates any signals (noises, ambient field, magnetization of the sample etc.) other than the 5 Hz field synchronized to the applied field and achieves about 30 pTrms noise level. After correction of drifting by linear fitting, we calculated the average values of the backgrounds and the signal Evolution temporelle du champ magnétique lunaire 107 4
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  with sample. Finally, we calculate the difference between the average values of the signal with sample and the background, named k value (Eq. A1), k = R !"#$%& − R !"# (A1), where k is the averaged k value, R indicate the mean value of the drift-corrected R, and R !"# is the mean of Rair1 and Rair2. This k value is dimensionless and proportional to the volume magnetic susceptibility χv in SI unit, thus !
  !! = ! (A2), where ! is the calibration constant of the instrument. We calibrated ! by measurements of terrestrial rocks with known magnetic susceptibilities and different weight (N = 22, 10 g to 800 g) at CEREGE laboratory. The average (αCEREGE) and the standard deviation of α (σα) is 78.9 and 13.2, respectively. Theoretically, α is a function of the distance between the center of the coil and the sensor; indeed, we found no dependency of α on the sample weights. Since α can be varied by disassembly and assembly of the instrument, we cross-calibrated the
  Contr ol Voltag
  LPF1 DIFF fc 2nd fc=160Hz
  Coil Current I
  instrument by same standard sample in both CEREGE and NASA, and αNASA is 81.1.
  POW
  I Monitor DIFF
  D/A 24bit
  A/D
  SET
  R
  16bit
  USB
  Coil
  PC
  B I Monitor Sensor Outpu t
  F G
  4th fc=400Hz LPF3
  BUF
  LPF2 2nd fc=2.5 kHz
  Figure S3: The block diagram of the data acquisition system. The computer (PC) running LabView program controls the entire procedure. A 16-bit D/A converter generates sinusoidal waves to control the coil current I whose intensity is monitored and limited by the series resistance RS. The magnetic field B is monitored by the on-axis flux-gate sensor (FG). The flux-gate output is digitized by a 24-bit A/D converter after buffering and filtering. The current monitor signal (IMonitor) is acquired simultaneously.
  108
  Evolution temporelle du champ magnétique lunaire Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  4
  S3.2. Error on magnetic susceptibility There are two main sources of error of the measurement. The first is the error in the estimation in the proportionality constant !, which can be equal to the standard deviation σα, though this can be overestimated due to the shape anisotropy of the terrestrial samples. The second is the error in k, which can be defined by the standard deviation (σk) of several measurements. Since we do not always conduct repeated measurements, however, we cannot obtain the actual σk. Instead, we estimated this by the k value for individual measurement (ki), respectively given by !! ! =
  !!! !! !! !!! !!!
  !"# !! = !!"#$%&_! − !!"#$%&'$_! (A3),
  where !!"#$%&_! is the i-th measurement of the sample, !!"#$%&'$_! is the interpolated background (baseline) at the i-th measurement (Fig. S4). Note that the average of ki is the same as k defined by Eq. (A1). In case of the repeated measurement (N = 2 or 3), we divide the average of σk by !. The χv and its estimated error of the sample are given by (!±! )
  !! ± !!! = (!±!! ) (A4), !
  where !
  !!! =
  ! ! !
  !
  !
  +
  !!
  !!
  !
  (A5),
  Then, the susceptibility χv is normalized by the mass (noted m) of the sample. Note that there is no error on the mass. The mass susceptibility !! is obtained by !! =
  !! !
  ±
  !!! !
  (A6).
  •
  •
  • •
  R •
  dri ft
  Rsa ki •
  R
  (s)
  Figure S4: A typical result of the susceptibility measurement. The x-axis is the time in second and the y-axis is the normalized magnetic field (ratio) corresponding to R in the text. Evolution temporelle du champ magnétique lunaire 109 4
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  The first five points correspond to the first background measurement (Rair1). We put the sample and start measurement at 13 s (Rsample). Finally, we remove the sample and continue the second background measurement (Rair2). There is no visible jumping in the background. The solid line is the estimated drift of the background. The ki value can be calculated by the distance from the baseline (bluish line). The entire measurement procedure takes 26 seconds. Another source of error specific to our discrete magnetic field measurement design is the possible effect of sample heterogeneity, in particular due to a more magnetic mantle surrounding the sample due to regolith contamination. This mantle being closer to the field probe it will bias our measurement toward higher values (also effect of the field being lower in the sample center than its periphery). S4. Calibration of MS2C instrument for discrete samples The MS2C susceptibility meter is a commercial instrument from Bartington company. It is designed to measure the magnetic susceptibility of rock cores with an inner diameter of 12 cm. It operates at a peak field of 250 μT and a frequency of 565 Hz. We calibrated this instrument using a set of 123 pebbles with masses ranging from 3.8 to 669 g, and total susceptibility ranging from 5.10-8 to 5.10-5 m3. Each pebble was measured using a KLY2 susceptibility meter from Agico, equipped with a large (65 cm3) coil. The pebbles were then measured with the MS2C. Each pebble was measured in three orthogonal positions with respect to the MS2C coil, the pebble being centered in the coil. This takes into account the variability in shape and the effect of sample anisometry on the measurements. The average of three measurements is then used for calibration (Figure S5). From the 123 pebbles data, we compute a calibration curve K = 8.30 x (R2=0.996, slope 95% confidence interval ±0.089) where K is the total magnetic susceptibility (in m3) and x is the MS2 reading (in CGS units). Because the MS2 operates at a frequency of 565 Hz, it was not suitable for samples wrapped in aluminum of set in aluminum containers. Only samples kept in Teflon bags could be measured with this instrument. The effect of the aluminum sample tags used at JSC was tested and shown to be negligible.
  110
  Evolution temporelle du champ magnétique lunaire Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  4
  Susceptibility (10-9 m3) measured with KLY2
  100000
  10000
  y = 8,304x R2 = 0,996
  1000
  100
  10 1
  10
  100
  1000
  10000
  MS2C reading (cgs units)
  Figure S5: Susceptibility measured with KLY2 vs. susceptibility measured with MS2C.
  Evolution temporelle du champ magnétique lunaire 111 4
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  S5. Comparison between measurements with our susceptibility meter and with MS2C instrument. 2.5
  Susceptibility with MS2 (10-6 m3)
  2.0 y = 1.03x - 0.02 R2 = 0.98 1.5
  1.0
  0.5
  0.0 0.0
  0.5
  1.0
  1.5
  2.0
  Susceptibility with susceptibilitimeter
  2.5
  3.0
  (10-6 m3)
  Figure S6: Magnetic susceptibility measured with MS2C vs. magnetic susceptibility measured with our instrument on the same samples. The error bars for measurements with MS2C are smaller than the symbols.
  S6. NRM and magnetic susceptibility (literature data). Sample Regolith breccia
  Reference NRM
  χ (10 m kg )
  Reference χ
  95.8 41.2 41.0 1000 31.0
  [1] [1] [2] [2] [3]
  38.02 42.66 35.48 17.78 4.17 11.48 -
  [12] [12] [12] [12] [12] [12] -
  -6
  10021.32 10048.55 14047.47 14049.28 14301.65 14313.29 14318.31 14318.38 14318.38 15015 15015
  112
  NRM (10 Am kg ) 2
  -1
  Evolution temporelle du champ magnétique lunaire
  -6
  3
  -1 Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks 15086.12 15459.95 15498.36 60016.54 60255 60255.26 65010 79135.1 Impact melt breccia 14066 14066.24 14066.28 14303.47 14305 14305.56 14305.83 14306 14306.33 14306.4 14311.45 14312.2 14321.78 14321.184 15418.41 15455 61156.11 61156.12 62235 62235.53 62235.53 62295 62295.56 62295 65015 66055.19 66055.10 67455.19 67915 68815.7 68815.17 72275.67 72275.2 73215 76055.5 76315.2 76315.2c
  120 69.5 58.0 8.70 1.80 8.20 8.30 9.40 2.80 22.4 67.5 36.0 110 105 147 7.80 11.1 79.0 140 3.20 57.0 123 -
  [2] [4] [2] [1] [1] [2] [1] [1] [2] [1] [1] [2] [2] [5] [1] [2] [6] [2] [7] [2] [2] [6] -
  10.23 12.59 8.51 9.12 75.86 15.14 28.18 22.91 3.02 2.00 1.91 1.12 5.50 0.39 9.55 1.82 1.58 2.51 6.61 0.28 16.22 4.57 42.66 4.27 1.12 4.68 4.79 3.24
  4
  [12] [12] [12] [12] [12] [12] [12] [12] [12] [12] [12] [12] [12] [12] [12] [12] [6] [12] [12] [12] [12] [6] [12] [12] [12] [12] [12] [12]
  Evolution temporelle du champ magnétique lunaire 113 4
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks 77017.45 77017.2 77035.29 77115 77135.2 77135.36
  Dimict breccia Impact melt rocks
  64475.7 64475.11 14310.69 60315.15 60315.47 60335.3 61016 66095.36 66095.39 68415.33 68415 68416 68416.23 10003 10003 10017 10017 10017 10017 10020 10020.4F 10024.22
  Basalt
  10047 10049 10049 10050 10050.33 10057.7 10057 10069 12002 12009.95 12017 12017 12018 12022 12022.52 12022 12022
  114
  27.0 8.80 24.0 3.30 73 88 34 16.0 2.00 2.00 4.60 2.79 0.88 5.50 35 7.53 54 16.8 16 11 23 65 64 5.70 5.70 27 6.34 14.21 4.38 26 2.90 5.78 11.26 15.85
  [2] [5] [2] [2] [5] [5] [2] [2] [2] [5] [2] [8] [2] [9] [9] [8] [2] [1] [2] [9] [9] [2] [5] [5] [2] [2] [6] [10] [10] [2] [2] [10] [10]
  Evolution temporelle du champ magnétique lunaire
  2.29 2.63 4.79 45.71 0.72 11.48 1.45 3.24 10.47 12.30 3.09 5.37 0.85 1.12 0.49 1.12 0.52 1.02 0.66 -
  [12] [12] [12] [12] [12] [12] [12] [12] [12] [12] [12] [12] [8] [12] [12] [6] [12] [8] [12] - Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks 12053.47 14053.48 14053 14053.35 14053.49 14072 15016a 15016b 15016 15016.29 15058.55 15076.5 15085.31 15475 15495 15499 15499.108 15499.21 15498.313e 15498.282c 15498.282t 15498.313k1 15498.313k2 15555.75 15555.204 15555 15555 15556 15556.37 15556.38 15595.13 15597.2 15597 15597 15597 70017 70017.26 70017.78 70035 70035.1 70135 70215 70215 70215.1 71505
  110 218 153 500 1.37 2.00 3.60 1.27 51.6 57 83.0* 35.66* 46.08* 46.28* 68.18* 3.26 21.12 1.14 0.54 5.00 2.82 66.0 19.88 51.5 8.79 51.00 36.00 9.10 19.5
  [2] [1] [1] [2] [6] [6] [2] [12] [6] [2] [11] [11] [11] [11] [11] [12] [12] [12] [6] [2] [10] [6] [12] [5] [6] [2] [2] [6] [6]
  1.12 9.77 6.31 0.87 0.83 0.41 2.00 0.71 1.26 0.94 0.72 0.95 0.95 0.66 1.13 0.85 0.81 1.12 1.95 1.95 0.65 0.72 2.04 2.88 2.01 2.40 2.51 4.54 1.10 1.14 1.00 0.91
  4
  [12] [12] [12] [6] [6] [12] [12] [12] [12] [8] [12] [12] [12] [12] [8] [12] [12] [12] [8] [12] [12] [6] [6] [8] [8] [6] [12] [8] [12] [6] [12] [6]
  Evolution temporelle du champ magnétique lunaire 115 4
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks 71567 72015.45 74275 74275.65 74275.65 74275.56 75035.37 75055.6
  Feldspathic rocks
  28.6 6.60 7.79 2.32 4.78 47.37 57 13 1.91 1.04 0.66 0.54 0.43 0.17 8.97 30.24 6.82 15.0 0.76
  75055 75075 78505 79155 10085.13 60015.67 60015.49 60015 60025 60215 65315 72415.2c 76230.4 76535.136 76535 76535 78155.2 78235.1 78236
  [6] [5] [6] [12] [6] [8] [2] [2] [13] [5] [2] [6] [6] [6] [13] [14] [14] [15] -
  1.55 1.23 2.29 1.11 0.76 0.50 2.88 0.83 2.32 17.78 0.041 0.025 0.025 0.032 0.0031 0.0016 0.44 1.91 0.72 1.51
  [6] [6] [12] [8] [12] [12] [12] [6] [8] [12] [8] [6] [12] [6] [8] [8] [12] [12] [12] [6]
  Table S2: NRM estimates derived graphically from figures (noted *). S7: Magnetic susceptibility measurements performed with SM30 instrument (Macke et al., 2014) Lithology Basalt RB Basalt Basalt Basalt Basalt Basalt IMB IMB 116
  Sample 10017* 10048 12002 12038* 12051* 12052* 14053* 14303* 14305*
  Mass (g) 13.74 9.35 35.38 6.73 12.19 10.11 8.10 22.26 155.81
  χ (10 m kg ) 0.75 38.55 1.10 1.27 0.83 1.03 10.55 2.82 3.42 -6
  Evolution temporelle du champ magnétique lunaire
  3
  -1
  error χ (10 m kg ) 0.12 5.94 0.22 0.20 0.17 0.21 1.63 0.56 0.58 -6
  3
  -1 Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks IMR IMB IMB IMB RB Fd rock IMB IMB IMB RB Basalt Basalt RB Fd rock Fd rock IMB IMR IMR IMB IMB IMB Fd rock IMB IMB IMB IMB IMB IMR IMB IMB IMB IMB DB IMR IMR IMB Basalt IMB IMB IMB IMB Basalt Basalt IMB IMB
  14310* 14311* 14321* 14321* 15015* 15418* 15455* 15455* 15455* 15459* 15555* 15556* 15565 60015* 60015* 60017* 60315* 61016 64435 64435 65015 65315 66055* 66055* 66055* 66055* 66055* 66095* 67015 67015 67015 67016 67915* 68415 68415 68815 70215* 72395* 73235* 73255* 73255* 75035 75055 76055* 76215*
  72.82 5.49 10.01 76.16 71.63 26.68 12.69 16.23 16.55 23.32 32.98 44.37 11.72 8.96 36.15 19.83 5.36 35.29 4.52 7.78 5.98 10.03 4.72 5.33 8.17 8.61 11.41 5.57 10.64 11.53 28.21 16.35 9.52 4.35 5.30 8.78 9.07 29.42 7.42 2.22 5.91 7.16 6.22 18.02 5.66
  1.11 1.71 1.88 1.79 3.45 1.06 2.62 1.66 0.49 8.02 0.96 1.50 8.46 0.04 0.27 0.88 16.51 5.87 0.99 0.85 6.10 0.024 5.07 4.76 2.80 7.18 4.98 12.94 2.05 3.14 2.53 0.58 1.92 0.80 1.33 4.21 1.53 1.76 2.06 2.07 2.82 0.61 0.74 2.95 2.49
  4
  0.22 0.34 0.38 0.30 0.69 0.21 0.40 0.26 0.083 1.24 0.19 0.25 1.30 0.006 0.045 0.15 2.54 0.97 0.15 0.13 1.03 0.004 0.78 0.73 0.43 1.11 0.77 1.99 0.32 0.48 0.42 0.09 0.30 0.14 0.20 0.65 0.31 0.30 0.32 0.41 0.43 0.10 0.13 0.45 0.38
  Evolution temporelle du champ magnétique lunaire 117 4
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks IMB IMB
  76315 77035*
  5.47 15.87
  3.35 6.90
  0.52 1.06
  Table S3: IMB: Impact melt breccia; IMR: Impact melt rocks; RB: Regolith breccia; DB: Dimict breccia; Fd rock: Feldspathic rocks. Samples analyzed in our study are noted *. Errors were estimated based on the comparison of results obtained in the laboratory with a SM30 instrument and with a KLY2 instrument on a large set of samples.
  S8. Comparison of ß measurements with literature paleointensity estimates. Samp le
  Age Paleointensity References (Ga) (μT)
  Method
  ß (Am )
  ß error (Am )
  IMB
  62235
  3.88
  86.17
  [2]
  sIRM
  12.91
  1.80
  IMR
  68415
  3.85
  54.23
  [2]
  sIRM
  11.80
  0.51
  Basalt
  71567
  3.75
  111
  [6]
  sIRM
  11.48
  0.30
  Basalt IMB
  70017
  3.70
  42
  [6]
  sIRM
  9.42
  1.10
  72275
  3.93
  19
  [48]
  ARM
  9.36
  0.71
  IMB
  65015
  3.98
  45.42
  [2]
  sIRM
  7.94
  0.60
  Basalt
  10003
  3.91
  6.79
  [2]
  sIRM
  5.94
  1.08
  Basalt
  10017
  3.56
  *45.15
  [9]
  sIRM
  5.87
  2.49
  Basalt
  12022
  3.11
  *6.62
  [10]
  ARM/sIRM
  5.71
  0.38
  IMR
  60315
  3.88
  40
  [49]
  single pTRM
  5.63
  0.37
  Basalt
  70035
  3.70
  37.6
  [2]
  sIRM
  5.57
  1.81
  IMB
  77017
  3.91
  4.12
  [50]
  sIRM
  4.66
  0.42
  IMB
  73235
  3.97
  1.10
  [51]
  Thellier-Thellier
  4.52
  1.23
  Regolith breccia
  15015
  1.00
  0.03
  [3]
  Thellier-Thellier
  4.23
  1.18
  Regolith breccia
  10018
  1.56
  0.65
  [8]
  Thellier-Thellier
  3.92
  1.07
  Basalt
  70215
  3.80
  20
  [6]
  sIRM
  3.88
  0.96
  Fd-rock
  60015
  3.50
  *24.13
  [7]
  ARM/sIRM
  3.64
  0.75
  IMB
  72215
  3.85
  41
  [52]
  shaw
  3.01
  0.35
  IMB
  72255
  3.85
  35
  [48]
  ARM
  2.57
  0.18
  Basalt
  12018
  0.65
  7.39
  [2]
  sIRM
  1.80
  0.90
  Regolith breccia
  60255
  1.70
  0.50
  [4]
  Thellier-Thellier
  1.23
  0.75
  Basalt
  74275
  3.85
  28
  [6]
  sIRM
  0.91
  0.24
  Lithology
  -1
  -1
  Table S4: IMB: Impact melt breccia; IMR: Impact melt rocks; Fd-rock: Feldspathic rock; * indicates the mean of two paleointensity values; ß: ratio of the Natural Remanent Magnetization to the magnetic susceptibility. IRM and ARM methods indicate normalization of the NRM by IRM and ARM, respectively, during progressive AF demagnetization.
  118
  Evolution temporelle du champ magnétique lunaire Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  4
  S9. Supplementary references References for Tables in Supplementary Material [1] D.W Collinson, S.K Runcorn, A. Stephenson and A.J Manson, Proceedings of the 3th Lunar Science Conference 3, 2343-2361 (1972) [2] S.M Cisowski, D.W Collinson, S.K Runcorn and A. Stephenson, Proc. Lunar Planet. Sci. Conf. 13th, A691-A704 (1983) [3] H. Wang, S. Mighani, B.P Weiss, D.L Shuster and K.V Hodges, Lunar and Planetary Science XLVIII 1439 (2017) [4] N. Sugiura and D.W. Strangway, Proc. Lunar Planet. Sci. Conf. 11th, 1801-1813 (1980). [5] A. Stephenson, D.W Collinson and S.K Runcorn, Proceedings of the 5th Lunar Conference, 2859-2871 (1974) [6] C. Cournède, J. Gattacceca and P. Rochette, Earth and Planetary Science Letters 331332, 31-42 (2012) [7] A. Stephenson and D.W Collinson, Earth and Planetary Science Letters 23, 220-228 (1974) [8] Our unpublished data [9] C. Suavet, B.P Weiss, W.S Cassata, D.L Shuster, J. Gattacceca, L. Chan, I. GarrickBethell, J.W Head, T.L Grove and M.D Fuller, PNAS 110, 21,8453-8458 (2013) [10] S.M Tikoo, B.P Weiss, W.S Cassata, D.L Shuster, J. Gattacceca, E.A Lima, C. Suavet, F. Nimmo and M.D Fuller, Earth and Planetary Science Letters 404, 89-97 (2014) [11] S.M Tikoo, B.P Weiss, D.L Shuster, C. Suavet, H. Wang and T.L Grove, Science advances 3 (2017) [12] P. Rochette, J. Gattacceca, A.V Ivanov, M.A Nazarov and N.S Bezaeva, Earth and Planetary Science Letters 292, 383-391 (2010) [13] K. Lawrence, C. Johnson, L. Tauxe and J. Gee, Physics of the Earth and Planetary Interiors 168, 71-87 (2008) [14] I. Garrick-Bethell, B.P Weiss, D.L Shuster, S.M Tikoo and M.M Tremblay, Journal of Geophysical Research : Planets 122, 76-93. (2017) [15] C. Cournède, I. Garrick-Bethell, V. Perera, B.P Weiss and R. Nelson, Lunar and Planetary Science XLVIII, 2648 (2017) [16] A.L Fagan, K.H Joy, D.D Bogard and D.A Kring, Earth Moon Planets 112, 59-71 (2014), DOI 10.1007/s11038-014-9437-7. [17] G. Eglinton, B.J Mays, C.T Pillinger, S.O Agrell and 17 others, The European Consortium, 217-219 (1974). [18] G. Turner, Phys. Chem. Earth. 10, 145-195 (1977). [19] F.J Stadermann, E. Hausser, E.K Jessberger, S. Lingner and D. Stöffler, Geochimica et Cosmochimica Acta 55, 2339-2349 (1991). [20] E.C. Alexander Jr and P.K Davis, Geochimica et Cosmochimica Acta 38, 911-928 (1974). [21] D. Stöffler, G. Ryder, B.A Ivanov, N.A Artemieva, M.J Cintala and R.A.F Grieve, Reviews in Mineralogy & Geochemistry 60, 519-596 (2006). [22] O. Eugster, P. Eberhardt, J. Geiss, N. Grogler, M. Jungck, F. Meier, M. Morgell and F. Niederer, Proc. Lunar Planet. Sci. Conf. 14th, B498-B512 (1984a). [23] T.J Bernatowicz, C.M Hohenberg, B. Hudson, B.M Kennedy and F.A Podosek, Proc. Lunar Planet. Sci. Conf. 9th, 905-919 (1978). [24] E.C Alexander Jr and S.B Kahl, Proceedings of the 5th Lunar Science Conference 2, 1353-1373 (1974). [25] J.A Hudgins, J.G Spray, S.P Kelley, R.L Korotev and S. Sherlock, Geochimica et Cosmochimica Acta 72, 5781-5798 (2008). [26] M.D Norman, R.A Duncan and J.J Huart, Geochimica et Cosmochimica Acta 70, 6032-6049 (2006).
  Evolution temporelle du champ magnétique lunaire 119 4
  Chapitre 4 : A survey of the NRM and the magnetic susceptibility of Apollo whole rocks
  [27] D. Phinney, S.B Kahl and J.H Reynolds, Proc. Lunar Sci. Conf. 6th, 1593-1608 (1975). [28] G. Turner and P.H Cadogan, Proc. Lunar Sci. Conf. 6th, 1509-1538 (1975a). [29] B. Dominik and E.K Jessberger, Earth and Planetary Science Letters 38, 407-415 (1978). [30] M.D Norman, C.Y Shih, L.E Nyquist, D.D Bogard and L.A Taylor, Lunar and Planetary Science Conference 38th, abs#1991 (2007). [31] O.A Schaeffer, R. Warasila and T.C Labotka, Lunar and Planetary Institute Tech Rept. 82-02, 123-125 (1982a). [32] E.K Jessberger, B. Dominik, T. Kirsten and Th. Staudacher, Lunar Sci. VIII (abs), 511-513 (1977a). [33] P.H Cadogan and G. Turner, Proc. Lunar Sci. Conf 7th, 2267-2285 (1976). [34] A. Stettler, P. Eberhardt, J. Geiss and N. Grogler, Earth and Planetary Science Letters 23, 453-461 (1974). [35] A. Stettler, P. Eberhardt, J. Geiss, N. Grögler and P. Maurer, Proceedings of the 4th Lunar Science Conference 2, 1865-1888 (1973). [36] J.C Huneke, E.K Jessberger, F.A Podosek and G.J Wasserburg, Proceedings of the 4th Lunar Science Conference 2, 1725-1756 (1973). [37] V.R Murthy, N.M Evensen, B.M Jahn and M.R Coscio, Geochimica et Cosmochimica Acta 35, 1139-1153 (1971). [38] R.A Cliff, C. Lee-Hu and G.W Wetherill, Proc. Lunar Sci. Conf. 2th, 1493-1502 (1971). [39] G. Turner, J.C Huneke, F.A Podosek and G.J Wasserburg, Earth and Planetary Science Letters 12, 19-35 (1971). [40] D.A Papanastassiou and G.J Wasserburg, Earth and Planetary Science Letters 11, 37-62 (1971). [41] T. Kirsten, P. Horn and J. Kiko, Proceedings of the 4th Lunar Science Conference 2, 1757-1784 (1973). [42] G.A Schaeffer and O.A Shaeffer, Proc. Lunar Sci. Conf. 8th, 2253-2300 (1977a). [43] T. Kirsten, J. Deubner, P. Horn, I. Kaneoka, J. Kiko, O.A Schaeffer and S.K Thio, Proc. Lunar Sci. Conf. 3th, 1865-1889 (1972). [44] L.E Nyquist, Proc. Lunar Sci. Conf. 7th, 1507-1528 (1976). [45] V.R Murthy and C. Coscio, Lunar Sci. VIII (abs), 706-708 (1977). [46] P. Horn, E.K Jessberger, T. Kirsten and H. Richter, Proc. Lunar Sci. Conf. 6th, 1563-1591 (1975). [47] W.R Premo and M. Tatsumoto, Proceedings of Lunar and Planetary Science 21, 89100 (1991). [48] S.K Banerjee and G. Swits, The Moon 14, 473-481 (1975). [49] A. Brecher, D.J Vaughan and R.G Burns, Proc. Lunar Sci. Conf. 4th, Acta 37, Suppl. 4, 2991 (1973) [50] A. Brecher; W.H Menke and K.R Morash, Proc. Lunar Sci. Conf. 5th, 2795-2814 (1974) [51] D.E Watson, E.E Larson and R.L Reynolds, Lunar Sci. 5, 827 (1974) [52] S.K Banerjee and J.P Mellema, Nature 260, 230-231 (1976)
  120
  Evolution temporelle du champ magnétique lunaire Chapitre Cinq Nouvelle évolution du champ magnétique lunaire
  Vue de la Terre depuis la Lune, mission Apollo 11 Copyright @ images.nasa.gov
  Evolution temporelle du champ magnétique lunaire 121 122 Evolution temporelle du champ magnétique lunaire Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  5
  Chapitre 5 Nouvelle évolution du champ magnétique lunaire Dans ce chapitre, deux études paléomagnétiques sont présentées : une analyse sur deux météorites lunaires et une analyse de vingt-sept échantillons lunaires.
  5.1 Cas des météorites Les échantillons Apollo sont les principaux témoins du champ magnétique lunaire passé. Mais les météorites lunaires peuvent aussi fournir des indications du champ passé. Pour cela, deux météorites ont été étudiées. La première est LAP 02205, une météorite basaltique non bréchique riche en plagioclases avec des veines de roche fondue, qui a été trouvée en Antarctique en 2002 dans la région de La Paz Icefield. Elle est datée à 2,87 ± 0,06 Ga par Ar/ Ar (Fernandes et al., 2009). Un échantillon de 219 mg a été étudié. La seconde est Northwest Africa 10782 (NWA 10782), une brèche de régolithe trouvée dans le désert saharien. Son âge est encore inconnu. Le souci majeur des météorites provenant d'Afrique du Nord-Ouest (météorites NWA) vient de l'utilisation d'aimants par les nomades qui collectent les météorites pour tester leur teneur en métal. Leur signal paléomagnétique est ainsi souvent oblitéré partiellement ou en totalité. Les données obtenues après analyses de ces deux météorites sont détaillées dans la Table 5.1. 40
  Mass (g)
  NRM 2 -1 (Am kg )
  Impact melt breccia LAP02205 0.219
  1.51E-05
  Regolith breccia NWA10782 42.07
  5.66E-05
  Sample
  39
  Range (LC)
  N
  MAD (°)
  6.09E-07 PARM PIRM
  2-12 2-12
  11 11
  9.22E-09 PARM PIRM
  0-6 0-6
  4 4
  χ (m3kg-1) Method
  Range (HC)
  N
  MAD (°)
  Paleofield (μT)
  Error (μT)
  8.8 8.8
  13-100 13-100
  33 33
  4.4 4.4
  13.8 16.1
  0.7 0.5
  5.1 5.1
  38-100 38-100
  11 11
  5 5
  196.9 92.7
  10.7 5.5
  Tableau 5.1 : Données magnétiques des deux météorites LAP05505 et NWA10782. NRM est l'aimantation naturelle rémanente, χ est la susceptibilité magnétique, PARM est la méthode de normalisation par l'ARM et PIRM est la méthode de normalisation par l'IRM. Les rangs LC et HC sont en mT. N est le nombre de paliers utilisés pour les calculs des composantes sur la NRM. L'analyse du diagramme de désaimantation de la NRM (Figure 5.1 gauche) et du REM' (Figure 5.1 droite) permet de vérifier si la météorite a été ré-aimantée si elle possède des valeurs de REM' autour de 0,1 (Gattacceca et Rochette, 2004).
  Evolution temporelle du champ magnétique lunaire 123 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire a) LAP02205 (0.22 g)
  -1
  N
  W
  10 1.E-01
  Up x 10-9 Am2 E x 10-9 Am2
  100 70
  REM'
  HC NRM
  -2
  10 1.E-02
  12 mT
  LC 2
  NRM
  S
  Down -3
  10 1.E-03
  b) NWA10782 (42.07 g) N Up x10-6 Am2
  0
  10
  20
  30
  40
  50
  60
  70
  80
  90
  100
  90
  100
  AF (mT)
  1 1.E+00
  NRM
  LC
  REM'
  6 mT
  -1
  10 1.E-01
  20
  HC
  W
  100
  38
  E x10-6 Am2 10-2
  1.E-02
  NRM
  S Down
  0
  10
  20
  30
  40
  50
  60
  70
  80
  AF (mT)
  Figure 5.1 : Résultats paléomagnétiques pour les météorites LAP02205 (a) et NWA10782 (b). Chaque échantillon présente son diagramme de désaimantation AF de la NRM (gauche). Les symboles noirs représentent la projection sur le plan horizontal (N-E) et les symboles blancs dans le plan vertical (UpE). Les composantes de faible (LC) et hautes (HC) coercivités sont représentées par des flèches gris clair et gris foncé, respectivement. Les variations du REM' (en échelle logarithmique) en fonction du champ AF des deux échantillons sont présentées (droite). Les droites grises représentent les intervalles HC de calcul de la paléointensité : 13-100 mT pour la météorite LAP et 38-100 mT pour la météorite NWA. Les deux météorites présentent deux composantes d'aimantation : une de faible champ coercitif LC et une composante d'aimantation de haut champ HC qui tend vers l'origine. La composante LC de la météorite LAP02205 est une composante visqueuse difficile à effacer en AF (composante qui tourne). La composante LC de la météorite NWA est aussi difficile à désaimanter et tourne sur presque 90°. De plus, sa courbe REM' reste à 0,1 à faible champ, ce qui indique une ré-aimantation probable par un petit aimant et que sa composante LC serait une IRM (voir explications Chapitre 3). Cette possible ré-aimantation a pu avoir lieu lors de sa collecte par exemple. Cette composante IRM est cependant complètement désaimantée dès 40 mT, avec une stabilisation du REM'. La méthode de détermination de paléointensité par champ alternatif (détaillée Chapitre 3) permet ainsi d'obtenir deux estimations de la paléointensité (PARM et PIRM) par météorite (Figure 5.2)
  124 Evolution temporelle du champ magnétique lunaire Chapitre 5 : Nouvelle évolution du champ magnétique lunaire a) LAP02205 4
  0.4
  DC 1 T
  DC 0.1 mT
  AF 100 mT
  0.3 AF 100 mT
  NRM lost (x10-8 Am2)
  3
  NRM lost (x10-9 Am2)
  HC (13.8 ± 0.7 μT) 2 AF 13 mT
  1
  HC (16.1 ± 0.5 μT) 0.2
  0.1
  AF 13 mT
  AF 2 mT
  AF 2 mT
  0
  0
  2
  6
  4
  8
  10
  0
  12
  10
  0
  IRM lost
  b) NWA10782
  DC 3 T
  DC 0.1 mT AF 100 mT
  3
  NRM lost (x10-6 Am2)
  NRM lost (x10-6 Am2)
  AF 38 mT
  2 AF 6 mT
  1
  0.1
  50
  AF 100 mT
  HC (92.7 ± 5.5 μT)
  3
  0
  40
  (x10-8 Am2)
  4
  HC (196.9 ± 10.7 μT)
  0
  30
  20
  ARM lost (x10-9 Am2)
  4
  5
  0.2
  0.3
  ARM lost
  0.4
  0.5
  (x10-6 Am2)
  0.6
  0.7
  AF 38 mT
  2 AF 6 mT
  1
  0 0
  10
  20
  30
  IRM lost
  40
  50
  60
  (x10-6 Am2)
  Figure 5.2 : Résultats des paléointensités des météorites LAP02205 (a) et NWA10782 (b). Les graphes de gauche montrent la NRM perdue pendant la désaimantation AF en fonction de l'ARM perdue à chaque palier croissant de désaimantation (Champ AF augmente jusqu'à 100 mT pour un champ DC de 0.1 mT). Les graphes de droite montrent la NRM perdue en fonction de l'IRM perdue à chaque palier croissant de désaimantation (Champ AF augmente jusqu'à 100 mT pour une IRM acquise à 1T – LAP02205 et 3T – NWA10782). Pour les 4 graphes, les points gris clair montrent la composante LC et les points gris foncé la composante HC, sur laquelle est calculée la paléointensité. Les points blancs des graphes de la météorite NWA10782 sont associés à la partie visqueuse MC, qui est mal effacée par champ alternatif. Cette analyse permet d'enrichir les données de champ magnétique lunaire, notamment à des âges où peu d'échantillons Apollo ont été rapportés. Les météorites de l'Antarctique sont intéressantes car, provenant d'expéditions scientifiques, elles n'ont en principe pas été réaimantées par des aimants lors de leur échantillonnage. Cependant, la quasi-totalité des météorites d'Afrique du Nord-Ouest (NWA) est contaminée par contact avec des aimants lors de la collecte ou des manipulations successives par les marchands de météorites. Des résultats peuvent cependant être obtenus si l'aimantation acquise lors du contact avec un aimant peut être effacée par champ alternatif.
