# A reappraisal of the metamorphic history of EH3 and EL3 enstatite chondrites

Eric Quirico, Michèle Bourot-Denise, C. Robin, Gilles Montagnac, Pierre Beck

► To cite this version:

Eric Quirico, Michèle Bourot-Denise, C. Robin, Gilles Montagnac, Pierre Beck. A reappraisal of the metamorphic history of EH3 and EL3 enstatite chondrites. Meteoritics and Planetary Science, 2011, 46 Supplement (S1), pp.A195. 10.1111/j.1945-5100.2011.01221.x. insu-00683399

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Abstracts

5112

RAMAN, FTIR, AND MÖSSBAUER SPECTROSCOPY OF OLIVINES FROM THE D'ORBIGNY METEORITE

Y. A. Abdu¹, M. E. Varela² and F. C. Hawthorne¹. ¹Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada. E-mail: abdu@cc.umanitoba.ca. ²ICATE-CONICET, Av. España 1512 Sur, CP J5402DSP, San Juan, Argentina.

**Introduction:** Olivine is a major basaltic mineral in high-temperature volcanic rocks and in stony meteorites. The D'Orbigny angrite contains large olivine crystals (up to ~1 cm in size) that are present throughout the rock, mimicking xenocrysts or forming olivinite (polycrystalline olivine) [1-3]. The olivinites are of particular interest, as very little is known about them. Here, we study three types of D'Orbigny olivines: honey olivine (HOI), green olivine (GOI), and olivinite (POI), by Raman, Fourier-transform infrared (FTIR), and Mössbauer spectroscopy, and compare their structural and spectral features.

**Experimental:** Raman spectroscopy measurements were done using a LabRAM ARAMIS confocal microscope (Horiba Jobin Yvon). A x100 objective microscope was used to focus the laser beam (532 nm excitation line) on the sample to a size of ~1 µm. FTIR spectra over the range (4000-400 cm⁻¹) were collected on KBr sample pellets using a Bruker Tensor 27 FTIR spectrometer. Transmission Mössbauer spectra were acquired at room temperature (RT) using a ⁵⁷Co(Rh) point source.

**Results and Discussion:** The chemical compositions of the three olivines were determined by electron microprobe analysis (EMPA). The Mg# [= Mg/(Mg + Fe)] is 0.64 for HOI, 0.85 for GOI, and 0.90 for POI.

Raman and FTIR spectroscopy.

The main feature of Raman spectra of olivines is a doublet with peaks located at ~850 cm⁻¹ (κ₁) and 820 cm⁻¹ (κ₂) due to the asymmetric and symmetric vibrations, respectively, of the SiO₄ group [4]. For D'Orbigny olivines, the κ₁ and κ₂ peaks occur at 850, 820 cm⁻¹ for HOI; 854, 823 cm⁻¹ for GOI; 856, 824 cm⁻¹ for POI, reflecting variability in composition. Using the ω (= κ₁-κ₂) - Mg# relation [3], the estimated Mg# for HOI, GOI, and POI are 0.70(4), 0.82(4), and 0.93(4), respectively, in close agreement with EMPA results. Similarly, the FTIR spectra of the three olivines do not show any differences except the composition dependence of the bands positions. Furthermore, no structural OH bands are observed in the OH-stretching region (3800-3000 cm⁻¹) for all samples.

Mössbauer spectroscopy.

The RT Mössbauer spectra of the HOI and GOI consist of two asymmetric absorption peaks, resulted from the overlap of two quadrupole doublets due to the presence of Fe²⁺ at the M1 and M2 octahedral sites. On the other hand, the spectrum of POI shows an additional weak doublet (CS = 0.35 mm s⁻¹, QS = 0.30 mm s⁻¹, relative area = 2%) which is attributed to Fe³⁺. Because Fe³⁺ could indicate a laihunite-like material similar to that observed in partially oxidized metasomatized mantle olivines [5], olivinites may record the increasing oxidizing conditions prevailing during the late metasomatic event that affect angrite formation.

**References:** [1] Kurat G. et al. 2001. Abstract #1753. 32nd Lunar and Planetary Science Conference. [2] Kurat G. et al. 2004. *Geochimica et Cosmochimica Acta* 68:1901-1921. [3] Varela M. E. et al. 2005. *Meteoritics & Planetary Science* 40:409-430. [4] Mouri T. and Enami M. 2008. *Journal of Mineralogical and Petrological Sciences* 103:100-104. [5] Banfield J. F. et al. 1992. *American Mineralogist* 77:977-986.

5444

A PRELIMINARY STUDY OF MAGMATIC VOLATILES IN ANGRITES

F. A. J. Abernethy¹, A. Verchovsky¹, I. A. Franchi¹ and M. M. Grady¹,². ¹PSSRI, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK. ²Department Mineralogy, The Natural History Museum, London SW7 5BD, UK. E-mail: F.A.J.Abernethy@open.ac.uk.

**Introduction:** Angrites are a rare group of achondrites notable for their critical silica undersaturation, lack of shock metamorphic features and ancient crystallization ages [1-3]. They are subdivided into plutonic or basaltic angrites on the basis of their inferred petrogenesis. Similar studies to those already carried out on HEDs and shergottites [4, 5] could be used to shed light on magmatic processes on the angrite parent body (APB). Here we present results from carbon and nitrogen stepped combustion experiments on the plutonic angrites Angra dos Reis (AdoR) and NWA 2999, and the basaltic angrite D'Orbigny.

**Results:** Magmatic volatiles are released once the silicate matrix starts to become plastic, usually between 800 and 1000 °C; depending on the CRE age of the specimen, it is possible for this to be mixed with a spallogenic component, the effects of which can be removed following correction. We have not yet applied this correction to the data but expect it to have little effect on either carbon or nitrogen abundance, and to reduce isotopic composition by <2‰. We also discount NWA 2999 from further consideration, because it contained large amounts of terrestrial organic carbon which swamped all other potential carbon-bearing components.

<table><thead><tr><th>Name</th><th>Total C<br>(ppm)</th><th>δ<sup>13</sup>C<br>(‰)</th><th>Magmatic C<br>(ppm)</th><th>δ<sup>13</sup>C<br>(‰)</th></tr></thead><tbody><tr><td>AdoR</td><td>801</td><td>-21.6</td><td>63</td><td>-21.4</td></tr><tr><td>NWA 2999</td><td>6990</td><td>-29.5</td><td>-</td><td>-</td></tr><tr><td>D'Orbigny</td><td>199</td><td>-8.8</td><td>9</td><td>-11.4</td></tr><tr><td>Name</td><td>Total N<br>(ppm)</td><td>δ<sup>15</sup>N<br>(‰)</td><td>Magmatic N<br>(ppm)</td><td>δ<sup>15</sup>N<br>(‰)</td></tr><tr><td>AdoR</td><td>15</td><td>+8</td><td>3</td><td>-2</td></tr><tr><td>NWA 2999</td><td>84</td><td>-2</td><td>-</td><td>-</td></tr><tr><td>D'Orbigny</td><td>12</td><td>+8</td><td>1</td><td>+14</td></tr></tbody></table>

**Discussion:** The overall ¹³C-enrichment of D'Orbigny compared to AdoR is partly the result of carbonates (δ¹³C approximately 0 to +5‰) lining vugs in the meteorite. There are also differences in magmatic carbon and nitrogen between the two meteorites. AdoR has a higher abundance of the component, with lighter isotopic composition. This is consistent with the petrogeneses of the meteorites, with D'Orbigny having outgassed during its ascent: McCoy et al. [6] calculated that the vesicles present in D'Orbigny could have formed from as little as approximately 12.5 ppm CO₂ dissolved in the magma. Stepped combustion analyses of further angrites should reveal whether there is a relationship between sample petrogenesis and magmatic volatiles.

**References:** [1] Mittlefehldt D. W. et al. 1998. In *Planetary materials*, Reviews in Mineralogy 36, Chapter 4. Mineralogical Society of America. [2] Mittlefehldt D. W. and Lindstrom M. M. 1990. *Geochimica et Cosmochimica Acta* 54:3209-3218. [3] Mittlefehldt D. W. et al. 2002. *Meteoritics & Planetary Science* 37:345-369. [4] Grady M. M. et al. 1997. *Meteoritics & Planetary Science* 32:863-868. [5] Grady M. M. et al. 2004. *International Journal of Astrobiology* 3:117-124. [6] McCoy T. J. et al. 2006. *Earth and Planetary Science Letters* 246:102-108.
PETROGRAPHIC EVIDENCE OF SHOCK METAMORPHISM IN
CR2 CHONDRITE GRO 03116

N. M. Abreu. Earth Science, Pennsylvania State University—DuBois, PA, USA. E-mail: abreu@psu.edu.

**Introduction:** CR chondrites are primitive meteorites that show a broad range of aqueous alteration features from nearly pristine [1] to fully altered [2]. However, evidence of any thermal metamorphism group is only beginning to emerge [3, 4]. To better understand the effects and the source of metamorphism in the CR parent body, an SEM/EPMA/TEM study of GRO 03116 is underway. GRO 03116 was chosen because O-isotopic analyses suggest that it has only suffered limited aqueous alteration [5], while Raman spectra shows that its organic materials have undergone some thermal metamorphism [4]. GRO 03116 is an Antarctic CR2 find of terrestrial weathering type C. Thus, disentangling its record of asteroidal versus terrestrial aqueous alteration merits careful analysis petrologic analysis.

**Results and Discussion:** GRO 03116 contains porphyritic type I and fewer type II chondrules, abundant opaques, a heterogeneous matrix and CAIs. These components are well defined texturally. Fine-grained layered assemblages and ferryhydrite veins, characteristic of terrestrial aqueous alteration [6], are also present. Chondrule mesostasis is often dendritic. Chondrule forsterites show no systematic enrichment in fayalite content at their edges. However, some chondrules exhibit complex chemical zoning. Opaques range from nearly intact kamacite grains with variable amounts of Ni and Co and terrestrial ferrihydrite haloes to terrestrial alteration assemblages. Matrix is significantly less abundant (approximately 21 vol% – this study, 24.3 vol% – [5]) than in other CR chondrites (> 30 vol% [7]) and elastic in texture. Matrix is rich in opaques, particularly magnetite and sulfides. In addition, some matrix regions appear very compact and have a foliation fabric. Some chondrules show signs of plastic deformation along the direction of matrix foliation. These chondrules display substantial textural integration with the surrounding metal and matrix. In addition, porous melt veins (approximately 200 µm across) containing angular olivine fragments and abundant, rounded, micron-sized magnetite and pentlandite grains have been identified.

**Conclusion:** GRO 03116 has undergone pervasive terrestrial weathering that overprinted its original record. However, preservation of some kamacite suggests limited preterrestrial aqueous alteration. TEM studies will be conducted to better establish the extent of alteration. The following petrologic observations also indicate limited thermal metamorphism, certainly lower than in CR GRA 06100 [3]: minor integration of chondrules and matrix, preservation of some mesostasis, low Fe content in type I chondrules, Coheterogeneity in metal, and presence of magnetite. Thus, preterrestrial alteration of GRO 03116 was probably dominated by shock metamorphism, as indicated by its deformed chondrules, comminuted matrix, and melt veins [8].

**Acknowledgments:** Funded by NNX11AH10G and AAS grants to NMA and conducted at the MRI—Penn State.

**References:** [1] Abreu N. M. and Brearley A. J. 2010. *Geochimica et Cosmochimica Acta* 74:1146–1171. [2] Weisberg M. K. and Huber H. 2001. *Meteoritics & Planetary Science* 42:1495–1503. [3] Abreu N. M. and Stanek G. L. 2009. Abstract #2393. 40th Lunar and Planetary Science Conference. [4] Briani et al. 2010. *Meteoritics & Planetary Science* 45:5234. [5] Schrader D. L. et al. 2011. *Geochimica et Cosmochimica Acta* 75:308–325. [6] Zolensky M. E. and Gooding J. L. 1986. [7] Weisberg M. K. et al. 1993. *Geochimica et Cosmochimica Acta* 57:1567–1586. [8] Sharp T. G. and DeCarli. 2006. In *MESS II*. pp. 653–677.

BAJADA DEL DIABLO, PATAGONIA, ARGENTINA: THE IMPACT
OF A SPLIT COMET?

R. D. Acevedo, C. Prezzi, M. J. Orgeira, J. Rabassa, H. Corbella, J. F. Ponce, O. Martinez, C. Vasquez, I. Subias, M. Gonzalez and M. Rocca. Conicet, Argentina. E-mail: acevedo@cadic-conicet.gob.ar.

An impact origin has been proposed for the almost 200 circular structures found in Bajada del Diablo (42° 46' to 43° S and 67° 24' to 45' W), Patagonia, Argentina [1], some of them partially obliterated by erosion or sediment accumulation, which range in diameter from 100 m up to 400 m.

The circular structures have been identified in two rock types: the Quñelaf eruptive complex (Miocene basalts) and Pampa Sastre Formation (Pliocene basaltic conglomerate).

Geological and geomorphological strong evidences of involvement on both geologic entities more over the Pleistocene pediment gravels and sands has been interpreted as the cause of the impact but lack either pieces of the impacting body like meteorites or definitive signs of shock metamorphism. Only samples of breccias randomly spread were collected. *In neither case*, however, was detected there any evidence of coesite. Most important diagnostic elements collected so far are tiny Fe-Ni lawrencite-bearing microspheres of 300 microns in average diameter, presumably originated by the impact, buried under post-impact sedimentary coverage.

To strengthen the hypothesis of impact through the geophysical survey magnetometric record of magnetic anomalies were performed. The magnetic anomalies show a circular pattern with magnetic lows in the circular structure's floors. Furthermore in the circular structure's rims, high-amplitude, conspicuous and localized (short wavelength) anomalies are observed. Such large amplitude and short wavelength anomalies are not detected out of the circular structures. For all used frequencies, the electromagnetic profiles show lower apparent electrical conductivities in the circular structure's floor, while the rims present notably higher values. Curvature attributes, analytic signal, horizontal gradient and Euler solutions were calculated for the magnetic data. 2.5 D magnetic models were developed across each one of the studied circular structures. Our results suggest that in the circular structure's floors up to 12 m of Pampa Sastre conglomerate would have been removed. On the contrary, the circular structure's rims exhibit high-amplitude, localized magnetic anomalies and higher apparent electrical conductivities, which would be related to the anomalous accumulation of basalt boulders and blocks remanently magnetized. The fact that such high-amplitude anomalies are not present out of the surveyed circular structures supports this hypothesis. The geomorphological, geological and geophysical features of the studied circular structures can only be explained by means of an extra-terrestrial projectile impact.

Even Fe-Ni microspheres with lawrencite and breccias have been found inside craters, nevertheless, the absence of direct evidences of a disintegrated asteroid opens the possibility to consider the event of a split comet with an ice nucleus hitting the patagonian surface in Pleistocene times.

The authors acknowledge the financial support by Conicet and DGPCyT-ME Provincia del Chubut (Argentinean Agencies) and National Geographic/Waitt and The Planetary Society.

**Reference:** [1] Acevedo R. D. et al. 2009. *Geomorphology* 110:58–67.
METEORITE IMPACT CRATERS IN SOUTH AMERICA: A BRIEF
REVIEW

R. D. Acevedo, M. Rocca, J. Rabassa and J. F. Ponce. Conicet,
Argentina. E-mail: acevedo@cadic-conicet.gob.ar.

**Introduction:** The first enumeration of impact craters sites in South America is presented here. Proximately twenty proven, suspected and disproven structures have been identified by several sources in this continent until now. But everyone events proposed here aren't really produced by impacts at all. About some of them reasonable doubts exist. Brazil leading the record containing almost half detected and following it in the list Argentina. In Bolivia, Peru, Chile, and Colombia only a few were observed. The rest of countries are awaiting for new discoveries.

The following possible and confirmed meteorite impact structures
have been reported for this continent, country by country:

ARGENTINA. Campo del Cielo (S 27°30', W 61°42'). Impact crater strewn field. At list twenty craters with an age of about 4000 years over sandy-clay sediments of Quaternary-Recent age. The impactor was an iron-nickel Apollo-type asteroid (meteorite type IA) and plenty of meteorite specimens survived the impact. Bajada del Diablo (S 42°45', W 67°30'). A very remarkable site of a new very large meteorite impact craters field. Almost 200 structures were identified there. Age is estimated between 0.13 and 0.78 Ma. Other possible craters are Rio Cuarto (S 32°52', W 64°14'), Islas Malvinas (S 51°00', W 62°00'), Salar del Hombre Muerto (S 25°12', W 66°55'), Antofalla (S 26°15', W 68°00'), La Dulce (S 38°14', W 59°12'), and General San Martin (S 38°00', W 63° 18').

BOLIVIA. Iturralde (S 12°35', W 67°38'). On Quaternary alluvial deposits, 8 km in diameter. Llica (S 19°49', W 68°19') 2.8 x 2.5 km, without specific geological information.

BRASIL. Araguainha Dome (S 16°46', W 52°59'). This is so far the largest well stated impact crater in South America. It is a 40-km diameter crater in Paleozoic sediments of the Paraná Basin. Serra da Cangalha (S 8°05', W 46°51'). Total diameter of multiple rings has been estimated at 12 km; in Paleozoic sediments in the Parnaiba Basin. Vergeao (S 26°50', W 52°10'). It is a 12.4 kilometer-diameter circular depression located on Cretaceous basalts and Jurassic/Triassic sandstones of the Sao Bento Group of Paraná Basin. Vista Alegre (S 25°57', W 52°41'). It is a 9.5 km-wide circular structure in the Parana State, and it is located on the Cretaceous basalts of the Serra Geral formation. Riachao (S 7°42', W 46°38'). 4.0 km diameter in sedimentary rocks of the Paranaiba Basin (sandstones from the Pedra de Fogo Formation). 200 Ma. Other possible craters are Gilbues (S 10°10', W 45°14'), Sao Miguel do Tapuio (S 5°38', W 41°24'), Cerro Jarau (S 30°12', W 56°33'), Inajah (S 8°40', W 51°00'), Piratininga (S 22°28', W 49°09'), and Colonia (S 23°52', W 46°42').

COLOMBIA. Rio Vichada (N 4°30', W 69°15'). Has a diameter of 50 km and it is the largest possible impact structure ever reported in the continental South America. Rocks exposed there include Precambrian meta-sedimentary and granitoid bodies with an extensive sedimentary Tertiary cover.

CHILE. Monturaqui (S 23°56', W 68°17') is emplaced in Jurassic granite rocks, overlain by a thin Tertiary-Quaternary ignimbrite sheet. The impacting asteroid was metallic: an iron-nickel object; 1 Ma.

PERU. Carancas (S 16°40', W 69°02'). H4-5 type ordinary chondrite fall on September 15, 2007 digging a crater of 15 m.

NWA 6588: NEW ANOMALOUS LL6 CHONDRITE

C. B. Agee. Institute of Meteoritics, University of New Mexico, Albuquerque, NM, USA. E-mail: agee@unm.edu.

**Introduction:** NWA 6588 was found in Morocco in 2008, with TKW of 2730 g. A freshly broken surface reveals a dark gray, polycrystalline rock, with numerous cleavage faces, macroscopic evidence of relict chondrules is obscure, overall appearance resembles a fine grained peridotite hand sample. It shows moderate desert weathering with some iron oxide staining. Currently, NWA 6588 is the only meteorite with the classification LL6-an.

**NWA 6588:** This new meteorite consists of 65% olivine: (Fa33.3 ± 0.2 Fe/Mn = 68 ± 3), 25% low-Ca pyroxene: (Fs27.0 ± 0.2 Wo2.2 ± 0.1 Fe/Mn = 40 ± 2), 5% albitic feldspar: (Or<sub>6.6</sub>Ab<sub>84.1</sub>An<sub>9.3</sub>), 3% sulfides: pentlandite (Fe = 17.6 Ni = 22.8 Co = 2.1 Si = 2.0 S = 55.3, atom%), stoichiometric pyrite (FeS<sub>2</sub>), 1% chromite (Fe# = 0.90, Cr# = 0.87), <1% Fe-oxide, <1% phosphate consistent with the whitlockite group (Fe# = 0.27, Na<sub>2</sub>O = 2.4 wt%). Silicate grain size is variable (5–300 µm) and relict chondrules are present. Albitic feldspar grain sizes reach up to 150 µm.

**Discussion:** This is an equilibrated, oxidized, ordinary chondrite with very uniform olivine, pyroxene, and plagioclase compositions. Fayalite (Fa), ferrosilite (Fs) content and Fe/Mn of olivines are extremely high, Fa and Fs are slightly above, or at the upper limits, recommended by [1] for LL chondrites. Although relict chondrules ranging from 500–2000 µm are observable in BSE, olivines and pyroxenes within chondrule domains are chemically indistinguishable from matrix olivines and pyroxenes. This indicates that chemical equilibrium has been reached while textural equilibrium in this meteorite was not completed. Finely disseminated pyrite and pentlandite (<1 µm) are found throughout NWA 6588, also common are larger pockets (>100 µm) of pyrite and pentlandite in grain boundary contact with each other, often occupying silicate-bounded triple junctions, suggesting exsolution from monosulfide phase. Pyrite shows fine lamellae. Fe-Ni metal is absent in NWA 6588, however minor amount of low-Ni, Fe-oxide is present and this is presumably the product of desert weathering which has replaced the original minor amounts of primary metal. A few large (>200 µm) irregular shaped phosphates with inclusions and intergrowths were observed.

**Metamorphic Temperatures:** The chromite/olivine geothermometer [2] gives a metamorphic equilibration temperature of approximately 740 °C, which is about 50 °C above the average for LL6. Therefore, peak metamorphic temperatures for the NWA 6588 parent body were presumably higher than 740 °C and perhaps very near the silicate solidus. This introduces the question of why there are no known "LL-achondrites" in existence (not LL-impact melt, of which a few examples exist). It seems remarkable that the LL chondrite parent body(s) reached peak temperatures just below the silicate solidus, but never exceeded it, and never produced partial melts or magmas that would be recognizable as primitive achondrites with oxygen isotope values consistent with being derived from an LL chondrite precursor.