  Evolution temporelle du champ magnétique lunaire 125 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  5.2 Etude des échantillons lunaires Après avoir analysé deux météorites lunaires, le but de cette étude a été d'obtenir de nouvelles valeurs de paléointensités afin de compléter les données récentes de la littérature et de mieux comprendre l'évolution de la dynamo lunaire ainsi que sa durée de vie. Cette étude concerne ainsi l'analyse en laboratoire de vingt-cinq échantillons Apollo (voir planches en Annexe 4) et de deux météorites lunaires. Neuf échantillons proviennent de la sélection établie à partir de la courbe de ß en fonction du temps obtenue à la fin du Chapitre 4. Les autres échantillons, étudiés de manière plus sommaire, proviennent de prêts par des laboratoires français (CRPG, IPGP, LMV), qui nous ont autorisés à réaliser nos analyses magnétiques avant la destruction des échantillons pour des mesures géochimiques. Les neuf échantillons reçus ont été découpés par scie à fil et tous les sous-échantillons sont mutuellement orientés (voir Chapitre 3). La susceptibilité magnétique et les propriétés d'hystérésis ont été mesurées pour tous les échantillons. La majorité des basaltes ont des grains multi-domaines (MD) tandis que les regolith breccia ont des grains superparamagnétiques (SP). La NRM de tous les échantillons a été désaimantée thermiquement ou par champ alternatif (AF). Douze échantillons présentent une composante stable HC et six une composante stable HT. L'utilisation des méthodes thermiques et AF, décrites dans le Chapitre 3, permet d'obtenir neuf nouvelles valeurs de paléointensité avec des méthodes AF et quatre nouvelles paléointensités avec la méthode IZZI. Les autres échantillons n'ont permis que d'obtenir six valeurs de limites supérieures de paléointensités, à partir de la méthode développée par Tikoo et al. (2012) détaillée dans le Chapitre 3. Ces résultats permettent d'affiner la connaissance du champ de surface lunaire en fonction du temps (Figure 5.3). Les mécanismes possibles de la dynamo sont ajoutés sur la figure. Des valeurs de champ relativement fortes sont obtenues après l'époque de champ fort (HFE) dont la limite est donnée dans la littérature à 3,56 Ga. Deux échantillons dans la HFE (dimict breccia 61015 à 3.81±0.04 Ga et IMR 65055 à 3.87±0.02 Ga) donnent des valeurs de limites supérieures de 7 et 11 μT. Cela suggère que le champ moyen pendant cette HFE a pu être plus faible (entre 9 et 52 μT) que la moyenne des paléointensités données à ~70 μT pour cet intervalle (Tikoo et al. 2014). Les données ne montrant pas de différence significative entre 4 et 3 Ga, la transition entre l'époque de champ fort et de champ faible semble se décaler plutôt après 3 Ga contre 3,56 Ga précédemment. De plus, le champ varie sur plus d'un ordre de grandeur à cette époque, cas similaire à la Terre où son Moment Virtuel Dipolaire varie d'un facteur 15 sur les 100 derniers millions d'années (e.g., Biggin et al., 2015). Un manque de données entre 3 Ga et 2 Ga fait que la transition entre la période de champ fort et celle de champ plus faible est mal contrainte (dynamique et chronologie). L'analyse de météorites serait une alternative pour enrichir les données de paléointensités. De plus, une dynamo faible semble prendre fin après 1 Ga, comme le confirment les quatre données de champ (> 1 μT) obtenues dans l'intervalle 1,56- 0,1 Ga ; montrant d'autres mécanismes possibles à l'origine de cette dynamo (intermittente ?).
  126 Evolution temporelle du champ magnétique lunaire EA H EEA |ARC R EH V IE|| WR R R ES E A RC H | RREES V IE WRC RES ES RC H REEV VIE IEW R ES E A RC H | R E VIE W RESEARCH | REVIEW
  E VIE WES E A RC H | R E VIE W R ES E A RC H || R V IER RE W
  3.7 Ga
  4.2 Ga
  1 Ga
  1.8 Ga
  2.5 Ga
  2.7 Ga
  3.5 Ga
  Paleointensities (μT)
  of anoth o of another geologic associatio of of another geologic association wi ern farsi eli ern farside anomalies: They er Chapitre 5 : Nouvelle évolution du champ magnétique lunaire 5 ern farside anomalies: lie on of another They geologic ass edge ofed e of the vast South Pole-A ern farside anomalies: edgeedge of the vast South Pole-Aitken of a ofofanother another geologic associati anothergeologic geologicassociatio associat of the large t the largest known impact crat th edge of the vast South the largest knownanomalies: impact crater on ern ern farside anomalies: They ernfarside farside anomalies: They ern They ld This the largest known impa Convection Convection This discovery has led to This discovery has led toPole-A two Précession Convection Impacts edgel edge ofofthe the vast South Pole-A edge thevast vast South Pole edge of South Convection thermique pure This discovery has thermique these fo for these anomalies asfor alternat fo for these as alternatives t thermique avec couverture thermochimique the the largest known impact cra theanomalies largest known impact cr the largest known impact cra for these anomalies as a pure plasma f p hydratée plasma fields hypothesis. Both pl plasma fields hypothesis. Both prop thermique Th fieldshas hypothesis Thisplasma discovery has ledto t This discovery led This discovery has led to er than et erfor than reflecting locally high er er than reflecting locally high Bpal eranomalies than reflecting local for these as Précession for these anomalies altern for these anomalies asasalterna alterna manifest manifestation of materials with loca m manifestation materials wit m manifestation of materi bis plasma fields hypothesis. Both plas plasma fieldsof hypothesis. Bo plasma fields hypothesis. Both that were in magnetized in thatthat were magnetized a dynamo 1000 that wer were magnetized in a dyn tf th er hig er th thanreflecting reflectinglocally locally hi ererthan than reflecting locally hig of these(71) proposals (71) of these proposals identifies th ofmanifestation these proposals (71) identi o of of these of materials wit manifestation materials man manifestation ofof materials wiw with the inferred locat withwith the the inferred locations of <4.0 inferred locations ow w with the that magnetized in thatwere were magnetized adyn dy that were magnetized ininaathat dyn dikes located within the dikes located within the SPA (Fig. 3). dikes located within SPA (F di d dikes of proposals (71) identi these proposals (71) iden great verticalthe thickness ofloc th ofofthese these proposals (71) ident great vertical thickness (30 km) o great vertical thickness (30 gr gkoo great ve enables them to produc with withthe theinferred inferredlocations locations with with the inferred locations 100 enables themthem to despite produce the stron alies the weak NR enables to produce the en e dikes located within the SPA (F enables dikes located within the SPA dike dikes located within SPA (F aliesalies despite theThe weak NRMthe ofproposa typica despite the second weak NRM ofal atkk great vertical thickness (30 alies desp great vertical thickness (30 grea great vertical thickness (30 TheThe second the proposal anomalies(72) are emp in th second proposal (72) enables them to produce the enables them to produce th The s enab enables them to produce the ejecta impact mel the anomalies are and in the expected alies despite the weak NRM of the anomalies are inthe the exp th tot alies despite the weak NRM alies anom alies despite the weak NRM of impactor (Fig. 3). The ejecta and impact melt derived fr 10 The second proposal (72) ejecta and impact melt deriv ej The second (7 proposal isproposal that typica Th The second proposal (72 ejecta ae impactor (Fig. 3). The key advan the anomalies are in the exp which far more F impactor (Fig. are 3). The key a im the anomalies are in the ex im the the anomalies are in the exp impacto proposal is that typical chondriti 4 times that ofde m and 10 ejecta and impact melt deri proposal is that typical chon p ejecta and impact melt p ejec ejecta and impact melt deri proposa which are far more Fe-rich, hav feldspathic rocks, resp impactor (Fig. 3). The key 4 are far more Fe-rich, impactor (Fig. 3). The key w imp w impactor (Fig. 3). The key which a times that of mare basalts andwhich 10 of relatively thin 4 osition proposal is that typical cho 4a proposal is that typical ch times that of mare ba and 10 an prop proposal is that typical cho 1 feldspathic rocks, respectively. Th and 10 to thousands of meters which are far more Fe-rich which arerocks, far more Fe-ric feldspathic respectivel fe whi which are far more Fe-rich fe have fundamen osition of relatively thin layers (perh 4should feldspat that of mare bs and 10 4 4times times that of mare and 10 osition relatively layers os properties on the and times thatthin of mare b and 10ofnetic to thousands of meters thick) of osition oos feldspathic rocks, respective Fig. 6. Paleointensity measurements of the lunar magnetic field using modern methods andto estithat from more recen feldspathic rocks, respectiv thousands of meters thick feld to feldspathic rocks, respective should have as fundamentally enhan t to osition ofofrelatively thin layers mated lifetimes of various lunar dynamos. Each point represents measurements of a single Apollo count for most of thous the oth osition relatively thin laye should have fundamentally es ositi sh osition of relatively thin layer netic properties on the surface. SPA should sample. Circles represent actual paleointensities, downward arrows represent upper limits on paleoalies. However, thusthick farh totothousands ofofmeters 0.1 6. Paleointensity measurements of the lunar magnetic field using modern methods and estithousands meters Fig. tothic netic properties onindicating the surface. nth tofrom thousands of meters thick as that more recent impacts, c n intensity, and right arrows represent upper limits on age. Green and blue points were measured using the identified tha netic pro should have fundamentally e should have fundamentally Fig. 6. Paleointensity measurements of the Each lunar magnetic field using modern methods and estimated lifetimesIRM of various lunar dynamos. point represents measurements of a single Apollo Fig. 6. Paleointensity Paleointensity measurements of thelimits lunar magnetic field using modern methods and estishou as that from more recent imp should have fundamentally e count for most of the other isolated as Fig. 6. measurements of the lunar magnetic field using modern methods and estiand Thellier-Thellier methods, respectively, and the upper were derived from the ARM method. a (top ~30 cm) of the so Fig. 6. Paleointensity measurements of the lunar magnetic field using modern methods and esti-on the netic properties assurface that f netic properties on the surfac sample. Circles of represent actual downward represent upper limits on paleomated lifetimes various lunar dynamos. Each point represents measurements ofbox a single Apollo mated lifetimes of various lunar dynamos. Each point represents measurements of aathus single Apollo netic Note the datum atpaleointensities, <7 Ma at the of extreme rightlunar of arrows the dynamos. figure.The shaded green encompasses the mean alies. However, no evidence netic properties on the surface count for most of the other isol co region isfar anomalously mated lifetimes various Each point represents measurements of single Apollo ci Fig. 6. 6. Paleointensity Paleointensity measurements of the lunar magnetic field using modern methods and estimated lifetimes of various lunar dynamos. Each point represents measurements of as aassingle Apollo that from more recent imp count for Fig. measurements of the lunar magnetic field using modern methods and estithat from more recent im paleointensity value the limits period 3.56 toGreen 4.25 Ga (central green line) and itsmethods estimated 2 represent SD uncertainty intensity, and right arrows actual represent upper on age. and blue points were measured using the Fig. 6.for Paleointensity measurements of the lunar magnetic field using modern methods and estiFig. 6. Paleointensity measurements of the lunar magnetic field using modern estisample. Circles represent paleointensities, downward arrows represent upper limits on and paleoBoth of these new pr sample. Circles represent actual paleointensities, downward arrows upper limits on paleoidentified indicating that at least the as th as that from more recent imp alies. However, thus far no evid al sample. Circles represent actual paleointensities, downward represent upper limits on paleoa mated lifetimes lifetimes of various lunar dynamos. Each point represents measurements ofofrepresent aarrows single Apollo count for most of the other iso sample. Circles represent actual paleointensities, downward arrows upper limits on paleoalies. Ho ofand various lunar dynamos. Each point represents measurements amethod. single Apollo (upper lower lines) (see table S6). The paleointensity value for 76535 green point) is measured count for most of the other is fields rather than IRMmated andand Thellier-Thellier methods, respectively, and the upper limits were derived from the ARM mated lifetimes of various lunar dynamos. Each point represents measurements of anamo single Apollo mated lifetimes of various lunar dynamos. Each point represents measurements of(leftmost aand single Apollo intensity, right arrows represent upper limits on age. Green and blue points were measured using the (top ~30 cm) of the southern fars intensity, and right arrows represent upper limits on age. Green blue points were using the coun count for most of the other iso identified indicating that at lea id 0.01 intensity, and right arrows represent upper limits on age. Green and blue points were measured using the sample. Circles represent actual paleointensities, downward arrows represent upper limits on paleoid alies. However, thus far no evi intensity, and right arrows represent upper limits on age. Green and blue points were measured using the currently not well constrained due to spurious demagnetization effects. Vertical dashed lines show sample. Circles represent actual paleointensities, downward arrows represent upper limits on paleoidentified the magnetic anomalie alies. However, thus far no e Note the datum at <7represent Ma at the extreme right of the figure.The shaded green box encompasses the mean sample. Circles represent actual paleointensities, downward arrows represent upper limits on paleosample. Circles actual paleointensities, downward arrows represent upper limits on paleoregion is anomalously iron metal-ri IRM and Thellier-Thellier methods, respectively, and the upper limits were derived from the ARM method. alies IRM and Thellier-Thellier Thellier-Thellier methods, respectively, and the upper limits were from the ARM alies. However, thus far no ev (top ~30 cm) ofmethod. thethat southern (t(t IRM and methods, respectively, and the upper limits were derived derived from the ARM method. intensity, and andestimated rightand arrows represent upper on age. Green and blue points were measured using the identified indicating at~30 lea lifetimes oflimits various proposed lunar dynamo mechanisms: purely thermal convection amethod. dynamo comes from IRM Thellier-Thellier methods, respectively, and theestimated upper limits were from the ARM intensity, right arrows represent upper limits on age. Green and blue points were measured using thewere (top identified indicating that at l paleointensity value for arrows themaximum period 3.56 to 4.25 (central green line) and its 2Green SD uncertainty intensity, and right arrows represent upper limits onencompasses age. andderived blue points measured using thethat Both of these new proposals req intensity, and right represent upper limits on age. Green and blue were measured using the iden Note the atin <7 the extreme right ofGa the figure.The shaded green box the mean identified indicating at le Note the datum at <7 Ma atthe the extreme right ofpoints the figure.The shaded green box encompasses the mean region isofanomalously iron me re Note the datum at <7 Ma at the extreme right of the figure.The shaded green box encompasses the mean IRMdatum and Thellier-Thellier methods, respectively, and upper limits were derived from the ARM method. a Ma dry at mantle (with and without an early thermal blanket surrounding the core); impact-driven mantle r many anomalies wit (top ~30 cm) of the souther Note the datum atS6). <7 Ma atpaleointensity the(central extreme rightrespectively, of figure.The green box encompasses the mean IRM and Thellier-Thellier methods, respectively, andmethods, the upper upper limits were derived from the ARM method. (upper and lower lines) (see table The value forthe 76535 (leftmost green point) is derived region is (topthe ~30 cm) ofthe the southe IRM and Thellier-Thellier and theshaded upper limits were from ARM method. namo fields rather than plasma fie IRM and Thellier-Thellier methods, respectively, and the limits were derived from the ARM method. (top paleointensity value for the period 3.56 to 4.25 Ga green line) and its estimated 2 SD uncertainty (top ~30 cm) of souther paleointensity value for the period 3.56 to 4.25 Ga (central green line) and its estimated 2 SD uncertainty rotational changes, purely thermal convection in a wet mantle; precession; and thermochemical core Both of these new proposa Nectarian (i.e., ~3.85 to Note the datum at <7 Ma at the extreme rightfor of the figure.The shaded green box encompasses the mean paleointensity value the period 3.56 to 4.25 Ga (central green line) and its estimated 2 SD uncertainty is anomalously iron mm Note the datum at <7 <7 Ma Ma at the extreme right ofMa the figure.The shaded green box encompasses the mean paleointensity value for the period 3.56 to 4.25shaded Ga (central green line)green and its estimated 2region SD uncertainty currently not well constrained due to spurious demagnetization effects. Vertical dashed lines show region is anomalously iron Both the magnetic anomalies. Yet more Note the datum at <7 at the extreme right ofpaleointensity the figure.The shaded green encompasses thepoint) mean Note datum at at the extreme of the figure.The green box encompasses the mean regio convection driven by core crystallization. The horizontal line shows the maximum surface field as region isof anomalously iron m (upper andthe lower lines) (see table S6). The paleointensity value for 76535 (leftmost point) is box (upper and lower lines) (see table S6). The for 76535 (leftmost green is pre-Nectarian (i.e., mor namo fields rather than plasm nno paleointensity value for (upper the period 3.56 toright 4.25 Ga (central green line) and its estimated 2value SDlunar uncertainty and lower lines) (see table S6). The paleointensity value for 76535 (leftmost green point) is Both these new proposa paleointensity value for the period 3.56 to 4.25 Ga (central green line) and its estimated 2 SD uncertainty 0.001 estimated maximum lifetimes of various proposed lunar dynamo mechanisms: purely thermal convection (upper and lower lines) (see table S6). The paleointensity value for 76535 (leftmost green point) is Both of these new propo a dynamo comes from the recent i estimated from Eq. 2. The lifetimes of the precession and thermochemical dynamos are highly uncertain. namo fie paleointensity value for the period 3.56 to 4.25 Ga (central green line) and its estimated 2 SD uncertainty paleointensity value for the period 3.56 to 4.25 Ga (central green line) and its estimated 2 SD uncertainty (Fig. 3) (74–76). These Bo Both of these new proposa currently not and welllower constrained duetable tonot spurious demagnetization Vertical dashedgreen lineseffects. show isVertical currently not well constrained duevalue toeffects. spurious demagnetization dashed lines show the magnetic anomalies. Yetth (upper lines)currently (see S6). The paleointensity for 76535 (leftmost point) namo fields rather than plasm well constrained due to demagnetization effects. Vertical dashed lines show t and lower lines) (see table S6). The paleointensity value for is 76535 (leftmost green point) is 4.5mantle 4lifetimes 3.5 3 The 2.5 2spurious 1convection 0.5 0tized in a (upper dry (with and without an early thermal blanket the core); impact-driven mantle Paleointensity data set is S6). from (14, 18, 50,(see 59–61, 63, 66) and also1.5 listed in table S6. namo fields rather than plas currently not well constrained due tosurrounding spurious demagnetization effects. Vertical dashed lines show of many anomalies within the inte any existing NRM (upper and lower lines) table S6). The paleointensity value for 76535 (leftmost green point) is (upper and lower lines) (see table paleointensity value for 76535 (leftmost green point) is the mag nam namo fields rather than plasm estimated maximum of various proposed lunar dynamo mechanisms: purely thermal estimated maximum lifetimes of various proposed lunar dynamo mechanisms: purely thermal convection currently not well constrained due to spurious demagnetization effects. Vertical dashedmechanisms: lines show purely a the dynamo comes from theYet rec aa magnetic anomalies. estimated maximum lifetimes of various proposed lunar dynamo thermal convection currently not estimated well constrained due to well spurious demagnetization effects. Vertical dashed lines show rotational changes, purely thermal convection in constrained a wet mantle; precession; and thermochemical core sheets that should have the magnetic anomalies. Ye Nectarian (i.e., ~3.85 to 3.92 Ga)the bas lifetimes of various proposed lunar dynamo mechanisms: purely thermal convection Fig. 6.in Paleointensity measurements the lunar magnetic field using modern methods and es currently not due tomechanisms: spurious demagnetization effects. Vertical dashed lines show currently well due to spurious demagnetization effects. Vertical dashed lines show Age (Ga) a dynam the magnetic anomalies. Yet a dry mantle not (with andconstrained without an early thermal blanket surrounding the core); impact-driven mantle inmaximum a dry dry mantle (withof and without an early early thermal blanket surrounding the core); impact-driven mantle estimated maximum lifetimes of mantle various proposed lunar dynamo purely thermal convection of many anomalies within the of a dynamo comes from the re in a (with and without an thermal blanket surrounding the core); impact-driven mantle cite Curie point to am persisted in a weakened state (surface field of to be able to power a dynamo this late in history or estimated maximum lifetimes of various proposed lunar dynamo mechanisms: purely thermal convection convection driven crystallization. Theproposed horizontal line shows the maximum lunar surface field as impact-driven dynamo comes from the (i.e., more than estimated maximum of various proposed lunar dynamo mechanisms: purely thermal convection inby acore drylifetimes mantle (with and without an early thermal blanket surrounding the core); mantle estimated maximum of various lunar dynamo mechanisms: purely thermal convection a~3.9 dy of many aathermochemical dynamo comes from the re rotational purely thermal convection inpurely alifetimes wet mantle; precession; and thermochemical core pre-Nectarian in a:changes, dry mantle (with and without an early thermal blanket surrounding the core); impact-driven mantle rotational changes, purely thermal convection in a wet mantle; precession; and core Nectarian (i.e., ~3.85 to 3.92 Ga of many anomalies within th N Figure 5.3 Evolution du champ magnétique (paléointensités) en fonction du (Ga) et des mated lifetimes of various lunar dynamos. Each point represents measurements of a single Apo peratures far more slo ~2 mT) until sometime after 3.3temps Ga. (Figs. 4 and 6). rotational changes, thermal convection in acore); wet mantle; precession; and thermochemical core N in a dry mantle (with and without an early thermal blanket surrounding the impact-driven mantle estimated from Eq. 2. The lifetimes of the precession and thermochemical dynamos are highly uncertain. of many anomalies within t (Fig. 3) (74–76). These impacts in a dry mantle (with and without an early thermal blanket surrounding the core); impact-driven mantle in a dry mantle (with and without an earlyhorizontal thermal blanket surrounding themantle; core); impact-driven mantle rotational changes, purely thermal convection inprecession; amaximum wet precession; and core oflike m of many anomalies within th Nectaria rotational changes, thermal convection in acrystallization. wet mantle; and thermochemical core convection driven by corepurely crystallization. The line shows the lunar surface field asthermochemical convection driven by core core crystallization. The horizontal line shows the maximum lunar surface field as Nectarian (i.e., ~3.85 to 3.92 G These results confirm some Apollo-era concluthicknesses, over athan per pre-Nectarian (i.e., more pr convection driven by The horizontal line shows the maximum lunar surface field as Paleointensity data set is from (14, 18, 50, 59–61, 63, 66) and is also listed in table S6. rotational changes, purely thermal convection in a wet mantle; precession; and thermochemical core p mécanismes de dynamo associés. Les symboles blancs indiquent les données récentes de la littérature et tized any existing NRM, producing Nectarian (i.e., ~3.85 to 3.92 sample. Circles actual paleointensities, downward arrows represent limits on pale The global context of crustal magnetism rotational changes, purely thermal convection in a and wet mantle; precession; andupper thermochemical core rotational changes, thermal convection inand alifetimes wet mantle; precession; thermochemical core convection driven byfrom core crystallization. The horizontal line shows the maximum lunar surface field as Nec Nectarian (i.e., ~3.85 toimpact 3.92 G convection driven bypurely core crystallization. The horizontal line shows the maximum lunar surface field as pre-Nect any impact-generated sions and refute others. Modern estimated fromrepresent Eq. 2. The lifetimes of the precession thermochemical dynamos are highly uncertain. estimated Eq. 2.paleomagnetic The of the precession and thermochemical dynamos are highly pre-Nectarian (i.e., more than (Fig. 3) should (74–76). These (F estimated from Eq. 2. The lifetimes the precession and thermochemical dynamos are highly uncertain. uncertain. convection driven by core core crystallization. The horizontal lineof shows thehorizontal maximum lunar surface field as (F sheets that have cooled fro pre-Nectarian (i.e.,as more th convection driven by core crystallization. The line shows the maximum lunar surface field convection driven by crystallization. The horizontal line shows the maximum lunar surface field as central basin anomalie Coincident with these new paleointensity exmeasurements have now demonstrated that aalso preles noirs, ceux de cette étude. Les cercles représentent les données de paléointensités obtenues en AF et estimated from Eq. 2. The lifetimes of the precession and thermochemical dynamos are highly uncertain. pre-Nectarian (i.e., more tha estimated from Eq. 2. The lifetimes of the precession and thermochemical dynamos are highly uncertain. (Fig. 3) Paleointensity data set is from (14, 18, 50, 59–61, 63, 66) and is listed in table S6. (Fig. 3) (74–76). These impac Paleointensity data set is from (14, 18, 50, 59–61, 63, 66) and is also listed in table S6. tized existing NRM, prod intensity, and right arrows represent upper limits on age. Green and points were measured using titt Paleointensity data set ispersisted from (14, 18, 50, 59–61, 63, 66) and is also listed incite table S6.any estimated from Eq. 2. The The lifetimes of the precession and thermochemical dynamos areblue highly uncertain. Curie point to ambient lunar in a weakened state (surface field of to be able to power a dynamo this late in history (Fig. 3) (74–76). These impa field like that expecte periments, there have also been important recore dynamo likely existed on the Moon at least estimated from Eq. 2. The lifetimes of the precession and thermochemical dynamos are highly uncertain. estimated from Eq. 2. lifetimes of the precession and thermochemical dynamos are highly uncertain. (Fig (Fig. 3) (74–76). These impac Paleointensity data set Les is from (14,set 18, 50, 59–61, is also listed table S6. listed Paleointensity data is from (14,63, 18,66) 50,and 59–61, 63, 66) in and is also in table S6. sheets tized any existing NRM, prod tized an losanges en 6). thermique. rectangles correspondent valeurs de limites supérieures. that should have coole sh Paleointensity setmethods, is from (14, 18, 50, 59–61, 63, 66)and and also listedafter in table S6. peratures more slowly (given ~2 mT) until 3.3S6. Ga.theis (Figs. 4 and cent investigations of lunar crustal during the Nectarian an asdata far back infrom time as 4.25 Ga, in contradiction to aux tizedfar any existing NRM, pro s IRMlesand Thellier-Thellier respectively, the upper limits were derived from the ARM metho Paleointensity data set is from (14, 18, 50, 59–61, 63, 66) and also listed in magnetic table S6. Paleointensity data set is (14, 18, 50, 59–61, 63, 66) and isissometime also listed in table tized tized any existing NRM, prod sheets that should have coole sheets th 4 cite Curie point to ambient persisted in a weakened state (surface field of to beThese able to power a dynamo this late in history ci persisted in a weakened state (surface field of to be able to power a dynamo this late in history anomalies. As discussed above, a keythicknesses, finding However, oncehave again, the hypothesis of able a latetoorigin (i.e., after 4.0 Ga). results confirm some Apollo-era concluover ashould period of shee >10 sheets that coo clt persisted in a weakened state (surface field of to be power a dynamo this late in history sheets that should have cool −1 cite Curie point tomthe ambient persisted in a the weakened state (surface field of theafter toand be able toOn power a dynamo this late in history The global context of crustal magnetism cite Cur persisted in a weakened state (surface field of to be able to power a dynamo this late in history Note the datum at <7 Ma at the extreme right of the figure.The shaded green box encompasses me peratures far more slowly (g ~2 mT) until sometime after 3.3 Ga. (Figs. 4 6). ) exceed sities (~1 A from Apollo-era measurements was disthe other hand, modern measurements have p ~2 mT) until sometime 3.3 Ga. (Figs. 4 and 6). any impact-generated field. The sions and refute others. Modern paleomagnetic cite3.3 Curie point toambient ambient persisted in ahistory weakenedpersisted state (surface fieldofof after to be able to power a dynamo this late in history history p ~2 mT) until Ga.far (Figs. andlate 6).power cite in asometime weakened state (surface field to be4this able to a dynamo this insometime cite Curie point to persisted in aconcluweakened state (surface field to be able to power peratures more slowly (g ~2 mT) until 3.3 Ga. (Figs. 4 and 6). covery of intense magnetic anomalies, endogenous lun confirmed the existence ofconcluain high-field (mean of late perature ~2several mT)after until sometime after 3.3thicknesses, Ga. (Figs. 4a dynamo and 6). These results confirm some Apollo-era over a of period of These results confirm some Apollo-era th central basinknown anomalies likely req Coincident with new paleointensity exmeasurements have now demonstrated that a 4.25 peratures farmore more slowly ~2 mT)(central until these sometime after 3.3 Ga. (Figs. 44value and 6). These results confirm some Apollo-era conclupaleointensity for the period 3.56 to Ga green line) and its estimated 2 SD uncertain t( pera ~2 mT) until sometime after 3.3 Ga. (Figs. 4 and 6). peratures far slowly ~2 mT) until sometime after 3.3 Ga. (Figs. and 6). These results confirm some Apollo-era concluthicknesses, over a period o includingof those on the southern farside, that flect the addition of Fe~77 These mT from six samples) epoch lasting from atconcluThe global context crustal magnetism The global context of crustal magnetism results confirm some Apollo-era thicknes any impact-generated field. sions and refute others. Modern paleomagnetic sions and refute others. Modern paleomagnetic Theimportant global context of crustal magnetism field like that expected from aan c periments, therecontext have alsoofbeen recore dynamo likely existed on theand Moon at least These results confirm some Apollo-era conclu-recognized thicknesses, over period The global crustal magnetism sions refute others. Modern paleomagnetic apparently require crustal materials with NRM It green has been suggeste least 3.85 (see to 3.56 Ga, paleomagnetic as originally These results confirm some Apollo-era concluthicao These confirm some Apollo-era concluthicknesses, over aaany period any impact-generated field. sions andresults refute others. Modern The global context of excrustal magnetism (uppermeasurements and lower lines) table S6). The paleointensity value for 76535 (leftmost point) The global context of crustal magnetism imp sions and refute others. Modern paleomagnetic central basin anomalies likel Coincident with these new paleointensity have demonstrated that a now ce cent investigations of thea crustal magnetic during the Nectarian and expre-Necta as far back in time asnow 4.25 Ga, in contradiction towe Coincident with these new paleointensity measurements have now demonstrated that alunar any impact-generated fiel sions and refute others. Modern paleomagnetic The global context of crustal magnetism The global context of crustal magnetism lack of anomalies in th larger than that of nearly all known Apollo during the Apollo era. However, now see that c Coincident with these new paleointensity exmeasurements have demonstrated that any sions andpaleomagnetic refute that others. paleomagnetic central basin anomalies field likel any impact-generated sions and refutehave others. Coincident with these new paleointensity exmeasurements nowModern demonstrated a Modern field like that expected from periments, there also been important recore likely on the Moon at least As discussed above, awith key finding However, once again, the inferred thedynamo hypothesis ofthe aexisted late (i.e., after 4.0 Ga). central Coincident these new paleointensity exmeasurements have now demonstrated a with fifib periments, there have also been important core dynamo likely existed on thethat Moon athave least central basin anomalies lik Coincident these new paleointensity exmeasurements haveorigin now demonstrated that aanomalies. samples (35). magnetic field maps from sins (<1 nT atrethe cent surf dynamo subsequently declined precipitously currently not well constrained due to spurious demagnetization effects. Vertical dashed lines sh periments, there have also been important recore dynamo likely existed on the Moon at least Coincident with these new paleointensity exmeasurements have now demonstrated that a Global field like that expected from periments, there have also been important recore dynamo likely existed on the Moon atthat least central basin anomalies like Coincident with these new paleointensity exmeasurements have now demonstrated a −1 ) exceed what ispreach sities (~1 Athe m from the Apollo-era measurements was important the dishand, modern measurements have cent investigations of the lunar crustal magnetic during Nectarian and as On far the backother in time as 4.25 Ga, infar contradiction to field lik thethere Lunar Prospector spacecraft, which only becentral impact melt lay tocore <∼4 mT byas 3.2 Ga. In particular, modern analyperiments, there have also been important redynamo likely existed on the Moon at least cent investigations of the lunar crustal magnetic du back in time as 4.25 Ga, in contradiction to field like that expected fr periments, have also been recore dynamo likely existed on the Moon at least cent investigations ofpurely the lunar crustal magnetic as core far in time asleast 4.25 inthe contradiction to investigations of the lunar crustal magnetic during the−3 Nectarian andindic pre as fardynamo back inses time asexisted 4.25 Ga, in contradiction to Ga,cent field periments, there have also beenthermal important redynamo likely existed on Moon atdynamo least field like that expected frod periments, there have also been important recore likely onback the Moon at −1 estimated maximum lifetimes of various proposed lunar mechanisms: convecti A lunar m ) may ~10 came available 6anomalies. years (8,of 29, 68), have now of samples 3.3 Gacontradiction or younger suggest that covery of investigations several intense magnetic anomalies, known endogenous materials theofexistence of4.25 athe high-field (mean oflate anomalies. As discussed above, aago key finding However, once again, the inf theconfirmed hypothesis aas late origin (i.e., after 4.0 Ga). As discussed above, a key finding H hypothesis of a origin (i.e., after 4.0 Ga). cent investigations the lunar crustal magnetic during t far back in time as 4.25 Ga, in contradiction to cent of the lunar crustal magnetic during the Nectarian and pr as far back in time as Ga, in to anomalies. As discussed above, a once key finding the hypothesis of a late origin (i.e., after 4.0discussed Ga). anomalies. As above, a and key finding However, again, the inH thefar hypothesis ofor aas late origin (i.e., after 4.0 Ga). cent investigations of the lunar crustal magnetic duri asGa, far back in time as 4.25 Ga, in investigations contradiction to cent ofsouthern theunique lunar crustal magnetic during the Nectarian and pre as back time in contradiction to −1 Fe-rich active by the confirmed the size intensity of the all4.25 of the Apollo-era paleointensity analyincluding those on the farside, that flect the addition of impact mT from sixinmost samples) epoch lasting from atmodern ) the exceed what sities (~1no A longer m from the4.0 Apollo-era measurements was the disOn~77 thethe other hand, modern measurements have si from the Apollo-era