**References:** [1] Grossman J. and Rubin A. 2006. Meteoritical Society Nomenclature Committee White Paper. [2] Wlotzka F. 2005. *Meteoritics & Planetary Science* 40:1673–1702.
## NWA 6601: INSIGHT INTO THE ORIGIN OF “PURE” IRON METAL IN EUCRITES

C. B. Agee. Institute of Meteoritics, University of New Mexico. E-mail: agee@unm.edu.

**Introduction:** NWA 6601 is an equilibrated, brecciated, main series eucrite that is permeated with shock melt and possesses nearly pure Fe-metal grains. Here we explore the origin of “pure” Fe-metal in this new meteorite and implications for processes on eucrite parent body.

**NWA 6601:** This new eucrite consists of ~60% total pyroxene, low-Ca pyroxene (Fs61.2 ± 0.9 Wo3.7 ± 1.0 Fe/Mn = 29 ± 1), high-Ca pyroxene (Fs26.6 ± 1.0 Wo44.5 ± 0.8 Fe/Mn = 30 ± 2), and ~30% plagioclase (Or₀.₅Ab₁₀.₁An₈₉.₄), with ubiquitous silica polymorph, Cr-spinel, ilmenite, troilite, and Fe-metal. Pyroxene shows exsolution lamellae and planar parting, fracturing, and microfaultings. Plagioclase often occurs as laths, some transitioning into impact melt pools or veins. Pyroxene and plagioclase grains are up to 500 microns in size and can appear texturally equilibrated, though some areas resemble fine grained cataclastite. Numerous impact melt veins and pools are present; some larger than 500 microns across, containing abundant silica grains, troilite crystallites and rafted basaltic clasts.

**Fe-metal in NWA 6601:** Fe-metal grains are conspicuous in NWA 6601 because of their relatively large size, uneven distribution, making up much less than 1% by volume. The Fe-metal occurs as distinct, equant or irregular clast-like grains, some up to 1 mm, found almost exclusively in cataclastic domains. The largest metal grain observed in our sample was bounded on all sides by plagioclase, though most are in multiphase contact. Troilite is commonly found nearby though rarely sharing a grain boundary with metal. None of the large metal grains occur in the impact melt veins or in the equilibrated domains of pyroxene and plagioclase. The metal is nearly pure iron, with Ni, Si, Mo below EMPA detection limits, low level of Co is detectable (~0.04 wt%).

**Formation of “Pure” Fe-metal in Eucrites:** Low-Ni metal (<1wt% Ni) is not uncommon in eucrites [e.g., 1, 2], however its origin and formation may not be uniform in Vesta. In contrast to NWA 6601, Camel Donga possesses “pure” Fe-metal that is very fine grained (5–20 µm), abundant (~2 wt%), and finely dispersed in silicate crystals, and hypothesized to have formed by impact induced reduction of pyroxene facilitated through S₂ volatile loss by breakdown of troilite [3]. We also see evidence for impact related formation of “pure” iron metal in NWA 6601. However troilite consumption and S₂-loss does not seem to be consistent with the ubiquity of this phase in the impact melt veins and pools which are swamped with troilite crystallites, while troilite is nearly absent in texturally equilibrated domains. In NWA 6601, the large, “pure” metal grains and most of the associated troilite give the appearance of being intruded into the texturally equilibrated domain as part of the impact process, not as in situ melt or reduction products— simply because of the paucity of feedstock troilite in equilibrated domains. Future work to determine the trace concentration and distribution of Ni, Co, Mo, and other siderophile elements may help shed more light on the origin of “pure” Fe-metal in NWA 6601 and impact processes on Vesta.

**References:** [1] Seddiki A. et al. 2007. Abstract #1049. 38th Lunar and Planetary Science Conference. [2] Wittmann et al. 2011. Abstract #1984. 42nd Lunar and Planetary Science Conference. [3] Palme H. et al. 1988. *Meteoritics* 23:49–57.

## ZIRCONIUM-HAFNIUM EVIDENCE FOR SEPARATE SYNTHESIS OF LIGHT NEUTRON-RICH NUCLEI

W. M. Akram¹, M. Schönbächler¹, P. Sprung², N. Vogel². ¹SEAES, The University of Manchester, M13 9PL. E-mail: waheed.akram@postgrad.manchester.ac.uk. ²Institute for Geochemistry and Petrology, ETH Zürich, Switzerland.

**Introduction:** Hafnium has four stable non-radiogenic isotopes (¹⁷⁷,¹⁷⁸,¹⁷⁹,¹⁸⁰Hf) that are produced by an s-/r-process combination. These isotopes have recently been shown to be homogeneously distributed in various carbonaceous chondrites, CAIs and the Earth [1] and this is in line with the observed s-/r-process homogeneity of Sm and Nd isotopes. In contrast, nucleosynthetic anomalies have been reported for lighter (s-/r-process) isotopes in bulk carbonaceous chondrites and their components (Ba, Ni, Cr, and Ti). This was interpreted as evidence for the synthesis of neutron-rich isotopes (via the r-process) in two distinct astrophysical sites for the light (Z ≤ 56) and heavy (Z > 56) nuclei, respectively [1]. Zirconium (Z = 40) and Hf (Z = 72) are both of similar refractory and geochemical nature, and therefore coincident isotope measurements of the two elements provide a good way of studying variable s-/r-process nucleosynthetic sites for light and heavy nuclei. Here we thus report new Zr high precision data for the samples previously analyzed for Hf isotopes.

**Analytical Technique and Results:** In this study, a terrestrial andesite (AGV-2), three carbonaceous chondrites (Murchison [CM2], Dar al Gani 137 [CO3], Dar al Gani 275 [CK4/5]) and four CAIs from Allende (CV3) were analyzed. Sample preparation and digestion is described in [1] and the Zr fractions obtained from [1] were passed through an additional anion exchange column to ensure adequate removal of Zr from the sample matrix. All five Zr isotopes (⁹⁰, ⁹¹, ⁹², ⁹⁴, ⁹⁶Zr) were measured on a Nu Plasma MC-ICPMS using the same analytical procedure as outlined in [3, 4].

Both Murchison and the CAIs display ⁹⁶Zr excesses, which are consistent with the values that we reported previously [3]. Furthermore, the new CAI data show identical ⁹⁶Zr enrichments (ε⁹⁶Zr = 2.0 ± 0.2) to those previously measured [2,3], suggesting that most CAIs formed from a reservoir with a homogeneous Zr composition, which is clearly distinct to that of the terrestrial standard. This is analogous to the relatively constant positive shifts observed in CAIs for the neutron-rich nuclide ⁵⁰Ti (ε⁵⁰Ti ~ +9) [5]. As ⁵⁰Ti is synthesized in supernovae, this link potentially indicates a common astrophysical origin for the two isotopes.

In conclusion, our data shows well-resolved ⁹⁶Zr excesses, in good agreement with previous data, but no variation for r-process dominated Hf isotopes (data obtained from the same sample digestion). Since both elements are produced through a combination of the s- and r-process, this new data further substantiate the decoupling of nucleosynthetic components. This is consistent with at least two distinct stellar sources for the production of neutron-rich isotopes that operated in different mass regimes.

**References:** [1] Sprung P. et al. 2010. *Earth and Planetary Science Letters* 295:1–11. [2] Schönbächler M. et al. 2003. *Earth and Planetary Science Letters* 216:467–481. [3] Akram. M W. et al. 2011. Abstract #1908. 42nd Lunar and Planetary Science Conference. [4] Schönbächler M. et al. 2004. *The Analyst* 129:32–37. [5] Leya I. et al. *The Astrophysical Journal* 702:1118–1126.
## H, C AND N ISOTOPE SYSTEMATICS OF CI, CM AND CR CHONDRITES: CLUES TO THE ORIGIN OF WATER

C. M. O'D. Alexander¹, R. Bowden², M. Fogel² and K. Howard³. ¹DTM, Carnegie Institution of Washington, Washington, DC 20015, USA. E-mail: alexander@dtm.ciw.edu. ²GL, Carnegie Institution of Washington, Washington, DC 20015, USA. ³IARC, The Natural History Museum, London SW7 5BD, UK.

**Introduction:** One explanation for the O isotopic composition of inner solar system bodies is that there was a massive influx of ¹⁶O-poor water ice from the outer solar system. This ice should have been very D-rich, like comets, and would have been accompanied by abundant organic matter. The O isotopic compositions of secondary minerals in chondrites suggest that the altering fluids were ¹⁶O-poor. Thus, one might expect these fluids to have been D-rich and, if alteration occurred in a closed system, the water content to be correlated to that of the organics, particularly the insoluble organic matter (IOM), and the bulk O isotopes. Previously, we showed in a suite of CMs that H contents and isotopic compositions correlated with petrologic indicators of alteration [1]. However, the H is not D-rich. Also, the O isotopic compositions of the CMs do not correlate with H content. We have since extended this bulk H (and C and N) work to additional CMs, as well as Orgueil and CRs.

**Methods:** The meteorites were crushed to <106 µm, and aliquots stored in a desiccator for days to weeks prior to analysis. The H, C and N abundances and isotopes were determined as described in [2].

**Results:** The bulk H results for the new CMs are consistent with our earlier results, although Bells (δD≈300‰), like Essebi, is clearly an outlier. Orgueil (δD≈80‰) is isotopically heavier than CMs, particularly CM1s, but is still much lighter than cometary water. The CRs generally have bulk H isotopic compositions that are heavy (δD≈580-760‰) and exhibit a limited range of H contents (~0.4-0.6 wt%). The one exception is the CR1 GRO 95577 (1.3 wt% and 270‰). There is no apparent correlation between bulk H contents and O isotopes.

**Discussion:** The bulk H measurements are a mix of components – primarily organics and hydrated silicates. The IOM is the most abundant known organic component. Subtraction of the D-rich IOM H from the bulks significantly reduces the δD values of the residual H. If it is assumed that the range of IOM δD values is due to isotopic exchange and the most D-rich IOM composition is subtracted, the residual δD values are reduced still further (CI -180‰, CMs -170 to -370‰, CRs 130-550‰, Bells -40 to 120‰, Essebi 70‰). The range within CMs and CRs may at least partially reflect isotopic fractionation associated with Fe-oxidation and/or the presence of an as yet unidentified H-bearing phase [1,2]. The H contents of CMs or CRs do not correlate with IOM contents or bulk O isotopes, suggesting that alteration did not occur in closed systems. Redistribution of water within the parent bodies could also produce some H isotopic fractionation.

**Conclusions:** There is little evidence for comet-like water in the CI s and CMs. The CRs are isotopically heavier, but this could be largely due to the oxidation of Fe. At present, it seems that the water accreted by the CI, CM and CR chondrites formed or re-equilibrated in the inner solar system. This may have significant implications for the transport and the isotopic evolution of materials in the nebula.

**References:** [1] C. M. O'D. Alexander et al. 2011. Abstract #1869. 42nd Lunar and Planetary Science Conference. [2] C. M. O'D. Alexander et al. 2010. *Geochimica et Cosmochimica Acta* 74:4417.