measurements was disOn the other hand, measurements have −1 anomalies. As discussed above, a key finding However, once again, the hypothesis ofand a late origin (i.e., after 4.0 Ga). anomalies. As discussed above, a key finding Howeve the hypothesis of a late origin (i.e., after Ga). in a dry mantle (with without an early thermal blanket surrounding the core); impact-driven man ) exceed what sities (~1 A m from the Apollo-era measurements was the disOn the other hand, modern measurements have si from the was the dis-Ga). On thehypothesis other hand, have anomalies. As discussed above, a(i.e., key finding How the of amodern lateapparently origin (i.e.,require after 4.0 Ga). anomalies. As discussed above, a Apollo-era key finding However, once again, the in the hypothesis of a as late origin (i.e., after 4.0 Ga). southern farside anomalies and theirmeasurements spatial after ~3.85 H ses Ga, from this late epoch, which ranged up measurements to apcrustal materials with NRM Itknown has been suggested (29) that leastOn 3.85 to 3.56 recognized covery of the several intense magnetic anomalies, endogenous lunar mat confirmed the existence of aoriginally high-field (mean ofupper wha sities (~1 Aconsistent m−1−1) exceed from Apollo-era measurements was theintense disthe other hand, modern measurements have covery of several magnetic anomalies, kn confirmed the existence of acovery high-field (mean ofwith sities (~1 from the Apollo-era measurements was the disOn the other hand, modern measurements have of several intense magnetic anomalies, known endogenous lunar ma confirmed the existence of a high-field (mean of correlation antipodes of four, or perhaps not with th parent values of 20 mT, are only limits on sitie from the Apollo-era measurements was the disOn the other hand, modern measurements have covery of several intense magnetic anomalies, k confirmed the existence of a high-field (mean of ) exceed what sities (~1 A m from the Apollo-era measurements was the disOn the other hand, modern measurements have rotational changes, purely thermal convection inalasting athose wet mantle; precession; and thermochemical co lack ofknown anomalies in the centers of than that of nearly all known Apollo the Apollo era. However, we now from see that including on the southern farside, that flect the addition ofofthat Fe-rich im ~77during mTconfirmed from samples) epoch lasting at covery of several intense magnetic anomalies, endogenous lunarpa m the existence of amT high-field (mean oflarger including those on the southern farside, fl ~77 from six samples) epoch from at covery several intense magnetic anomalies, known confirmed the existence of a high-field (mean of dynamo from even five (69), ofof the eight youngest basins. Howthe lunar field and therefore do not require including those on the southern farside, that flect thecore addition Fe-rich im ~77 mT six from six samples) epoch lasting from at covery of several intense magnetic anomalies, kno confirmed the existence of a high-field (mean of covery of several intense magnetic anomalies, known endogenous lunar ma confirmed the existence of a high-field (mean of including those on the southern farside, that fle ~77 mT from six samples) epoch lasting from at samples (35).require Global magnetic field maps from sins (<1 nTthe at the surface, sugges the 3.85 dynamo subsequently declined precipitously including those on the southern farside, that flect addition offlect Fe-rich ~77 mT from six samples) epoch lasting from atas apparently crustal materials with NRM It has been suggested (29) least to 3.56 Ga, as originally recognized apparently require crustal materials with NRM least 3.85 to 3.56 Ga, originally recognized ever, the acquisition of the first high-resolution 3.56 to 3.85 Ga (Imbria dynamo at this time. Finally, a recent analysis convection driven by core crystallization. The horizontal line shows the maximum lunar surface field including those on the southern farside, that the ~77 mT from six samples) epoch lasting from at apparently require crustal materials with NRM It has been suggested (29) least 3.85 to 3.56 Ga, as originally recognized those onthat the southern that flect ~77 mT from sixfrom samples) epoch lasting fromspacecraft, including those onatthe including southern farside, flect thefarside, addition of Fe-rich im ~77mT mTbyfrom six In samples) epoch lasting apparently require crustal materials with NRM least 3.85 to 3.56 Ga,atas originally recognized the Lunar Prospector which only becentral impact melt layer has an N to <∼4 3.2toof Ga. particular, modern analyglobal topography data in 1993 by the Clemenbasalts above). In a potentially very young lunar sample (66) apparently require crustal materials with NRM It has been(see suggested (29 least 3.85 3.56 Ga,However, as originally recognized lack of anomalies in the larger than that of nearly all known Apollo during the Apollo era. However, we now see that la larger than that of nearly all known Apollo during the Apollo era. However, we now see that apparently require crustal with NRM Itcent has least 3.85 3.56 Ga, as originally recognized of anomalies in the cent larger that of nearly all known Apollo during the Apollo era. we now see that −3lack −1 apparently require crustal materials with NRM It least 3.85 to 3.56 Ga, as originally recognized apparently require crustal materials with NRM Itm has been suggested (29) least 3.85 to 3.56 Ga, asto originally recognized la larger than that ofmaterials nearly all known Apollo during the Apollo era. However, wethan now see that estimated from Eq. 2. The lifetimes of the precession and thermochemical dynamos are highly uncerta A ) may indicate that the ~10 came available 6 years ago (8, 29, 68), have now ses of samples 3.3 Ga or younger suggest that tine laser altimeter (70) led to the identification bital data sets are cons hints that the dynamo may have nevertheless lack of anomalies in theofcen larger than that of nearly allfield known Apollo during the Apollo era.declined However, we now see thatwe samples (35). Global magnetic field maps from sins (<1 nT the surface, su the dynamo subsequently precipitously samples (35). Global magnetic field maps si the dynamo subsequently declined precipitously samples (35). Global maps from sins (<1 nTat atApollo thefrom surface, the dynamo subsequently declined lack larger than of nearly alllonger known Apollo during Apollo era.precipitously However, now see that larger than that of nearly all known during the Apollo we now see that lack of anomalies in theoflack cen larger than that of magnetic nearly allthat known Apollo during the Apollo era.the However, we now seeera. thatHowever, samples (35). Global magnetic field maps from ssa the dynamo subsequently declined precipitously no active by the time the confirmed the unique size and intensity of the most or all of the Apollo-era paleointensity analysamples (35). Global magnetic field maps from sins (<1 nT at thelayer surface, dynamo subsequently declined precipitously theprecipitously Lunar Prospector spacecraft, which only central impact melt has to <∼4the bydata 3.2by Ga. InGa. particular, modern analythe Lunar Prospector spacecraft, which only beces to <∼4 mT modern byprecipitously 3.2 Ga. In 59–61, particular, modern analyPaleointensity set isIn from (14, 18, 50, 63, 66) and issamples also listed inbetable S6. the Lunar Prospector spacecraft, which only becentral impact melt layer has tomT <∼4 mT 3.2 particular, analysamples (35). Global magnetic field maps from sins (<1 the dynamo subsequently declined (35). Global magnetic field maps from sins the dynamo subsequently declined precipitously samples (35). Global magnetic field maps from sins (<1 nT at the surface, the dynamo subsequently declined the Lunar Prospector spacecraft, which only bec to <∼4 mT by 3.2 Ga. In particular, modern analysouthern farside anomalies and their spatial (i.e., after ~3.85 Ga). However, thi ses from this late epoch, which ranged up to ap−3 −1−1 the Lunar Prospector spacecraft, which only be- ago central impact melt layertha ha <∼4 mT by 3.2 Ga. Inyounger particular, modern analy1246753-8 5 DECEMBER 2014 •Ga VOLor 346 ISSUE 6214 m ) )may indicate ~10 came 6 years ago (8, 29, 68), have now ses ofto samples 3.3 Ga orGa suggest that ~is came available 6 years (8,−3A 29, 68), have now ses of samples 3.3 younger suggest that A m may indicate tha ~10 came available 6the years ago (8, 29, 68), have now ses of samples 3.3 or suggest that Lunar Prospector spacecraft, which only central to <∼4 mTses by 3.2 Ga. In particular, modern analythe Lunar Prospector spacecraft, which only betoyounger <∼4 mTmodern by 3.2 Ga. Inor particular, modern analytheavailable Lunar Prospector spacecraft, which only becentral impact melt layercent has to <∼4 mT of by 3.2 Ga. In particular, analy−3 −1becorrelation with antipodes of four, or perhaps not consistent with the strong ev parent values 20 mT, are only upper limits on ~ came available 6 years ago (8, 29, 68), have now of samples 3.3 Ga younger suggest that A active mactive indicate ~10 came available 6 years ago (8,intensity 29, 68), have now ses samples 3.3 Gapaleointensity or younger suggest thatconfirmed −3 nt− −3 intensity −1 ) may nono longer by the the unique sizeavailable and of the most or allof the Apollo-era analyconfirmed the size and ofindicate the most or all of the Apollo-era paleointensity analylonger by time confirmed the unique size and intensity of the most orfield all of the3.3 Apollo-era paleointensity analy~10 A ~10time came available 6unique years ago (8, 29, have now of3.3 samples 3.3 Ga even or suggest younger suggest that 668), years ago (8, 29, 68), have now ses of Ga samples Ga orApollo-era younger that A68), mfrom ) may thn ~10 came available 6came years ago (8, 29, have now ses ofof samples orses younger suggest that core dynamo paleomagnetic five (69), of the eight youngest basins. Howthe most lunar and therefore do not require a confirmed the unique size and intensity of the most or all of the paleointensity analynoafter longer active byHoweve the tim confirmed the unique size and intensity of the (i.e., or late all ofepoch, the Apollo-era paleointensity analysouthern farside anomalies and their ~3.85 sesdynamo from which ranged to southern farside anomalies and their spatial (i.e., after ~3.85 Ga). Howeve ses from this late epoch, which ranged upapto southern farside and their spatial (il ses this late epoch, which ranged up to appersisted in aintensity weakened state (surface field to be able tothis power a Apollo-era dynamo this late inaphistory no confirmed thespatial unique sizeintensity and intensity theno most or allup oflate the Apollo-era paleointensity analyno longer active by the time confirmed the unique size and of anomalies the most or this all oftime. the paleointensity analylonge confirmed the unique size and of theofGa). most or all the Apollo-era paleointensity analyever, theranged acquisition of the first high-resolution 3.56 to 3.85 Ga (Imbrian-aged) Apo at Finally, afrom recent analysis southern farside anomalies and their spatial ( sesof from this epoch, which up to apsouthern farside anomalies and their spatial (i.e., after ~3.85 Ga). Howe ses from this late epoch, which ranged up tomT, apcorrelation with antipodes of four, or perhaps not consistent with the stro parent values of 20 mT, are only upper limits on correlation with antipodes of four, or perhaps not consistent with the stro parent values of 20 mT, are only upper limits on correlation with antipodes of four, or perhaps n parent values of 20 are only upper limits on southern farside anomalies and their spatial (i.e., ses from thisoflate epoch, which up tosouthern apsouthern farside anomalies and their spatial after ~3.85 Ga). Howev ses from thisvery latefrom epoch, which ranged up to apfarside anomalies and(i.e., their spatial (i.e., afte ses this late epoch, which ranged upranged to apglobal topography data in 1993 by the Clemenbasalts (see above). In fact, the sam aparent potentially young lunar sample (66) correlation with antipodes of four, or perhaps n parent values 20 mT, are only upper limits on ~2 mT) until sometime after 3.3 Ga. (Figs. the 4 ofand 6). correlation with antipodes of four, or perhaps not consistent with the str values of 20 mT, are only upper limits on core dynamo from paleoma of(70) the youngest basins. Howthethat lunar field and therefore do not require atine core from paleomag even five (69), of the eight youngest basins. Howlunar field and dolunar require a therefore co even five (69), of the eight youngest basins. Howthe field and dofive not(69), require aeight with of dynamo four, or perhaps not parent values of 20 mT, are even only upper limits on correlation with antipodes four, or perhaps not consistent with theifcon stro parent values oftherefore 20 mT, are only upper limits on laser altimeter led tocorrelation theofwith identification bital data sets are consistent the hints the dynamo may have nevertheless correlation antipodes of four, or perhaps not parent values ofnot 20 mT, are only upper limits on c even five (69), of antipodes the eight youngest basins. Howthe lunar field and therefore dothe notacquisition require aeight core dynamo from paleom even five (69), of the youngest basins. Howthe lunar field and therefore do not require a ever, of the first high-resolution 3.56 to 3.85 Ga (Imbrian-aged dynamo at this time. Finally, a recent analysis ever, the acquisition ofaeight the first high-resolution 3.56 to dynamo 3.85 Ga (Imbrian-aged dynamo thisconfirm time. Finally, a recent analysis ever, the acquisition of the first high-resolution 3. dynamo atnot this time. Finally, ado recent analysis These results some Apollo-era core even five (69), of the eight youngest basins. How-core the lunar fieldrequire and therefore not require core from paleoma even five (69), of the youngest basins. Howtheatlunar field and therefore do aconcludyn even five (69), of the eight youngest basins. Howthe lunar field and therefore do not require a ever, the acquisition of the first high-resolution 3 dynamo at this time. Finally, a recent analysis ever, the acquisition of in the first high-resolution 3.56 to 3.85 Ga (Imbrian-ag dynamo at very this time. Finally, a recent analysis global topography data 1993 by the Clemenbasalts (see above). In fact, fact, th of a potentially very young lunar (66) global topography data in 1993 by the Clemenbasalts (see above). In th a potentially young (66) global topography data inhigh-resolution 1993to by the ba oflunar a potentially very young lunar sample (66) The global context of crustal magnetism ever, the acquisition of the first high-resolution 3.56 dynamo atsample this time. Finally, a the recent analysis ever, acquisition ofthe the first high-resolution 3.56 3.85 GaClemen(Imbrian-age dynamo atothers. this time. Finally, asample recent analysis ever, acquisition of the first 3.56 to dynamo at this time. Finally, a recent analysis sions of and refute Modern paleomagnetic global topography data in 1993 by the Clemenb3 of a potentially very young lunar sample (66) global topography data in 1993 by the Clemenbasalts (see above). In fact, of a potentially very young lunar sample (66) 127 Evolution temporelle du champ magnétique lunaire 1246753-8 5 DECEMBER 2014 VOL 346 ISSUE 6214 tine tine laser altimeter (70) to the bital data sets aresciencemag.o consistent hints thatdynamo the dynamo may have nevertheless laser altimeter (70) ledled to the identification data sets are consistent hints that the may have nevertheless tine laser altimeter (70)inbital led to identification bit( hints that the dynamo may have nevertheless global topography 1993 byClementhe Clemenbasa of a•lunar potentially very young lunar sample (66) global topography data in 1993 byidentification the Clemenbasalts (see above). In fact, of a potentially young sample (66) global topography data indata 1993 by thethe basalts ofdynamo avery potentially very young lunar sample (66) tine laser altimeter (70) led to the identification b hints that the dynamo may have nevertheless tine laser altimeter (70) led to the identification bital data sets are consisten hints that the may have nevertheless Coincident with theseto new paleointensity e measurements have now demonstrated that alasernevertheless tine altimeter led to the identification hints thatnevertheless themay dynamo have tine altimeter led tolaser the identification data sets are consistent hints that the dynamo have tine(70) laser altimeter (70) led(70) thebital identification bitalbita dat hints that may the dynamo havemay nevertheless 1246753-8 5 DECEMBER • 346 VOLMoon 346 ISSUE 6214 science periments, there have also been important core dynamo likely existed on the at least 1246753-8 5 DECEMBER 2014 •2014 VOL ISSUE 6214 sciencem 1246753-8 5 DECEMBER 2014 • VOL 346 ISSUE 6214 1246753-8 5 DECEMBER 2014 • VOL 346 ISSUE 62142014 • VOL 346 ISSUE 6214 scienc 1246753-8 5 DECEMBER 1246753-8 5 DECEMBER 2014 • VOL 346 ISSUE 6214 DECEMBER • contradiction VOL 346 ISSUE 6214 science investigations of the lunar crustal magne as far back1246753-8 in time 1246753-8 as5 4.25 Ga,2014 in to ISSUE cent 5 DECEMBER 2014 • VOL 346 6214 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  Article en préparation pour EPSL
  New paleointensities of Apollo samples and the evolution of lunar surface magnetic field with time Camille Lepaulard1, Jérôme Gattacceca1, David Cébron2, Sonia Tikoo3, Christopher McDonald4, Brent Turrin3. 1
  Aix Marseille University, CNRS, IRD, Coll. France, INRA, CEREGE, Aix-en-Provence,
  France. 2
  Université Grenoble Alpes, CNRS, ISTerre, Grenoble, France.
  3
  Department of Earth and Planetary Sciences, Rutgers University, 610 Taylor Road,
  Piscataway, NJ 08854, USA. 4
  School for Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-6004,
  USA.
  Abstract Paleomagnetic analyses of Apollo samples and studies of lunar crustal magnetic anomalies indicate that the Moon had a magnetic field in the past, generated by a core dynamo. To better constrain the evolution of the lunar magnetic field with time, and hence the processes responsible for the lunar dynamo generation, we present in this paper a paleomagnetic study of twenty-seven lunar samples. We obtained nine paleointensity estimates with methods based on alternating field demagnetization, and four paleointensity estimates with methods based on thermal demagnetization. Six other samples provided only upper limits on the paleointensity. Six paleointensities in the 13-57 μT range between 3.85 and 3.04 Ga confirm the existence of a surprisingly strong field with Earth-like intensity in the early Lunar history and suggests that this high field epoch ended after 2.9 Ga, significantly younger than previously proposed. Two samples in the high field epoch provides lower values than others, suggesting that the mean field during this high field epoch may have been lower than the average paleointensities proposed previously for this interval. Four samples provided paleointensities that cannot be accounted by the lunar crustal field, in the 1.11 to 2.72 μT range in the low field epoch. It suggests that the lunar dynamo was still active in fairly recent geologic times (<2 Ga, and possibly even after 1 Ga), with surface fields of ~1 μT indicating a weak state of the dynamo compared to the period older than 2.9 Ga.
  128 Evolution temporelle du champ magnétique lunaire Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  5
  1. Introduction Paleomagnetic analyses of Apollo samples (Fuller and Cisowski, 1987), and studies of lunar crustal magnetic anomalies (Hood et al., 1981; 2011) indicate that the Moon had a magnetic field in the past. This magnetic field could have been generated by a core dynamo (Wieczorek et al., 2006) or generated or amplified by impact-generated plasma (Hood and Artemieva, 2008). More recent analyses have shown that at least some paleomagnetic results require the existence of a long lived (> days) strong (several dozens μT) magnetic field, which requires the existence of a past lunar core dynamo. Geophysical and geochemical data show the presence of a lunar core with a liquid outer core of ~350 km radius, with a 240 km radius solid inner core, compatible with a core dynamo (Newsom, 1984; Wieczorek et al., 2006; Weber et al., 2011). Paleomagnetic results show that the dynamo was active from 4.25 Ga to 3.56 Ga (so called high-field epoch), with surface magnetic fields in 30-110 μT range, and an average field of ~70 μT (Garrick-Bethell et al., 2009; Cournède et al., 2012; Shea et al., 2012; Tikoo et al., 2012; Garrick-Bethell and Weiss, 2013; Suavet et al., 2013). The evolution of the magnetic field after 3.56 Ga is poorly constrained with only twelve paleointensity estimates, including seven upper limits (Tikoo et al., 2014; Tikoo et al., 2017). After 1.5 Ga, only three paleointensities (5 μT for regolith breccia 15498 and two lower values at 0.4 μT and 0.01 μT for regolith breccia 15015 and 15465 respectivly) are available to constrain the shutdown of the lunar magnetic field (Tikoo et al., 2017a; Wang et al., 2017). The 5 μT paleointensity (Regolith breccia 15498) is higher than the present day lunar magnetic remanent surface field, indicating the existence of a dynamo at 1.5 Ga. But the transition between the high field period and the weak field period is not documented. Mantle precession (Dwyer et al., 2011) and core crystallization (Zhang et al., 2013; Laneuville et al., 2014) could explain a persistence of a dynamo until 3.56 Ga. Local surface impacts could generate a dynamo only before 3.76 Ga (Suavet et al., 2013). To better constrain the evolution of the lunar magnetic field with time, and hence the processes responsible for the lunar dynamo generation, we present in this paper a paleomagnetic study of twenty-seven lunar samples. 2. Samples and methods 2.1 Samples Nine samples were studied in details after selection based on preliminary measurements of Natural Remanent Magnetization and magnetic susceptibility of large samples in the Lunar Sample Laboratory Facility (LSLF) at Johnson Space Center (JSC), Houston (Lepaulard et al., submitted). The selection criteria were a strong Natural Remanent Magnetization (NRM); availability of radiometric age and absence of previous paleomagnetic studies. The following samples were studied: three basalts (Apollo samples 12005, 12021 and 15529), one impact melt breccia (IMB, 14169), three regolith breccias (10018, 15505 and 61195), one dimict breccia (61015) and one impact melt rock (65055). The allocated samples, with masses around 2 g, were cut into mutually oriented small sub-samples with a diamond wire saw (Figure S1 in Supplementary material). A total of thirty-four subsamples were studied, with masses in the 55 to 699 mg (median mass 145 mg). Evolution temporelle du champ magnétique lunaire 129 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  Another set of eighteen samples was received on temporary loan from different research laboratories (IPG Paris, LMV Clermont-Ferrand, CRPG Nancy) for magnetic measurements before destructive geochemical analyses, following the opportunistic strategy used by Cournède et al. (2012). This set of samples contains thirteen basalts (Apollo samples 10003, 10017, 12012, 12016, 12018, 14078, 15555, 70017, 70135, 74255, 74275 and 75075; meteorite LAP 02205), one regolith breccia (meteorite NWA 10782) and four feldspathic rocks (Apollo samples 60015, 60215, 62255 and 65315). Among these samples, eight had never been studied for magnetism (12012, 12016, 14078, 62255, 74255, 75075, and the two meteorites). In this study, the samples have mass ranging from 25.7 mg to 42.1 g with a median mass of 157 mg. Some of the studied samples had already been dated, and their ages are given in Table 1. Seven of these samples had been dated using the 40Ar/39Ar method, four using the 87Rb/87Sr method, and one (62255) using the 143Nd/147Sm method. Mare basalts 12005, 12012, 12016 and IMB 14169 have no radiometric age and we use in the following an age estimate that is the average age for Apollo 12 olivine and ilmenite basalts (3.22±0.04 and 3.17±0.02 Ga respectively) and Apollo 14 IMB (3.85±0.03 Ga). Feldspathic rocks 60215, 65315 and NWA10782 have never been dated and cannot be readily compared to well dated rocks, so we have no age estimates for these samples. We observed thin sections of the nine samples we obtained from the Curation and Analysis Planning Team for Extraterrestrial Materials (CAPTEM) to look for shock evidence in the studied rocks (Figure S2). There is only evidence of a maximal pressure < 5 GPa with an exception for regolith breccia 61195. This breccia is named "shocked breccia", with a matrix containing abundant glass spheres. Some glass spheres are recrystallized and some clasts (plagioclase, pyroxene, olivine) exhibit shock features (Meyer et al., 2009 – compendium). For the other samples, obtained in loan from other French laboratories, we had no thin sections available, but sample description (Meyer et al., 2009 - Compendium) and online thin sections images available through the virtual microscope project (www.virtualmicroscope.org) show no evidence for shock except for two samples. Feldspathic rock 65315 contains olivine grains with undulose extinction but no shock melting is observed. Basalt 10017 shows undulose extinction in olivine grains. 2.2 Geochronology We performed radiometric dating of five samples whose paleomagnetic results were satisfactory. Two basalts (12005 and 15529) were dated using 40Ar/39Ar method at Rutgers University. One impact melt breccia (14169) and two regolith breccias (10018, 15505) were dated at Arizona State University using 40Ar/39Ar laser probe dating (Mercer et al., 2015). These analyses were performed on irradiated polished sections after characterization by scanning electron microscopy and electron dispersive spectroscopy mapping. Results are still being processed as of October 4, 2018.
  130 Evolution temporelle du champ magnétique lunaire Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  5
  2.3 Magnetic measurements Most magnetic measurements were performed at CEREGE (Aix-en-Provence, France). The paleomagnetism of eight sub-samples was analyzed at Rutgers University (New Jersey, USA), and two sub-samples (from 10018 and 15529) were analyzed at MIT paleomagnetism laboratory (Cambridge, USA). The magnetic susceptibility and the magnetic anisotropy of all samples were measured using a MFK1. Hysteresis measurements were conducted on all samples, with a Princeton Micromag Vibrating Sample Magnetometer (VSM) at room temperature. Maximum field of 1T was applied. Parameters obtained with the hysteresis loops allow us to calculate the coercive force (Bc), the remanent coercive force (Bcr) and the ratio of saturation remanent magnetization (Mrs) to saturation magnetization (Ms). All remanence measurements performed in CEREGE were conducted with a SQUID magnetometer (2G Enterprises, model 755R, with noise level of 10-11 Am2). Remanence measurements performed at Rutgers were conducted with a SQUID magnetometer (2G Enterprises, model 755, with noise level of 10-12 Am2). The NRM of forty sub-samples was measured and stepwise demagnetized using an alternating field up to 200 mT. Each demagnetization step was repeated three times and measurements were averaged to reduce the effect of spurious effects during AF demagnetization. ARM (Anhysteresis Remanent Magnetization) was acquired using a PAM1 anhysteretic magnetizer. Like the NRM demagnetization, each step of ARM demagnetization was performed and measured three times. IRM (Isothermal Remanent Magnetization) was acquired in a pulse magnetizer MMPM9, in a field of 1 or 3 T. Like for the NRM, each step of IRM demagnetization was performed and measured three times. Thermal demagnetization was only performed on the nine samples. Both at MIT and CEREGE, heating was done under controlled oxygen fugacity in an ASC oven, using a system described in Suavet et al. (2014). Viscous magnetization experiment (Enkin and Dunlop, 1988) was performed on the nine bulk samples received from CAPTEM to estimate its maximum contribution to the NRM. The acquisition of VRM (Viscous Remanent Magnetization) in a field of 30 μT was monitored over a two-week period, after which its decay was monitored over a similar period. 2.4 Paleointensity measurements We used first non-destructive AF-based methods with normalization by ARM (Stephenson and Collinson, 1974) and IRM (Gattacceca and Rochette, 2004) to determine paleointensities values for samples with stable HC component. These estimates, noted PARM and PIRM, are calculated along the AF interval over which the HC component is defined. For the ARM-based method, we used the calibration factor f'=1.34 estimated on lunar rocks by Stephenson and Collison (1974). For the IRM-based method, we used a calibration factor a=3000 μT (Gattacceca and Rochette, 2004). Both methods have an error of about a factor two because the calibration constant f' and a depend on the exact magnetic mineralogy and grain size (Tikoo et al., 2012). When the NRM is a thermoremanent magnetization, the most robust method for paleointensity determination is the method based on thermal demagnetization (Thellier and Thellier, 1959). We used the IZZI method of Yu and Tauxe,. (2004), with pTRM
  Evolution temporelle du champ magnétique lunaire 131 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  checks every two temperature step. Heating was performed under oxygen fugacity at IM/IW-1 using a system similar to that described in Suavet et al. (2014). 3. Magnetic properties Rock magnetism results are summarized in Table 1 and Table S1. Magnetic susceptibility and other properties depending on the magnetic mineral concentration are correlated to impact processing, though incorporation of meteoritic metal, as already evidenced in previous studies (e.g., Rochette et al., 2010; Lepaulard et al. submitted). We could determine the VRM acquisition and decay rates for five samples only (Table S1). The maximum VRM contribution to the NRM of these samples was determined, using a worst-case scenario where the samples have been stored in a fixed position with respect to the geomagnetic field at the Lunar Facility since their return from the Moon. In these five samples, the VRM could make up only a relatively small fraction of the NRM (maximum of 24% for regolith breccia 10018). Hysteresis properties were measured for twenty-six samples (Table 1, Figure 1). Basalts have multidomain magnetic carriers (Figure 1), in agreement with previous studies (Cournède et al., 2012; Tikoo et al., 2012; Gattacceca et al., 2010). One IMB (14169-B2a), one regolith breccia (61195-B1) and one dimict breccia (61015-B5) also have multidomain behavior, as observed in most lunar rocks whose magnetic properties are dominated by kamacite grains (Fuller et Cisowski, 1987). One feldspathic rock (60015) and one basalt (LAP 02205) have pseudo-single domain (PSD) behavior. Two regolith breccia (10018 and 15505) have a significant superparamagnetic contribution, as retrieved in other regolith breccia (Fuller et Cisowski, 1987). Superparamagnetic iron grains in regolith breccia reflect the properties of the lunar soil from which they originate as superparamagnetic iron is abundant in lunar soils from the in situ reduction of iron-bearing phases (Fuller et Cisowski, 1987). 1 1.00 SD PSD
  SP
  Mrs/Ms
  0.10
  MD
  0.01
  0.001 0.00 1
  10 Bcr/Bc
  132 Evolution temporelle du champ magnétique lunaire
  100 Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  5
  Figure 1: Dunlop-Day plot (Mrs/Ms versus Bcr/Bc) of hysteresis parameters for various lunar lithologies: basalts (circles), feldspathic rocks (triangles), regolith breccia (box), Impact Melt Breccia (cross), Impact Melt Rock (diamonds) and dimict breccia (star) in open symbols. Horizontal and vertical black lines divide the plot into regions representing single domain (SD), pseudo-single domain (PSD), superparamagnetic (SP) and multidomain (MD) states. subsample
  Sample
  Mass (mg)
  X (m3kg-1)
  Mrs (Am2kg-1)
  Ms (Am2kg-1)
  Bcr (mT)
  Br (mT)
  Slope corr. (Am2kg-1T-1)
  Age (Ga)
  Error (Ga)
  Dating method
  Dating ref.
  Lab. orgin
  Basalt 10003
  9
  74.3
  8.54E-07
  4.09E-04
  1.40E-01
  44.38
  0.68
  -1.40E-04
  3.82
  0.03
  Ar-Ar
  [1]
  LMV
  10017
  bulk
  550
  8.39E-07
  9.45E-04
  1.79E-01
  92.72
  2.72
  -4.63E-04
  3.04
  0.007
  Ar-Ar ‡
  [2]
  IPGP
  12005
  B2
  169
  1.00E-06
  6.32E-04
  1.52E-01
  15.63
  0.88
  -3.06E-04
  3.17
  0.02
  Ar-Ar
  [3]
  LSLF
  B3
  134
  9.79E-06
  5.00E-04
  1.12E-01
  19.15
  1.415
  -2.22E-04
  12012
  bulk
  290
  9.23E-07
  5.14E-04
  2.08E-01
  39.37
  1.59
  -2.75E-04
  3.22
  0.04
  Ar-Ar †
  [3]
  IPGP
  12016
  bulk
  340
  1.12E-06
  4.38E-04
  1.03E-01
  17.03
  1.28
  -1.76E-04
  3.17
  0.02
  Ar-Ar †
  [3]
  IPGP
  12018
  295
  996
  1.02E-06
  -
  -
  -
  -
  -
  0.65
  -
  Ar-Ar ‡
  [4]
  CRPG
  12021
  B1
  533
  1.58E-06
  7.15E-04
  1.03E-01
  2.89
  0.27
  -8.99E-05
  3.3
  0.1
  Rb-Sr
  [5]
  LSLF
  14078
  14
  25.7
  6.93E-07
  5.56E-04
  2.08E-01
  30.13
  2.40
  -8.00E-04
  3.89
  0.02
  Rb-Sr
  [6]
  LMV
  15529
  A1
  96
  8.04E-07
  6.32E-04
  9.91E-02
  27.10
  2.48
  -2.07E-04
  3.18
  0.004
  Ar-Ar
  [7]
  LSLF
  B2
  352
  9.98E-07
  6.88E-04
  1.14E-01
  28.49
  1.93
  -2.19E-04
  15555
  1085
  58.8
  1.13E-06
  5.12E-04
  1.14E-01
  23.52
  1.25
  -1.57E-04
  3.21
  0.06
  Ar-Ar ‡
  [1]
  LMV
  70017
  578
  112.2
  2.01E-06
  1.43E-03
  3.33E-01
  7.32
  0.77
  -1.20E-03
  3.72
  0.12
  Ar-Ar ‡
  [8]
  LMV
  70135
  107
  31.7
  4.54E-06
  2.42E-03
  7.74E-01
  10.42
  0.93
  -3.51E-03
  3.75
  0.09
  Rb-Sr
  [9]
  LMV
  74255
  221
  63.4
  1.95E-06
  2.51E-03
  3.05E-01
  9.73
  1.52
  -2.31E-03
  3.83
  0.06
  Rb-Sr
  [9]
  LMV
  74275
  358
  80.3
  1.11E-06
  1.51E-03
  2.39E-01
  65.06
  2.37
  -1.16E-03
  3.85
  0.08
  Ar-Ar
  [1]
  LMV
  75075
  217
  57
  2.32E-06
  2.11E-03
  3.69E-01
  5.93
  1.15
  -1.97E-03
  3.74
  0.04
  Ar-Ar
  [1]
  LMV
  LAP02205
  bulk
  219.4
  6.09E-07
  2.42E-03
  2.57E-02
  71.58
  18.85
  -1.59E-04
  2.87
  0.06
  Ar-Ar
  [10]
  IPGP
  B5
  88
  1.41E-05
  1.78E-02
  1.24E-01
  14.77
  1.32
  -4.29E-03
  3.81
  0.036
  Ar-Ar
  [11]
  LSLF
  Ar-Ar ‡
  [1]
  LMV
  -
  -
  LMV
  Nd-Sm
  [12]
  LMV
  -
  [3]
  LMV
  Dimict breccia 61015
  Feldspathic rock 60015
  820
  2132
  4.09E-08
  2.68E-04
  7.76E-03
  20.01
  5.45
  -7.87E-04
  4.09
  0.06
  60215
  61
  1981
  3.10E-09
  -
  -
  -
  -
  -
  -
  -
  62255
  295
  1034
  5.01E-09
  -
  -
  -
  -
  -
  4.43
  0.025
  65315
  161
  2048
  1.57E-09
  1.01E-05
  3.61E-05
  45.08
  25.02
  -2.73E-07
  -
  -
  73
  2.34E-06
  3.78E-03
  5.72E-01
  31.74
  2.01
  2.16E-03
  3.85
  0.03
  Ar-Ar †
  [3]
  LSLF
  699
  9.53E-07
  8.45E-04
  2.35E-01
  36.07
  1.41
  -2.44E-03
  3.87
  0.02
  Ar-Ar ‡
  [13]
  LSLF
  Impact melt breccia 14169
  B2a
  Impact melt rock 65055
  C
  Regolith breccia 10018
  A2a
  145
  2.92E-05
  1.29E-01
  1.52E+00
  42.16
  6.12
  -5.79E-01
  1.56*
  0.43
  ArTr
  [14]
  LSLF
  15505
  A1b
  80
  1.31E-05
  1.00E-01
  8.49E-01
  70.44
  9.23
  -2.44E-01
  0.1*
  0.01
  ArTr
  [14]
  LSLF
  61195
  B1
  253
  8.01E-06
  6.09E-03
  8.42E-01
  30.12
  1.23
  -2.69E-02
  3.41
  0.43
  ArTr
  [14]
  LSLF
  NWA10782
  bulk
  42070
  9.22E-09
  -
  -
  -
  -
  -
  -
  -
  -
  IPGP
  -
  Evolution temporelle du champ magnétique lunaire 133 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  Table 1: Lithological classification of samples is based on classification of Heigen et al. (1991). LAP02205 is an unbrecciated basalt meteorite and NWA10782 is a regolith breccia meteorite. Mrs: saturation remanent magnetization; Ms: saturation magnetization; Bcr: the remanent coercive force; Bc: the coercive force. Samples never dated or with no stable plateau of dating are noted -. All samples noted ‡ were recalculated with the calibration constant from Steiger and Jäger (1977). Samples in datation with the laser probe 40Ar/39Ar method have an age with symbol *. Samples with an average age from the Apollo group lithology (based on Stöffler et al., 2006) are noted †. ArTr indicates model closure age determined from the trapped 40Ar/39Ar ratio (Fagan et al., 2014). LMV: Laboratoire Magmas et Volcans (Clermont-Ferrand, France). IPGP: Institut du Physique du Globe de Paris (Paris, France). LSLF: Lunar Sample Laboratory Facility in Johnson Space Center (Houston, USA). CRPG: Centre de Recherche Pétrographiques et Géochimiques (Nancy, France). [1] Turner (1977) [2] Suavet et al. (2013) [3] Stoffler et al. (2006) [4] Stettler et al. (1973) [5] Cliff et al. (1971) [6] McKay et al. (1978) [7] Us in Rutgers
  [8] Phinney et al. (1975) [9] Nyquist et al. (1975) [10] Fernandes et al. (2009) [11] Norman et al. (2006) [12] Boyet et al. (2009) [13] Jessberger et al. (1977) [14] Fagan et al. (2014)
  4. Results 4.1 Geochronology The two analyzed splits of 12005 produced disturbed release-spectra. They do not fill the criteria commonly used to define a plateau. The higher temperature step seems to indicate that the 36Ar is dominated by a cosmogenic source. The only conclusion is that this sample is older than ~2.8 Ga (Figure 2a, Text S3a). For this sample, we will use the average age for Apollo 12 ilmenite basalts: 3.17±0.02 Ga (Stöffler et al., 2006) The single studied split of 15529 produced a disturbed release-spectra defined on 45 steps. The mid-to-high temperature steps is made by up of ~62% of the total 39Ar release, and gave a plateau age of 3175±4 Ma. The integrated and total fusion age is defined at 3091±10 Ma (Figure 2b, Text S3b). These ages are calculated assuming a nebular 40Ar/36Ar value of 1.10-4 for the trapped component.