## A STUDY OF THE MINERALOGY AND TEXTURES OF BASALT FINES FROM APOLLO 12 REGOLITH SAMPLE 12023-, 155

L. Alexander¹,², J. F. Snape²,³, I. A. Crawford¹,², K. H. Joy³,⁴,⁵ and R. Burgess⁶. ¹Birkbeck College, London, UK. E-mail: l.alexander@bbk.ac.uk. ²CPS at UCL-Birkbeck, London. ³Department of Earth Sciences, UCL, London, UK. ⁴CLSE, The Lunar and Planetary Institute, USRA, Houston, TX 77058, USA. ⁵The NASA Lunar Science Institute. ⁶SEAES, University of Manchester, UK.

**Introduction:** The Apollo 12 (A12) mission landed in the Eastern region of Oceanus Procellarum (OP). Crater size-frequency distribution measurements [1] indicate that some of the youngest lava flows on the Moon occur within OP and it is possible that young basalt fragments may have been incorporated into the regolith at the A12 landing site. This study aims to search for such fragments by examining basaltic fines from the A12 landing site and determining their composition. Samples will be dated using Ar-Ar dating. Here we present initial petrographic results for 12023-, 155, consisting of 12 fines ~2 mm in diameter.

**Methods:** Each individual grain was split in two with the larger split retained for petrographic analysis and the smaller split allocated for radiometric dating. Textures were examined using a JEOL JXA-8100 electron microscope with an Oxford Instrument INCA energy dispersive system (EDS) to produce backscattered electron (BSE) images and elemental maps. Modal mineralogies were obtained from BSE images and elemental maps using imaging software to identify the phases. Bulk compositions were calculated using multiple raster beam analyses (RBA) across the samples that were corrected for differences in phase densities in accordance with [2]. These methods have been previously tested on known samples [3] and found to be in good agreement.

**Results:** Initial results indicate that these fines are generally typical of A12 low-Ti basalts [4], composed of pyroxene (48-60%), plagioclase (30-45%) and ilmenite (3-4%), with varying amounts of olivine (0-12%) and accessory silica and Cr-spinel. Traces of Fe-Ni metal and troilite occur rarely (<1%). Textures are varied but are mainly holocrystalline and subophitic. Highly zoned pyroxenes are the major mafic phase, which often enclose anhedral olivines. Plagioclase varies from fine subhedral laths to larger blocky textures, occasionally intrafasciculate, up to 1 mm in length. Ilmenite commonly occurs as fine laths of varying lengths (50-600 µm) and also as irregular grains in some samples, while spinels are small (<100 µm) and zoned. Silica is present as randomly distributed, anhedral crystals. We have also identified a coarse-grained pyroxene cumulate in which ilmenite is absent.

**Discussion:** Most basalt fines have modal mineralogies consistent with pigeonite or feldspathic A12 basalt classifications [4], but exceptions are noted, which may require contributions from separate lava flows. Detailed mineral compositions (ongoing) will complete the chemical analysis of these samples. Results can then be evaluated together with Ar-Ar dating to understand the petrogenesis and source regions of basaltic grains collected at the A12 site.

**References:** [1] Hiesinger H. et al. 2003. *Journal of Geophysical Research* 108, E7. [2] Warren P. H. 1997. Abstract #1497. 28th Lunar and Planetary Science Conference. [3] Snape J. F. et al. 2011. Abstract #2011. 42nd Lunar and Planetary Science Conference. [4] Papike J. J. et al. 1998. Lunar samples. Reviews in Mineralogy, vol. 36, pp 5-1-5-234.
5440

## MAGNESIUM ISOTOPES IN THE ULTRAREFRACTORY CAIS EFREMOVKA 101.1: EVIDENCE OF OPEN SYSTEM BEHAVIOR

J. Aléon¹, J. Marin-Carbonne², K. D. McKeegan² and A. El Goresy³.
¹CSNSM, CNRS/IN2P3/University of Paris Sud, Bat 104, 91405 Orsay Campus, France. ²Earth and Space Science department, UCLA, Los Angeles, CA, USA. ³Bayerisches Geoinstitut, Bayreuth, Germany. E-mail: Jerome.Aleon@csnsm.in2p3.fr.

Efremovka 101.1 is an unusual compact type A calcium-aluminum-
rich inclusion (CAIs) with several lithological subunits that may once
have been individual free-floating CAIs in the early solar nebula [1]. All
subunits share an ultrarefractory rare earth elements abundance pattern
and an ultrarefractory fassaitic clinopyroxene enriched in Sc and Zr is
locally observed [1]. E101.1 is surrounded by a well preserved Wark-
Lovering rim (WLR) and a partially preserved olivine-rich accretionary
rim (AR) In order to understand the formation of this enigmatic object
and the implications for isotopic fluctuations in the nebular gas we
started an exhaustive isotopic study of E101 using the UCLA IMS1270
ion microprobe combining O [2], Mg (this work) and Si isotopes [3].
Eighty five high precision Mg isotope analyses, spatially associated with
O isotope analyses whenever possible, were obtained using a
approximately 15 nA, approximately 30 µm O⁻ beam and simultaneous
detection of ²⁴Mg, ²⁵Mg, ²⁶Mg and ²⁷Mg on four Faraday cups.

Mg isotopic analyses of E101.1 reveal a highly complex distribution of both Mg isotopic mass fractionation and ²⁶Mg excesses due to ²⁶Al decay. As a whole, different regions in the CAIs have different Mg isotope systematics. This includes the rims, the different lithological subunits of possible xenolithic origin, as well as different domains defined using O isotopes [2]. δ²³Mg values range from -6.5‰ (melilite in subinclusion 1) to +6.7 ‰ (spinel clusters in the host) with values in the rims clustering around 0‰. Initial ²⁶Al/²⁷Al ratios in subcomponents range from supracanonical (7.1 × 10⁻⁵ in the Sc-Zr-fs) down to 1.8 × 10⁻⁵ in disturbed melilite from the host. Olivine in the AR is enriched in ²⁶Mg by approximately 0.16‰. A more complete description of the isotope systematics will be presented at the meeting. To summarize, Mg isotopes suggest open-system behavior of Mg with evaporation as well as incorporation of light Mg possibly from the digestion of xenolithic inclusions or by direct condensation from nebular gas. The lightest components, sinuous fragments and possibly subinclusion 1, can probably be considered as xenoliths. E101.1 likely had an initial canonical ²⁶Al/²⁷Al ratio but secondary disturbances resulted in both higher and lower than canonical ratios in specific components. Mg isotopes in the WLR are in agreement with previous studies suggesting a younger age although some spinels having a larger ²⁶Mg excess may be derived from the interior. Comparison with O isotopes will also be presented at the meeting.

**References:** [1] El Goresy et al. 2002. *Geochimica et Cosmochimica Acta* 66:1459–1491. [2] Aléon et al. 2010. *Meteoritics & Planetary Science* 45:A7. [3] Marin-Carbonne et al. 2011. This conference.

5244

## 3-D IMAGING OF MINERAL INCLUSIONS IN EXTRA-TERRESTRIAL CHROMITE USING SYNCHROTRON RADIATION X-RAY TOMOGRAPHIC MICROSCOPY

C. Alwmark¹,², B. Schmitz¹, S. Holm¹,², F. Marone³, M. Stampanoni³,⁴. ¹Department of Geology, University of Lund, Sölveg. 12, SE-22362 Lund, Sweden, E-mail: carl.alwmark@geol.lu.se. ²Institute for Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, CH-8092 Zürich. ³Swiss Light Source, Paul Scherrer Institute, 5232 Villigen, Switzerland ⁴Institute for Biomedical Engineering, University and ETH Zürich, CH-8092 Zürich.

**Introduction:** In ordinary chondrites, chromite is the only common mineral that survives long-term exposure on Earth. However, a recent study [1] showed that chromite grains from ordinary chondrites contain small inclusions of the matrix minerals of the meteorite and that information about the silicate matrix of the original meteorite can be derived from these inclusions. Thus, the inclusions are an important tool in classification of fossil extraterrestrial chromite used for characterizing the past influx of material to Earth (e.g., [2]), but have, due to their small nature, previously been difficult to locate. The main aim of this study is to, with the help of synchrotron radiation X-ray tomographic microscopy (SRXTM), establish a fast and nondestructive protocol in which chromite grains can be scanned and inclusions located. SXRTM will also allow quantitative and morphological studies of both host chromite grains and inclusions in three dimensions.

**Material and Methods:** 385 chromite grains from eight equilibrated (type 4-6) ordinary chondrites of different groups (H, L, LL) were searched for inclusions. The grains were stacked in capillaries and then analyzed using SRXTM at the TOMCAT beamline of the Swiss Light Source at the Paul Scherrer Institute [3]. An automatic sample exchanger integrated at the beamline [4] enabled unattended measurements over the course of 26 h. The tomographic reconstructions were performed using a highly optimized routine based on a Fourier transform method and a gridding procedure [5].

**Results and Discussion:** Analysis of the reconstructed data reveals that inclusions are readily distinguished down to a resolution of <1 µm and that almost 2/3 of all grains contained one or more inclusions. The number of inclusions within each chromite grain varies from 1 – > 300. The average number of inclusions in samples of petrographic type 4 is about twice as high (33 inclusions per grain) compared to types 5 and 6 (15 inclusions per grain, respectively). The size of the inclusions range from <1 to 2.4 × 10⁵ µm³. A comparison of the average size of the largest inclusion from each chromite grain shows that petrographic type 6 chromite grains have an average inclusion size >20 times larger than that of grains from type 4 and 5.

**Conclusion:** The method applied in this study to identify inclusions in chromite grains, using SRXTM, allows for large amounts of grains to be analyzed in a short time with high spatial and contrast resolution. The method is nondestructive and does not affect the properties of the material analyzed, i.e., grains of interest can be recollected and further studied. Furthermore, the size and frequency distribution of inclusions can be used to assess the petrographic type of the precursor meteorite.

**References:** [1] Alwmark C. and Schmitz B. 2009. *Geochimica et Cosmochimica Acta* 73:1472–1486. [2] Schmitz B. et al. 2003. *Science* 300:961–964. [3] Stampanoni M. et al. 2006. *Proceedings of SPIE* 6381. [4] Mader K. et al. 2011. *Journal of Synchrotron Radiation* 18:117–124. [5] Marone F. et al. 2010. *Proceedings of SPIE* 7804.
5077

## SEARCH FOR Q IN SARATOV (L4)

Sachiko Amari¹ and Jun-ichi Matsuda². ¹Laboratory for Space Sciences and the Physics Department, Washington University, St. Louis, MO 63130-4899, USA. E-mail: sa@wuphys.wustl.edu. ²Department of Earth and Space Science, Graduate School of Science, Osaka University, Osaka 560-0043, Japan.

**Introduction:** Q, for quintessence, carries most of heavy noble gases in primitive meteorites [1]. It is most likely carbonaceous matter [2], however the exact nature remains elusive for over three decades. On the other hand, Q-gases in meteorites from various compositional types have been extensively studied [3, 4]. Since Q is less susceptible to thermal metamorphism than presolar diamond [3], meteorites with a higher metamorphic grade (≥3.7) contain Q but not diamond. For this reason, we chose Sararov (L4) to study Q and its noble gases.