  134 Evolution temporelle du champ magnétique lunaire Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  5
  a) 40Ar/39Ar Spectrum of basalt 12005
  b) 40Ar/39Ar Spectrum of basalt 15529
  Figure 2: Ar/Ar step-heating spectrum for samples 12005 (a) and 15529 (b) and curve associated the apparent age (Ma) versus cumulative % 39Ar released. (a) There are two spectrum of two small pieces of basalt 12005 (left, 12005 22936 ; right, 12005 22934).
  4.2 Demagnetization of the NRM Stepwise demagnetization of the NRM allowed isolating stable magnetization components in the fifty-two analyzed samples, using principal component analysis (Kirschvink, 1980). Results are detailed in Table 2. A low coercivity (LC) magnetization component is obtained for forty samples with unblocking up to a maximum of 17 mT. Low temperature (LT) magnetization component is obtained for twelve samples, with unblocking temperature up to 330°C. A high coercivity (HC) magnetization component was identified in twelve samples (Figure 3a-g). A high temperature (HT) component is obtained for six samples (Figure 3h-k). Most of these HC and HT components are origin trending (Figure 3), and are interpreted as the characteristic remanent magnetization (ChRM). For samples with HC/HT components and multiple mutually-oriented samples (10018, 15505, 15529), the directions show a satisfactory grouping for such small subsamples (median mass 150 mg) with precision parameters k ranging from 3 to 46 (Figure 4). Most samples (34 out of 52) have only LC/LT components and have unstable behavior at high AF field and high temperature (Figure 5). Evolution temporelle du champ magnétique lunaire 135 136 Evolution temporelle du champ magnétique lunaire
  S
  17
  44
  44
  HC 40
  5 mT
  LC
  HT
  E Down
  580
  250
  LT
  NRM
  N x 10-8 Am2
  NRM
  100°C
  NRM
  S Down
  HC
  100
  W
  60
  NRM
  NRM
  W
  HT
  460
  Up x10-9 Am2
  -9
  S Down
  -9
  2
  LC
  200
  LT
  HC
  NRM
  NRM
  NRM
  W
  NRM
  E x 10-9 Am2
  12 mT
  2
  E x 10 Am
  Up x 10 Am
  100
  NRM
  N
  70
  f) LAP 02205 (219.4 mg)
  W
  HC
  8 mT
  i) 10018-B1b (68.4 mg)
  S Down
  630
  N
  Ex
  LC
  NRM
  b) 10017 (550 mg)
  10-9 Am2
  N Up x 10-9 Am2
  E x 10-8 Am2 NRM
  h) 15505-A3b (213 mg)
  20
  Up x 10-8 Am2
  S Down
  220
  N
  e) 10018-B4 (80 mg)
  5 mT
  LC
  NRM
  W Up x 10-8 Am2
  W
  W
  NRM
  a) 15529-A2 (96 mg)
  S
  100
  N
  S
  100
  45
  -8
  19
  Down
  E x 10-9 Am2
  Up x 10-9 Am2
  2
  NRM
  W
  NRM
  E Down x 10 Am
  HC
  9 mT
  LT
  520
  200°C
  HT
  580
  N
  NRM
  NRM
  LC
  NRM
  W
  NRM
  LC
  2 mT
  W
  E x 10-9 Am2
  Down
  HC
  Up x 10-9 Am2
  20
  S Down
  450
  HT
  200°C
  LT
  N Up x10-9 Am2
  6 mT
  E x10-6 Am2
  NRM
  E x10-9 Am2
  NRM
  k) 14169-B1a (129 mg)
  S
  14
  N
  38
  S Down
  100
  HC
  LC
  N Up x10-6 Am2
  d) NWA 10782 (42.07 g)
  g) 12012 (290 mg)
  4
  E x 10-9 Am2
  Up x 10-9 Am2
  S Down
  450
  N
  j) 61195-A1a (129 mg)
  2
  W Up x 10 Am -8
  2
  c) 15505-A2a (96 mg)
  5 Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  Figure 3: NRM demagnetization data for 11 lunar samples. Orthogonal projection plots of stepwise alternating field demagnetization data. Dark symbols represent the projection into the horizontal plane (N-E or W-N) and open symbols into the vertical plane (Up-E or Up-N). (a-g) NRM vector of 7 samples during AF demagnetization. Low coercivity (LC) and high coercivity (HC) magnetization components are labeled and represented with light gray and dark gray arrows respectively. (h-k) NRM vector of 4 samples during thermal demagnetization. Low thermal (LT) and high thermal (HT) magnetization components are labeled and represented with light gray and dark gray arrows respectively. Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  subMass sample (mg)
  NRM Method (Am2.kg-1)
  Basalt 10003
  9
  74.3
  2.79E-06
  10017
  bulk
  550
  7.53E-06
  12005
  B2 B3 B4 C1 bulk
  169 134 144.5 198 290
  1.53E-05 4.29E-05 2.26E-05 1.10E-05 9.55E-06
  AF AF AF Thermal P(ARM) P(IRM)
  bulk 295 A1 C B1 14 A2 B2
  340 996 287 220 533 25.7 434 352
  7.88E-06 3.53E-05 1.13E-06 1.16E-05 1.25E-05 3.79E-05 2.44E-06 8.32E-06
  AF AF Thermal AF AF AF P(ARM) P(ARM) P(IRM)
  C1 C2 15555 1085 70017 578 70135 107 74255 221 74275 358 75075 217 LAP02205 bulk
  438 381 58.8 112.2 31.7 63.4 80.3 57 219.4
  1.34E-05 2.57E-05 7.86E-06 1.99E-05 5.55E-05 7.84E-05 2.32E-06 4.74E-05 1.51E-05
  Thermal Thermal AF AF AF AF AF AF P(ARM) P(IRM)
  Dimict breccia 61015 B2 B3 B5 B6
  79.6 87 88 55
  2.70E-05 9.51E-05 4.15E-04 1.82E-04
  Feldspathic rock 60015 820 60215 61 62255 295 65315 161
  2132 1981 1034 2048
  Sample
  N
  MAD (°) Range (HC;HT)
  N
  MAD (°)
  Paleofield (μT)
  Error (μT)
  f
  g
  q
  D (%)
  E (%)
  0-4 mT
  4
  5.8
  4-100 mT*
  35
  43.5
  -
  -
  -
  -
  -
  -
  -
  2-8 mT 2-8 mT 0-2 mT 0-7 mT 2-9 mT 2 mT-250°C 2-14 mT 2-14 mT 2-13 mT 2-8 mT 2mT-150°C 0-3 mT 0-12 mT 0-14 mT 2-17mT 0-16 mT 0-16 mT 150-280°C 25-200°C 0-3 mT 0-7 mT 0-7 mT 0-9 mT 0-8 mT 0-4 mT 2-12 mT 2-12 mT
  7 7 2 8 8 6 13 13 12 7 4 4 12 14 16 16 16 4 6 3 7 7 9 8 4 11 11
  6.7 6.7 0.1 8.5 9.5 8.5 14.3 14.3 22.4 6.5 6.9 9 28.5 6.6 13.7 14 14 6 4 13.8 27.3 11.7 14.8 23.6 5.6 8.8 8.8
  11-100 mT 11-100 mT 4-100 mT* 7-100 mT* 9-100 mT* 250-540°C* 14-100 mT 14-100 mT 15-100 mT* 9-180 mT* 200-500°C* 3-100 mT* 13-100 mT* 14-55 mT* 17-100 mT 16-100 mT 16-100 mT 300-480°C* 200-360°C* 3-100 mT* 7-100 mT* 7-100 mT* 10-100 mT* 9-100 mT* 13-100 mT* 13-100 mT 13-100 mT
  35 35 28 42 30 10 31 31 30 33 9 46 26 20 38 23 23 7 5 34 32 32 28 30 26 33 33
  4.2 4.2 37.9 44.0 39.9 35.8 24.4 24.4 32.8 35.5 33.7 37.5 36.9 43.1 22.7 23.6 23.6 9.1 38.4 43.0 27.0 41.5 47.4 45.1 36.3 4.4 4.4
  48.97 44.33 < 22 < 11 21.05 25.84 < 15 < 22 10.00 13.99 25.01 13.77 16.09
  1.92 2.15 2.43 2.91 1.26 3.55 7.39 0.66 0.48
  -
  -
  -
  -
  -
  -
  42.7 5.2 70.3 44 -
  69.3 73.2 73.8 100.7 -
  Thermal AF AF AF
  2mT-150°C 0-5 mT 2-12 mT 0-12 mT
  4 6 10 12
  5.5 3.4 8.9 2.1
  150-330°C* 6-100 mT* 12-100 mT* 12-14 mT*
  5 43 20 3
  30.2 37.9 30.9 36.2
  < 22 < 11
  -
  -
  -
  -
  50 30.7
  38.3 72.1
  1.91E-06 6.46E-08 1.75E-07 8.64E-08
  AF AF AF AF
  0-8 mT 0-5 mT 0-10 mT 2-14 mT
  8 5 10 13
  9.9 16 12.8 22.1
  9-100 mT* 5-80 mT* 11-100 mT* 15-100 mT*
  30 31 28 23
  40.6 39.5 36.4 39.6
  <7
  -
  -
  -
  -
  14.1
  66.2
  Impact melt breccia 14169 B1a 129 B2a 73 B2b 118
  6.90E-05 6.65E-05 3.55E-05
  Thermal AF AF
  2mT-330°C 0-10 mT 0-4 mT
  8 10 5
  4.3 7.6 2.7
  360-580°C 10-90 mT* 11-100 mT*
  9 24 38
  19.2 39.5 38
  57.81 <7 <7
  0.53 -
  0.67 -
  1.3 -
  90.8 51
  38.9 57.1
  Impact melt rock 65055 A1 B2 C
  387 279 699
  3.85E-06 3.57E-06 4.75E-06
  Thermal AF AF
  25-200°C 0-7 mT
  6 7
  12.8 14.5
  250-580°C* 1-100 mT* 8-100 mT*
  12 47 21
  33.5 33.8 38.1
  < 15
  -
  -
  -
  <7
  -
  -
  -
  -
  1.9 29
  24 84.2
  Regolith breccia 10018 A2a
  145
  1.48E-04
  58 68.4 68.4 80
  1.28E-05 8.65E-05 9.23E-05 1.99E-04
  C1
  132
  1.15E-04
  A1b
  80
  4.04E-04
  A2a
  96
  1.30E-04
  A3c2
  187
  4.05E-05
  3.1 3.1 0.2 1.7 14.8 5.6 5.6 6.6 6.6 4.7 4.7 11.7 11.7 2.7 2.7 5.7
  6 7 5 13 13 10 10 9 9 13 13 22 22
  17.7 17.7 14.3 29.6 12.7 22.6 22.6 10.5 10.5 20.0 20.0 14.9 14.9 29.0 29.0
  1.72E-04 1.26E-05 4.03E-05 7.96E-06 5.66E-05
  6 6 4 3 3 5 5 5 5 7 7 5 5 5 5 4
  24-100 mT 24-100 mT 360-610°C 300-525°C 320-560°C 38-100 mT 38-100 mT 34-80 mT 34-80 mT 45-150 mT 45-150 mT 40-100 mT 40-100 mT 16-90 mT 16-90 mT
  213 129 253 225 42070
  0-6 mT 0-6 mT 100-300°C 100-300 °C 25-150°C 0-5 mT 0-5 mT 0-5 mT 0-5 mT 0-12 mT 0-12 mT 0-5 mT 0-5 mT 2-6 mT 2-6 mT 100-250°C
  21 21
  A2b B1a B1b B4
  P(ARM) P(IRM) Thermal Thermal Thermal P(ARM) P(IRM) P(ARM) P(IRM) P(ARM) P(IRM) P(ARM) P(IRM) P(ARM) P(IRM) Thermal
  25-200°C 2-7 mT 0-5 mT
  6 6 6
  11.9 30.0 12.7
  11 6 32 44
  12.5 26.3 41.1 25.0
  0.21 0.17 0.29 0.12 0.22 0.60 0.34 0.11 0.09 0.52 0.35 0.23 0.17 0.09 0.06 0.23
  Thermal AF AF P(ARM)
  300-580°C 250-450°C 7-100 mT* 5-100 mT*
  1.36 1.04 1.43 0.75 1.04 2.25 1.26 2.02 1.39 1.72 1.33 1.51 1.14 0.60 0.38 2.72 13.35 <7 <7
  2.76 -
  5.1 5.1
  38-100 mT 38-100 mT
  11 11
  5 5
  196.9 92.7
  10.7 5.5
  0.5 1.2 1.1 4.4 1.4 -
  116.7 22.9
  4 4
  1.0 0.69 0.69 0.89 0.77 -
  57 21
  0-6 mT 0-6 mT
  0.76 0.27 0.34 0.42 0.37 -
  -
  -
  12012 12016 12018 12021
  14078 15529
  15505
  A3b A1a B1 B3 NWA10782 bulk 61195
  AF P(ARM)
  Range (LC;LT)
  5
  P(IRM)
  P(IRM)
  15.63 -
  Evolution temporelle du champ magnétique lunaire 137 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  Table 2: Results of all lunar samples analyzed in this study. AF: alternating field method. PARM: paleointensity result obtained with ARM normalization (see text); PIRM: paleointensity result obtained with IRM normalization (see text). LC: Low coercivity component in mT; LT: low temperature component in Celsius degrees; HC: high coercivity component in mT used for paleointensity calculation (see text). *: HC range for samples with no stable component, interval of calculation of the upper limit value. HT: high temperature component in Celsius degrees used for paleointensity calculation (see text). *: HT range for sample with no stable component. N: number of steps for the characteristic component or component used for upper paleointensity limit. Samples with lower paleointensity limit have no satisfactory results of paleointensity recognized. D= difference in % and E= error in %, calculated only for samples with an upper paleointensity limit (see text). Italic values of E and D are for samples with unsuccessful upper limit recognized (D and E > 100%) and have a lower limit of paleointensity (noted ">"). B
  a) 10018
  A
  b) 15505
  0 Mean direction k = 5.8 0α95 = 27.3°
  HC / HT
  0
  HC / HT
  HC / HT
  A3c2 A3b
  B1a
  A2a
  270
  90
  270
  A1b
  90
  B1b C1 B4 B1b
  B1a
  270
  A2b
  90
  A2a
  C1 B4
  A2b
  180
  c) 15529
  0
  HC
  Mean direction k = 3.2 α95 = 61.8°
  180
  AF Thermal Lower Hemisphere H
  A2a
  180
  Upper Hemisphere H
  Mean direction k = 45.7 α95 = 37.8°
  270
  90
  B2 A2
  180
  Figure 4: Magnetization components observed for mutually oriented samples of (a) 10018, (b) 15505 and (c) 15529. HC (circles) and HT (squares) components directions are in an equal stereographic projection. Filled symbols (solid lines) represent directions in the lower hemisphere and open symbols (dashed lines) in the upper hemisphere.
  138 Evolution temporelle du champ magnétique lunaire Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  5
  a) 61015-B5 (88 mg)
  N Up x10-8 Am2 W
  5
  N
  Up x10-9 Am2
  E x10-8 Am2
  3 mT NRM
  100
  W
  LC
  E x10-9 Am2
  9 mT
  9 mT
  S
  Down
  NRM
  S Down
  b) 65055-A1 (387 mg)
  N Up x10-9 Am2 NRM
  LT 250 150°C
  W
  E x10-9 Am2
  NRM 580
  S Down
  Figure 5: NRM demagnetization data for samples 61015 (AF-method) and 65055 (thermal method) on an orthogonal projection plots of stepwise alternating field demagnetization data. Dark symbols represent the projection into the horizontal plane (N-E) and open symbols into the vertical plane (Up-E). (a) These samples are an example of samples with only a low coercivity (LC) magnetization component (a) / a low thermal (LT) magnetization component (b) that are represented in light gray arrow, as all others samples with no ChRM component (see text). (a) A zoom on the HC part of sample 61015 is delimited by a dotted box to see the zig-zag behavior of demagnetization vector after 9 mT.
  4.3 Paleointensities determinations After paleointensities experiments, we obtained nine satisfactory paleointensity estimates with AF-based methods (Figure 6a-g), and four satisfactory paleointensity estimates with the Thellier-Thellier method (Figure 7a-d). These results are given in Table 2 and summarized in Table 3. For all samples that did not show stable HC components (Figure S4), Tikoo et al. (2012) has developed a method to estimate an upper limit for the paleointensity by imparting ARM in decreasing DC fields and determining the lowest DC field that can be retrieved by normalization with a reference ARM acquired in a stronger field (200 μT in this study). We used fidelity parameters (the difference noted D and the error noted E) used in Tikoo et al. (2014) to estimate the upper limit for the paleointensity (Figure S5, Table 2). Over the twentyEvolution temporelle du champ magnétique lunaire 139 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  eight samples analyzed with this protocol, only six give satisfactory upper limit results, with D and E < 100% (Table 3). a) 10017 (550 mg) 6
  0.8
  DC 0.1 mT
  DC 1 T AF 100 mT
  0.6
  HC (48.9 ± 1.9 μT) NRM lost (x10-8 Am2)
  NRM lost (x10-9 Am2)
  4
  AF 11 mT
  2
  AF 100 mT
  HC (44.3 ± 2.2 μT) 0.4
  AF 11 mT
  0.2 AF 8 mT
  AF 8 mT
  10017
  AF 2 mT
  0
  2
  0
  4 ARM lost (x10-9 Am2)
  6
  0
  10017
  AF 2 mT
  8
  0
  10
  40
  30
  20 IRM lost (x10-8 Am2)
  b) 10018-B4 (80 mg) 0.15
  DC 0.01 mT
  AF 38 mT
  1
  0.5
  AF 2 mT
  10018-B4 0.5
  0
  1.5
  1
  HC (1.3 ± 0.3 μT)
  AF 5 mT
  10018-B4
  AF 2 mT
  0 2
  AF 100 mT
  AF 38 mT
  0.1
  0.05
  AF 5 mT
  0
  DC 3 T
  AF 100 mT
  HC (2.3 ± 0.6 μT)
  NRM lost (x10-7 Am2)
  NRM lost (x10-8 Am2)
  1.5
  0
  40
  60 IRM lost (x10-7 Am2)
  20
  ARM lost (x10-8 Am2)
  80
  100
  c) 12012 (290 mg) 3
  0.3
  DC 0.1 mT
  DC 1 T
  AF 100 mT
  AF 100 mT
  AF 15 mT
  AF 15 mT
  HC (25.8 ± 2.9 μT) NRM lost (x10-8 Am2)
  NRM lost (x10-9 Am2)
  HC (21.1 ± 2.4 μT) 2
  AF 2 mT
  1
  12012
  0 0
  1
  2
  3
  4
  0.2
  AF 2 mT
  0.1
  12012
  0
  5
  0
  ARM lost (x10-9 Am2)
  140 Evolution temporelle du champ magnétique lunaire
  2
  4
  6
  8
  IRM lost (x10-8 Am2)
  10
  12
  14 Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  5
  d) 15505-A2a (96 mg) 1.5
  0.15
  DC 0.02 mT HC (1.5 ± 0.2 μT)
  HC (1.1 ± 0.2 μT)
  AF 100 mT
  AF 40 mT
  AF 100 mT
  NRM lost (x10-7 Am2)
  NRM lost (x10-8 Am2)
  1
  DC 1 T
  0.5
  0.1
  AF 40 mT
  0.05
  AF 5 mT
  0 0
  LC
  AF 5 mT
  15505-A2a
  AF 2 mT
  0.5
  1
  1.5 2 ARM lost (x10-8 Am2)
  2.5
  15505-A2a
  AF 2 mT
  0
  3
  0
  20
  40
  60
  80
  100
  IRM lost (x10-7 Am2)
  e) LAP 02205 (0.22 mg) 4
  0.4
  DC 0.1 mT
  AF 100 mT
  3
  0.3 NRM lost (x10-8 Am2)
  NRM lost (x10-9 Am2)
  HC (13.8 ± 0.7 μT)
  2 AF 13 mT
  1
  AF 100 mT
  HC (16.1 ± 0.5 μT)
  0.2
  0.1
  AF 2 mT
  0
  DC 1 T
  0
  AF 2 mT
  LAP02205 2
  4
  6
  8
  10
  AF 13 mT
  0
  12
  0
  LAP02205 10
  ARM lost (x10-9 Am2)
  20
  30
  40
  50
  IRM lost (x10-8 Am2)
  f) NWA 10782 (42.07 g) -6
  3.10 4.E-03
  -6
  4.10 4.E-03
  DC 0.1 mT
  DC 3 T
  AF 100 mT
  3.E-03
  HC (196.9 ± 10.7 μT)
  2.E-03 AF 6 mT
  1.10-6
  AF 100 mT
  AF 38 mT
  AF 38 mT
  NRM lost (Am2)
  NRM lost (Am2)
  2.10-6
  -6
  2.10 2.E-03
  AF 6 mT
  1.10-6
  1.E-03
  0 0.E+00 0 0.E+00
  HC (92.7 ± 5.5 μT) 3.10-6
  3.E-03
  1.E-03
  1.10-7 1.E-04
  2.10-7 2.E-04
  3.10-7 3.E-04
  -7
  4.10 4.E-04
  5.10-7 5.E-04
  6.10-7 6.E-04
  ARM lost (Am2)
  -7
  7.10 7.E-04
  0 0.E+00 0.E+00 0
  -5 1.10 1.E-02
  2.10-5 2.E-02
  3.10-5 3.E-02
  4.10-5 4.E-02
  5.10-5 5.E-02
  6.E-02 6.10-5
  IRM lost (Am2)
  g) 15529-A2 (434 mg) 2.5
  DC 0.1 mT AF 100 mT
  2
  NRM lost (x10-9 Am2)
  HC (10.00 ± 1.26) 1.5
  1 AF 17 mT
  0.5 AF 1 mT
  15529-A2
  0 0
  5
  10 ARM lost
  15
  20
  (x10-9 Am2)
  Evolution temporelle du champ magnétique lunaire 141 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  Figure 6: ARM and IRM paleointensity experiments for 7 samples. Left : Shown is NRM lost during AF demagnetization as a function of ARM lost during stepwise ARM decay (AF field increasing to 100 mT, DC bias field = 0.01, 0.02 or 0.1 mT) at an equivalent AC field. Right : Shown is NRM lost during AF demagnetization as a function of IRM lost during stepwise IRM decay (AF field increasing to 100 mT, IRM field = 1 or 3 T) at an equivalent AC field. Data corresponding to the LC component are in light gray and data from higher AF levels (HC) are shown in dark gray. White points corresponding to an overlap between the LC and HC components. B
  A
  4
  14 1.4E-08
  10018-A2b (58 mg)
  15505-A3b (213 mg)
  NRM
  NRM 12 1.2E-08
  3 NRM remaning (x10-8 Am2)
  NRM remaning (x10-9 Am2)
  10 1.0E-08
  8 8.0E-09
  6 6.0E-09
  4 4.0E-09
  360°C
  1.43 ± 0.29 μT
  0 0 0.0E+00 0 0.0E+00
  300°C
  1
  610°C
  580
  0 2 2.0E-09
  6 6.0E-09
  4 4.0E-09
  0
  1
  2 3 pTRM gained (x10-8 Am2)
  pTRM gained (x10-9 Am2)
  C
  NRM
  4
  5
  20
  D
  100 90
  2.72 ± 0.23 μT HT
  HT
  2 2.0E-09
  2
  61195-A1a (129 mg)
  14169-B1a (145 mg) NRM 15
  NRM remaning (x10-10 Am2)
  NRM remaning (x10-10 Am2)
  80 70 60 50 40 30
  10
  250°C 5
  13.4 ± 2.8 μT
  20 360°C
  57.8 ± 15.6 μT
  10
  450°C
  580°C 0
  0 0
  5
  10
  15
  20
  0
  pTRM gained (x10-10 Am2)
  10
  20
  30
  40
  50
  60
  pTRM gained (x10-10 Am2)
  Figure 7: Thellier-Thellier paleointensity experiment results for samples (a) 10018, (b) 15505, (c) 14169 and (d) 61195. Shown is the NRM remaining versus partial TRM grained. Paleointensity (in gray on each figure) is calculated for each sample on the HT component, which is represented in light gray arrow. pTRM checks are represented in green triangles.
  5. Discussion 5.1 Variation of lunar surface field with time Together with selected literature data (Table S2), our paleointensity estimates (Table 3) allow discussing the evolution of lunar surface field intensity with time (Figure 8). Overall, the field intensity decreases with time since 4 Ga, and two main epochs (high field and low field epochs) can be distinguished. 142 Evolution temporelle du champ magnétique lunaire Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  5
  We provide six paleointensities in the 13-57 μT range, between 3.85 and 3.04 Ga. This confirms the existence of a surprisingly strong field with Earth-like intensity during the high field epoch. We obtain a lower value for 10017 (46.65±3.28 μT) than the value obtained (71 μT) by Suavet et al. (2013) and we considerate a magnetization age at 3.04 Ga due to the Ar/Ar plateau age obtained in his study. The result from 10017 (46.65±3.28 μT at 3.04±0.01 Ga) and even more the results from LAP 02205 (14.93±1.64 μT at 2.87±0.06 Ga) extend the high field epoch to younger ages than previously proposed by Tikoo et al. (2014). She advocated a steep decline of the lunar field between 3.56 Ga and 3.19 Ga mostly based on the weak paleointensity upper limits obtained for Mare basalts 15597 and 12022 (< 7 μT and < 4 μT, respectively). The pristine anorthosite 65315 provides an upper limit of 7 μT but has not been dated. Two samples in the high field epoch (dimict breccia 61015 at 3.81±0.04 Ga and IMR 65055 at 3.87±0.02 Ga) provide paleointensity upper limits of 7 to 11 μT. We have calculated the average paleointensities values between 4.5 and 3.5 Ga and between 4.5-2.9 Ga. We used paleointensities equal to zero (resp. equal to the upper limit itself) to derive a lower (resp. upper) bound for the average paleointensity. In the first interval, paleointensities values are ranging from 51±38 to 52±36 (n=12). In the second interval, values are ranging from 29±34 to 37±31 μT (n=29). These numbers suggests that the mean field during the 4.5-3.5 Ga interval may have been lower, around 50 μT, than the average paleointensities of ~70 μT proposed for this interval (Tikoo et al. 2014). The literature dataset may be biased towards strong paleointensity samples that may have been more likely to be selected for study in view of their high NRM. These very strong paleointensities in the 50 to 100 μT range may represent only the upper range of a variable field with lower values at 7 and 11 μT. These values remain remarkably high and do not solve the disagreement between dynamo modeling and paleomagnetic results. Over the 3.5-2.8 Ga interval, the average paleointensities ranges from 14±22 to 27±22 μT (n=17). These values are lower than the 50 μT value obtained in the previous interval (4.53.5 Ga). Our data, with upper limits of 7 to 22 μT obtained in the 3.8-3.2 Ga interval, and paleointensities in the 10 to 50 μT range in the 3.5-2.9 Ga interval, indicate that the lunar surface field may have had a relatively stable behavior over the whole period 4 Ga to 2.9 Ga, with a field intensity varying between < 4 μT and 100 μT. The theory of an intermittent dynamo has been advanced (Evans et al., 2017), which could also explain the lower values. But this < 4 μT-100 μT range may also simply represent the expected variability for a dynamo generated field. Such a wide range is observed in the Earth paleointensity record, with variation of the Earth Virtual Dipole Moment by a factor ~15 over the last 100 Myr (eg., Biggin et al. 2015). In the low field epoch, we obtain robust paleointensity estimates for regolith breccia 10018 indicating fields of 1.55±0.47 μT (AF methods, n=6) and 1.07±0.34 μT (ThellierThellier method, n=3) at 1.56±0.43 Ga. A paleointensity of ~2 μT is obtained from regolith breccia 15505 dated at 0.1±0.01 Ga. The future new radiometric age of 15505 (using 40Ar/39Ar) should validate this surprising result. Together with the literature dataset, a total of five paleointensity estimates in the 0.01 to 7.4 μT range is available in the low field epoch. The result on 15465 at 0.01±0.01 μT (Wang et al., 2017) is an outlier in this dataset, and the only one compatible with a remanent lunar surface field. The remaining four samples provide paleointensities that cannot be accounted by the lunar crustal field, as we know it today. Direct Evolution temporelle du champ magnétique lunaire 143 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  surface field measurements at Apollo 12, 14, 15 and Apollo 16 landing sites gave local field intensities between 3 and 327 nT (Dyal et al., 1974), and electron reflectometry data show that most of the lunar surface has crustal field below 50 nT. In view of the kilometer scale spatial variability of the crustal field (Dyal et al., 1970; Dyal and Daily 1978) it is probable that the field may be locally stronger than this, but these strong fields have to be rare at the surface and it is unlikely that they can account for the paleointensities > 1 μT observed in four out of five Apollo rocks in this low field epoch. These four samples suggest that the lunar dynamo was still active in fairly recent geologic times (<2 Ga, and possibly even after 1 Ga), with surface fields of ~1 μT indicating a weak state of the dynamo compared to the period older than 3 Ga. Data are notably lacking between 3 Ga and 1.5 Ga, because volcanism is rare during this period, and regolith breccia from this period are unlikely to be preserved up to the present day because of surface reworking.
  Sample
  Age (Ga)
  error (Ga)
  10017 10018
  3.04 1.56
  0.01 0.43
  12012 15505
  3.22 0.1
  0.04 0.01
  15529 14169 61195 NWA10782 LAP02205
  3.18 3.85 3.41 2.87
  0.004 0.03 0.43 0.06
  Age (Ga) 3.17 3.17 3.30 3.81 3.87
  error (Ga) 0.02 0.02 0.10 0.04 0.02
  Sample 12005 12016 12021 61015 65315 65055
  Method AF AF Thermal AF AF Thermal AF Thermal Thermal AF AF
  N 2 1 1 2 1 2
  N
  Average paleointens ity (μT)
  2 6 3 2 6 1 3 1 1 2 2
  46.65 1.55 1.07 23.45 1.11 2.72 16.33 57.81 13.35 144.82 14.93
  s.d (μT) 3.28 0.47 0.34 3.39 0.53 0.23 7.77 15.63 2.76 73.71 1.64
  Paleofield (μT) < 11 < 15 < 22 < 11 <7 <7
  Table 3: Paleointensities and upper limit results obtained. N is the number of values used for average for each Apollo rock.
  144 Evolution temporelle du champ magnétique lunaire Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  5
  1000
  Paleointensities (μT)
  100
  10
  1
  0.1
  0.01
  0.001 4.5
  4
  3.5
  3
  2.5 2 Age (Ga)
  1.5
  1
  0.5
  0
  Figure 8: Paleointensities evolution with time (Ga). Open symbols come from literature data and dark symbols come from this study. Circles indicate paleointensities values, obtained by AF method; diamonds obtained by thermal method and rectangles indicates upper limits values.
  5.2. Confronting dynamo models to the paleomagnetic dataset Dynamo generation models can be confronted to this increased paleointensity dataset. So far, the origin of the lunar dynamo has been attributed to either buoyancy or inertia forces. In the former case, dynamo is due to thermal convection, or thermo-solutal convection if a crystallizing inner core is present (Evans et al., 2017). In the latter case, it is due to mechanical stirring by impact (Le Bars et al., 2011) or precession (Dwyer et al., 2011). Simulations of convection driven dynamos are studied since Glatzmaier & Roberts (1995), with validated scaling laws to extrapolate them to planetary relevant parameters (Christensen and Aubert, 2006). These laws show that such dynamo fields are too weak for the high-field epoch, but are compatible with the low field epoch (Evans et al. 2017). By contrast, simulations of mechanically driven dynamo were scarce hitherto (Tilgner, 2005; Reddy et al., 2018), preventing reliable extrapolations to the Moon. Thus, scaling laws of convection driven dynamos have been tentatively adapted, giving fields too weak for the high-field epoch (Dwyer et al., 2011; Le Bars et al., 2011). However, many precession dynamos have been recently performed in spherical shells, confirming that the scaling laws remain elusive (Cébron et al., subm., https://arxiv.org/abs/1809.05330), preventing any predictions of early lunar surface field. Interestingly, these simulations exhibit strongly intermittent dynamos, with an energy varying of a factor 10-100 on 0.1-1 magnetic time (the lunar core magnetic time is ~200 years). These simulations also suggest that large-scale planetary magnetic fields are unlikely to be Evolution temporelle du champ magnétique lunaire 145 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  produced by a precession dynamo in a spherical core. But this question remains open since planetary cores are not exactly spherical, and the relevant turbulent dissipation regime has barely been reached. The former point is key since a topographic coupling at the Core-Mantle Boundary does not vanish with vanishing viscosity. One can expect much larger dynamo fields in an ellipsoidal lunar core. To conclude, the low-field epoch is compatible with convection driven dynamos. Intermittent mechanical driven dynamo may explain the high-field epoch, provided higher field can be generated by turbulent dynamo simulations in ellipsoids. 6. Conclusion We conducted paleomagnetic analyses on twenty-seven lunar rocks totaling fifty-two samples and subsamples (fifty Apollo samples and two lunar meteorites). A stable ChRM was isolated in twenty-eight sub-samples. Paleointensity determinations (using both AF and thermal methods) provided ten paleointensity estimates. Six other samples provided only an upper limit for the paleointensity. All together this new dataset confirms the dichotomy between a high field epoch and a younger low field epoch for the Moon. However, it suggests that the high field epoch ended after 2.9 Ga, significantly younger than previously proposed, and that the average paleointensity over this period may have been significantly lower (< 70 μT) and the field more variable than previously thought. The weak field epoch, starting at least at 1.5 Ga is characterized by paleointensity of ~1 μT and may last even after 1 Ga. The transition between the end of the high field period (after 2.9 Ga) and the low field period (after 2 Ga) is currently not recorded in the paleointensity dataset. Lunar meteorites may offer the possibility to fill this gap in the knowledge of the lunar surface field. Recent dynamo simulations exhibit strongly intermittent dynamos, with an energy varying of a factor 10-100 on 0.1-1 magnetic times. This low-field epoch is compatible with convection driven dynamos. Acknowledgements This work was funded by the Agence Nationale de la Recherche (projet Maglune, ANR-14CE33-0012). The authors are grateful to Frédéric Moynier (IPGP Paris), Maud Boyet (LMV Clermont-Ferrand), Evelyn Furi (CRPG Nancy) and Ryan (Johnson Space Center, NASA, Houston) for the temporal allocation of all Apollo and lunar meteorites samples analyzed in this study. References Christensen, U. R., Aubert, J., 2006. Scaling properties of convection-driven dynamos in rotating spherical shells and application to planetary magnetic fields. Geophys. J. Int., 166, 97–114, doi:10.1111/j.1365-246X.2006.03009.x. Cournède, C., Gattacceca, J., Rochette, P., 2012. Magnetic study of large Apollo samples: possible evidence for an ancient centered dipolar field on the Moon. Earth Planet. Sci. Lett. 331–332, 31– 42. Dunlop, D. J., 2002a. Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc): 1. Theoretical curves and tests using titanomagnetite data, J. Geophys. Res., 107(B3), 2056, doi:10.1029/2001JB000486.