**Experimental:** We started from ~7.2 g of Saratov. First it was alternately treated HF-HCL and HCL to remove silicates. Then sulfur was extracted with $CS_2$. Noble gases in bulk Saratov and the HF-Hcl residue AC were analyzed by step-wise heating, and were already reported [5]. The Ne data points of AC lie on a mixing line between spallogenic Ne and Ne-Q in Saratov on a Ne 3-isotope plot. AC was subjected to colloidal separation, yielding the colloidal fraction AE, and the non-colloidal fraction AF. AE is black, while AF is dark brown, suggesting that essentially all carbonaceous matter in AC was concentrated in AE (Fig. 1). AE was further subjected to two successive colloidal separations, yielding the colloidal fractions AG and AI, and the non-colloidal fraction AJ. Since AE is most likely devoid of spallogenic Ne, we will be able to precisely determine Ne-Q in Saratov from the daughter fractions of AE. If we assume all Xe in AC is concentrated in AE, from the mass balance, the $^{132}$Xe concentration of AE is calculated to be $20,100 \times 10^{-10} \text{ cm}^3 \text{ STP/g}$. If further separations are successful, the Xe concentrations of one or two of the daughter fractions (AG, AI, and AJ) can be even higher. We will report noble gas data on these colloidal and non-colloidal fractions.

**References:** [1] Lewis R. S. et al. 1975. *Science* 190:1251-1262. [2] Ott U. et al. 1981. *Geochimica et Cosmochimica Acta* 45:1751-1788. [3] Huss G. R. et al. 1996. *Geochimica et Cosmochimica Acta* 60:3311-3340. [4] Busemann H. et al. 2000. *Meteoritics & Planetary Science* 35:949-973. [5] Matsuda J. et al. 2010. *Meteoritics & Planetary Science* 45:361-372.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Saratov fractions AE (left) and AF (right).</figcaption></figure>

5196

## LEAD ISOTOPIC AGE OF THE QUENCHED ANGRITE NORTHWEST AFRICA 1296

Y. Amelin¹ and A. J. Irving². ¹Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia. E-mail: yuri.amelin@anu.edu.au. ²Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA.

**Introduction:** Northwest Africa 1296 [1] is a quenched angrite with similar composition to the quenched angrites D'Orbigny and Sahara 99555 that were dated with the Pb-Pb method at 4564.42 ± 0.12 Ma [2] and between 4564.58-4564.86 Ma [3, 4], respectively (dates calculated with $^{238}$U/$^{235}$U = 137.88). Here we describe the Pb-isotopic systematics of the angrite NWA 1296. Determination of U-Pb ratios is in progress.

**Techniques:** NWA 1296 is a very fine-grained rock, therefore separation of minerals by hand picking, as was done for other coarser-grained angrites, was impractical. Instead, we focused on separation of minerals on the basis of solubility in acids. Various size fractions of crushed sample were gently leached in 0.5M $HNO_3$ or 0.3M HBr (first leachates) in order to extract phosphate minerals, then leached in hot concentrated $HNO_3$ and HCl (second leachates), dissolved and analyzed following [5].

**Results:** Residues after acid leaching of NWA 1296 whole rock contain 9-17 ppb Pb—lower than in D'Orbigny (18-30 ppb) and Sahara 99555 (27-34 ppb). The measured $^{206}$Pb/$^{204}$Pb ratios are between 280 and 1150 (330-4990 after blank subtraction). The six most radiogenic analyses with blank-corrected $^{206}$Pb/$^{204}$Pb > 1000 yield consistent $^{207}$Pb/$^{206}$Pb dates with a weighted average of 4564.20 ± 0.45 Ma, MSWD = 0.89 ($^{238}$U/$^{235}$U = 137.88). These analyses yield uniform $^{232}$Th/$^{238}$U model ratios of 8.87 ± 0.05 (2 SD). Less radiogenic residue analyses are more dispersed, probably due to the presence of two nonradiogenic Pb components.

The first and second leachates contained 200-600 ppb and 23-75 ppb Pb, respectively. Lead in the leachates (with $^{206}$Pb/$^{204}$Pb up to 174 in the first leachates and up to 725 in the second leachates) is sufficiently radiogenic to attempt age calculation. The $^{232}$Th/$^{238}$U model ratios in the leachates are uniform (3.78 ± 0.04 in the first leachates, 3.70 ± 0.15 in the second leachates) and distinct from the value in the residues, suggesting that most U in the rock is contained in one moderately soluble mineral, or in two minerals with different solubility but similar Th/U ratios. A combined Pb-Pb isochron for the first and second leachates yields 4565.13 ± 0.55 Ma, MSWD = 92. The isochron passes between the points for primordial Pb and modern crustal Pb, implying the presence of two nonradiogenic Pb components.

**Discussion:** The residue model age of 4564.20 ± 0.45 Ma gives a minimum estimate of the crystallization age. This value is consistent with the ages of D'Orbigny and Sahara 99555. The distribution of Th/U model ratios between soluble and insoluble minerals is also similar among these meteorites. These observations further support their close genetic connection.

Interpretation of the leachate isochron date depends on identification of the principal U-carrying mineral in NWA 1296. If U is contained in a Ca-phosphate such as merrillite, apatite or silico-apatite, then the isochron date corresponds to the closure of radiogenic Pb migration in this mineral. Comparison of the residue and leachate dates indicates very rapid cooling, which can be estimated quantitatively if the dates are better constrained, and the grain size of the U-carrying mineral is determined.

**References:** [1] Jambon A. et al. 2005. *Meteoritics & Planetary Science* 40:361-375. [2] Amelin Y. 2008. *Geochimica et Cosmochimica Acta* 72:221-232. [3] Connelly J. N. et al. 2008. *Geochimica et Cosmochimica Acta* 72:4813-4824. [4] Amelin Y. 2008. *Geochimica et Cosmochimica Acta* 72:4874-4885. [5] Amelin Y. et al. 2010. *Earth and Planetary Science Letters* 300:343-350.
5197

U-Pb SYSTEMATICS OF THE ULTRAMAFIC ACHONDRITE NORTHWEST AFRICA 5400

Y. Amelin¹ and A. J. Irving². ¹Research School of Earth Sciences, The Australian National University. Canberra, ACT, Australia. E-mail: yuri.amelin@anu.edu.au. ²Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA.

**Introduction:** Northwest Africa 5400 is a metal-bearing ultramafic achondrite petrologically similar to brachinites, but with oxygen isotopic composition indistinguishable from that of the Earth [1]. A previous attempt to determine the formation age of this meteorite using the $^{53}$Mn-$^{53}$Cr system [2] yielded an internal isochron with zero slope and elevated $\epsilon(^{53}Cr)$ of 0.44, corresponding to the age younger than 4541 Ma. Here we report U-Pb data for NWA 5400.

**Techniques:** A portion of coarsely crushed meteorite was leached in warm 7M HCl to remove weathering products (mostly iron hydroxides). Three fractions of clinopyroxene with variable abundance of inclusions and turbidity, one fraction of olivine and one fraction of orthopyroxene were hand-picked from this acid-washed material. These fractions, and three whole rock fractions and their leachates were analyzed using the procedures of [3].

**Results:** Acid-washed minerals and rocks contain between 0.13–0.72 ppb U and between 1.2–7.7 ppb Pb. Their Pb isotopic compositions, with $^{206}$Pb/$^{204}$Pb between 9.35 and 13.69, plot along the mixing line between primordial Pb [4] and modern terrestrial crustal Pb. The least radiogenic Pb isotopic compositions are indistinguishable, within error, from primordial Pb. The $^{206}$Pb/$^{204}$Pb ratios are correlated with $^{238}$U/$^{204}$Pb. If interpreted as an isochron, this correlation corresponds to the date of 1570 Ma.

The whole rock contains 13–32 ppb of acid leachable U, and 49–173 ppb of acid leachable Pb. The most easily soluble Pb in whole rocks is similar to modern terrestrial crustal Pb; the Pb extracted with hot concentrated acids is slightly less radiogenic. No correlation between $^{206}$Pb/$^{204}$Pb and $^{238}$U/$^{204}$Pb ratios is observed for the leachates.

On a Pb-Pb isochron diagram, all residue and leachate data points plot along a single line that yields a date of 4478 ± 55 Ma if interpreted as a single-stage isochron.

**Discussion:** The U-Pb isotopic systematics of NWA 5400 are explained by mixing between initial primordial Pb and modern crustal Pb introduced by terrestrial weathering. Acid-soluble uranium was also accumulated during weathering. The linear arrays in U-Pb and Pb-Pb isotopic space are mixing lines with no chronological significance. This meteorite contains chlorapatite, a potentially suitable mineral for U-Pb dating, but it would be necessary to completely remove weathering products by a nonacidic treatment before dating of apatite can be attempted.

Although the U-Pb data reported here do not constrain the crystallization age of NWA 5400, they rule out a terrestrial origin: the initial Pb isotopic composition is far more primitive than the least radiogenic terrestrial Pb [5]. Instead, these data are consistent with origin from a body that either differentiated very early, or escaped an extensive loss of volatile elements. We suggest that the evolution of the NWA 5400 parent body differed from that of the Earth, Moon and parent bodies of angrites and eucrites, but was possibly akin to evolution of some of the parent bodies of iron meteorites, where the most primitive Pb is found.

**References:** [1] Irving A. J. et al. 2009. Abstract #2332. 40th Lunar and Planetary Science Conference. [2] Shukolyukov A. et al. 2010. Abstract #1492. 41st Lunar and Planetary Science Conference. [3] Amelin Y. et al. 2010. *Earth and Planetary Science Letters* 300:343–350. [4] Tatsumoto M. 1973. *Science* 180:1279–1283. [5] Frei R. and Rosing M. T. 2001. *Chemical Geology* 181:47–66.

5089

AN ANISOTROPY OF MAGNETIC SUSCEPTIBILITY STUDY OF THE STAC FADA MEMBER SUEVITE: CONSTRAINTS ON THE IMPACT CRATER LOCATION

K. Amor, J. Taylor, S. P. Hesselbo and C. MacNiocaill. Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR. E-mail: kena@earth.ox.ac.uk.

The Stac Fada Member (SFM) of the 1.177Ga [1] Stoer Group sediments (NW Scotland) has recently been reinterpreted as a proximal ejecta blanket [2]. Field observations of suevite injected between bedding planes of the underlying sandstones suggest a strong component of lateral movement during ejecta emplacement, consistent with the notion of fluidized deposition by surge-type flows rather than simple ballistic sedimentation. If this is the case then the ejecta may be expected to retain a fabric or lineation indicative of flow direction, which when measured at several dispersed geographical locations may point towards the impact crater. Anisotropy of magnetic susceptibility (AMS) has been used to determine directional information in geological materials subjected to flow or stress.

We measured the AMS and frequency dependence of susceptibility of 99 cores taken from 20 oriented blocks at four locations in the SFM in NW Scotland. All locations show an increasing frequency dependence of susceptibility towards the top of the deposit indicating an increasing proportion of fine-grained magnetic particles, which is consistent with an ejecta blanket origin. A weak anisotropy was detected of 1–2%, which is also a characteristic of pyroclastic flow deposits. The data shows a lineation radiating away from a common center. Coupled with observations of other directional field data which is in good agreement with the AMS lineations, we are able to propose a location for the impact crater in the middle of the North Minch Basin between Stornoway and the Stoer Peninsula.

**References:** [1] Parnell J. et al. 2011. *Journal of the Geological Society* 168:349–358. [2] Amor K. et al. 2008. *Geology* 36:303–306.
ON THE POSSIBILITY OF A LATE PLEISTOCENE
EXTRATERRESTRIAL IMPACT: LA-ICP-MS ANALYSIS OF THE
BLACK MAT AND USSELO HORIZON SAMPLES

A. V. Andronikov, D. S. Lauretta, I. E. Andronikova and R. J.
Maxwell. Department of Planetary Sciences, Lunar and Planetary
Laboratory, The University of Arizona, Tucson, AZ, 85721 USA. E-mail:
andron@lpl.arizona.edu.

**Introduction:** A dark layer of organic-rich material contemporaneous with the onset of the Younger Dryas (YD) cooling (12.9 ka) has been identified in North America (black mat; BM) and Western Europe (Usselo Horizon; UH). The following main hypotheses on the origin of this layer exist: (1) it formed by water-transported organic material; (2) it is the result of a heavy deposition of algae in a shallow fresh-water reservoir; (3) it formed in response to periods of spring-fed stream activation when groundwater oxidized organic material; (4) it resulted from wood fires and decomposition of charred wood; or (5) it resulted from the impact of a comet or asteroid [1, 2]. Most BM sequences contain a thin (2–5 cm) basal pitch-black layer likely corresponding to the lower YD boundary (LYDB). The UH sequences are represented by dark charcoal-rich layers within aeolian sands [3].

**Discussion:** Trace element concentrations in the BM (Arizona) and UH (Holland and France) samples were studied using LA-ICP-MS. Trace element compositions of the LYDB layers which directly underlie the BM, and the BM itself are different: the BM displays trace element concentrations similar to the average continental crust, while the LYDB layers are strongly enriched in REE (up to 800 × CI chondrite) and relatively depleted in Ta, Nb, Zr, and Hf (down to 30 × CI). Such a difference in compositions can point to a sharp change in the conditions of sedimentation just before LYDB layer deposition. LYDB, BM and UH samples display 2–5 times higher concentrations of PGES than the sediments underlying BM sequences. Additionally, LYDB samples display a positive correlation between Ni and Ir accompanied by an Os-to-Ir ratio of 1:1 and overall higher concentrations of both Os and Ir. In contrast, UH samples do not display any correlation between Os and Ir, and BM samples have an Os-to-Ir ratio of 1:2, which is more typical for terrestrial sediments [4]. Overall, UH samples display trace element characteristics that are a mixture between those typical of the BM and the LYDB. Trace element distributions and relations for samples of LYDB may be consistent with incorporation of the material of ET origin and could, therefore, support the hypothesis that an impact occurred shortly before the beginning of the YD cooling 12.9 ka.

**References:** [1] Firestone R. B. et al. 2007. *Proceedings of the National Academy of Science* 104:16016–16021. [2] Haynes C. V. et al. 2010. *Proceedings of the National Academy of Science* 107:4010–4015. [3] Kaiser K. et al. 2009. *Boreas* 38:591–609. [4] Agiorgitis G. and Wolf R. 1984. *Chemical Geology* 42:277–286.

ASTEROID IMPACT VARIATIONS OF NRM AND REM IN
TARGET BASALTS OF LONAR CRATER, INDIA

Md. Arif¹, N. Basavaiah¹, S. Misra² and K. Deenadayalan¹. ¹Indian Institute of Geomagnetism, Navi Mumbai-410218, India. E-mail: mdarifkrl@gmail.com. ²School of Geological Sciences, University of KwaZulu Natal, Durban-4000, South Africa.

**Introduction:** Around 50 ka old Lonar crater (dia. approximately 1.8 km), India, is completely excavated in approximately 65 Ma Deccan Traps by an oblique impact of a chondritic asteroid that stuck the preimpact target from the east at an angle of 30–45° [1, 2 and references therein]. We report here some rock magnetic experimental results on the possible relationships between variations of natural remanent magnetization (NRM) and REM (=NRM/saturation isothermal remanent magnetization [SIRM]) of the target basalts with the direction of impact.

**Experimental Procedures:** Oriented drill cores collected from around the Lonar crater rim and adjoining areas were cut into two to three specimens of 2.2 cm height in the laboratory. The NRM and SIRM imparted at 1 T by a Molspin pulse magnetizer, were measured using a Molspin and JR-6 spinner magnetometers.

**Results:** The unshocked target basalts (n = 44) at ≥2 km E of crater rim have mean NRM of approximately 3.87 Am⁻¹ [standard deviation (SD): 1.24 Am⁻¹]. The shocked targets [2] between approximately 1–2 km uprange of crater to the west (n = 207) show a general increase in average NRM to approximately 11.24 Am⁻¹ (SD: 9.58 Am⁻¹). The targets from N, NW, W, and SW sectors of crater rim (n = 203) and an eastern sector (n = 27) show average NRM of approximately 5.62 Am⁻¹ (SD: 2.22 Am⁻¹) and approximately 3.89 Am⁻¹ (SD: 1.28 Am⁻¹), respectively. The targets from SE (n = 51) and S (n = 34) sectors show a wide variation of NRM with an average of approximately 18.2 Am⁻¹ (SD: 14.3 Am⁻¹). A few samples in the N, SE and S sectors, however, show very high NRM (approximately 100–350 Am⁻¹). The REM of unshocked target basalts (n = 7) are <0.79% with an average of approximately 0.42%. The basalts from around the crater rim (n = 45) except the western rim sector (n = 12) have REM mostly <1.5% with an average of approximately 0.40%, and only 22% of shocked basalts show higher REM with an average of approximately 3.67%. Further it is found that samples with still higher REM occur only on the western rim sector and to the west adjacent to the crater in the uprange. The samples from this area (n = 39) have 36% data of REM between 1.97 and 12.65% with an average of approximately 5.10%; the rest of the samples, however, have REM <1.5% with an average of approximately 0.49%.

**Discussions:** The NRM of shocked basalts is in general increases due to impact. Although the NRM does not show much change over the unshocked target either in the downrange (no increment) or uprange (approximately one and half time increment) on the east-west plane of impact, a significant increase (approximately 5 times) is seen oblique to the plane and direction of impact. The shocked targets at approximately 1.5 to 2 km west of the crater rim in uprange also show a significant increase (approximately 3 times) in NRM. Most of very strongly shocked target basalts (approximately 71%) from around the crater rim and adjacent to the west of the rim have an average REM of approximately 0.44%, which is very similar to unshocked targets; the rest of the sample set, mostly from the western sector of the crater rim and to the west adjacent to the crater in uprange show an average REM of approximately 5.10% with values up to approximately 12 times higher over the unshocked targets.

**References:** [1] Fredriksson et al. 1973. *Science* 180:862–864. [2] Misra et al. 2010. *Geological Society of America Bulletin* 122:563–574. [3] Sengupta et al. 1997. *Revista de Fisica Aplicada e Instrumentacao* 12:1–7.
5137

IMPACT EJECTA TEMPERATURE PROFILE ON THE MOON –
WHAT ARE THE EFFECTS ON THE Ar-Ar DATING METHOD?

N. Artemieva¹,² and V. A. Fernandes³. ¹Planetary Science Institute E-mail: artemeva@psi.edu. ²Institute for Dynamics of Geospheres, Moscow. ³Museum of Natural History, Berlin.

**Introduction:** The $^{40}$Ar/$^{39}$Ar method is widely used for the acquisition of impact related cooling ages. Several of the Apollo and lunar meteorite samples dated (e.g., [1]) have demonstrated that there is a decoupling between K-Ar reset ages and their shock related petrographic features: partial to total resetting of the K-Ar system occurs even at low shock pressures. This suggests that the Apollo samples, after an impact event, were kept in a warm environment which permitted the partial to total resetting of the K-Ar clock. Here we present preliminary model results for the formation and evolution of shock-heated ejecta blanket produced during a large impact event on early and modern Moon, and its effects on pre-existing cold rock fragments.

**Numerical Model:** We model impact cratering on the Moon with the 3D hydrocode SOVA [2] complemented by the ANEOS equation of state for geological materials [3]. We use two thermal profiles within the target: cold modern Moon with a thermal gradient of 2 K km$^{-1}$ and hot ancient Moon with a thermal gradient of 15 K km$^{-1}$ [4]. To define ejecta distribution on the surface, we use a method of ballistic continuation on a sphere [5]. The evolution of post-depositional temperature profile within the ejecta blanket is estimated via one-dimensional thermal conductivity equation.

**Results:** At any distance from the crater ejecta are a mixture of materials compressed to various shock pressures (from partial vaporization, $P > 150$ GPa, to weakly compressed fragments) and excavated from various depths. At distal sites, melt and highly compressed materials from the crust prevail; near-crater ejecta consist mainly from unshocked materials. At the current cold Moon, antipodal ejecta have a temperature range of 400–600 K, while proximal ejecta have only slightly elevated $T$ of 250–300 K. If the early Moon was substantially hotter (thermal gradient of 15 K km$^{-1}$ during the first 0.2–0.5 Gyr), an average ejecta temperature reaches 600 K at distances >2400 km from a crater center, and is above 400 K at smaller distances.

Ejecta cooling time $\tau$ after ejecta deposition depends strongly on its thickness $H$ ($\tau \sim H^2$), which, in turn, increases quickly with a projectile size $D_{pr}$ increase ($H \sim D_{pr}$). Thus, gas losses due to an elevated post-deposition temperature is not as important for small impacts (with fast cooling rate), but may lead to a partial or total resetting of the K-Ar system in large, basin-forming impacts.

Considering plagioclase closure temperature is <573 K [e.g., 8], a rock fragment kept under this temperature for >20 years within an ejecta blanket [9] will likely have its K-Ar systematics reset and little to no petrographic shock related features.

**Future work:** (1) high-resolution model of an impact, resulting in a map of ejecta distribution around the crater (thickness $H$ and temperature $T$ as a function of distance and azimuth); (2) cooling time as a function of $T$ and $H$; (3) Ar diffusion and estimates of gas loss.

**References:** [1] Fernandes et al. 2008. Workshop Early Solar System Impact Bombardment, Abstract #3028. [2] Shuvalov. 1999. *Shock Waves* 9:381–390. [3] Thompson and Lauson. 1972. *SC-RR-61 0714*. Sandia National Laboratories p. 119. [4] Spohn et al. 2001. *Icarus* 149:54–65. [5] Dobrovolskis. 1981. *Icarus* 47:203–219. [6] Cassata et al. 2010. *Geochimica et Cosmochimica Acta* 73:6600–6612. [7] Shuster et al. 2010. *Earth and Planetary Science Letters* 290:155–165.

5065

MODELING THE FORMATION OF THE GLOBAL K/Pg LAYER

N. Artemieva¹,² and J. Morgan³. ¹Planetary Science Institute. E-mail: artemeva@psi.edu. ²Institute for Dynamics of Geospheres, Moscow. ³Imperial College, London.

**Introduction:** The K/Pg boundary [1, 2] is widely recognized as a global ejecta layer from the Chicxulub impact crater [3]. However, some properties of this layer (thickness, shocked quartz distribution, chemical composition) are inconsistent with the idea that ejecta was transported around the globe purely on a ballistic path.