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  Dunlop, D. J., 2002b. Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc): 2. Application to data for rocks, sediments, and soils, J. Geophys. Res., 107(B3), 2057, doi:10.1029/2001JB000487. Dyal, P., Parkin, C.W., Sonett, C.P., 1970. Apollo 12 magnetometer: Measurements of a steady magnetic field on the surface of the moon. Science 196, 762-764. Dyal, P., Parkin, C.W., Daily, W.D., 1974. Magnetism and the interior of the Moon. Rev. Geophys. Space Phys. 12. doi: 10.1029/RG012i0 04p0 0568. Dyal, P., Daily, W.D., 1978. Crustal magnetic scale sizes measured at Apollo landing sites. Lunar and Planetary Science IX, 273-275. Dwyer, C.A., Stevenson, D.J., Nimmo, F., 2011. A long-lived lunar dynamo driven by continuous mechanical stirring. Nature 479, 212–214. Enkin, R.J., Dunlop, D.J., 1988. The demagnetization temperature necessary to remove viscous remanent magnetization. Geophys. Res. Lett. 15, 514–517. Evans, A.J., Zuber, M.T., Weiss, B.P., Tikoo, S.M., 2014. A wet, heterogeneous lunar interior: lower mantle and core dynamo evolution. J. Geophys. Res., Planets 119, 1061–1077. Evans, A.J, Tikoo, S.M., Andrews-Hanna, J.C., 2017. The case against an early lunar dynamo powered by core convection. Geophys. Res. Lett. 45, 98-107. doi: 10.1002/2017GL075441. Fernandes V. A., Burgess R., Morris A., 2009. 40Ar-39Ar age determinations of lunar basalt meteorites Asuka 881757, Yamato 793169, Miller Range 05035, LaPaz Icefield 02205, Northwest Africa 479, and basaltic breccia Elephant Moraine 96008. Meteoritics & Planetary Science 44, 805–821. Fuller, M., Cisowski, S.M., 1987. Lunar paleomagnetism. Geomagnetism 2, 307–455. Gattacceca, J., Rochette, P., 2004. Toward a robust normalized magnetic paleointensity method applied to meteorites. Earth Planet. Sci. Lett. 227, 377–393. Garrick-Bethell, I., Weiss, B.P., Shuster, D.L., Buz, J., 2009. Early lunar magnetism. Science 323, 356– 359. Glatzmaier, G.A, Roberts, P.H., 1995. A three-dimensional convective dynamo solution with rotating and finitely conducting inner core and mantle. Physics of the Earth and Planetary Interiors 91, 6375. Hood, L.L., Russell, C.T., Coleman Jr., P.J., 1981. Contour maps of lunar remanent magnetic fields. J. Geophys. Res. 86, 1055–1069. Hood, L.L., Artemieva, N.A., 2008. Antipodal effects of lunar basin-forming impacts: initial 3D simulations and comparisons with observations. Icarus 193, 485–502. doi: 10.1016/j.icarus.2007.08.023. Hood, L., et al. , 2011. Initial mapping and interpretation of lunar crustal magnetic anomalies using lunar prospector magnetometer data. J. Geophys. Res. 106, 27825–27839. Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of palaeomagnetic data. Geophys. J. Roy. Astron. Soc. 62, 699–718. Le Bars, M., Wieczorek, M.A., Karatekin, Ö., Cébron, D., Laneuville, M., 2011. An impact-driven dynamo for the early Moon. Nature 479, 215–218. Lepaulard, C., Gattacceca, J., Uehara, M., Rochette, P., Quesnel, Y., Macke, R., Kiefer, W., 2018. A survey of the lunar remanent magnetization and magnetic susceptibility of Apollo whole rocks. Submitted in Physics of the Earth and Planetary Interiors. Mercer, C.M., Young, K.E., Weirich, J.R., Hodges, K.V., Jolliff, B.L., Wartho, J-A., Soest, M.C., 2015. Refining lunar impact chronology through high spatial resolution 40Ar/39Ar dating of impact melts. Sci. Adv.1, 1:e1400050. doi: 10.1126/sciadv.1400050.
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  148 Evolution temporelle du champ magnétique lunaire Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  5
  Supplementary material Table S1 Sample (bulk)
  Mass (g)
  VRM contribution (%)
  10018 12005.0 12021.0 14169,0-A 14169,0-B 15505,0-A 15505,0-B 15529.0 61195.0 65055,4-A 65055,4-B 65055,4-C
  2.3 2.1 2.28 0.96 1.15 1.83 0.37 2.09 2.09 2.27 1.11 0.70
  24 0.11 16 5 1 -
  Table S1 : Parameters of magnetic properties. VRM contribution is estimate by the ratio of the maximum rate of ending VRM on the NRM value. Some samples, noted -, have no results or were not measured.
  Table S2 Sample
  Age (Ga)
  Error age (Ga)
  10017 10020 10049 12002 12008 12009 12017 12022 14053 15015 15016 15465 15498 15556 15597 70017 70215 71505
  3.04 3.72 3.56 3.21 3.03 3.11 3.19 3.05 3.85 1 3.31 1.19 1.32 3.33 3.30 3.72 3.75 3.85
  0.01 0.01 0.01 0.06 0.07 0.07 0.07 0.04 0.05 0.02 0.08 0.14 0.43 0.10 0.20 0.12 0.04 0.04
  Paleointe nsity (μT) 71 66 77 50 7 7 37 4 20 0.40 37 0.01 5 75 7 42 20 95
  Error max (μT) 71 66 77 50 20 0.50 0.01 2 -
  Error min (μT) 35.50 33 38.50 25 10 0.20 0.01 2 -
  42 20 95
  21 10 47.50
  Method sIRM sIRM sIRM sIRM Upper limit Upper limit Upper limit Upper limit sIRM Th-Th Upper limit ARM Th-Th Upper limit Upper limit sIRM sIRM sIRM
  References [1] [2] [1] [3] [4] [4] [5] [6] [3] [7] [5] [7] [8] [5] [6] [3] [3] [3]
  Lithology basalt basalt basalt basalt basalt basalt basalt basalt basalt regolith breccia basalt regolith breccia regolith breccia basalt basalt basalt basalt basalt
  Evolution temporelle du champ magnétique lunaire 149 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  71567 74275 76535
  3.75 3.85 4.19
  0.10 0.08 0.02
  111 28 92.5*
  111 28 92.50
  55.50 14 46.25
  sIRM sIRM sIRM
  [3] [3] [9]
  basalt basalt feldspathic rock
  Table S2: sIRM and ARM errors are calculated based on the factor 2 obtained with PARM and PIRM methods (see text). Value with asterick is an average of values in Garrick-Bethell 2009 and 2017.
  [1] Suavet et al. (2013) [2] Shea et al. (2012) [3] Cournède et al. (2012) [4] Tikoo et al. (2017b) [5]Tikoo et al. (2012)
  [6] Tikoo et al. (2014b) [7] Wang et al. (2017) [8] Tikoo et al. (2017a) [9] Garrick-Bethell et al. (2017)
  S1. Sample preparation After magnetic measurement in laboratory, each bulk sample was fixed with low temperature wax on an aluminum plot to be cut with a diamond wire saw lubricated with ethanol (Figure S1, left) down to subsample size around 3x3x3 mm or smaller depending on the NRM intensity of the original bulk sample. All subsamples were mutually oriented. Bulk sample 10018 (2.3 g)
  B A
  C
  Sample 10018-B
  Sample 10018-B1 (140 mg)
  D
  a
  1
  b
  2 3 mm
  3 4 N Down
  3 mm
  N
  N
  E
  E
  E Down
  Down
  Figure S1: Preparation of bulk sample 10018.
  After cutting, subsamples were fixed with wax on a quartz disk for AF demagnetization or placed into a quartz cylinder and for thermal demagnetization. S.2 Microscopy analyze of nine thin section We present pictures of thin sections of our nine samples (Figure S2). Basalt 15529 (Figure S2f) contains troilite grains, which contain small metallic iron. This antiferromagnetic mineral troilite (FeS) is known to alter during thermal heating (Garrick-Bethell and Weiss, 2010). It could explain why AF demagnetization provided HC components in samples 15529-A2 and 15529-B2 whereas thermal demagnetization failed to isolate a stable HT component (samples 15529-C1 and C2). Two textures are shown into the dimict breccia 61015 (Figure S2-g). A 150 Evolution temporelle du champ magnétique lunaire Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  5
  melt matrix vein is included into crystallized matrix. Regolith breccia 61195 (Figure S2-h) has broken minerals, which are not well distinguishable from the matrix.
  a) Regolith breccia 10018
  b) Basalt 12005
  c) Basalt 12021
  d) Impact melt breccia 14169
  e) Regolith breccia 15505
  f) Basalt 15529
  Evolution temporelle du champ magnétique lunaire 151 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  g) Dimict breccia 61015
  h) Regolith breccia 61195
  i) Impact melt rock 65055
  Figure S2: Detailed pictures obtained of the nine thin sections (a, i) View is in natural light and red arrows indicate yellow troilite grains, which contain metallic iron inside (white grains). Scale bar is in white on each picture.
  S.3 39Ar/40Ar dating We present complementary information for the dating of two Apollo samples: 12005 and 15529. S3a: 12005 Both splits of this sample produced disturbed release-spectra. Release-spectra for samples 22934 22936 are defined by 19-steps and 13-steps, respectively. The release pattern is indicative of Ar-loss. Neither sample split meets the criteria commonly used to describe/define a plateau. The high temperature steps, however suggest that age of sample 12005 is older that ~2.8 Ga. When cast on an isotope correlation diagram the low temperature steps indicate absorption of terrestrial atmospheric Ar. The higher temperature steps indicate that the 36Ar is dominated by a cosmogenic source. This sample may not be readily datable.
  152 Evolution temporelle du champ magnétique lunaire Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  5
  Figure S3a: Isochron for Sample 12005.
  S3b: 15529 This sample produced a disturbed release-spectra, defined by 45-steps. A 29-step plateau age of 3175 ±4 Ma, however is defined by the mid-to-high temperature steps and is made up of ~62% of the total 39ArK released. The integrated/total fusion age is 3091 ±10 Ma. These ages are calculated assuming a nebular 40Ar/36Ar value of 1x10-4 for the trapped component (Table S3a). Table S3a: Summary the plateau and integrated age data. Sample
  ID
  15529 22938 15529 22933
  J
  ±1s
  2.13x10-2 5.0x10-5 2.13x10-2 5.0x10-5
  Integ. Age (Ma)
  Erro r
  3091 3203
  10 16
  Table S3b : Summary of isochron results Sample Run ID Age (Ma) ±1s 15529 22938 3163.06 6.8 15529 22933 3215.5 34
  40
  Plat. Age (Ma)
  3175 3222
  ±1s
  5 8
  MSW D
  Steps
  1.1 K-AM 0.9 N-Z
  nsteps
  29 13
  n-tot
  % Gas
  45 27
  61.7 51.7
  Ar/36Artr ±1s MSWD n 0.4 0.04 1.247 36 0.4 0.1 0.177 13
  When cast on an isotope correlation diagram (Figure S3a), the first 9-steps are notable in that they do not lay on the linear array defined by the remaining 36-steps. The remaining steps form a well-defined array that corresponds to age of 3163±6 Ma and indicates a 40Ar/36Ar ratio of 0.33±0.04 for the "trapped" Ar component (Table S3b). The 9-low temperature steps lie along an array that indicates an isochron age of 1656±60 Ma and an initial 40Ar/36Ar ratio of 64±3. The cause of the displacement of the low-temperature array from the high-temperature array is not clear at this time. However, when the isotope data are cast on a 37ArCa/36Ar by 38Ar/36Ar isotope correlation diagram (Figure S3b) the 38Ar/36Ar ratios cluster about a value of 1.6, making it clear that the 38Ar and 36Ar in the trapped component is cosmogenic in origin, not terrestrial. Evolution temporelle du champ magnétique lunaire 153 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  Figure S3b: Isochron for Sample 15529.
  S.4 Demagnetization of the NRM We present here the demagnetization data for all samples analyzed in laboratory.
  154 Evolution temporelle du champ magnétique lunaire Chapitre 5 : Nouvelle évolution du champ magnétique lunaire A2) 10018-B1a (68 mg)
  A1) 10018-A2b (58 mg) -8
  W Up x10 Am
  N
  W S
  HT
  HT
  NRM
  E x10-9 Am2
  560 340
  N x10-8 Am2
  630
  A3) 10018-B1b (68 mg)
  N Up x10-9 Am2
  2
  5
  W
  Up x10-9 Am2 E x 10-9 Am2
  630
  460
  HT
  225
  200
  NRM
  200°C 100 °C
  LT
  LT
  LT
  NRM
  NRM
  S Down
  100 °C
  S Down
  E Down
  A5) 12021-A1 (287 mg)
  A4) 12005-C1 (198 mg)
  A6) 14169-B1a (129 mg)
  W Up x10-10 Am2
  N Up x10-9 Am2
  N Up x10-9 Am2 NRM
  NRM NRM
  LT
  500
  N x10-10 Am2
  S
  LT
  200°C
  W 450
  250°C
  E x10-9 Am2
  HT
  200°C LT
  E x10-9 Am2
  W NRM
  NRM
  580
  S Down
  W Up x 10 Am
  S Down
  A9) 15529-C2 (381 mg)
  A8) 61195-A1a (129 mg)
  A7) 15505-A3b (213 mg) -8
  NRM
  E Down
  N
  2
  N Up x10-8 Am2
  Up x 10-9 Am2 NRM
  NRM
  NRM LT
  520 580
  W
  100°C
  450
  LT
  HT
  250
  S
  W
  200°C
  E x10-8 Am2
  NRM
  NRM
  NRM
  580
  -8
  N x 10 Am
  360
  580
  200°C
  LT
  HT
  E x 10-9 Am2
  2
  S Down
  E Down
  S Down
  A11) 15529-C1 (438 mg)
  A12) 65055-A1 (387 mg) N Up x10-9 Am2
  N Up x10-9 Am2
  A10) 61015-B2 (279 mg)
  280°C
  W
  NRM
  Up x10-9 Am2 NRM
  LT
  LT
  250
  S
  500
  540
  150°C
  150°C
  250°C 480
  LT
  W
  N x10-9 Am2
  E x10-9 Am2
  NRM NRM
  NRM
  E Down
  580
  W
  NRM
  E x10 Am -9
  S Down
  2
  S Down
  Evolution temporelle du champ magnétique lunaire 155 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  B1) 10003 (74 mg)
  B2) 10017 (550 mg) N
  N Up x10-10 Am2 LC
  NRM
  B3) 10018-A2a (145 mg)
  Up x 10 Am -9
  N
  W
  8 mT
  NRM
  E x10 Am -8
  2
  B5) 10018-C1 (132 mg)
  Up x 10-8 Am2
  W
  N Up x10-8 Am2
  E x 10-8 Am2
  E x10-8 Am2
  W 80
  220
  140 NRM
  HC
  LC
  NRM
  HC
  22
  40
  NRM
  HC
  20
  10
  HC
  2 mT
  B4) 10018-B4 (80 mg)
  N Up x10-8 Am2
  2
  NRM
  7 5 mT
  17
  E x10-10 Am2
  W
  60
  5 mT
  100
  W
  E x 10 Am -9
  2
  5 mT
  LC
  LC
  NRM
  S Down
  LC
  NRM
  NRM
  S Down
  B6) 12005-B2 (169 mg)
  N Up x10 Am -9
  100
  W
  B8) 12005-B4 (145 mg)
  B7) 12005-B3 (134 mg)
  2
  4
  S Down
  B9) 12012 (290 mg)
  N Up x10-9 Am2
  W Up x10-9 Am2
  E x10-9 Am2
  NRM
  S Down
  S Down
  B10) 12016 (340 mg)
  Up x 10-9 Am2
  N NRM
  NRM
  W Up x10-9 Am2
  N x10-9 Am2
  S
  2 mT
  NRM
  2 mT
  5 mT
  E x10-9 Am2
  W
  LC
  N x10-9 Am2
  S
  LC
  6 mT
  14
  NRM 7 mT
  HC
  LC NRM
  LC
  W
  E x 10-9 Am2 NRM
  E Down
  LC NRM
  NRM
  S Down
  -8
  W
  E x10 Am
  S Down
  N Up x 10-9 Am2
  N Up x10-9 Am2
  N Up x10-9 Am2
  2
  B15) 14169-B2a (73 mg)
  B14) 14078 (26 mg)
  3 mT
  LC
  LC 2
  E x10-9 Am2 NRM
  NRM
  W
  E x10 Am
  E x10-10 Am2
  W 7 mT
  LC
  N Up x10 Am
  N Up x10-9 Am2 4
  LC
  LC
  NRM
  LC
  W
  44
  E x10-9 Am2
  W E x10-8 Am2
  HC
  HC
  E x10-9 Am2
  5 mT
  45
  NRM
  N
  N
  NRM
  Down
  100
  B25) 61015-B6 (55 mg)
  N
  Up x10 Am -9
  E x10-9 Am2
  W 2
  NRM
  16 mT
  160
  W
  E x10
  -10
  Am
  B26) 60215 (61 mg)
  2
  4 mT LC
  NRM
  6
  3 mT
  S Down
  S Down
  N Up x10 Am -9
  E x10-9 Am2 Up
  LC
  N x10 Am
  S
  Down
  E Down
  B30) 65055-B2 (279 mg)
  B29) 62255 (295 mg)
  B28) 61195-B3 (225 mg) -9
  NRM
  NRM
  S
  2
  7 mT
  100
  LC
  4 mT
  W Down
  NRM
  B27) 61195-B1 (253 mg)
  N x10-10 Am2
  S
  E x10-9 Am2
  W
  LC LC
  S Down
  W Up x10-10 Am2
  NRM
  NRM
  HC
  LC
  NRM
  NRM
  NRM
  E x10-9 Am 2
  W
  Up x10-9 Am2
  N S
  N x10-9 Am2
  NRM
  Up x10-10 Am2
  100
  S Down
  NRM
  B24) 61015-B3 (87 mg)
  Up x10-9 Am2
  17
  N
  E Down x 10-8 Am2
  B23) 60015 (820 mg)
  LC
  B21) 15529-B2 (352 mg)
  S
  S Down
  B22) 15555 (59 mg)
  HC
  19
  100
  W
  E x 10-9 Am2
  100 44
  NRM
  HC
  LC
  120
  S Down
  8
  NRM
  W
  4 mT
  9 mT
  90
  16
  NRM
  3 mT
  N Up x 10-9 Am2
  NRM
  NRM NRM
  B20) 15529-A2 (434 mg)
  B19) 15505-A3c2 (187 mg)
  W Up x 10-8 Am2
  2
  8 mT
  NRM
  S Down
  B18) 15505-A2a (96 mg)
  B17) 15505-A1b (80 mg)
  Down
  S
  S Down
  NRM -8
  NRM
  NRM
  S Down
  B16) 14169-B2b (118 mg)
  3
  13
  NRM
  NRM
  E x 10-9 Am2
  W
  2
  5 mT
  Down
  LC
  6 -9
  170 mT
  N Up x10-9 Am2
  Up x10-10 Am2
  N
  NRM
  W
  NRM
  NRM
  5
  S
  Down
  B13) 12021-C (220 mg)
  B12) 12021-B1 (533 mg)
  N Up x10-8 Am2
  LC
  NRM
  NRM
  Down
  E
  B11) 12018 (996 mg)
  S
  B31) 65055-C (699 mg) N Up x10-9 Am2
  N Up x10-9 Am2
  2
  W Up x10-10 Am2
  NRM
  NRM NRM
  NRM
  100
  LC
  W
  4
  4 mT
  E x10-9 Am2
  9
  E
  W Down
  S
  E x10-9 Am2
  W
  NRM LC
  E x10-9 Am2
  W 5
  N x10-10 Am2
  Up x10-9 Am2
  LC
  4 mT LC
  2 mT
  100 NRM
  S Down
  NRM
  NRM
  S
  NRM
  E
  Down
  156 Evolution temporelle du champ magnétique lunaire
  100 mT
  S Down
  NRM
  S
  Down Chapitre 5 : Nouvelle évolution du champ magnétique lunaire B32) 65315 (161 mg) W
  B33) 70017 (578 mg)
  Up x10-10 Am2
  B34) 70135 (107 mg)
  NRM
  LC
  24
  9 mT
  N x10-10 Am2
  NRM
  W
  100 mT
  W
  E x10-9 Am2
  Down
  Up
  NRM
  W
  E
  Down
  Up x10-9 Am2
  LC
  N
  Up x10
  Am
  E Down
  S
  E Down
  B37) 75075 (217 mg)
  B39) NWA10782 (42 g)
  B38) LAP02205 (219 mg)
  W Up x10-9 Am2
  2
  N x10-9 Am2
  NRM
  S
  -10
  5 mT
  NRM
  S
  NRM
  B36) 74275 (358 mg)
  LC
  2
  LC
  NRM
  N W
  100
  7 mT
  N Up x10-6 Am2
  Up x 10-9 Am2 E x 10-9 Am2 NRM
  NRM
  S W
  E x10
  Am
  6 mT
  HC
  LC
  2
  LC
  70
  N x10-9 Am2
  LC
  -10
  Up x10-9 Am2
  NRM
  11 mT
  S
  B35) 74255 (221 mg)
  N x10-9 Am2
  N x10-9 Am2
  5
  NRM NRM
  12 mT
  NRM
  S Down
  20
  HC
  LC NRM
  E Down
  NRM
  W
  2
  S
  Down
  100
  38
  E x10-6 Am2
  NRM
  S Down
  Figure S4: NRM demagnetization data. Orthogonal projection plots of stepwise alternating field demagnetization data. Dark symbols represent the projection into the horizontal plane (N-E or W-N) and open symbols into the vertical plane (Up-E or Up-N). (A1-A12) NRM vector of twelve samples during thermal demagnetization. Low thermal (LT) and high thermal (HT, when existing) magnetization components are labeled and represented with light gray and dark gray arrows respectively. (B1-B39) NRM vector of thirty-nine samples during AF demagnetization. Low coercivity (LC) and high coercivity (HC, when existing) magnetization components are labeled and represented with light gray and dark gray arrows respectively.
  S.5 Paleointensity fidelity parameters Figure S5: Retrieved paleointensities and paleointensity fidelity limit tests, performed in CEREGE (blue curves) and in Rutgers (purple curves). Recovered paleointensities (left) versus applied laboratory field (μT) for a) two basalt 12005; b) two basalt 12021; c) two dimict breccia 61015 and d) two impact melt rock 65055. R2 value is noted. Difference (middle) and error (right) for retrieved paleointensities versus applied laboratory field (μT) for the same sample. The dashed horizontal line represents the 100% difference (error) between the applied field and the retrieved paleointensity (see text).
  Evolution temporelle du champ magnétique lunaire 157 Retrived paleointensity (μT)
  Retrived paleointensity (μT)
  0
  10
  R2 = 0.489
  158 Evolution temporelle du champ magnétique lunaire
  0
  50
  100
  150
  200
  0
  10
  b) Basalt 12021
  0
  20
  40
  60
  80
  100
  30
  12005,B3
  40
  20 30 Applied laboratory field (μT)
  12021,B1
  Applied laboratory field (μT)
  20
  12005,B4
  40
  50
  Difference (%)
  50
  R2 = 0.094
  R2 = 0.018
  12021,C
  R2 = 0.964
  0%
  50%
  100%
  150%
  200%
  250%
  300%
  350%
  400%
  Difference (%)
  a) Basalt 12005
  0%
  200%
  400%
  600%
  800%
  1000%
  1200%
  1400%
  0
  0
  10
  10
  12005,B3
  12021,C
  20 30 Applied laboratory field (μT)
  12021,B1
  20 30 Applied laboratory field (μT)
  12005,B4
  40
  40
  50
  Error (%) 50
  0
  0%
  200%
  400%
  600%
  800%
  1000%
  1200%
  0%
  50%
  100%
  150%
  200%
  250%
  300%
  350%
  400%
  450%
  Error (%)
  0
  10
  10
  12005,B3
  12021,C
  20 30 Applied laboratory field (μT)
  12021,B1
  20 30 Applied laboratory field (μT)
  12005,B4
  40
  40
  50
  50
  5 Chapitre 5 : Nouvelle évolution du champ magnétique lunaire Retrived paleointensity (μT)
  Retrived paleointensity (μT)
  0
  10
  0
  10
  20
  30
  40
  50
  0
  10
  61015,B3
  20 30 Applied laboratory field (μT)
  65055,C
  65055,B2
  20 30 Applied laboratory field (μT)
  d) Impact melt rock 65055
  0
  20
  40
  60
  61015,B6
  40
  40
  50
  R2 = 0.142
  R2 = 0.186
  50
  R2 = 0.045
  R2 = 0.989
  Difference (%)
  Difference (%)
  c) Dimict breccia 61015
  0
  0%
  50%
  100%
  150%
  200%
  250%
  300%
  350%
  400%
  450%
  500%
  0%
  50%
  100%
  150%
  200%
  250%
  0
  10
  10
  65055,B2
  20 30 Applied laboratory field (μT)
  65055,C
  20 30 Applied laboratory field (μT)
  61015,B6
  40
  40
  61015,B3
  50
  50
  Error (%) Error (%)
  80
  0%
  20%
  40%
  60%
  80%
  100%
  120%
  140%
  160%
  0%
  50%
  100%
  150%
  200%
  250%
  0
  0
  10
  10
  65055,B2
  20 30 Applied laboratory field (μT)
  65055,C
  20 30 Applied laboratory field (μT)
  61015,B6
  40
  40
  61015,B3
  50
  50
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire 5
  Evolution temporelle du champ magnétique lunaire 159 5
  Chapitre 5 : Nouvelle évolution du champ magnétique lunaire
  160 Evolution temporelle du champ magnétique lunaire Conclusion générale
  Conclusion générale Ce travail de thèse a consisté en l'analyse des propriétés magnétiques d'échantillons lunaires (collection Apollo et météorites) afin de mieux comprendre l'évolution du champ magnétique lunaire au cours du temps. En particulier, le manque de données après 3 Ga ne permettait pas de bien contraindre l'intensité du champ pour la période 3 Ga-1 Ga, ni l'âge de l'arrêt de la dynamo lunaire. A travers l'étude de l'aimantation rémanente naturelle et de la susceptibilité magnétique de 161 roches Apollo, nous avons pu enrichir les bases de données sur les propriétés magnétiques des roches lunaires. Nos valeurs d'aimantations, mesurées sur des échantillons de masse moyenne de 500g se révèlent plus représentatives que les valeurs classiquement obtenues sur de petits échantillons (100 mg) qui surestiment les valeurs d'aimantations d'un facteur 3. Ces nouvelles données d'aimantation sont donc plus appropriées pour alimenter les interprétations des anomalies magnétiques lunaires. Les données de la susceptibilité magnétique ont montré une dépendance à la lithologie, les roches impactées étant plus magnétiques que les roches feldspathiques. Cela confirme que les processus d'impact lunaire sont le principal facteur contrôlant la quantité de minéraux ferromagnétiques dans les roches lunaires, avec signal magnétique plus fort dû à l'incorporation de métal météoritique. Enfin, le rapport de la NRM sur la susceptibilité a permis de tester un critère de détermination grossier de la paléointensité afin de sélectionner des échantillons pour des analyses en laboratoire. Un nouveau jeu d'échantillons lunaires a été demandé afin de compléter cette étude, sur des échantillons (10016, 12010, 14305, 66035 et 15505) qui présentaient de fortes valeurs de NRM/X et donc, potentiellement, de fortes paléointensités, à des âges variés. La seconde étude a permis de déterminer des valeurs de paléointensités à partir de 25 roches Apollo et 2 météorites lunaires. Un système de chauffes sous fugacité d'oxygène contrôlée a été développé au laboratoire de magnétisme des roches du CEREGE afin de limiter l'altération des échantillons lunaires lors de l'utilisation de protocoles de détermination de paléointensité de type Thellier-Thellier. Nous avons vu également dans ce travail de thèse que la connaissance de l'âge d'aimantation de la roche est très importante pour contraindre l'évolution du champ magnétique lunaire. C'est pourquoi des datations récentes et plus précises sont nécessaires, comme celles réalisées sur 5 échantillons lunaires lors de ce travail. Un total de 10 nouvelles données de paléointensités et de 6 valeurs de limites supérieures a été obtenu. Parmi elles, deux paléointensités ont été obtenues sur deux météorites lunaires, montrant qu'il est possible d'obtenir de nouveaux jeux de données et ouvre la voix vers l'analyse de nouveaux types d'échantillons. Ces nouvelles données, associées à celles de la littérature récente, confirment la dichotomie entre une époque de champ magnétique lunaire fort et une époque plus jeune de champ magnétique plus faible. Ces données suggèrent que l'époque de champ fort a duré plus longtemps que précédemment proposé, jusqu'à 2,9 Ga environ (au lieu de 3,56 Ga) et avec un champ moyen plus faible que précédemment proposé (50 μT contre 70 μT). La variabilité des intensités dans cette époque est également forte, les valeurs allant de <7 μT à 100 μT. Des simulations récentes de dynamo proposent une dynamo intermittente dont l'énergie produite varie d'un facteur 10 à 100, pouvant expliquer les variations d'intensités obtenues dans l'époque de champ fort par exemple.
  Evolution temporelle du champ magnétique lunaire 161 Conclusion générale L'époque de champ faible débute entre 2,9 Ga et 1,5 Ga et persiste peut-être jusqu'à des périodes récentes (datations en cours) avec une paléointensité moyenne de 1,5 μT. Ce champ faible mais long peut être expliqué par une dynamo de précession et par une dynamo convective de type thermochimique. Concernant les modèles de dynamo, des améliorations sont attendues sur les paramètres de bases utilisés lors des modélisations. Par exemple, la simulation d'une dynamo mécanique à partir d'une géométrie non sphérique permettrait de tester les intensités de champ magnétiques produits en surface (génération d'un champ plus fort ?). De plus, les incertitudes liées à l'intérieur de la Lune et à son évolution thermique sont un frein pour les obtenir des prédictions précises de la fin de la dynamo lunaire. De nombreuses études sont encore à prévoir afin d'avoir suffisamment de données fiables et des modèles de dynamo complets afin de comprendre l'évolution de l'ancien champ magnétique lunaire.
  162
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  174 Evolution temporelle du champ magnétique lunaire ANNEXES
  ANNEXES Annexe 1 : Carte géologique de la Lune Annexe 2 : Article Lepaulard et al., 2019 Annexe 3 : Planches photographiques des échantillons analysés dans cette étude. Annexe 4 : Cartes réalisées au MEB de trois échantillons Annexe 5 : Diagrammes de désaimantation de la NRM Annexe 6 : Article Uehara et al., 2017
  Evolution temporelle du champ magnétique lunaire 175 ANNEXES
  Annexe 1 Carte géologique de la Lune Je présente ici la carte géologique de la Lune, face visible (Figure A1-haut) et face cachée (Figure A1-bas) afin de se rendre compte de la répartition des différentes lithologies lunaires ainsi que leur âge.
  176 Evolution temporelle du champ magnétique lunaire ANNEXES
  Figure A1 : Carte géologique de la Lune. Haut) Carte de la face visible et légende associée. Bas) Carte de la face cachée. D'après Heiken et al., 1991.
  Evolution temporelle du champ magnétique lunaire 177 ANNEXES
  Annexe 2 Article accepté le 18 Novembre 2018 - MAPS
  A Paleozoic age for the Tunnunik impact structure Camille Lepaulard1, Jérôme Gattacceca1, Nicholas Swanson-Hysell2, Yoann Quesnel1, François Demory1 , Gordon Osinski3,4.
  1
  Aix Marseille Univ, CNRS, IRD, Coll France, INRA, CEREGE, Aix-en-Provence, France Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA 3 Dept. of Earth Sciences, Centre for Planetary Science and Exploration, University of Western Ontario, 1151 Richmond St., London, ON, N6A 5B7, Canada 4 Dept. Physics & Astronomy, University of Western Ontario, 1151 Richmond St., London, ON, N6A 5B7, Canada 2
  Corresponding author: Camille Lepaulard (lepaulard@cerege.fr) Abstract We report paleomagnetic directions from the target rocks of the Tunnunik impact structure, as well as from lithic impact breccia dikes that formed during the impact event. The target sedimentary rocks have been remagnetized after impact-related tilting during a reverse polarity interval. Their magnetization is unblocked up to 350°C. The diabase dikes intruding these sediments retained their original magnetization which unblocks above 400°C. The impact breccia records a paleomagnetic direction similar to that of the overprints in the target sedimentary rocks. The comparison of the resulting virtual geomagnetic pole for the Tunnunik impact structure with the apparent polar wander path for Laurentia combined with biostratigraphic constraints from the target sedimentary rocks is most consistent with an impact age in the Late Ordovician or Silurian, around 430 to 450 Ma, soon after the deposition of the youngest impacted sedimentary rocks. Our results from the overprinted sedimentary rocks and diabase dikes imply that the post-impact temperature of the studied rocks was about 350°C. 1 Introduction Hypervelocity impacts are a major process for the evolution of planetary surfaces, including the Earth. Terrestrial impact events have significantly influenced the biosphere in numerous ways including, on one hand, the development of habitats for primitive life forms (Reimold and Koeberl, 2008; Cockell et al., 2005) and, on the other hand, some extinctions such as the Cretaceous-Paleogene mass extinction (Schulte et al., 2010). Depending on the 178 Evolution temporelle du champ magnétique lunaire ANNEXES
  size of the impactor, impact events significantly modify regional geology and may lead to the formation of ore deposits (Ames et al., 1998). However, on Earth, plate tectonics and erosion can obscure or completely erase the geologic record of impact structures. Currently, around 190 impact structures are identified on Earth, including thirty in Canada, but the age of approximately half of these impacts is unknown (Jourdan et al., 2012). Dating of impact structures is crucial for assessing correlations (or the lack thereof) between impact events and biological or geological changes, and to constrain the flux of crater-forming bodies to the Earth through time. While most of the large terrestrial impact structures (> 6 km) on Earth have likely already been documented (Hergarten and Kenkmann, 2015), discovery of additional impact structures is still possible. In 2010, the Tunnunik structure, a ~28 km diameter impact structure was identified in the Canadian Arctic (Dewing et al., 2013). The age of this structure is presently not constrained more precisely than between 0 and ca. 450 Ma. The purpose of this study is to provide a refined estimate for the age of this impact structure. In the absence of suitable material for radiometric dating, we utilize paleomagnetic dating of target rocks that were remagnetized by the impact (Deutsch and Schärer, 1994; Pilkington and Grieve, 1992) and of the lithic breccia that formed during the impact event (Fairchild et al., 2016). 2 Geological setting, sampling, methods Geological setting The Tunnunik impact structure is located on Victoria Island in the Canadian High Arctic. The impact structure is deeply eroded and has been identified by the presence of shatter cones, sedimentary rocks steeply dipping quaquaversally (compared to the generally relatively flat regional bedding), and concentric faults (Dewing et al., 2013). The target rock sequence, exposed outside of the impact structure, consists of sub-horizontal sedimentary rocks, mostly dolostones and mudstones, ranging from the Neoproterozoic (Wynniatt formation of the Shaler supergroup) to the Late Ordovician - Early Silurian (?) (Thumb Mountain and Allen Bay formations). Following Newman and Osinski (2016), we group these sedimentary rocks into the Shaler Supergroup (Neoproterozoic), Mount Phayre, Victoria Island, Thumb Mountain and Allen Bay formations in ascending stratigraphic order. Neoproterozoic diabase dikes associated with the ca. 720 Ma Franklin Large Igneous Province which intrude the Neoproterozoic sedimentary rocks are also present within the target rock (Dewing et al., 2013; Heaman et al. 1992). Lithic impact breccias, composed of sedimentary clasts up to centimeter size set in a silt-sized matrix, are encountered in the form of dikes and sills up to a few decimeters thick intruding the sedimentary target rocks, especially the Mount Phayre Formation and Shaler Supergroup rocks.