**Numerical Model:** We model the Chicxulub impact with the 3D hydrocode SOVA [4] complemented by the ANEOS equation of state for geological materials [5]. In three separate stages we model: (1) the impact and initial ejection of material [6]; (2) the ballistic continuation of ejecta on a spherical earth [7]; and (3) ejecta re-entry into the atmosphere (at an altitude of 200 km re-entering tracers are replaced by real particles with a size-frequency distribution in accordance with their maximum shock pressure; the interaction of these particles with the atmosphere is modeled using a multi-phase approximation).

**Results:** Up to distances of 1000–1500 km from the crater, ballistic ejecta is deposited rapidly. At larger distances, the mass of ejecta and atmosphere are comparable, hence, their interaction becomes significant. Re-entering ejecta create strong winds in the upper atmosphere and these winds disperse small fragments (molten spherules and shocked quartz grains of <1 mm in diameter) preferentially downrange. For two-three hours after the impact, these dispersed ejecta travel up to a few thousand km from their re-entry site (final deposition through the dense lower atmosphere or ocean may take days or weeks). This mechanism is much more intense than observed for volcanic aerosols in stable atmospheric flows. Similar phenomena (atmospheric skidding) has been suggested for re-entering ejecta after the Shoemaker-Levy 9 Comet impact on Jupiter [8], but has never been modeled before.

**Conclusions:** Numerical modeling of the interaction of ejecta with the atmosphere allows us to reproduce the constant thickness of the distal K/Pg layer, as well as the presence of low-velocity ejecta from the crystalline basement (shocked quartz) around the globe.

**References:** [1] Alvarez L. et al. 1980. *Science* 208:1095–1108. [2] Smit J. 1999. *Annual Review of Earth and Planetary Science* 27:75–113. [3] Hildebrand A. R. et al. 1991. *Geology* 19:867–871. [4] Shuvalov V. 1999. *Shock Waves* 9:381–390. [5] Thompson S. L. and Lauson H. S. 1972. *SC-RR-61 0714*. Sandia Nat. Lab. 119 p. [6] Artemieva N. and Morgan J. 2009. *Icarus* 201:768–780. [7] Dobrovolskis A. 1981. *Icarus* 47:203–219. [8] Colgate S. A. and Petschek A. G. 1985. *LA-UR-84-3911*. Los Alamos Nat. Lab.
POST-FALL SURFACE MODIFICATION FEATURES OF IRON METEORITES FOUND BY THE OPPORTUNITY ROVER AND THEIR IMPLICATIONS FOR MARTIAN WEATHERING PROCESSES

J. W. Ashley¹, M. P. Golombek², P. R. Christensen¹, S. W. Squyres³, T. J. McCoy⁴, C. Schröder⁵, I. Fleischer⁶, J. R. Johnson⁷, K. E. Herkenhoff⁷ and T. J. Parker². ¹Mars Space Flight Facility, Arizona State University. ²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. ³Department of Astronomy, Cornell University, Ithaca, NY, USA. ⁴Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA. ⁵Center for Applied Geoscience, Eberhard Karls University, Tübingen, Germany. ⁶Institut für Anorganische und Analytische Chemie, Johannes Gutenberg-Universität, Mainz, Germany. ⁷Astrogeology Science Center, U. S. Geological Survey, Flagstaff, AZ, USA. E-mail: james.ashley@ser.asu.edu.

**Introduction:** Including Heat Shield Rock [1] (officially renamed Meridiani Planum [2]), a total of six large, iron meteorites have been found to date by the Opportunity rover at Meridiani Planum, Mars. In order of discovery, the five additional meteorites have been unofficially named Block Island, Shelter Island, Mackinac Island, Oileán Ruaidh, and Ireland [3]. In addition to Pancam and Navcam imaging, surface targets on Block Island, Shelter Island and Oileán Ruaidh have been examined using two or more of the rover arm instruments (Alpha-Particle X-ray Spectrometer, Mössbauer Spectrometer, and Microscopic Imager).

**Observations:** These meteorites show a remarkable variety of surface textures, morphologic features, and coatings that convey their post-fall alteration history, and thus provide insights into Martian weathering processes. Among the more notable of these are (1) a marked dichotomy in surface roughness on Block Island suggestive of partial burial and exposure, (2) a large pit lined with delicate iron protrusions suggestive of inclusion removal on Block Island by a gentle process, (3) expression of the Widmanstätten pattern in four of the meteorites, suggestive of differential mechanical abrasion, (4) a spatially discontinuous iron oxide-rich coating elevated in Mg and Zn relative to its substrate, and (5) large-scale cavernous weathering in two of the meteorites. In addition, the presence of regmaglypts on several of these meteorites suggest that some areas altered much less than other areas since their atmospheric passage and arrival on Mars.

**Discussion:** Cross-cutting relationships between the oxide coating and Widmanstätten pattern, together with elevated Mg and Zn abundances, show the coating to be the result of post-fall processes and not a fusion crust. If the coating is the result of water exposure (possibly ice), a sequence of two periods of abrasion with oxide production between them is indicated. The morphologies of iron protrusions in the Block Island cavity are similarly suggestive of water exposure (possibly acidic) more than of wind abrasion, but the type of inclusive mass removed is unknown. The cavernous weathering features in Shelter Island and Mackinac Island may also have been caused by acidic corrosion. However, the extent to which mechanical abrasion may contribute to the final appearance of these meteorites remains an unknown, but clearly present (and potentially testable), component of their alteration history.

**References:** [1] Schröder C. et al. 2008. *Journal of Geophysical Research* 113:doi:10.1029/2007JE002990. [2] Connolly H. C. et al. 2006. *Meteoritical Bulletin* #90, *Meteoritics & Planetary Science* 41:1383–1418. [3] Ashley J. W. et al. 2011. *Journal of Geophysical Research* 116: doi:10.1029/2010JE003672.

GEOLOGIC MAPPING, MORPHOLOGY, AND DATING OF THE KING CRATER REGION, LUNAR FAR SIDE

J. W. Ashley,¹B. R. Hawke², N. DiCarlo¹, A. C. Enns¹, H. Hiesinger³, M. S. Robinson¹, H. Sato¹, E. J. Speyerer¹, C. H. van der Bogert³, R. V. Wagner¹, K. E. Young¹ and the LROC Science Team. ¹Lunar Reconnaissance Orbiter Camera, School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-3603. ²Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI. ³Institut für Planetologie, Westfälische Wilhelms-Universität, Münster, Germany. E-mail: james.ashley@asu.edu.

**Introduction:** Since its discovery on the lunar farside, King crater (5.0°N and 120.5°E) has been a site of fascination to the lunar science community; and recently, as a Constellation site of interest, become appealing to the goals of human space exploration. The crater-forming impact interacted in a complex way with pre-existing topography, and a variety of impact processes can be explored in the morphology of the resulting features. In particular, the King crater impact melt pond (~17 km diameter; located in nearby Al-Tusi crater) provides opportunities to study large volumes of once-molten material of dominantly anorthositic composition. The Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) and Narrow Angle Camera (NAC) [1] imaged King crater from a nominal 50 km altitude at pixel scales of 100 meters and 0.5 meter, respectively. Images were used to create detailed geologic maps for the region at both scales, with the NAC emphasis on the melt pond. Digital elevation models (DEMs) were also derived from both WAC [2] and NAC [3] images for the area.

**Discussion:** The high-resolution NAC images are particularly useful for augmenting previous mapping efforts [e.g., 4], showing fine details within the main melt pond that raise questions on melt pond dynamics and evolution. These include both positive and negative relief features, craters with anomalous morphologies (involving hummocky floors, irregular outlines, and boulder associations), and complex flow morphologies. NAC images also show (1) interactions of melt with slump blocks in the crater wall terraces, (2) anomalous mounds and other morphologic complexities in the crater floor melt deposits, and (3) details of deceleration lobes in the distal ejecta blanket. Crater counts on the extended ejecta blanket [e.g., 5] show that King crater is likely to be ~1 Ga in age.

Based on pre-existing topography assumptions, an 175 m depth was estimated for the main melt pond using the NAC DEM, which also reveals kilometer-scale topographic depressions, particularly across the central and eastern portions of the pond. Vertical displacements within these depressions from their inferred original equipotential surfaces range from 15 to 20 meters. The perimeters of these downwarped zones correlate with the occurrence of negative relief features (~10 to 100 m in length). The sagging may have occurred as the result of contraction and/or compaction within the melt both during and following cooling. Some negative relief features in the melt pond appear to show evidence of collapse into subsurface voids, and are likely the result of melt withdrawal, indicating some mechanism for local drainage while significant volumes of melt remained mobile.

**References:** [1] Robinson M. S. et al. 2010. *Space Science Review* 150:81–124. [2] Scholten et al. 2011. Abstract #2046. 42nd Lunar and Planetary Science Conference. [3] Tran T. et al. 2010. ASPRS CaGIS Specialty Conference. [4] Wilhelms D. E. and El-baz F. 1977. *USGS*, Map I-948. [5] Ashley et al. 2011. Abstract #2437. 42nd Lunar and Planetary Science Conference.
5083

NOBLE GASES IN COSMIC SPHERULES AND MICROMETEORITES FROM THE TRANSANTARCTIC MOUNTAINS

B. Baecker¹, C. Cordier², L. Folco², M. Trieloff³, J. A. Cartwright¹ and U. Ott¹. ¹Max Planck Institute for Chemistry, Joh.-J.-Becher-Weg 27, D-55128 Mainz, Germany. E-mail: b.baecker@mpic.de. ²MNA, Siena, Italy. ³University of Heidelberg, Germany.

**Introduction:** At MPI Mainz we have initiated a comprehensive survey of noble gases in micrometeorites. While He and Ne in micrometeorites are generally dominated by solar contributions, the inventory of heavy primordial noble gases is not as well characterized. We hope to fill this gap with particular focus on the diagnostic isotopic composition of Xe. While our study will eventually include material from a variety of collections (e.g., the CONCORDIA collection [1]), our first results are from "large" PNRA (Programma Nazionale delle Ricerche in Antartide, Italy) micrometeorites collected from the Victoria Land, Transantarctic Mountain micrometeorite traps [2].

**Samples and Analytics:** 51 micrometeorites (MM's)—26 cosmic spherules (CS), 11 scoriaceous (Sc), 14 unmelted (Un)—in the 600–800 µm size range were extracted from traps #45 b and c on the top of Miller Butte, and split into aliquots for combined petrographic and noble gas analyses. For the latter, aliquots were heated using a CO₂ laser. The analyses were performed using our multicollector Nu Instruments noble gas mass spectrometer [3].

**Preliminary Results:** At present, we have obtained noble gas results for three CS and one Sc micrometeorites. Our Ne and Ar results (corrected for isobaric interferences, but not extraction blank) are listed in Table 1.

As found in previous studies (e.g., [4, 5]) noble gas abundances in CS are mostly low and essentially have a terrestrial air signature (e.g., $^{20}$Ne/$^{22}$Ne ~9.8, $^{21}$Ne/$^{22}$Ne ~0.029). In contrast, the Sc micrometeorite shows $^{20}$Ne/$^{22}$Ne and (even clearer) $^{21}$Ne/$^{22}$Ne ratios higher than air, indicative of the presence of solar and spallogenic Ne. Using the production rates for IDPs from table 3 in [6], the spallogenic $^{21}$Ne of ~1.6 × 10⁻⁹ cc/g would be produced at 1 AU by SCR primaries in ~0.17 Ma, for production by GCR primaries, the inferred exposure would be ~4 Ma. The concentration of trapped (solar) $^{20}$Ne of ~5 × 10⁻⁷ cc/g is about an order of magnitude lower than the geometric mean for Un micrometeorites from Yamato Mountains and Dome Fuji, but distinctly higher than in CS (see table 4 in [4]). The low $^{40}$Ar/$^{36}$Ar ratio for the Sc micrometeorite (~128) indicates the presence of trapped $^{36}$Ar, at an abundance of ~6 × 10⁻⁸ cc/g. This is most likely "planetary" type Ar, carried primarily by phase Q [7].