  Evolution temporelle du champ magnétique lunaire 179 ANNEXES
  Sampling Three main rock types were sampled for paleomagnetic analysis: impact breccia, Neoproterozoic to Upper Ordovician-lower Silurian sedimentary rocks, and Neoproterozoic diabase dikes. For impact breccia sampling, we focused on facies dominated by the finegrained matrix (clast content below 10 vol%) and with clast size below 1 mm. A total of 29 paleomagnetic sites were sampled, mostly located inside the impact structure (Figure 1, Table S1). Sampling was performed by drilling of 2.5 cm-diameter cores using a gas-powered drill, or more rarely through collection of oriented blocks. Samples were oriented using magnetic and sun compasses.
  Figure 1. Location of sampled sites. Color codes indicate the sampled formation. Numbered sites are the ones that provided interpretable paleomagnetic results. The red circle is the proposed limit of the impact structure from Newman and Osinski (2016). The purple contour delineates the area where shatter cones have been found. The background image is an Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model (GDEM) Version 2 product. The elevation ranges from 0 (white) to 283 m (black). Except for the regional map on the upper left, coordinates are expressed in meters in the Universal Transverse Mercator (UTM) Zone 12 projection with the World Geodetic System (WGS) 84 datum. For the zoomed-in area (upper right), the background image is to a mosaic of orthophotographic images (Digital Globe Quickbird data).
  180 Evolution temporelle du champ magnétique lunaire ANNEXES
  Methods The magnetic mineralogy was studied by measurement of susceptibility versus temperature (Figure 2A), stepwise thermal demagnetization of saturation isothermal remanent magnetization (sIRM) that is more sensitive to the presence of pyrrhotite (Figure 2B), and measurements of S-300 ratio that is the IRM obtained after applying a 3 T field and then a back field of − 0.3 T normalized to the IRM acquired in 3 T. The low field magnetic susceptibility (noted χ in m3.kg-1) was measured for all samples using an AGICO apparatus, either with an MFK1 or a KLY-2 kappabridge instrument (with sensitivity of 5x10-13 m3), depending on sample size. The MFK1 kappabridge operates at 200 Am-1 peak field and at a frequency of 976 Hz. The KLY-2 kappabridge operates at 425 Am-1 peak field and at a frequency of 920 Hz. Thermomagnetic curves were obtained with the use of the MFK1 instrument coupled with a CS3 furnace. Estimates of Curie temperatures were computed as the inflection points of the susceptibility versus temperature curves. Thermal demagnetization of sIRM (imparted with 3 T pulse with a MMPM9 pulse magnetizer) was conducted using an MMTD furnace. Hysteresis properties of the diabase dikes were measured with a MicroMag 3900 vibrating sample magnetometer. All rock magnetism measurements were performed at CEREGE. All remanence measurements were performed with a SQUID magnetometer (2G Enterprises, model 760R, with noise level of 10-11 Am2) in a magnetically shielded room, with an attached automated 3-axis alternating field degausser system (with a maximum peak field of 170 mT). For the majority of samples, the natural remanence was studied (around 100 specimens) by stepwise thermal demagnetization using a MMTD80 furnace. For each studied site, two pilot samples were demagnetized, one using thermal demagnetization up to 600°C, one using alternating field (AF) demagnetization up to 120 mT. The most efficient demagnetization method was then used for the remaining samples. Demagnetization data were analyzed through principal component analysis and summarized with Fisher statistics using PaleoMac software (Cogné, 2003). Paleomagnetic results are summarized in Table 1. Figure 2 (next page). Intrinsic magnetic properties of studied rocks. A) Susceptibility versus temperature for a diabase dyke sample (site TUN32, heating and subsequent cooling curves are shown), and an impact breccia sample (site TUN20). B) Thermal demagnetization of sIRM as a function of temperature. The red box indicates the unblocking temperature range of the impact-related magnetization of the target sedimentary rocks. The blue box indicates the unblocking temperature range of the pre-impact magnetization of the diabase dikes. The postimpact temperature excursion is constrained between these two boxes. The Curie temperature of pyrrhotite (325°C) is indicated by the thick vertical line.
  Evolution temporelle du champ magnétique lunaire 181 Magnetic susceptibility (x10-9 m3)
  ANNEXES
  A 6 diabase dike 4
  2
  impact breccia 0 1.0 pyrrhotite Tc
  Saturation remanence (normalized)
  B 0.8
  0.6
  impact breccia
  0.4
  Shaler Supergroup
  0.2
  Victoria Island diabase dike
  0
  100
  200
  300
  400
  500
  600
  Temperature (°C) Figure Table 1 (next page). Site-mean paleomagnetic directions from2 the Tunnunik impact structure. Site TUN32# is for Shaler Supergroup sediments baked by the diabase dike at site TUN32. These samples have four components of magnetization (A, B, C, D, see text). SLat and SLong, site latitude and longitude; N/n, number of samples used in the computation of the mean direction / number of samples stepwise demagnetized; ChRM unblocking temperature, the temperature interval of the high temperature component used for calculation; Dg and Ds, declination before and after tilt correction; Ig and Is, inclination before and after tilt correction; kg and ks (noted Kg and Ks for the average of VGPs) precision parameter before and after tilt correction; α95g and α95s (noted A95g and A95s for the average of VGPs) are semi-angle of aperture of the cone where the true mean direction lies with 95% confidence before and after tilt correction; Strike, strike of the site; Dip, dip of the site; *, Strike and Dip range from 162/16 to 190/38 (site TUN20), from 213/60 to 193/70 (site TUN33); For impact breccia and diabase dikes sites, strike and dip are for the intruded rocks; dp is the semi-axis of the confidence ellipse about the VGP along the great circle path from site to VGP; dm is the semi-axis of the confidence ellipse perpendicular to that great-circle path. NRM, average natural remanent magnetization; χ, average magnetic susceptibility. *The average impact is computed from sites TUN07, 20, 21, 29, 31, 33, 36 (see text).
  182 Evolution temporelle du champ magnétique lunaire Average diabase dikes
  Diabase dikes
  baked Shaler Supergroup
  Average impact direction*
  Shaler Supergroup
  Victoria Island
  Impact Breccia
  Formation
  TUN32#-A TUN32#-B TUN32#-C TUN32#-D TUN30 TUN32 TUN35
  TUN20 TUN21 TUN07 TUN29 TUN31 TUN33 TUN36
  Site
  72.4827 72.4827 72.4827 72.4827 72.4729 72.4827 72.4839
  72.4973 72.4965 72.4363 72.4732 72.4831 72.4832 72.4662
  -113.9410 -113.9410 -113.9410 -113.9410 -113.9320 -113.9410 -113.9379
  -114.0209 -114.0198 -113.8668 -113.9503 -113.9321 -113.9392 -113.9665
  Slat (°N) Slong (°E)
  15/15 5/5 6/9 7/7 5/5 9/9 6/6 7/7 4/4 2/2 4/4 4/4 6/6 5/6 8/8 3/3
  N/n
  200-345 345-370 370-450 450-520 465-590 500-590 500-590
  260-590 400-530 250-310 270-350 250-350 250-330 230-330
  ChRM unblocking temperatures (°C) 66.6 61.2 83.1 85.4 72.4 82.5 87.6 77 58.6 97.3 104.2 93.8 98.4 110 102.3 103.5
  Dg (°) -15.2 -7.8 11.2 -5.0 -9.3 -0.5 -10.9 -5.5 -3.1 60.2 -62 -61.5 -32.7 -35.9 -29.6 -32.8
  Ig (°) 112.5 51.8 6.6 52.4 13.7 50.1 8.7 37.2 67.3 190 654 33.3 10.8 48 18.5 194
  kg 3.6 10.7 28.1 8.4 21.5 7.3 24.0 10 11.3 18.2 3.6 16.2 21.3 11.2 13.2 8.9
  α95g (°) 66.9 60.8 84.3 85.9 70.6 53.8 81.4 72.7 42.1 297.1 111 106.1 98.1 110.2 101.5 103.1
  Ds (°) 9.5 15.8 12.3 22.3 20.4 46.6 -8.0 17.2 28.3 -9.8 7.4 6.4 23.2 33.7 32.8 29.6
  Is (°) 67.9 51.8 6.6 49.9 13.7 11.1 8.7 16.8 67.3 190 654 33.3 10.8 48 18.5 108.8
  ks 4.7 10.7 28.1 8.6 21.5 16.2 24.0 15.2 11.3 18.2 3.6 16.2 21.3 11.2 13.2 11.9
  α95s (°)
  207 207 207 207 184 207 213
  * 162 95 170 194 * 89
  Strike
  70 70 70 70 56 70 66
  * 24 6 28 35 * 36
  Dip 0.6 4.8 34.1 341.2 354.2 343.3 340 349.3 358.9 145 339 348.3 333.6 323.1 329.3 328.7
  -0.6 4.6 7.5 -1 0.7 2 -4.5 1.2 55.6 47 -44.9 -41.4 -19.5 -24.9 -18.8 -21.1
  Longg (°) Latg (°)
  dm (°)
  1.9 3.7 5.4 10.8 14.4 28.5 4.2 8.4 10.9 21.7 3.7 7.3 12.3 24.3 Kg=53.3 A95g=8.3 10.7 21.5 20.9 27.6 4.3 5.6 19.2 24.7 13.6 24.1 7.5 13 8.1 14.6 Kg=187 A95g=9
  dp (°)
  356.9 1.9 339.7 336.5 351.6 2.7 345.5 350.3 14.1 128.7 315 319.8 324.8 311.5 319.8 318.7
  Longs (°)
  11.4 16.2 7.7 12.3 15.9 37.1 -1.3 14.3 31.3 3.1 -2.6 -1.7 9.1 11.7 12.9 11.3
  Lats (°)
  dm (°)
  NRM (Am2kg-1) 2.4 4.7 9.37E-07 5.7 11 5.11E-07 14.5 28.6 2.92E-08 4.8 9.1 2.28E-07 11.8 22.5 9.54E-08 13.4 20.8 1.32E-07 12.2 24.2 5.43E-08 Ks=27,9 A95s=11.6 14.4 24.9 3.99E-06 9.3 18.4 5.19E-06 1.8 3.6 3.99E-06 8.2 16.2 3.99E-06 12 22.7 2.75E-04 7.3 12.7 1.87E-04 8.4 14.9 2.72E-04 Ks=140 A95s=10,5
  dp (°)
  not measured not measured not measured not measured 1.14E-05 9.30E-06 1.62E-05
  6.61E-08 7.39E-08 -8.21E-10 1.08E-08 1.72E-08 9.35E-09 2.10E-08
  χ (m3kg-1)
  ANNEXES
  Evolution temporelle du champ magnétique lunaire 183 ANNEXES
  3 Results In the impact breccia, thermomagnetic experiments reveal a Curie temperature of 585°C, indicating the presence of magnetite (Figure 2A). Pyrrhotite is also present as indicated by significant unblocking of sIRM around 280-320°C (Figure 2B), although its signal is hidden by the magnetite contribution in the susceptibility curve (Figure 2A). The presence of pyrrhotite is confirmed by the S-300 ratio of -0.84 that indicates the significant contribution of a high coercivity mineral. The impact breccia samples display two magnetization components that are equally evidenced by thermal (Figure 3A) and AF demagnetization (Figure 3B). The low-temperature/low-coercivity component corresponds to the present local field. The high-temperature (isolated above ~200-350°C and up to the Curie temperature of magnetite at 585°C), high-coercivity (above 15 mT) component is origintrending and is interpreted as the characteristic remanent magnetization (ChRM). In the diabase dikes, the Curie temperature of ~570°C indicates the presence of magnetite, with a contribution of maghemite identified by an irreversible decay at around 350°C (Figure 2A). The diabase displays two components of magnetization, better evidenced by thermal (Figure 3C) than by AF demagnetization (Figure 3D). The high-temperature component, which is origin-trending, is unblocked between 400 and 590°C. The magnetic mineralogy of rocks from Victoria Island and Shaler Supergroup is a mixture of pyrrhotite and magnetite. Pyrrhotite is evidenced by significant unblocking temperatures of sIRM around 300-320°C (Figure 2B), and the S-300 ratio of -0.79 that indicates the significant contribution of a high coercivity mineral. Pyrrhotite has previously been reported for the Cambrian-Ordovician sedimentary sequence in the region (e.g., Quesnel et al., 2013). The absence of field-dependence of low field magnetic susceptibility suggests a small pyrrhotite grain size (Worm et al., 1993), as classically found in many pyrrhotitebearing sedimentary rocks (e.g., Rochette et al., 1992). Magnetite is indicated by the significant fraction of the sIRM unblocked above 330°C and up to 580°C (Figure 2B). Among the sedimentary rocks studied for paleomagnetism, 16 out of 21 sites showed unstable remanence during demagnetization and did not provide interpretable results, except for four sites from the Shaler Supergroup and for one site from the Victoria Island formation. These seven sedimentary rocks (dolostones and mudstones) display two components of magnetization, evidenced by thermal demagnetization (Figure 3E-F) where AF demagnetization (not shown) fails to separate both components. The origin-trending hightemperature component is unblocked between 290 and 350 °C (310°C for the only successful Victoria Island site, the remanence being too weak above this demagnetization step).
  Figure 3 (next page). Orthogonal projection plot of stepwise demagnetization data before tiltcorrection. Representative samples of the 3 main formations are presented: A-B) Lithic impact breccia, C-D) diabase dike, E) Shaler Supergroup, F) Victoria Island formation. G) Shaler Formation sample collected 25 cm away from the diabase dike of site TUN32. For clarity the four components of magnetization recorded in this sample and discussed in the text are highlighted by colored arrows.
  184 Evolution temporelle du champ magnétique lunaire ANNEXES
  a) Lithic impact breccias, AF demagnetization
  b) Lithic impact breccias, thermal demagnetization
  N Up
  Site TU20, sample TU2006
  N Up
  Site TU20 sample TU2005A
  170°C
  14 mT 28 12
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  d) Diabase dykes, thermal demagnetization N Up
  Site TU35, sample TU3502b
  Site TU35, sample TU3505a
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  32
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  NRM Scale: 10
  -6
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  f) Victoria Island sedimentary rocks Site TUN07, sample TUN0707a thermal demagnetization
  e) Shaler Supergroup sedimentary rocks N Up
  N Up
  Site TU33, sample TU3305a thermal demagnetization 310 350
  W
  250
  250°C
  290
  E
  Scale: 10 -9 A.m2
  170 230
  W
  310
  E
  Scale: 10 -10 A.m2 120°C
  NRM
  S D ow n
  S Down
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  g) Shaler Supergroup sediments baked by diabase dike N Up
  Site TU32, sample TU3207a 25 cm away from diabase dyke
  345 450 C-D 500
  B
  W 370
  NRM
  330
  A
  310°C
  Scale: 10 -8 A.m2
  E
  FIGURE 3
  S Down
  Evolution temporelle du champ magnétique lunaire 185 ANNEXES
  4 Discussion It is crucial to determine the age of the remanent magnetizations identified in these rocks relative to the impact. The breccia dikes were formed during the impact event and their magnetic remanence would have been acquired both by the clasts and the fine-grained matrix as they cooled following emplacement and associated frictional heating. It has been shown that impact-related tilting occurs quite rapidly, taking place over a timescale shorter than several minutes in mid-sized impact craters (Fairchild et al., 2016). Therefore, the impact breccia is expected to carry a thermoremanent magnetization acquired when the breccia cooled following impact-related tilting. Such a timing of impact acquisition has been shown for clastic breccia dikes within the Slate Island impact structure (Fairchild et al., 2016). However, because frictional heating in the breccia dikes is very localized (Fairchild et al., 2016), the target sedimentary rocks and the diabase dikes may or may not have been remagnetized by the impact. N
  N
  a)
  Shaler Supergroup sediments
  tilt corrected
  in situ
  E
  E W
  W
  S
  S N
  b)
  ChRM
  in situ
  E
  W
  LT diabase dikes (N=3)
  S
  N
  c)
  baked Shaler Supergroup sediments at site TUN32 A
  C
  B
  W
  D in situ
  N
  A
  C E
  W
  E
  B
  tilt corrected diabase dike at site TUN32
  diabase dike at site TUN32
  S
  Figure 4
  186 Evolution temporelle du champ magnétique lunaire
  S
  D ANNEXES
  Figure 4. Equal area stereographic projections. Open (closed) symbols are for upper (lower) hemisphere directions. A) Directions of the 24 samples from 4 sites of the Shaler Supergroup sediments before (left) and after (right) tilt correction. The red symbols are the low temperature components (isolated between 280 and 330°C) of the four samples collected within 25 cm of the diabase dike of site TUN32. Blue star is the overall mean direction (samples from site TUN32 not included). B) Low temperature directions (grey symbols) and ChRM directions for the seven sites from the impact breccia, Shaler Supergroup and Victoria Island Formation listed in Table 1, before tilt correction. Also shown is the average ChRM direction for the 3 diabase dikes (square), and the average present day magnetic field (red star). C) Directions of the four components of magnetization from the Shaler Supergroup sediments collected within 25 cm of the diabase dike at site TUN32. Also shown is the ChRM direction of the diabase dike at this site (in red).
  A paleomagnetic tilt test was used to estimate the relative timing of magnetization and impact tilting (Tauxe and Watson, 1994). This test was performed at site levels and not within sites because bedding was almost constant at each individual site. As expected, for all studied lithologies, the low-coercivity / low-temperature components fail the tilt test and were therefore acquired following tilting of the beds. The direction corresponds to the present day local magnetic field such that it is interpreted to be a recent viscous overprint (Figure 4C). For the impact breccia, the tilt test is also negative for the high coercivity components and progressive tilt test indicate a maximum grouping of the directions at 0% untilting. The ChRM of the breccia is interpreted as a full thermoremanent magnetization (TRM) acquired as the breccia cooled shortly after breccia emplacement and impact tilting. For the Shaler Supergroup, the tilt test is also negative, as illustrated by the much larger scattering after tilt correction (Figure 4A-B). Only one site from the Victoria Island formation provided paleomagnetic results suitable for interpretation. The tilt-corrected paleomagnetic direction for this site is compatible with that from the Shaler Supergroup and we considered it as a post-tilting impact related magnetization as well. The possible processes to account for posttilting remagnetization of these sedimentary rocks are thermal magnetization during cooling from impact-related heating, shock magnetization during pressure release immediately after passage of the impact shock wave, or chemical magnetization resulting from post-impact hydrothermal activity (e.g., Zylberman et al., 2017). Shock remanent magnetization is excluded as the source of the impact-related magnetization because the release of the shock wave occurs before tilting, so that a shock remanence would pass a tilt test. Both partial or total TRM from impact-related heating and chemical remanent magnetization (CRM) from impact-related hydrothermal activities can result in a remagnetization allowing the structure to be paleomagnetically dated (Pilkington et al., 1992). In view of the unblocking temperatures of the natural remanent magnetization (NRM) in the sedimentary rocks (290°C to 350°C), a CRM could be explained by the formation of pyrrhotite during hydrothermal activity (e.g., Osinski et al., 2005). However, because the maximum unblocking temperature of 350°C is slightly higher than the Curie temperature of pyrrhotite (320-325°C), we favor a partial thermoremanent magnetization acquired during post-impact cooling from a peak temperature of 350°C. For the three diabase dikes, the tilt test is inconclusive because the beddings of the intruded sediments are similar for the three sites. The ChRM direction is significantly different (before tilt correction) to the average direction computed from the Shaler Evolution temporelle du champ magnétique lunaire 187 ANNEXES
  Supergroup, Victoria Island formation and impact breccia, suggesting that they have not been remagnetized by the impact (Figure 4B). Their virtual geomagnetic poles (VGPs, after tilt correction) are closer to paleomagnetic poles from other igneous rocks of the Franklin Large Igneous Province (Palmer et al., 1983; Denyszyn et al. 2009) than the impact breccia site VGPs. Poorly developed shatter cones have been described in the diabase dikes (Dewing et al., 2013), implying shock pressures above ~2 GPa during the impact event. Such pressure level is well below the pressure-induced magnetite magnetic transition at 12-16 GPa (Ding et al., 2008). Moreover, the coercivity of remanence of samples from the diabase dikes is 39 mT (our measurements, average of 3 samples, one per studied dike), and significant shock remagnetization is also unlikely for magnetite with this range of coercivity of remanence (Bezaeva et al., 2010). To test further whether or not the diabase dikes have preserved their primary TRM, we collected 4 oriented cores of the Shaler supergroup sedimentary rocks that were intruded and baked by the dike, at distances from 3 to 25 cm from the dike wall. In addition to a low temperature viscous component, these samples exhibit four magnetization components unblocked over the following temperature range: between 250 and 345°C, between 345 and 370, between 370 and 450°C, and above 450°C (Table 1, Figure 3G). These components are called A, B, C, D in the following for clarity. Among the three higher temperature components components B and D are identical and close to the reverse polarity ChRM of the diabase dikes (Figure 4C). Component C is antipodal to components B and D. These three components are interpreted as partial TRMs acquired successively during cooling from above 500°C following the diabase dike intrusion. During cooling, the geomagnetic field changed from reverse to normal and back to reverse polarity, leading to the record of antipodal directions D, C, and B successively. Such a record of successive polarity intervals has already been observed in slow cooling sedimentary rocks (e.g., Rochette et al. 1992). Component A blocked up to 345°C has a direction different from the other 3 components related to the dike intrusion but undiscernible (before tilt correction) from the other remagnetized Shaler Supergroup directions. It is interpreted as a partial TRM acquired following the impact. This result indicates that the post-impact temperature in these rocks was 345°C, in close agreement with the ChRM peak unblocking temperatures measured in the other Shaler Supergroup sediments within the impact structure. Component A is not recorded in the diabase dikes because these rocks have minimal blocking capacity below 350°C. The impact breccia, Shaler Supergroup formation, and Victoria Island formation all have post-tilting impact-related ChRM. These seven paleomagnetic directions are similar (Figure 4B) and the mean pole calculated from these sites is: 349.3°E, 1.2°N, angular radius of 95% confidence cone A95 = 8.3°, and Fisher precision parameter K = 53.3 (Table 1). The sites may have slightly different magnetization ages due to different blocking temperatures and varied cooling history and as a result may partially average out secular variation. However, the conservative interpretation is that the combined pole represents a snapshot of the paleomagnetic field shortly following impact that does not average out secular variation and should be considered a virtual geomagnetic pole rather than a mean paleomagnetic pole.
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  Comparison of this impact pole with the running mean Apparent Polar Wander Path (APWP) for North America (Torsvik et al., 2012; Besse and Courtillot, 1991) shows that it is closest to the 520, 510 and 500 Ma mean paleomagnetic poles (Figure 5).
  Figure 5. Comparison of the running mean APWP of Laurentia (Torsvik et al., 2012) with the Tunnunik virtual paleomagnetic pole which is the average of the virtual geomagnetic poles of the impact breccia, Shaler Supergroup Formation and Victoria Formation sites. Ages of the running mean APWP poles are labeled from the Cambrian to the Early Devonian. The running mean poles are shown with the calculated 95% confidence circle (A95) associated with the mean of studies that fall within a 20 Myr sliding window. The poles for 520, 460, 450, 390, 380, 360 and 350 Ma are shown with no such confidence circle as there are no studies within the sliding window and the position of the pole for that age is the result of linear interpolation by Torsvik et al. (2012).
  Following 500 Ma, the angular distance of the Tunnunik VGP becomes progressively greater from the geographic poles implied by the running mean poles of Torsvik et al. (2012) and the paleogeographic model of Torsvik and Cocks (2017) and its modification by Swanson-Hysell and Macdonald (2017) (Figure 6A). From 410 Ma onward to the present, the angular distance between the Tunnunik VGP and the geographic pole exceeds 45o. Deviations between the geographic pole and a virtual geomagnetic pole are expected to arise through secular variation of the geomagnetic field. However, secular variation of the geomagnetic field typically leads to geomagnetic pole positions that are close to the geographic pole. For Evolution temporelle du champ magnétique lunaire 189 ANNEXES
  example, random draws from the paleosecular variation model TK03.GAD (Tauxe and Kent, 2004) for a site at the equator will be within 20o of the geographic pole 90% of the time and within 30o of the geographic pole 95% of the time. The further away an impact VGP is from a pole of a given age, the less likely it is to have formed at that age (see similar discussion in Hervé et al., 2015 and Carporzen and Gilder, 2006). In order to estimate the probability of the Tunnunik VGP having arisen through secular variation throughout the Phanerozoic, the approach developed by Fairchild et al. (2016) was used: the angular distance between the Tunnunik VGP and the paleopoles is compared to that of VGPs randomly sampled from the TK03.GAD secular variation model (Tauxe and Kent, 2004). For this analysis, we simulated 105 VGPs from the TK03.GAD model in 10 million year increments at the paleogeographic position of the crater. Seeking to quantify the probability of an angular deviation between the paleopole and the Tunnunik VGP as large as that determined at each of these times (Figure 6A), we calculated the percentage of simulated VGPs that are at this angular distance or greater from the pole (Figure 6B and 6C). While the Tunnunik VGP is closest to North America's pole path at ca. 500 Ma, this analysis shows that the likelihood of a VGP with the angular distance as large as the deviation of the paleopole of the Tunnunik VGP is greater than 5% at every simulated time during the Ordovician, using the minimum angle given the uncertainty of the VGP. However, given the increasing angular distance of the Tunnunik VGP from Silurian paleopoles (from 420 Ma onward), the likelihood of a VGP at such a distance is less than 5% and continues to be a low probability event all the way to the present. The magnetostratigraphy scale for the 500-420 Ma time interval (e.g., Hounslow et al., 2016), although incomplete, shows multiple reversals and does not allow to better constrain the age of the reverse polarity remagnetization observed at Tunnunik structure.
  190 Evolution temporelle du champ magnétique lunaire ANNEXES
  angle between Tunnunik pole and the modeled geographic pole
  a) 100 80
  60
  40 Torsvik and Cocks (2017) paleogeographic model Swanson-Hysell and Macdonald (2017) Ordovician modification Torsvik et al. (2012) running mean APWP
  20
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  likelihood of this angle or greater from pole (%)
  b) 100
  400 stratigraphic age constraint
  300 time (Ma)
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  Torsvik and Cocks (2017) paleogeographic model Swanson-Hysell and Macdonald (2017) Ordovician modification Torsvik et al. (2012) running mean APWP
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  80
  TK03.GAD as secular variation 60
  40
  20 5 percent likelihood
  0
  500
  400
  300 time (Ma)
  200
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  0
  Figure 6. A) Angle between Tunnunik impact VGP and paleopoles as a function of time for the Torsvik et al. (2012) APWP, the paleogeographic models of Torsvik and Cocks (2017) and its Ordovician modification by Swanson-Hysell and Macdonald (2017). B) likelihood for a VGP to occur at an angular deviation from the paleopole equal or greater to that observed for the Tunnunik impact VGP for a Fisherian (k=20) distribution of VGPs. C) likelihood for a VGP to occur at an angular deviation from the paleopole equal or greater to that observed for the Tunnunik impact VGP for VGPs drawn from the TK03.GAD secular variation model.
  Geologic constraints on the timing of the impact come from the age of early Paleozoic shallow-marine sedimentary rocks within the Tunnunik impact structure. Extensive mapping of the shatter cone distribution at the impact structure reveal the youngest rocks with shatter cones to be those of the Victoria Island Formation (Osinski and Ferrière, 2016). Conodont biostratigraphy has revealed an Early Ordovician age (ca. 487 to 470 Ma) for the Victoria Island Formation (Dewing et al., 2013; Dewing et al., 2015). Faulting associated with impact crater development extends beyond the central region that contains shatter cones and deforms rocks of the Thumb Mountain Formation (Dewing et al., 2013; Newman and Osinkski, 2016). Evolution temporelle du champ magnétique lunaire 191 ANNEXES
  Conodonts and a fossil assemblage of crinoids, corals, gastropods, and cephalopods indicate a Late Ordovician age for the Thumb Mountain Formation (ca. 458 to 445 Ma; Dewing et al., 2013). Overlying the dolostone of the Thumb Mountain is a poorly exposed dolostone unit that Dewing et al. (2013) tentatively assigned to the Allen Bay Formation. Further to the northwest on the Arctic Platform at Devon Island, biostratigraphy on carbonates of the Allen Bay Formation have revealed it to have an age that spans from the Late Ordovician into the Silurian (Thorsteinsson and Mayr, 1987). Mapping of the impact structure has largely left rocks assigned to the Thumb Mountain and Allen Bay Formations undivided (Dewing et al., 2013; Newman and Osinski, 2016). Currently the most robust constraint on the maximum age of the impact event is that the development of the impact structure led to faulting of the Thumb Mountain Formation and therefore occurred after ca. 458 Ma. Regional normal faults taken to have formed in association with Early Cretaceous extension cut across the impact structure were interpreted to indicate that the crater formed before ca. 130 Ma (Dewing et al., 2013), but this constraint is rather speculative. The position of the Tunnunik VGP is most consistent with an Ordovician age for the Tunnunik impact. While secular variation of the geomagnetic field naturally results in VGPs that differ from the mean paleopole position, the large angle between the Tunnunik VGP and paleopoles 420 Ma and younger makes a deviation of this magnitude a low probability event (Figure 6). The combined paleomagnetic, stratigraphic and biostratigraphic constraints collectively suggest a Late Ordovician to Silurian age of ca. 430-450 Ma, only shortly after deposition of the youngest impacted sediments. 5 Conclusions This study focuses on constraining the age of the recently discovered Tunnunik impact structure. Paleomagnetic directions obtained from impact breccia and sedimentary rocks from the target sequence indicate the acquisition of a magnetization following impact-related tilting during a reverse polarity interval. In contrast, the diabase dikes intruding these sediments have retained their original magnetization unblocked above 400oC. The comparison of the resulting paleomagnetic pole with the apparent polar wander path for North America shows it to be close to Cambrian poles and strongly suggests a Paleozoic age for the crater. The likelihood of the observed impact direction becomes very small (<5% likelihood) if it was acquired after ca. 430 Ma suggesting an age of impact that is older than 430 Ma. Combined with the stratigraphic constraints these data indicate an impact age between 450 and 430 Ma, in the Late Ordovician or Silurian Period, soon after deposition of the youngest impacted sedimentary rocks. This old age opens the possibility that ejecta from the Tunnunik crater may have been preserved in the surrounding basin and may be found in the sedimentary rocks of the Arctic platform. The target sedimentary rocks were remagnetized during post-impact cooling. The preservation of pre-impact magnetization unblocked above 400°C in the diabase dikes, and the constraints from the paleomagnetic results obtained on the Shaler Supergroup sediments baked by the intrusions of the diabase dikes indicate collectively that the peak temperature of the thermal excursion associated with the impact was 350°C at the structural level of the studied rocks. 192 Evolution temporelle du champ magnétique lunaire ANNEXES
  Acknowledgments This project was partly funded by the French Polar Institute IPEV (Institut Paul Emile Victor). The authors are grateful to the Polar Continental Shelf Project for logistical support. The France-Berkeley Fund initiated collaborative research between J.G. and N.L.S.-H. We thank William Zylberman, Cassandra Marion-Beauchamp, and Racel Sopoco for their help in the field during sampling. References Ames, D.E., Watkinson, D.H. and Parrish, R.R. (1998), Dating of a regional hydrothermal system induced by the 1850 Ma Sudbury impact event, Geology, 26, 447–450. Besse, J. and V. Courtillot (1991), Revised and Synthetic apparent polar wander path of the African, Eurasian, North American and Indian plates, and True polar wander since 200 Ma, J. Geophys. Res., 96, 4029–4050. Bezaeva, N.S., Gattacceca, J., Rochette, P., Sadykov, R.A., and V.I. Trukhin (2010). Demagnetization of terrestrial and extraterrestrial rocks under hydrostatic pressure up to 1.2 GPa. Physics of the Earth and Planetary Interiors, 179, 7-20. Carpozen, L. and S.A. Gilder (2006), Evidence for coeval Late Triassic terrestrial impacts from the Rochechouart (France) meteorite impact, Geophysical research letters, v.33, L19308, doi: 10.1029/2006GL027356. Cockell, C. S., Lee, P., Broady P., Lim, D., Osinski, G., Parnell, J., Koeberl, C., Pesonen, L., and Salminen, J., 2005. Effects of Asteroid and Comet Impacts on Lithophytic Habitats – A Synthesis. Meteoritics and Planetary Science, 40, 12, 1-14. Cogné, J.P (2003), PaleoMac: A MacintoshTM application for treating paleomagnetic data and making plate reconstructions, Geochemistry, Geophysics, Geosystems, 4, 1, 1007, doi: 10.1029/2001GC000227. Denyszyn, S. W., Halls, H. C., Davis, D. W., and Evans, D. A. (2009). Paleomagnetism and U–Pb geochronology of Franklin dykes in High Arctic Canada and Greenland: a revised age and paleomagnetic pole constraining block rotations in the Nares Strait region. Canadian Journal of Earth Sciences, 46(9), 689-705. Deutsch A., Schärer U. (1994) Dating terrestrial impact events. Meteoritics, 29, 301-322. Dewing, K., Pratt, B.R., Hadlari, T., Brent, T., Bédard, J. and R.H.Rainbird (2013), Newly identified "Tunnunik" impact structure, Prince Albert Peninsula, northwestern Victoria Island, Arctic Canada, Meteoritics & Planetary Science, 48, 2, 211-223, doi: 10.1111/maps.12052. Dewing, K., Hadlari, T., Rainbird, R.H., and Bédard, J.H., 2015. Phanerozoic geology, northwestern Victoria Island, Northwest Territories; Geological Survey of Canada, Canadian Geoscience Map 171 (preliminary), scale 1:500 000. doi:10.4095/295530 Ding Y., Haskel D., Ovchinnikov S. G., Tseng Y. C., Orlov Y. S., Lang J. C., Mao H. (2008) Novel Pressure induced magnetic transition in magnetite (Fe3O4) Physical Review Letters, 100, 045508. Evolution temporelle du champ magnétique lunaire 193 ANNEXES
  Fairchild, L.M., Swanson-Hysell, N.L. and S.M. Tikoo (2016), A matter of minutes: Breccia dike paleomagnetism provides evidences for rapid crater modification, Geological Society of America, 44, 9, 723-726, doi: 10.1130/G37927.1. Heaman L. M., LeCheminant A. N., and Rainbird R. H. 1992. Nature and timing of Franklin igneous events, Canada: Implications for a Late Proterozoic mantle plume and the breakup of Laurentia. Earth and Planetary Science Letters 109:117–131. Hergarten, S. and T. Kenkmann (2015), The number of impact craters on Earth: any room for further discoveries ?, Earth and Planetary Science Letters, 425, 187-192. Hervé, G., Gilder, S.A., Marion, C.L., Osinski, G.R., Pohl, A., Petersen, N. and P.J.Sylvester (2015), Paleomagnetic and rock magnetic study of the Mistastin Lake impact structure (Labrador, Canada): implications for geomagnetic perturbation and shock effects, Earth and Planetary Science Letters, 417, 151-163, doi:10.1016/j.epsl.2015.02.011. Hounslow, M.W (2016), Geomagnetic reversal rates following Palaeozoic superchrons have a fast restart mechanism, Nature communication, doi: 10.1038/ncomms12507. Jourdan, F., Reimold, W.U. and A. Deutsch (2012), Dating terrestrial impact structures, Elements, 8, 49-53, doi: 10.2113/gselements.8.1.49. Newman, J.D. and G.R. Osinski (2016), Geological mapping of the Tunnunik impact structure, Victoria Island, Canadian High Arctic, paper presented at the 47th Lunar and Planetary Science Conference, Houston. Osinski, G.R. and L. Ferrière (2016), Shatter cones: (Mis)understood?, Sciences advances, 2, 8, doi: 10.1126/sciadv.1600616. Osinksi, G.R., Lee, P., Parneell J., Spray, J. G., Baron, M. (2005) A case study of impactinduced hyrothermal activity: the Haughton impact structure, Devon Island, Canadian High Arctic, Meteoritics & Planetary Science, 40, 1859–1877. Palmer, H.C., Baragar, W.R.A., Fortier, M. and Foster, J.H. (1983), Paleomagnetism of Late Proterozoic rocks, Victoria Island, Northwest Territories, Canada, Canadian Journal of Earth Sciences, 20, 1456-1469, doi: 10.1139/e83-131. Pilkington M., and R.A.F. Grieve (1992). The geophysical signature of terrestrial impact craters. Review of geophysics 30, 161-181. Quesnel, Y., Gattacceca, J., Osinski, G.R. and P. Rochette (2013), Origin of the central magnetic anomaly at the Haughton impact structure, Canada, Earth and Planetary Science Letters, 367, 116-122. Reimold, W.U. and C. Koeberl (2008), Catastrophes, extinctions and evolution: 50 years of impact cratering studies, Golden Jubilee Memoir of the Geological Society of India, 66, 69-110. Rochette, P., Jackson, M., and C. Aubourg (1992). Rocj magnetism and the interpretation of anisotropy of magnetic susceptibility. Review of Geophysics 30, 209-226. Schulte, P. and 40 co-authors (2010), The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary. Science, 327, 1214-1218. Swanson-Hysell, N. L. and F. A. Macdonald (2017), Tropical weathering of the Tacnic orogeny as a driver for Ordovician cooling, Geology, doi:10.1130/G38985.1 Thorsteinsson, R. and U. Mayr. (1987), The sedimentary rocks of Devon Island, Canadian Arctic Archipelago, Geological Survey of Canada Memoir, 411, 182. 194 Evolution temporelle du champ magnétique lunaire ANNEXES
  Tauxe, L. and D.V. Kent (2004). A simplified statistical model for the geomagnetic field and the detection of shallow bias in paleomagnetic inclinations: Was the ancient magnetic field dipolar? In J. Channell, D. Kent, W. Lowrie, & J. Meert (Eds.), Timescales of the Paleomagnetic Field, 145, 101–116. Washington, D.C.: AGU. Tauxe, L. and G.S. Watson (1994), The fold test – an eigen analysis approach, Earth and Planetary Science Letters, 122, 331-341. Torsvik, T.H. and L.R.M. Cocks (2017), Earth history and palaeogeography: Cambridge, UK, Cambridge University Press, 311, doi: 10 .1017/9781316225523. Torsvik, T.H., Van Der Voo, R., Preeden, U., Mac Niocaill, C., Steinberger, B., Doubrovine, P.V., Van Hinsbergen, D.J.J., Domeier, M., Gaina, C., Tohver, E., Meert, J.G., McCausland, P.J.A. and L.R.M. Cocks (2012), Phanerozoic polar wander, palaeogeography and dynamics, Earth Science Reviews, 114, 325-368, doi: 10.1016/j.earscirev.2012.06.007. Worm, H.-U., Clark, D., Dekkers, M.J. (1993) Magnetic susceptibility of pyrrhotite: grain size, field and frequency dependence. Geophys. J. Int., 114:127-137. Zylberman, W., Quesnel, Y., Rochette, P., Osinski, G.R., Marion, C. and J. Gattacceca, (2017), Hydrothermally-enhanced magnetization at the center of the Haughton impact structure?, Meteoritics and Planetary Science, 1-19, doi:10.1111/maps.12917.
  Evolution temporelle du champ magnétique lunaire 195 ANNEXES
  Supplementary material
  Site
  Slat (°N)
  Slong (°E)
  I/O crater
  TUN20 TUN21 TUN34 TUN58 TUN12 TUN30 TUN32 TUN35 TUN17 TUN50 TUN51 TUN56 TUN14 TUN15 TUN16 TUN13 TUN29 TUN31 TUN33 TUN36 TUN21 TUN22 TUN23 TUN28
  72.4973 72.4965 72.4821 72.4769 72.1422 72.4729 72.4827 72.4839 72.3326 72.4836 72.4825 72.4759 72.1503 72.1513 72.1516 72.1420 72.4732 72.4831 72.4832 72.4662 72.4966 72.4971 72.4985 72.5192
  -114.0209 -114.0198 -113.9376 -113.8936 -112.51387 -113.9320 -113.9410 -113.9379 -114.39646 -113.93269 -113.9292 -113.9063 -112.5226 -112.5230 -112.52317 -112.5206 -113.9503 -113.9321 -113.9392 -113.9665 -114.0198 -114.0200 -114.0200 -114.01946
  Inside Inside Inside Inside Outside Inside Inside Inside Outside Outside Outside Outside Outside Outside Outside Outside Inside Inside Inside Inside Inside Inside Inside Inside
  distance from center (km) 3.3 3.3 0.5 3 61 0.7 0.5 0.7 22.6 14.9 15.2 12.3 6 61 61 61 0.6 0.7 0.5 1.6 3.3 3.4 3.6 5.3
  Tan dolostone unit of Mount Phayre formation
  TUN11
  72.1643
  -112.6376
  Outside
  56
  Victoria Island
  TUN03 TUN07 TUN10 TUN18
  72.4567 72.4363 72.2800 72.2321
  -113.85526 -113.8668 -113.02338 -114.08812
  Inside Inside Outside Outside
  3.8 5.2 38 28
  Formation
  Impact Breccia
  Mafic dikes
  Allen Bay
  Clastic unit of Mount Phayre formation
  Shaler Supergroup
  Stripy unit of Mount Phayre formation
  paleomagnetic result yes yes no no yes yes yes yes no no no no no no no no yes yes yes yes no no no no no no yes no no
  Table S1 - list of sampled sites. Site TUN12 is from a 150 m wide mafic dike located 60 km away from the impact structure. It paleomagnetic direction (D=58°, I=-4°, a95=8.4, k=84, n=5) is different from the three diabase dykes sampled in the impact structure, but this can be easily accounted for by secular variation of the geomagnetic field since the emplacements of these dykes are not necessarily strictly coeval.
  196 Evolution temporelle du champ magnétique lunaire ANNEXES
  Annexe 3 Trois planches photographiques des échantillons analysés en laboratoire dans cette étude.
  Planche 1 10018 (2.3 g)
  12021 (2.3 g)
  12005 (2.1 g)
  1 cm
  1 cm
  14169-B (1.2 g)
  15505-A (1.8 g)
  1 cm
  1 cm
  15529 (2.1 g)
  1 cm
  1 cm
  61195 (2.1 g)
  61015 (2.2 g)
  1 cm
  1 cm
  65055-B (1.1 g)
  65055-A (2.3 g)
  1 cm
  65055-C (0.7 g)
  1 cm
  1 cm
  Evolution temporelle du champ magnétique lunaire 197 ANNEXES
  Planche 2 10003 (0.07 g)
  14078 (0.03 g)
  1 cm
  1 cm
  60015 (2.1 g)
  60215 (1.9 g)
  1 cm
  65315 (2.1 g)
  1 cm
  74255 (0.06 g)
  1 cm
  1 cm
  70017 (0.1 g)
  1 cm
  74275 (0.08 g)
  1 cm
  198 Evolution temporelle du champ magnétique lunaire
  15555 (0.06 g)
  1 cm
  62255 (1.03 g)
  1 cm
  70135 (0.03 g)
  1 cm
  75075 (0.06 g)
  1 cm ANNEXES
  Planche 3 10017 (0.6 g)
  1 cm
  12016 (0.34 g)
  1 cm
  12012 (0.3 g)
  1 cm
  12018 (0.9 g)
  1 cm
  Evolution temporelle du champ magnétique lunaire 199 ANNEXES
  Annexe 4 Partie 1 :
  Résultat de la datation standard Argon des échantillons 12005 et 15529. 1) 12005 sous-échantillon 22935
  sous-échantillon 22936
  200 Evolution temporelle du champ magnétique lunaire ANNEXES
  2) 15529 sous-échantillon 22933
  sous-échantillon 22938-01
  Partie 2 :
  Cartes de trois échantillons (10018, 14169 et 15505) réalisées au microscope électronique à balayage pour des datations Ar/Ar par sonde laser, présentant les sites à analyser. La première carte est, pour chaque échantillon, réalisée en lumière Evolution temporelle du champ magnétique lunaire 201 ANNEXES
  naturelle et la seconde carte montre les teneurs en aluminium, potassium et magnésium contenues par les échantillons. 1) 10018
  202 Evolution temporelle du champ magnétique lunaire ANNEXES
  Histogramme d'assemblage de la brèche : âge moyen calculé sur 1-2 Ga
  Evolution temporelle du champ magnétique lunaire 203 ANNEXES
  2) 14169
  Histogramme d'assemblage de la brèche : âge moyen calculé sur tout l'intervalle.
  204 Evolution temporelle du champ magnétique lunaire ANNEXES
  3) 15505
  Evolution temporelle du champ magnétique lunaire 205 ANNEXES
  Histogramme d'assemblage de la brèche : âge moyen calculé sur l'intervalle 0-1 Ga.
  206 Evolution temporelle du champ magnétique lunaire ANNEXES
  Annexe 5 Diagrammes de désaimantation de la NRM. A2) 10018-B1a (68 mg)
  A1) 10018-A2b (58 mg)
  N
  W S
  HT
  HT
  NRM
  E x10-9 Am2
  560 340
  N x10-8 Am2
  630
  A3) 10018-B1b (68 mg)
  N Up x10-9 Am2
  W Up x10-8 Am2
  W
  Up x10-9 Am2 E x 10-9 Am2
  630
  460
  HT
  225
  200
  NRM
  200°C 100 °C
  LT
  LT
  LT
  NRM
  NRM
  S Down
  100 °C
  S Down
  E Down
  A5) 12021-A1 (287 mg)
  A4) 12005-C1 (198 mg)
  A6) 14169-B1a (129 mg)
  W Up x10-10 Am2
  N Up x10-9 Am2
  N Up x10-9 Am2 NRM
  NRM NRM
  LT
  500
  N x10-10 Am2
  S
  LT
  200°C
  W 450
  250°C
  E x10-9 Am2
  HT
  200°C LT
  E x10-9 Am2
  W NRM
  NRM
  580
  S Down
  W Up x 10 Am
  S Down
  A9) 15529-C2 (381 mg)
  A8) 61195-A1a (129 mg)
  A7) 15505-A3b (213 mg) -8
  NRM
  E Down
  N
  2
  N Up x10-8 Am2
  Up x 10-9 Am2 NRM
  NRM
  NRM LT
  520
  100°C
  580
  W
  450
  LT
  HT
  250
  W
  200°C
  E x10-8 Am2
  NRM
  NRM
  NRM
  580
  S
  360
  580
  200°C
  LT
  HT
  E x 10-9 Am2
  N x 10-8 Am2
  S Down
  E Down
  S Down
  A11) 15529-C1 (438 mg)
  A12) 65055-A1 (387 mg) N Up x10-9 Am2
  N Up x10-9 Am2
  A10) 61015-B2 (279 mg)
  280°C
  W
  NRM
  Up x10-9 Am2 NRM
  LT
  LT
  250
  S
  500
  540
  150°C
  150°C
  250°C 480
  LT
  W
  N x10-9 Am2
  E x10-9 Am2
  NRM NRM
  NRM
  E Down
  580
  W
  NRM
  E x10-9 Am2 S Down
  S Down
  Evolution temporelle du champ magnétique lunaire 207 208 Evolution temporelle du champ magnétique lunaire
  W
  W
  W
  S Down
  2 mT
  LC
  NRM
  NRM
  4
  100
  S
  LC
  N Up x10-8 Am2
  Down
  3 mT
  E x10
  -10
  Am
  2
  E x10-9 Am2
  NRM
  NRM
  E x10-8 Am2
  S Down
  B11) 12018 (996 mg)
  LC
  2 mT
  NRM
  NRM
  N Up x10-9 Am2
  B6) 12005-B2 (169 mg)
  17
  N Up x10-10 Am2
  B1) 10003 (74 mg)
  W
  S
  W
  60
  8 mT
  NRM
  100 -9
  5
  E
  NRM
  NRM
  LC
  2
  NRM
  NRM
  W
  HC
  170 mT
  E x10-9 Am2
  N Up x10-9 Am2
  S Down
  10
  S Down
  140
  W
  W
  LC
  S Down
  5 mT
  LC
  N Up x10-9 Am2
  40
  E x10-9 Am2
  NRM
  NRM
  E x10-9 Am2
  S Down
  5 mT
  HC
  W
  NRM
  NRM
  W
  NRM
  20
  NRM
  LC
  NRM
  NRM
  E x 10-8 Am2
  S
  14
  LC
  7 mT
  13
  S
  N
  E x 10-9 Am2
  Down
  HC
  Up x 10-9 Am2
  B14) 14078 (26 mg)
  LC
  2 mT
  N
  B9) 12012 (290 mg)
  5 mT
  B4) 10018-B4 (80 mg) Up x 10-8 Am2
  S Down
  220
  N
  N Up x10-9 Am2
  B8) 12005-B4 (145 mg)
  NRM
  B13) 12021-C (220 mg)
  NRM
  W
  NRM
  LC
  5 mT
  E x10-8 Am2
  NRM
  B3) 10018-A2a (145 mg) N Up x10-8 Am2
  N x10-9 Am2
  2
  B12) 12021-B1 (533 mg)
  Down
  LC
  W Up x10-9 Am2
  6 mT
  2
  E x 10 Am
  S Down
  NRM
  -9
  Up x 10 Am
  B7) 12005-B3 (134 mg)
  HC
  LC
  N
  B2) 10017 (550 mg)
  NRM
  NRM
  S Down
  HC
  80
  Down
  22 7
  LC
  5 mT
  W
  NRM
  NRM
  E x10-8 Am2
  7 mT
  3
  S Down
  6
  LC
  N Up x 10-9 Am2
  NRM
  E x 10-9 Am2
  NRM
  B15) 14169-B2a (73 mg)
  E Down
  LC
  N x10-9 Am2
  W Up x10-9 Am2
  B10) 12016 (340 mg)
  E x10-10 Am2
  Up x10-10 Am2
  S
  W
  B5) 10018-C1 (132 mg)
  N Up x10-8 Am2
  ANNEXES W
  NRM
  NRM
  W
  W
  3 mT
  100
  LC
  NRM
  4
  NRM
  S Down
  N Up x10-9 Am2
  W
  HC
  16
  -9
  LC
  E x10-9 Am 2
  LC
  S
  NRM
  NRM
  Up x10 Am 4 mT
  2
  LC
  Down -9
  W Down
  NRM
  E
  100
  N x10-9 Am2
  S
  100
  45
  -8
  E Down x 10 Am
  HC
  19
  2
  9 mT
  S
  NRM
  NRM
  S
  E x10-9 Am2 Up
  2
  LC
  4 mT 9
  E
  100
  Down
  W Up x10
  -10
  NRM
  LC
  N x10-10 Am2
  Am
  LC
  S
  HC
  4 mT
  W
  S
  N
  6
  NRM
  NRM
  NRM
  S Down
  100 mT
  LC
  LC
  S
  17
  44
  44
  100
  16 mT
  W
  HC
  NRM
  NRM
  NRM
  Down
  E x10-9 Am2
  N Up x10-9 Am2
  S
  LC
  5
  2 mT
  NRM
  E x10-9 Am2
  N x10-10 Am2
  B31) 65055-C (699 mg)
  E Down
  LC
  E x 10-9 Am2
  Up x10-9 Am2
  S Down
  HC
  100
  N Up x 10-9 Am2
  S Down
  N
  7 mT
  100
  W Up x10-10 Am2
  B26) 60215 (61 mg)
  E x10-9 Am2
  N Up x10-9 Am2
  5 mT
  NRM
  NRM
  B20) 15529-A2 (434 mg)
  B21) 15529-B2 (352 mg)
  NRM
  W
  B30) 65055-B2 (279 mg)
  Down
  E x10-9 Am2
  Up x10-9 Am2
  W
  E x10-9 Am2
  Down
  90
  N Up x10-9 Am2
  B19) 15505-A3c2 (187 mg)
  B25) 61015-B6 (55 mg)
  NRM
  W
  2
  NRM
  NRM
  W
  N
  NRM
  NRM
  LC
  B29) 62255 (295 mg)
  4 mT
  NRM
  N x10 Am
  -9
  4
  B18) 15505-A2a (96 mg) W Up x 10-8 Am2
  B24) 61015-B3 (87 mg)
  E x10-8 Am2
  B28) 61195-B3 (225 mg)
  S Down
  NRM
  160
  NRM
  Up x10 Am
  2
  NRM
  NRM
  W
  LC
  4 mT
  N
  -9
  B23) 60015 (820 mg)
  2
  S Down
  120
  E x10-9 Am2
  E x10-10 Am2
  Up x10-10 Am2
  S Down
  N
  W
  8 mT
  B17) 15505-A1b (80 mg)
  N Up x10-8 Am2
  E x10 Am
  NRM
  NRM
  B27) 61195-B1 (253 mg)
  LC
  LC
  B22) 15555 (59 mg)
  S Down
  8
  3 mT
  N Up x10-9 Am2
  B16) 14169-B2b (118 mg)
  ANNEXES
  Evolution temporelle du champ magnétique lunaire 209 W
  S
  E Down
  Up x10
  -10
  210 Evolution temporelle du champ magnétique lunaire
  7 mT
  LC
  Up x10-10 Am2
  S Down
  N
  B36) 74275 (358 mg)
  NRM
  LC
  NRM
  9 mT
  W
  B32) 65315 (161 mg)
  NRM
  NRM
  Am
  2
  E x10
  -10
  Am
  2
  N x10-10 Am2
  S
  Down
  W
  NRM
  11 mT
  S
  LC
  E Down
  W Up x10 Am
  -9
  LC
  2
  B37) 75075 (217 mg)
  24
  NRM
  N x10 Am
  -9
  B33) 70017 (578 mg) 2
  NRM
  NRM
  Up
  2
  S
  LC
  NRM
  NRM
  W
  2
  LC
  12 mT
  HC
  70
  B38) LAP02205 (219 mg)
  NRM
  NRM
  S
  100
  N
  S
  Down
  Ex
  10-9 Am2
  NRM
  W
  NRM
  LC
  5 mT
  38
  S Down
  100
  HC
  LC
  20
  6 mT
  N Up x10-6 Am2
  E x10-6 Am2
  B39) NWA10782 (42 g)
  N x10-9 Am2
  Up x10-9 Am2
  E Down
  W
  B35) 74255 (221 mg)
  10-9 Am2
  NRM
  NRM
  Up x
  Up x10-9 Am2
  Down
  N x10-9 Am2
  E x10-9 Am2
  E
  2
  W
  100 mT
  -9
  N x10 Am
  B34) 70135 (107 mg)
  ANNEXES ANNEXES
  Annexe 6 REVIEW OF SCIENTIFIC INSTRUMENTS 88, 104502 (2017)
  A spinner magnetometer for large Apollo lunar samples M. Uehara,1,a) J. Gattacceca,1 Y. Quesnel,1 C. Lepaulard,1 E. A. Lima,2 M. Manfredi,3 and P. Rochette1 1 CNRS,
  Aix Marseille Univ, IRD, Coll France, CEREGE, Aix-en-Provence, France of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 3 CGI, 92097 La Défense, France 2 Department
  (Received 21 April 2017; accepted 10 October 2017; published online 27 October 2017) We developed a spinner magnetometer to measure the natural remanent magnetization of large Apollo lunar rocks in the storage vault of the Lunar Sample Laboratory Facility (LSLF) of NASA. The magnetometer mainly consists of a commercially available three-axial fluxgate sensor and a handrotating sample table with an optical encoder recording the rotation angles. The distance between the sample and the sensor is adjustable according to the sample size and magnetization intensity. The sensor and the sample are placed in a two-layer mu-metal shield to measure the sample natural remanent magnetization. The magnetic signals are acquired together with the rotation angle to obtain stacking of the measured signals over multiple revolutions. The developed magnetometer has a sensitivity of 5 ⇥ 10 7 Am2 at the standard sensor-to-sample distance of 15 cm. This sensitivity is sufficient to measure the natural remanent magnetization of almost all the lunar basalt and breccia samples with mass above 10 g in the LSLF vault. Published by AIP Publishing. https://doi.org/10.1063/1.5008905
  I. RATIONALE
  The Moon has no global magnetic field today. However spacecraft observations have shown that large portions of the crust are magnetized (e.g., Refs. 1 and 2). Paleomagnetic studies of samples returned by the Apollo program have also shown that some of these rocks carry a significant remanent magnetization that was acquired on the Moon.3 It is now rather firmly established that the Moon once had a global magnetic field generated by a dynamo mechanism in a molten metallic core.4 However, crucial questions remain to be answered such as the intensity of the lunar paleofield, its geometry, and the exact timing of the dynamo onset and turn-off. Answering these questions would ultimately shed light on the interior structure of the Moon, on the processes that allowed dynamo generation, and would provide the dynamo theory with a robust test case. A fairly large number of samples (74) were studied in the 1970s, soon after their return from the Moon (see Ref. 3 for a review). A new series of more refined paleomagnetic and thermochronology studies have been performed in the 2010s.5–9 All together, about 71 different Apollo rocks (for a total of 90 samples) have been studied for paleomagnetism. This represents only 5% of the 1402 individual returned during the Apollo program. All these studies (with the exception of Cournède et al.6 ) were performed on small chips (usually <1 g) allocated for detailed laboratory work that generally include sub-sampling and study of even smaller fragments using high-sensitivity Superconducting Quantum Interference Device (SQUID) magnetometers.10–12 Therefore, these paleomagnetic studies imply destructive and time-consuming sub-sampling of the original Apollo rocks by curators and
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  processors at NASA. Consequently, an exhaustive paleomagnetic study of the Apollo collection appears out of reach using the standard procedure. II. SPECIFICITIES AND INTEREST OF THE PROPOSED MEASUREMENTS
  With the aim of making an exhaustive magnetic survey of the Apollo rocks, we adopted the following strategy: perform simple magnetic measurements (Natural Remanent Magnetization, NRM) of the whole, unprocessed sample directly in the Lunar Sample Laboratory Facility (LSLF) storage facility, without any subsampling or demagnetization, thus reducing sample preparation and handling to a minimum that is acceptable for curators. Measuring large whole samples has other advantages in addition to its non-destructive quality. First, lunar rocks can be heterogeneously magnetized, especially the breccias that make up a large fraction of the Apollo collection. Indeed, different parts of a lunar breccia (matrix, clasts of various lithologies, and melt) can have strongly contrasted magnetic properties and also different paleomagnetic direction if the magnetization of the clasts has survived the assembly of the breccia. Second, the study of small sub-samples increases the apparent effect of possible remagnetization during sample return or processing. Some samples have been shown to be partially and locally remagnetized by exposure to fields up to several mT during the return flight from the Moon (e.g., Ref. 13). Others have been locally heated during cutting with band saw.14 Studying whole large samples will minimize the bulk effect of these magnetic contaminations, given that they can dominate the signal when studying small samples that may come from the area that has been heated by exposure to a strong field. The aim of our study is chiefly to perform an exhaustive survey of the NRM of Apollo rocks to identify the
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  key samples that can then be studied in details in the laboratory using standard paleomagnetic techniques. Therefore, we needed to develop a magnetometer that could measure the magnetic moment of whole unprocessed Apollo samples directly in their storage facility, while complying with NASA curatorial constraints. The main mass of Apollo samples is kept in a storage vault at the LSLF at Johnson Space Center (NASA) in Houston, USA. Samples are stored in the vault as whole rocks packed in multilayered Teflon bags (about 5–30 cm in size) filled with pure nitrogen gas to avoid oxidation and contamination. Sample mass ranges from <1 g to about 5 kg. In this study, we focused on samples above 50 g, corresponding to about 15 cm3 . The Apollo collection contains about 200 of such samples. Among them, only about 40 have been studied for paleomagnetism so far, indicating that an exhaustive survey will likely bring new valuable information.
  III. INSTRUMENTAL CONSTRAINTS
  Although modern commercial SQUID magnetometers are perfectly adapted for detailed paleomagnetic studies of lunar rocks, they can typically only accommodate samples up to about 10 cm3 (about 30 g) and are not portable, making them unsuitable for the proposed measurements. We need a magnetometer optimized for fast and efficient measurements of whole lunar rocks in the vault. The instrumental precision and accuracy are not the main constraints, since this instrument is mostly designed for the purpose of triage of samples for further more refined analyses in the laboratory. There are five technical challenges for the development of a magnetometer which is able to measure the NRM of unprocessed Apollo rocks in situ in their storage vault. The first is the limitations imposed by the curatorial constraints. As mentioned, samples must remain in their original packaging to avoid any chemical contaminations and time-consuming repackaging by NASA processors. Moreover, many mechanical components and chemical compounds (gear, cam, slider, electric motor, metal ball bearings, oils, etc.) cannot be used in the vault to avoid chemical contamination. This limitation requires that the magnetometer must use very simple mechanisms. The second constraint is the wide range of the expected magnetic moments to be measured due to the variety of sample size and nature. Depending on the lithology, the NRM is expected to vary from weak (norite, anorthosite, ⇠10 7 Am2 /kg) to relatively strong (basalt, ⇠10 5 Am2 /kg; impact melt breccias, ⇠10 4 Am2 /kg).6 Because we focus mostly on samples that range from 40 g to 4 kg in mass, the variation between the weakest and the strongest samples can be in the order of 104 , requiring a wide dynamic range. The third constraint is sensitivity, which must be good enough to allow the measurement of the NRM of most Apollo rocks with mass above 50 g. The fourth constraint is portability. To be allowed access to the lunar vault, the magnetometer should be dismountable, compact, and easy to reassemble in the vault. The fifth constraint is processing speed because hundreds of samples must be measured. Working in the vault requires the continuous presence of a NASA lunar curator and/or
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  processor and represents a heavy load in terms of personnel use. Measuring 100 samples in a week, including the initial setup and final disassembly of the magnetometer, implies that the measurement time (including sample handling) has to be 10 min/sample at most. A spinner magnetometer can satisfy all these requirements. They have been already used for the study of large samples such as whole meteorite stones15 and archaeological artifacts.16 This type of magnetometer consists of a fixed magnetic field sensor, a sample on a rotating stage, magnetic shields enclosing the sensor and the sample, and an encoder detecting the rotation angle. The sample, ideally carrying a magnetization equivalent to a single dipole, generates sinusoidal signals for the radial and the tangential components of the field at the sensor position as the rock is rotated about the vertical axis. By changing the sample's orientation at least two times, we can estimate the three components of magnetic moment. We can adjust the sensor-to-sample distance to measure samples of various sizes and achieve large dynamic ranges. Moreover, we can improve its signal-to-noise (S/N) ratio by stacking the data during multiple revolutions.17,18 In this paper, we describe a portable spinner magnetometer developed specifically to measure the magnetic moments of large unprocessed Apollo samples in the LSLF vault. Furthermore, we present pilot data processing using the result of the actual
  FIG. 1. Schematic illustrations of the magnetometer in the top view opening the top cover (a) and the side view showing the interior by a broken-out section of the shield (b). A two-layered mu-metal shield (1) enclosing a threeaxis fluxgate sensor (7) mounted on a sensor holder (8) that can slide on a rail (3), and a sample (4) on a rotating table (5). The sensor-to-sample distance is adjusted by changing the position of the pin (9) fixed on bores (10). The user can rotate the table by a handle (14) and the rotation angle is measured by an optical encoder unit (13). The power supply and the outputs of the sensor are connected to the outer signal conditioning unit via feedthrough (2) and a connector (12). The mu-metal shield and the entire system is mounted on an aluminum plate (6) supported by aluminum feet (11). The coordinate system is shown in the figures. The horizontal and vertical lines in (b) show rotation axis and the horizontal centerline of the sensor, respectively.
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  measurement of 133 Apollo samples during the first round of measurements in the LSLF, in addition to performance tests in our laboratory. IV. DESCRIPTION OF THE MAGNETOMETER
  Figure 1 shows schematic illustrations of the spinner magnetometer for the large Apollo samples. A commercial three-axis fluxgate magnetic field sensor (Mag-03MS100, Bartington Instruments, Ltd.) and a rotating sample stage are enclosed in a two-layer mu-metal magnetic shield (550 mm in diameter and 500 mm in height). The interior of the magnetometer can be accessed by opening the top lids of the mu-metal shields. To minimize stray fields, all of the holes penetrating both the inner and outer shields are arranged not to be coaxially positioned, except for the 11 mm bore for the spindle and the 4 mm hole for the feedthrough. The residual magnetic field, which is mainly the stray field resulting from small gaps between the outer and the main cylinder of the shield, is lower than 20 nT for all three components of the magnetic field, as evaluated during 10 successive opening and closing operations of the mu-metal shield. The samples are enclosed in a cube made of transparent acrylic resin (PMMA) plates welded by solvents. Cubes with different dimensions (5, 7, 10, 12, 15, 17, and 20 cm sides) were used to best fit various sample sizes and shapes. Samples are kept tight in the cubes using Teflon films and/or PMMA rings. The cubes have center marks on the surfaces that help locating the sample at the center. For samples with anisometric shapes, we recorded the shape and position in the cube for the later more refined analyses. As shown in Fig. 1(b), the center of the cube is at the intersection of the spindle of the stage and the horizontal centerline of the fluxgate sensor (hereafter call "stage center"). When using the smaller
  cubes, acrylic resin spacers are used to keep the cube center at the stage center. The distance between the stage center and the sensor (d) is adjustable according to the magnetic moment intensity and the size of the sample. The sensor holder can be fixed by an aluminum pin on the guide rail that has bores at d = 15, 16, 17.5, 20, 22.5, 25, 27.5, and 30 cm. The sensor can be moved as close as d = 5 cm from the sample center by using a PMMA extension plate. The sample stage is revolved manually using an aluminum handle directly connected to the spindle via an aluminum coupling mechanism. The target turning speed is about 1 revolution per 10 s (0.1 Hz), which is slower than other magnetic-sensor-equipped spinner magnetometers (5–7 Hz).19,20 To avoid chemical contamination, lubricant-free Teflon bearings were used. All the other metallic parts are made of aluminum, except for the mu-metal shield, which is never in contact with the PMMA cubes containing the samples. Figure 2 shows the schematic operation diagram of the magnetometer. The fluxgate sensor has output noise spectral densities of about 9 pTrms Hz 0.5 at 1 Hz for three components and the orthogonality errors are <0.1 , according to the manufacturer specifications. The rotation angle of the spindle is measured by an optical encoder connected to a digital input/output interface (NI 9403, National Instruments Corp.). The resolution of the encoder is 512 pulses-per-revolution and the maximum position error is 0.167 . The digital backend of the encoder can handle rotation speed up to several thousand rotations per minutes. Moreover, the index signal forces the decoder's counter to reset, preventing propagation of counting the error. A four-channel 24-bit A/D converter unit with ±10 V measurement range [analog-to-digital converter (ADC); NI 9239, National Instruments Corp.] samples all three channels (X-, Y-, and Z-axis) simultaneously after amplified
  !) is detected by the 3 axis fluxgate sensor FIG. 2. Schematic diagrams of the magnetometer. The magnetic field from the magnetic moment of the sample (m (FG) at the distance d connected to the signal conditioning unit (SCU3) that filters high frequency noises and amplitude at a gain of 1000. The output of the three magnetic field components (BX , BY , BZ ) is simultaneously digitized by 3 channels of a 24-bit A/D converter (NI9239). The encoded rotation angle of the sample table (✓) is decoded by a decoder IC connected to a parallel I/O unit (NI 9403) and converted in a relative angle. The zero position is where the index of the encoder exactly faces the fluxgate sensor. The resolution is 512 steps per a revolution. The A/D unit and the I/O unit are mounted to a USB chassis (NI cDAQ-9174) and connected to a PC via USB port. A LabVIEW program controls the quasi-real-time routine (RT main loop) and treats the data every 20 ms (50 Hz).
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  (Gain = 1000) and conditioned by a signal conditioning unit (SCU; SCU-3, Bartington Instruments, Ltd.). The analog and digital front end units are mounted on the universal serial bus (USB) chassis (NI cDAQ-9174, National Instruments Corp.) that can realize a synchronous operation of the mounted units. Figure 2 also shows a block diagram of the data acquisition software. The entire acquisition process is controlled by a 64-bit LabVIEW (National Instruments Corp.) program running on a laptop personal computer (PC). Since the revolution speed of the sample is variable, the synchronization between the encoder position and the sensor signals is important to measure the magnetic field distribution around the sample accurately. The program has two parallel threads working as a real-time routine. The first thread controls the sampling and simple low-pass filtering. To avoid the problem of aliasing, the ADC oversamples the signals at 50k samples per second (sps) that is 500 times faster than the cutoff frequency of the SCU's second ordered low-pass filter ( fc = 100 Hz). The digitized data stream is stored in a buffer and re-sampled at 50 sps by averaging of the buffered 1000 samples, which plays as a digital low-pass filter that removes signals above 50 Hz. This data stream is double buffered not to drop any data during unexpected heavy forward processes. The second thread records the position of the optical encoder through communications with the decoder that returns the position of the optical encoder's index mark with a 512 pulse-per-revolution resolution. The standard direction of rotation is defined as clockwise (CW). The gating of the sampling and the acquisition of the encoder position is triggered by the shared 50 Hz software trigger, realizing a synchronous measurement of the magnetic field and the sample position. The resolution of the optical encoder (512 positions/revolution) and the data acquisition frequency (50 Hz) is optimized for the target rotation speed (0.1 Hz) as it gives about 500 samples during 1 revolution in 10 s. The maximum instantaneous rotation speed that will not be affected by the SCU's low-pass filter is 1000 /s, which is five times faster than the instantaneous rotation speed in the actual measurements (see Appendix A). The data are recorded along with timestamps of 1 ms precision for the purpose of the post-acquisition filtering processes. Finally, the dataset is saved on the hard disk. V. THEORY OF OPERATION
  This magnetometer measures the magnetic fields around the rotating sample. For simplicity, we consider a dipole moment vector m = (mx , my , mz ) at the center of the sample cube. We define the sample coordinates as follows. We defined the north, east, and down surfaces of the sample cube that, respectively, correspond to x-, y-, and z-axis directions (Fig. 2). Using declination D and inclination I, this vector can be written as m = (m cos D cos I, m sin D cos I, m sin I), where m = |m|. We can observe sinusoids that are functions of the rotation angle ✓ due to the rotation of m. The Y-, X-, and Z-axis of the fluxgate sensor measure the radial, tangential, and vertically downward components of the field, respectively (Fig. 2). The observed magnetic field vector B(✓) is given by
  FIG. 3. An example of signal treatment procedure. The sample No. 12018.15, which was collected by the Apollo 12 mission, is one of the most weakly magnetized samples. The magnetic field is measured at the position 1, observed at d = 15 cm and rotated 32 revolutions. All magnetic field intensities are relative to the initial value. (a) The first step is the drift and jump correction by a high-pass filter. The original signal is converted in nT and plotted as a distribution over the absolute rotation angle (right chart). The small ripples have a wavelength of 360 corresponding to sinusoidal signals generated by revolutions of the sample. Large drifting (400 pT) and jumping (100 pT) are observable, which have been removed by the filtering (left chart). (b) The stacked data (solid dots) compiled for a single revolution (360 ) and its FFT low-pass filtered result (solid lines) after drift and jump corrections. (c) A prediction (solid black lines) with a dipole model after an inversion calculation involving data obtained at other two positions (position 2 and 3). The predicted dipole is m = 1.5 ⇥10 6 Am2 , I = 54 , and D = 162 .
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  m cos I sin (D + ✓) 8 > BX (✓) = ⇥ > > > 4⇡ d3 > > > > > 2m cos I cos(D + ✓) < BY (✓) = . (1) ⇥ > 3 > 4⇡ d > > > > m sin I 1 > > > BZ (✓) = ⇥ 3 : 4⇡ d Note that the CW rotation of the stage makes the scanning direction of the sample counter clockwise (CCW), resulting in a negative sign of the term sin(D + ✓). We defined the origin of the rotation ✓ when the sample north points toward the sensor. We can calibrate the stage north by measuring a point source placed on the north notch of the stage (Fig. 2); the position where |BX + BY | becomes maximum corresponds to the north (✓ = 0). It is important to note that the waveform of the BZ component is constant and BX and BY components are sinusoidal. Unfortunately, our magnetometer cannot measure Bz directly due to the DC offsets. Thus, we change the sample position in three different rotation axis; around z-axis (position 1), y-axis (position 2), and x-axis (position 3). The acrylic cubic sample holders have been checked for precise orthogonality to ensure the accuracy of these orthogonal rotations. This operation enables us to measure all three components of the moment m as sinusoidal signals and solves the problem of the DC offsets. For this reason, the DC component is not considered in the post-processing, and the chart always starts from 0 nT at the beginning of the measurement. The encoder angle is sampled at a fixed frequency (50 Hz) that is asynchronous to the optical encoder's movement (Fig. 2). This asynchronous sampling makes a quantization error between the actual direction ✓ and the apparent encoder angle ✓ enc (n) = n ⇥ 2⇡/N, where n is the encoder count (n = 0, 1, . . . = N 1) and N is the number of the pulses per revolution, yielding the resolution of the encoder ✓ = 2⇡/N (rad). This quantization error ✓ err = ✓ ✓ enc (n) is randomly distributed in the range 0  ✓ err < ✓, which makes a signal error given by err(✓, ✓ err ) = B(✓ + ✓ err ) B(✓). This is akin to quantization noise. The worst-case signal error is approximately given by substituting ✓ for ✓ err . For an encoder with a good resolution (e.g., N > 50), this worst-case signal error is err (✓, ✓) = ✓ ⇥ a(✓ enc (n)),
  maximum when we measure a dipole magnetic field with an amplitude A, given by B(✓) = A sin(✓) (see Appendix A). The first remedy to reduce this error is simply increasing the encoder's resolution N. The second is simultaneous acquisition of the optical encoder and the ADC to keep the same ✓ value during the measurement because this error is a sort of a phase error. The third is calculating an average during the passage between two positions, improving the worst-case error in half (⇡A/N, see Appendix A); this technique is eventually realized by the oversampling method (Fig. 2). To conclude, the current system with N = 512 has a worst-case error of ⇡/512 = 0.61% of the amplitude, which is 6 pT for a typical A = 1 nT signal. This is below the output noise density of the fluxgate sensor and far below the ambient noise (several tens of pT), indicating that it is negligible in our system. One of the advantages of the spinner magnetometer is that the signal-to-noise (S/N) ratio can be improved by postprocessing. This spinner magnetometer conducts a box-car integration (stacking) of the magnetic field signals whose reference signal is the encoder output. By filtering and stacking of the data over multiple revolutions, we can decrease the noise, which is not synchronized with the rotation of the sample, unlike the periodic signal resulting from the magnetization of the sample.17,18 We developed Python scripts using the Scipy library (www.scipy.org) that conducts three steps of post-processing. Figure 3 shows an example of the post processing using the dataset of the Apollo 12 sample (No. 12018.15) measured in the LSLF vault. The first step is removing low-frequency noise components whose frequencies are lower than that of the revolution (drifting and baseline jumping) due to temperature drifts and disturbance of the ambient field, which may be dominant in the untreated signal [left-side chart of Fig. 3(a)]. To remove this low-frequency noise, we subtract the baseline from the signal. The baseline is estimated by the application of a Savitzky–Golay filter with 1st order polynomial fitting and a 32-point window. The baseline for the first and last 32 points, where we cannot apply this filter, is estimated by a linear approximation. The right-side chart of Fig. 3(a) shows the signals after subtracting the baseline, indicating the successful removal of the targeted noises. The second step is stacking [Fig. 3(b)]. In the stacked result, we can roughly identify the sinusoidal curve buried in high-frequency noises. The third step is the low-pass filtering by a fast Fourier transformation (FFT). Since we try to explain the magnetic
  (2)
  where a(✓ enc (n)) is the slope of the signal at the nth encoder position. This worst-case error can reach 2⇡A/N at the
  TABLE I. Alternating field (AF) demagnetization result of StdBlockNo13, showing intensity, declination, and inclination obtained by the prediction given by the inversion results of the developed spinner magnetometer and the observation by the SQUID magnetometer. The intensity and angular errors (|Dir|) between the predictions and the SQUID vector moments are also shown. The angular error is in absolute values. Prediction AF (mT) 0 5 10 20 80
  Int (Am2 ) 4.90 ⇥ 10 3.50 ⇥ 10 2.30 ⇥ 10 1.30 ⇥ 10 5.00 ⇥ 10
  6 6 6 6 7
  Observation SQUID
  Dec (deg) Inc (deg) 200 200 208 238 254
  40 46 44 42 34
  Int (Am2 ) 4.69 ⇥ 10 3.44 ⇥ 10 2.15 ⇥ 10 1.29 ⇥ 10 4.70 ⇥ 10
  6 6 6 6 7
  Error
  Dec (deg) Inc (deg) 201 198 211 229 242
  37 44 42 40 31
  Int (Am2 ) 2.09 ⇥ 10 6.47 ⇥ 10 1.53 ⇥ 10 1.30 ⇥ 10 3.03 ⇥ 10
  7 8 7 8 8
  |Dir| (deg) 3.5 2.6 2.6 7.3 10.4
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  field by a single dipole source at the stage center in this paper, we do not use the high frequency components. The high-frequency components shorter than 100 wavelength, which can be originated fine-scaled magnetic structure, highfrequency noises, or non-dipole component,20 are removed by the FFT filter after smoothing by a weak Savitzky–Golay filter with 1st order polynomial fitting and a 11-point window. The solid line in Fig. 3(b) is the waveform after this FFT filtering. Since the stacked waveform is averaged over multiple periods, it is enough continuous at both ends to carry
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  out FFT. These stacked and filtered waveforms are used for the inversion to predict the dipole source parameters. This last step consists in a standard least-squares inverse approach to find the best-fitting set of the 3 unknown parameters: dipole moment intensity, inclination, and declination. The dipole is assumed to be centered. Indeed, our results show that 90% of the samples show magnetic field measurements "coherent" (i.e., less than 20% of error between predictions and observations) with a dipolar source located at the center of the sample, though the rest 10% of the samples contain quadrupole or higher harmonics probably due to the very anisotropic shape or heterogeneous composition like lunar impact breccia. The off-centered dipole may also be the source of non-dipole character.20 During the measurements, the LabVIEW program displays the raw data after stacking with error bars (±standard deviation) as a plot versus rotation angle ✓. Note that we visualize a result of stacking without filtering to reduce the central processing unit (CPU) load and keep the real-time routine. The program also shows the estimated sinusoidal curve and the noise level, which are calculated by FFT results of the observed signal. The noise will reduce with stacking inversely with the square root of the number of revolutions. The user can stop the rotations of the sample when the quality value cannot improve any further by adding revolutions. The nominal revolution time is about 8 turns (1.5 min) and thus the noise is reduced by 65% (=8 0.5 ) theoretically.17 The user can also check the skewness of the sinusoidal curve, which can originate from the shape effect or inhomogeneity of NRM and increase the sensor-to-sample distance to reduce those multipole components.
  VI. PERFORMANCE OF THE MAGNETOMETER
  FIG. 4. A demonstration of the magnetometer performances using a standard sample that is a small (0.98 g) fragment of a basalt rock. (a) The standard sample is demagnetized by an alternating field (AF) up to 80 mT and measured by the developed spinner magnetometer (solid square symbols, after prediction using 3 positions) and the SQUID magnetometer (open circle symbols). The error of the prediction is also shown in percent of the SQUID results. Our data predict the actual dipole moment measured by SQUID magnetometer in 0.8%–7.0% of overestimation. (b) The observations after data treatments at position 1 (dots) and the predictions of a dipole model using three positions (black lines) at 80 mT AF demagnetization step. The predicted dipole moment is m = 5.0 ⇥10 7 Am2 , I = 34 , and D = 254 .
  Table I and Fig. 4(a) show the result of a demagnetization experiment at the CEREGE laboratory (Aix-en-Provence, France) using the large sample spinner magnetometer and a commercially available SQUID magnetometer with an inline alternating field (AF) demagnetizer (2G Enterprise, model 760R). A small terrestrial basalt fragment (0.98 g), which can be considered as a quasi-dipole source, is enclosed in a 1 in. cubic plastic capsule. The sample is measured with the spinner magnetometer using a three-position scheme (i.e., rotation around x-, y-, and z-axis), and then, it is also measured with the SQUID magnetometer and demagnetized by the AF. We continue this sequence up to 80 mT AF demagnetization field to check for the effect of variable magnetic moment intensity. In view of the high precision of the SQUID magnetometer (2 ⇥ 10 11 Am2 ),21 and its cross calibration with other magnetometers in our laboratory (including a JR5 spinner magnetometer from AGICO, Inc.), we consider that the moment intensity measured with this instrument is close enough to the actual magnetic moment intensity for the intensity range in this study (10 6 Am2 ). The predicted intensities of the dipole moment using our spinner magnetometer are in close agreement with the actual dipole moments, though there is some overestimation between 0.8% and 7.0% [Fig. 4(a)]. Since the amount of the overestimation is not a function of the
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  intensity of the magnetic moment, it seems that this error is not due to the noise but other factors such as positioning error when we replace the samples at each step. It is notable that the error in the direction is also small (from 3 to 10 , Table I). The cubic shape of the sample capsule can constrain the tilt (inclination) of the sample but let freely rotate horizontally (declination) during the repeated placing of the sample. This may explain the larger error in declination (from 1 to +12 ) than in inclination (from +2 to +3 ). We estimated the repeatability error of this instrument by five repeated measurements of this basalt sample. Due to our operational schedule, we conducted this experiment within a magnetically shielded room of the CEREGE laboratory but without the mu-metal shield of the instrument. This configuration increases the background field and noise by a factor of ten. The sample was saturated in a 1 T magnetic field generated by a pulse magnetizer (model MMPM-9, Magnetic Measurements, Ltd.). The standard deviation for the five measurements is 3% of the average magnetization of the sample (Table II). The semi-angle of aperture of the 95% confidence cone (↵95 )22 is 1.7 , which gives one angular standard deviation (±1 ) of 2.2 . These results indicate a satisfactory repeatability of this instrument. We also conducted a series of measurements at four different sensor-to-sample distances. The result shows similar variability as for the repeatability test (Table III). This indicates that the error due to the different distance is within the error due to the repeatability. Overall, the intensity and directions provided by the instrument are precise within 3% and 2 , respectively, and likely better than that when using the mu-metal shielding. Figure 4(b) shows the example of the severe S/N ratio condition of sample demagnetized by 80 mT AF. The peakto-peak noise at CEREGE experiment is 250 pTp-p and that at NASA [Fig. 3(b)] is 203 pTp-p , that is, 20% weaker than in CEREGE. Carefully observing the result at LSLF, there is no spike noise such as the one visible in the result at CEREGE. This low noise environment at the LSLF vault is due to the fact that the vault itself is equivalent to a closed stainlesssteel capsule which acts as a good electromagnetic shield. As demonstrated by a previous study, it is hard to recover the signal buried in a strong noise. Using these background TABLE II. A result of repeated measure of sample StdBlockNo13, showing intensity, declination, and inclination obtained by the prediction given by the inversion results. The mean value, the standard deviation, the semi-angle of aperture of the 95% confidence cone (↵95 ), and the angular standard deviation (✓ 65 ) of the magnetic moment vectors are also shown. No. run
  Int (Am2 )
  1 2 3 4 5
  8.95 ⇥ 10 9.37 ⇥ 10 9.61 ⇥ 10 9.53 ⇥ 10 9.39 ⇥ 10
  5
  Mean Standard deviation ↵95 ✓ 65
  9.37 ⇥ 10 2.56 ⇥ 10  
  5
  5 5 5 5
  6
  Dec (deg)
  Inc (deg)
  2.5 0.1 1.4 0.6 0.7
  1.5 2.3 0 2.1 3.8
  1.1  1.8 2.1
  1.9 
  TABLE III. Result of the measurement of sample StdBlockNo13 at different sensor-to-sample distance, showing intensity, declination, and inclination obtained by the prediction given by the inversion results. The mean value, the standard deviation, the semi-angle of aperture of the 95% confidence cone (↵95 ), and the angular standard deviation (✓ 65 ) of the magnetic moment vectors are also shown. Distance (mm)
  Int (Am2 )
  80 100 130 160
  9.47 ⇥ 10 9.29 ⇥ 10 9.54 ⇥ 10 1.01 ⇥ 10
  5
  Mean Standard deviation ↵95 ✓ 65
  9.59 ⇥ 10 3.36 ⇥ 10  
  5
  5 5 4
  6
  Dec (deg)
  Inc (deg)
  0.5 0.1 1.1 1.8
  91.3 91.8 91.6 94.0
  0.5  1.7 2.2
  92.4 
  noise data, we try to estimate the worst S/N ratio for which we can still recover the signal. The S/N ratio is defined as (root mean square amplitude of signal)/(standard deviation of noise). We can assume that the forward model using Eq. (1) and the estimates by the SQUID measurement can be the actual signal without noise. The noise can be estimated by the difference between this forward model and the observed signal after stacking. Because the S/N ratio for BX is simply half that of BY [Eq. (1)], we consider only BY now. The amplitude of BY given by the SQUID measurement is 16.7 pTrms (47.7 pTp-p ) and the standard deviation of the noise is 33.5 pTrms , giving S/N = 0.50. We can also calculate the S/N ratio for the Apollo 12 sample (No. 12018.15) in the same manner but using the predicted dipole moment as a signal. The Apollo sample [Fig. 3(b)] shows the signal amplitude of BY = 36.8 pTrms (104.5 pTp-p ) and the noise of 28.4 pTrms , giving S/N = 1.30 that is better S/N ratio than at the CEREGE laboratory. This is because (1) the difference in the intensity of the magnetic moment and (2) the background at NASA vault is about 15% quieter than at CEREGE. Therefore, we can estimate that the demagnetization experiment at CEREGE [Fig. 4(b)] was performed in worse conditions than the operations that took place at LSLF and that this test demonstrates that our magnetometer can recover the signal from, at least, the condition S/N = 0.5. The detection limit for the magnetic moment can be defined by the point where the observed signal (in root mean square amplitude) becomes equal to the standard deviation of noise. Figure 5 shows the estimation of the detection limit for BY at different noise floors at S/N = 1. Since our magnetometer can adjust the sample to sensor distance d, the sensitivity for the magnetic moment m and the detection limit is a function of the distance and the noise floor. Because our magnetometer can recover the signal from S/N = 0.5 condition and the noise at NASA is 30 pTrms , we can measure the magnetic moment above 15 pT noise-floor line in Fig. 5. This figure also plots typical magnetic moment of three major moon rock types at a given weight, according to the previous study of the natural remanent magnetization (NRM) of Apollo samples measured by SQUID.6 At d = 20 cm, we can measure most of breccia rocks down to 10 g, whereas small (several
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  FIG. 5. Detection limits of the magnetometer for different sensor noise floors (15 pT, 30 pT, 60 pT, and 90 pT). The solid lines indicate where the peakto-peak intensity of the sinusoidal signal by a rotation of a dipole moment m observed at distance d becomes equal to the given noise intensity. Note that the inclination of the dipole is horizontal making the largest amplitudes. The magnetic moment at the hatched region is undetectable due to the weak signal below noise floor of the sensor. The calculated intensities of natural remanent magnetizations (NRMs) of different masses and types of moon rocks are also shown. The NRMs of the moon rocks are given by a previous study.6 The samples measured in this study are also shown (a = 12 018.15 at NASA, b = StdBlockNo13 at CEREGE; measurements data shown in Figs. 3 and 4, respectively).
  tens of grams) basalt rocks having slightly weaker NRM need to approach at d = 15 cm. Even when the background noise increases by a factor of 6 (90 pT line in Fig. 5), we can safely measure those types of rocks that have relatively strong NRMs, if the sample is heavier than about 50 g. Norite and anorthosite rocks, which are generally very weakly magnetized, need a sensor-to-sample distance of about d = 10 cm to measure >100 g samples and even down to d = 5 cm for samples below 100 g. With these detection limits, we could actually measure almost every breccia and basalt sample in the Apollo collection, except those that are stored in steel containers. In Eq. (1), we consider only the sinusoidal output produced by a homogeneously magnetized spherical sample that generates a dipole field.19 However, assuming that the sample holder is completely filled by a sample and homogeneously magnetized, such a cubic sample does not generate a dipole magnetic field. To evaluate this shape effect, we calculate the signal from a homogeneously magnetized cubic sample based on the calculation by Helbig23 in addition to a dipole source (see Appendix B). Figure 6(a) shows the half-cycle of the calculated signals expressed as linkage coefficients equivalent to B/m, showing M-shaped waveforms. The distance (r) is equal to the length of the edge of the cube (a). The magnetic signal is reduced at the angles where the peaks of the dipole field are located (✓ = 0 for the radial component and ✓ = 90 for the tangential component), and the amount of error becomes maximum at these angles (Fig. 7). Figure 7 shows the error of the
  FIG. 6. (a) The half-cycle of the calculated signals according to Helbig,23 expressed as linkage coefficient for the radial and tangential components. The distance (r) equals to the length of the edge of the cube (a). The sinusoidal curves indicate the signal from the dipole source and the M-shaped deformed curve is the signal from a cubic homogeneously magnetized sample. The difference between the signal from the cube and the dipole source becomes maximum at the peak of the sinusoidal curve (0 for the radial component and 90 for the tangential component). (b) The maximum error normalized by the amplitude of the dipole field (normalized error) as a function of the distance normalized by the length of the edge (r/a). A fitting curve is also shown. The inset of (b) shows the geometry of the samples and the sensor.
  signal normalized by the amplitude of the dipole field at different distances, and Fig. 6(b) shows the plots of the errors at ✓ = 0 (90 ) of the radial (tangential) components as functions of the normalized distance (r/a). The error reduced rapidly by increasing the distance by a factor of (r/a) 3.9 . The normalized error becomes acceptable (3.7%) at r/a = 1.5, ignorable (1%) at r/a = 2.1, and negligible (0.26%) at r/a = 3. Thus, as
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  be explained by a dipole field. Detailed analyses of the harmonics will help us to reveal the origin of the heterogeneous magnetizations.17–20
  VII. CONCLUSION
  In order to measure the remanent magnetization of large bulk samples, we have developed a spinner magnetometer equipped with a three-axis flux gate sensor and a large sample table enclosed within a two-layer mu-metal magnetic shield resulting in a residual field of about 20 nT. The adjustable sensor position (5–30 cm) enables the measurement of small (5 cm cube) to large (20 cm cube) samples with acceptable deformation of the sinusoidal signals. By means of the stacking technique of the signal, the experiments demonstrate that this instruments can measure weak (17 pTrms ) sinusoidal signals for S/N = 0.5. This performance indicates that the magnetometer can measure magnetic moments of about 5 ⇥ 10 7 Am2 at the standard sample to sensor distance d = 15 cm. This detection limit corresponds to the NRM of about 10 g of lunar basalt or breccia. Because we focused on the samples that range from 22 g to 4.7 kg in mass, this magnetometer can cover theoretically all of the basalt and breccia samples that we are interested in. We have already conducted a first visit to NASA and measured 133 samples in 4 working days, demonstrating an optimized mechanism and workflow of this magnetometer. In this study, we used the simplest magnetization model (single dipole source at the stage center). However, due to the possible anisometric shape and/or off-center positioning in the cubes and/or lithological heterogeneities, the actual sample may have off-centered and/or multiple dipole(s) that cannot be explained by this simple magnetization model. In the future studies, we will customize the model for the individual samples by integrating other information (e.g., shape and lithology) to explain the magnetic field distribution around such heterogeneously magnetized samples. This spinner magnetometer is also able to measure other large and precious samples, e.g., whole meteorites and archeological artifacts, without destructive sampling. FIG. 7. The errors due to the shape effect of a cubic-shaped sample at different normalized sensor-to-sample distances (r/a). The ranges of the rotation angle are limited to quarter cycles from the peak position of the dipole field [0 for the radial component (a); 90 for the tangential component (b)]. The values are normalized by the amplitude of the dipole field. The curves for r/a = 1 are reduced to 0.3 of the original curves.
  a rule of thumb, a distance farther than r/a 1.5 is recommended to reduce the shape effect. In the actual measurement of the Apollo samples, 62% of the samples were measured at distance farther than r/a = 1.5% and 93% of them were measured with r/a 1.25, based on the size of the cubic sample holder. Since the sample is always smaller than the holder, the actual r/a ratio is better than the value computed from the holder size. Therefore, we estimate that the deformation of the signal due to the shape effect is small in our study. In fact, as mentioned in Sec. V, most of the measured signals can
  ACKNOWLEDGMENTS
  This project was made possible by a seed funding from the Programme National de Planétologie (INSU-CNES) and subsequent funding by the Agence Nationale de la Recherche (Grant No. ANR-14-CE33-0012). We are greatly indebted to the lunar sample curators and processors (Ryan Zeigler, Andrea B. Mosie, Darvon Collins, and Anthony Ferrell) at NASA Johnson Space Center for their time, patience, and understanding. J.G. acknowledges funding from People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (No. FP7/2077-2013) under REA Grant Agreement No. 298355. E.A.L. would like to thank NASA Grant Nos. NNA14AB01A and NNX15AL62G and NSF Grant No. DMS-1521765 for partial support. M.U. wishes to thank Ateliers Mécaniques de Précision in Eguilles for their fine mechanical processing.
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  APPENDIX A: ERRORS DUE TO SAMPLING
  Equation (1) indicates that the radial and tangential component of a dipole moment can be observed as a sinusoidal curve given by B(✓) = A sin(✓). When we use an encoder with a resolution of N positions per revolution, the encoder resolution is ✓ = 2⇡/N and the apparent encoder angle is ✓ enc (n) = n ⇥ ✓ = 2⇡n/N. According to Eq. (2) the worst-case signal error (quantization error) for the observation of B(✓) at n-th encoder position becomes err (✓, ✓) = B (✓ + ✓ err ) B (✓) = ✓ ⇥ a(✓ enc (n)) d (A sin (✓)) (n ✓) = ✓A cos (n ✓). = ✓⇥ d✓ (A1) The absolute value of this signal error becomes maximum of A ⇥ ✓ = 2⇡A/N when |cos(n ✓)| = 1. Using the averaging technique, we average the signal between nth and (n + 1)-th encoder position to represent the magnetic field when ✓ is in the range of ✓ enc (n)  ✓ < ✓ enc (n + 1). The stacking technique increases the number of measurements to be averaged. The averaged signal at this ✓ is given by ⌅ (n+1) ✓ 1 B̄ (✓) = A sin (✓) d✓. (A2) ✓ n ✓ For a large enough number N, we can use sin( ✓/2) ⇠ ✓/2 and to approximate this integration, A {cos ((n + 1) ✓) + cos (n ✓)} ✓ ! ! ! 2A 2n + 1 ✓ ✓ = sin ✓ sin ⇡ A sin n ✓ + . ✓ 2 2 2
  B̄ (✓) =
  (A3)
  Therefore, the averaged signal for this ✓ can be approximated to B(✓ + ✓/2). This equation indicates that the averaging technique also improves the quantization error in the angular position by the convergence of the averaged signal towards B(✓ + ✓/2). The worst-case signal error for Eq. (A3) is given when ✓ is at ✓ enc (n) or just before ✓ enc (n + 1). The worst-case error for ✓ enc (n) is given by ! ✓ B̄ (✓) B (✓ enc (n)) = A sin n ✓ + A sin (n ✓) 2 ! ! ✓ ✓ = 2A sin cos n ✓ + 4 4 ! ✓ 1 ⇡ A cos (n + ) ✓ . (A4) 2 4 The worst-case signal error is, therefore, approximately half of the no-averaging case given by Eq. (A1). A similar error can occur due to the second ordered lowpass filter with a cutoff frequency of 100 Hz built-in in the signal conditioning unit when the rotation speed is too fast. We suppose that the error of the low-pass filter can be acceptable (86.5% of the final value) after 2⌧ s, where ⌧ is the time constant of this filter (about 2.5 ms). The sample rotates 2⌧v when the rotation speed is given by v /s when the error diminished to an acceptable amplitude. Thus, if the resolution of the encoder ( ✓ = 360 /N) is smaller than 2⌧v, the effect of the
  low-pass filter is not observable. Such critical rotation speed vc = ✓/2⌧ = 360/(2N⌧) = 141 /s. According to our measurement of instantaneous rotation speed in the actual measurement, we rotated the sample generally slower than vc . However, for short periods, the rotation speed sometimes reaches up to 2vc . This makes a similar effect from the quantization error discussed above, resulting in an error of [1 exp( 2)] ⇥ err[✓, ✓ ⇥ floor(2⌧v/ ✓)] = 0.135 ⇥ ✓ ⇥ floor(2⌧v/ ✓) using Eq. (A1). The function floor(x) returns the integer part of x. This error is 0.135 ⇥ 2 ✓ when v = 2vc . Thus, the estimated error due to the low-pass filter is about 27% of the quantization error, which can be ignored. This error becomes comparable to the quantization error when v becomes 7.4 vc = 1044 /s, which is equivalent to 2.9 Hz sample rotation frequency, for our combination of low-pass filter (⌧ = 2.5 ms) and encoder (N = 512). This is five times faster than the actual rotation speed. Therefore, the low-pass filter with a cutoff frequency of 100 Hz used in our system does not modify the waveform of the signal from the sample. APPENDIX B: MAGNETIC FIELD AROUND CUBIC SAMPLES
  We calculated the magnetic field around homogeneously magnetized isotropic (spherical) and cubic samples using the linkage tensor between a homogeneously magnetized body and the magnetic field given by Helbig.23 The linkage tensor can be regarded as a normalized, dimensionless magnetic field intensity. We assumed a magnetization moment directed to +x and located at the origin of a three dimensional Cartesian coordinate system. We considered the distribution of the magnetic field in the x-y plane. According to Helbig,23 at the position (x, y, z), the distance from the dipole (u, v, w) = (x 0, y 0, z 0) and the linkage tensors for the dipole field generated by an isotropic body are given by d gxx =
  2u2
  and d gxy =
  v2 e5
  w2
  3uv , e5
  (B1)
  where the superscript d indicated the dipole, the subscript xx and xy, respectively, indicates the contribution of the +x directed magnetization to the x and y components of the magnetic field, and e2 = u2 + v2 + w2 . The three-fold integration of Eq. (B1) yields the linkage tensors for a cubic sample, which has been already given by Eq. (4) in Helbig. Some calculated values in the first quadrant have been given in Table I of Helbig.23 However, the equations of Helbig do not reproduce the calculated values; they also cannot be applied to our calculation directly due to some problems. We used the equation modified after Eq. (4) in Helbig,23 ⇥⇥⇥ u ⇤ a/2 ⇤ y b/2 ⇤ z c/2 v*w ) xx+a/2 gxx = arcsin ( p p y+b/2 z+c/2 2 2 2 2 |u| u +v * u +w and
  gxy =
  ⇥⇥⇥
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  where a, b, and c are the length of the sides parallel to the x-, y-, and z-axis, respectively. The added term u/|u| in gxx gives the sign of u to extend the function to other quadrants. Note that (u, v, w) = (x, y, z) for the moment placed at the origin. The absolute value of w in gxy , which can be found in the original equation, is a typographical error, since the equation replaced |w| with w successfully reproduces the calculation results given in Table I of Helbig.23 Finally, the magnetic fields can be expressed in polar coordinates by gxr (r, ✓) = gxx (x, y, z) ⇥ cos(✓) + gxy (x, y, z) ⇥ cos(✓) and gxt (r, ✓) = gxy (x, y, z) ⇥ cos(✓) gxx (x, y, z) ⇥ sin(✓), which, respectively, indicates the radial and tangential contributions at the position (x, y, z) = [r ⇥ cos(✓), r ⇥ sin(✓), 0]. The calculation has been conducted with Maxima (http://maxima.sourceforge.net). 1 M. E. Purucker and J. B. Nicholas, J. Geophys. Res.: Planets 115(E12), 007,
  doi:10.1029/2010je003650 (2010). Tsunakawa, H. Shibuya, F. Takahashi, H. Shimizu, M. Matsushima, A. Matsuoka, S. Nakazawa, H. Otake, and Y. Iijima, Space Sci. Rev. 154, 219 (2010). 3 M. Fuller and S. M. Cisowski, Geomagnetism 2, 307 (1987). 4 B. P. Weiss and S. M. Tikoo, Science 346, 1246753 (2014). 5 I. Garrick-Bethell, B. P. Weiss, D. L. Shuster, and J. Buz, Science 323, 356 (2009). 6 C. Cournède, J. Gattacceca, and P. Rochette, Earth Planet. Sci. Lett. 331-332, 31 (2012). 7 E. K. Shea, B. P. Weiss, W. S. Cassata, D. L. Shuster, S. M. Tikoo, J. Gattacceca, T. L. Grove, and M. D. Fuller, Science 335, 453 (2012). 2 H.
  8 S.
  M. Tikoo, B. P. Weiss, W. S. Cassata, D. L. Shuster, J. Gattacceca, E. A. Lima, C. Suavet, F. Nimmo, and M. D. Fuller, Earth Planet. Sci. Lett. 404, 89 (2014). 9 J. Buz, B. P. Weiss, S. M. Tikoo, D. L. Shuster, J. Gattacceca, and T. L. Grove, J. Geophys. Res.: Planets 120, 1720, doi:10.1002/2015je004878 (2015). 10 W. S. Goree and M. Fuller, Rev. Geophys. 14, 591, doi:10.1029/rg014i004p 00591 (1976). 11 J. L. Kirschvink, R. E. Kopp, T. D. Raub, C. T. Baumgartner, and J. W. Holt, Geochem., Geophys., Geosyst. 9, Q05Y01 (2008). 12 T. A. T. Mullender, T. Frederichs, C. Hilgenfeldt, L. V. de Groot, K. Fabian, and M. J. Dekkers, Geochem., Geophys., Geosyst. 17, 3546, doi:10.1002/2016gc006436 (2016). 13 G. Pearce, W. Gose, and D. Strangway, in Proceedings of the Lunar Science Conference (supplement 4, Geochimica et Cosmochimica Acta) (Pergamon, 1973), Vol. 3, p. 3045. 14 H. Wang and B. P. Weiss, in AGU Meeting Abstract #167372, 2016. 15 M. Funaki, M. Koshita, and H. Nagai, Antarct. Meteorite Res. 16, 220 (2003), available at http://polaris.nipr.ac.jp/⇠library/publication/AMR. html. 16 E. Thellier, in Methods in Palaeomagnetism, edited by D. W. Collinson, K. M. Creer, and S. K. Runcorn (Elsevier, Amsterdam, 1967). 17 L. Molyneux, Geophys. J. Int. 24, 429 (1971). 18 M. Kono, Y. Hamano, T. Nishitani, and T. Tosha, Geophys. J. Int. 67, 217 (1981). 19 D. W. Collinson, Rev. Geophys. 13, 659, doi:10.1029/rg013i005p00659 (1975). 20 K. Kodama, Geochem., Geophys., Geosyst. 18, 434, doi:10.1002/2016gc 006615 (2017). 21 M. Uehara, J. Gattacceca, P. Rochette, F. Demory, and E. M. Valenzuela, Phys. Earth Planet. Inter. 200-201, 113 (2012). 22 R. Fisher, Proc. R. Soc. A 217, 295 (1953). 23 K. Helbig, J. Geomagn. Geoelectr. 17, 373 (1965).
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