**Acknowledgments:** We acknowledge support by DFG through SPP 1385 (Project OT 171/5-1) and PNRA.

**References:** [1] Duprat J. et al. 2007. *Advances in Space Research* 39:605–611. [2] Rochette P. et al. 2008. *Proceedings of the National Academy of Sciences* 105:18206–18211. [3] Ott U. et al. 2010. *Meteoritics & Planetary Science* 45:A158. [4] Osawa T. et al. 2010. *Meteoritics & Planetary Science* 45:1320–1339 [5] Marty B. et al. 2005. *Meteoritics & Planetary Science* 40:881–894. [6] Kehm K. et al. 2006. *Meteoritics & Planetary Science* 41:1199–1217. [7] Lewis R. S. et al. 1975. *Science* 190:1251–1262.

<table><caption>Table 1. Ne and Ar results for MM's. Sizes are given in µm. Weights are given in µg, noble gas amounts—not corrected for blank—in 10⁻¹² cc units. Uncertainties in the last digits.</caption><thead><tr><th>Sample</th><th>Size</th><th>Weight</th><th><sup>22</sup>Ne</th><th><sup>20</sup>Ne/<sup>22</sup>Ne</th><th><sup>21</sup>Ne/<sup>22</sup>Ne</th><th><sup>36</sup>Ar</th><th><sup>40</sup>Ar/<sup>36</sup>Ar</th></tr></thead><tbody><tr><td>CS 45c21</td><td>720 × 556</td><td>337.9</td><td>0.41</td><td>9.11 (37)</td><td>0.029 (2)</td><td>0.40</td><td>319 (17)</td></tr><tr><td>CS 45c17</td><td>630 × 476</td><td>126.3</td><td>0.16</td><td>9.34 (1.01)</td><td>0.038 (6)</td><td>2.36</td><td>280 (9)</td></tr><tr><td>CS 45c16</td><td>465 × 324</td><td>105.1</td><td>0.21</td><td>8.85 (85)</td><td>0.027 (4)</td><td>0.98</td><td>292 (10)</td></tr><tr><td>Sc 45b22</td><td>268 × 191</td><td>9.2</td><td>0.49</td><td>10.46 (45)</td><td>0.062 (4)</td><td>1.04</td><td>128 (5)</td></tr></tbody></table>

5365

CONSTRAINTS ON COSMIC-RAY EXPOSURE HISTORY OF THE VACA MUERTA MESOSIDERITE FROM NOBLE GAS ANALYSIS OF NEUTRON-IRRADIATED SAMPLES

K. Bajo¹,², H. Sumino² and K. Nagao². ¹Natural History Sciences, Hokkaido University. E-mail: bajo@ep.sci.hokudai.ac.jp. ²Geochemical Research Center, Graduate School of Science, University of Tokyo.

**Introduction:** Mesosiderites have unique and interesting textures, and their formation mechanism and histories have been investigated from petrological, mineralogical, geochemical, and chronological points of view. Bajo and Nagao [1] proposed a complex cosmic-ray exposure history and extreme depletion of volatile elements such as noble gases and halogens of Vaca Muerta. Here we present halogen and Ba concentrations of Vaca Muerta and discuss on cosmic-ray exposure condition based on noble gas analysis of neutron-irradiated samples?

**Experimental:** Two samples of different textures, one is metal-silicate mixture (VM-B) and another eucritic pebble (VM-P) [1], were investigated. Measured Ar, Kr, and Xe isotope compositions of neutron-irradiated samples were decomposed into each component derived from nuclear reactions of several elements (K, Ca, Cl, Br, I, Ba and U) and intrinsic noble gas components in order to determine the $^{39}$Ar-$^{40}$Ar age and concentrations of the parent elements. The Hb3gr interlaboratory $^{39}$Ar-$^{40}$Ar age standard ([2], [3]) was used to determine conversion factors of Cl, K and Ca to relevant Ar isotopes and neutron fluence. Bromine, I, Ba and U concentrations were determined from concentrations of neutron-produced Kr and Xe isotopes, neutron capture cross sections, and the neutron fluence. The halogen concentrations were calculated from neutron-produced $^{38}$Ar, $^{80}$Kr, and $^{128}$Xe after subtracting cosmogenic and trapped components from measured concentrations of these isotopes.

**Results and Discussion:** Bromine and I concentrations for VM-P were less than 10 ppb and lower than those of VM-B (< 5 ppb), which is consistent with volatile depletion in VM-P suggested by [1]. However, the low Cl concentrations and abundant cosmic-ray-produced noble gases allow us to obtain upper limits of Cl concentrations to be 50 ppm. Cosmogenic $^{131}$Xe/$^{126}$Xe ratio determined for the samples suggests the shielding depth (in the parent body) to be 150 g cm⁻². Ba concentration determined for the neutron-irradiated VM-P sample was 2.7 ppb. Adopting the estimated shielding depth and the cosmic-ray exposure age of 140 Myr [1], 2.5 × 10⁻¹² cm³ STP/g of cosmogenic $^{131}$Xe is estimated under 4π irradiation geometry [4]. This estimation agrees well with the observed value of (4 ± 3) × 10⁻¹² cm³ STP g⁻¹ [1]. This indicates that the cosmogenic $^{131}$Xe was mainly produced in meteoroid during transit from its parent asteroid to the Earth, and the progenitor of Vaca Muerta had been located in a deep layer of the parent body.

**References:** [1] Bajo K. and Nagao K. 2011. *Meteoritics & Planetary Science* 46:556–573. [2] Turner G. et al. 1971. *Earth and Planetary Science Letters* 10:227–234. [3] Roddick J. C. 1983. *Geochimica et Cosmochimica Acta* 47:887–898. [4] Hohenberg C. M. et al. 1978. *Proceedings*, 9th Lunar and Planetary Science Conference, pp. 2311–2344.
## TRACE ELEMENT GEOCHEMISTRY OF ENSTATITE CHONDRITES: THE SO-CALLED "REFRACTORY LITHOPHILE" ELEMENTS

J. A. Barrat¹, B. Zanda², C. Bollinger¹ and C. Liorzou¹. ¹Université Européenne de Bretagne, UBO-IUEM, CNRS UMR 6538, Place Nicolas Copernic, 29280 Plouzané Cedex, France. E-mail: barrat@univ-brest.fr. ²MNHN & CNRS UMR7202, 61, rue Buffon, 75005 Paris, France.

We report on the trace element abundances of 15 enstatite chondrites (EL3: MAC 88136, EL6: Eagle, Hvittis, Pillistfer, ALHA 81021, LON 94100, EH3: Kota Kota, Sahara 97096, Sahara 97123, Sahara 97158, EH4: Indarch, EH IMB: Abee, Galim-b, Saint Sauveur, EH impact melt: LAP 02225), the unique enstatite meteorite Itqiy and two ordinary chondritic materials associated with enstatite meteorites (Galim-a and a Cumberland falls chondritic clast). Our ICP-MS procedure (adapted from [1]) allows the determination of abundances and ratios of refractory elements (e.g., REEs + Y, Th, U, Nb, Ta, Sr, Ba) in chondrites with a good reproducibility (<3% for abundances, < 1.5% for ratios such as La/Sm, Eu/Eu*, Zr/Hf or Nb/Ta). In agreement with previous studies (e.g., [2-4]), our OC, EL3, EH3, EH4, and some of our EH-IMB (Abee and Saint Sauveur) samples display basically chondritic abundances and ratios. The situation is clearly different for all the other EC samples. The EL6 samples are all light REE depleted and display a small negative Eu anomaly ((La/Lu)n = 0.64-0.77, Eu/Eu* = 0.75-0.89). Their patterns are alike those of an oxidized dark inclusion from Sahara 97158, and the LAP 02225 impact melt. Furthermore, Galim-b display a more pronounced light REE depletion ((La/Lu)n = 0.30-0.40), a small negative Yb anomaly and a deep negative Eu anomaly (Eu/Eu* = 0.18-0.29). Finally, Itqiy displays a strong light REE depletion ((La/Lu)n = 0.12) and a deep negative Eu anomaly (Eu/Eu* = 0.034), in agreement with [5]. The light REE depletion shown by the impact melted ECs is accompanied by Nb, Sr, Eu, and Ba depletions. Striking correlations between La/Lu, Nb/Ta, and Eu/Eu* are obtained. We performed a series of leaching experiments on various ECs (EL3, EH3, EH-IMB, EL6) in order to constrain the concentrations of trace elements in leachable sulfides and in silicates (residues). The proportions of REEs and Nb (among others) in the leachable phases (mainly sulfides) are clearly affected by the metamorphic history of the meteorites. For example, the Nb budget is largely controlled by the sulfides in EL6 chondrites, not in EL3s.

The ranges of abundances and ratios of the so-called "lithophile refractory" elements in ECs can be explained by impact-induced mobilization of sulfides and possibly plagioclase [2]. Vaporization and losses of some of these elements upon impact cannot be directly rejected, and could account for some non-chondritic values inferred for planetary bodies (e.g., the Nb/Ta ratio of the Earth).

**References:** [1] Barrat J. A. et al. 2008. *Meteoritics & Planetary Science* 43:1759-1775. [2] Rubin et al. 2009. *Geochimica et Cosmochimica Acta* 73:1523-1537. [3] Kallemeyn G. W. and Wasson J. T. 1985. *Geochimica et Cosmochimica Acta* 49:261-270. [4] Kallemeyn G. W. and Wasson J. T. 1985. *Geochimica et Cosmochimica Acta* 50:2153-2164. [5] Patzer A. et al. 2001. *Meteoritics & Planetary Science* 36:1495-1505.

## SIGNIFICANCE OF OPAL IN UREILITES—DELIVERY OF H₂O TO THE INNER SOLAR SYSTEM?

A. D. Beard¹, H. Downes¹,², K. T. Howard². ¹Birkbeck/UCL Research School of Earth Sciences, Birkbeck, University of London, Malet Street, London WC1E 7HX, U.K. E-mail: a.beard@ucl.ac.uk. ²Department of Mineralogy, Natural History Museum, Cromwell Rd, London, UK.

**Introduction:** Numerous fragments of opal (SiO₂.nH₂O) have previously been reported in five internal chips of polymict ureilite EET 83309 [1]. The largest fragments are >300 µm in the longest dimension and show clear banding. The bands are terminated by the adjacent olivine clasts, indicating that the opal clasts were not formed *in situ*. Opal is also found in contact with ureilitic olivine, forming a single clast with a terrestrial weathering rim around both minerals. It also occurs as thin (10 µm) rims completely surrounding suessite (Fe₃Si) grains, and can also contain inclusions of schreibersite. It shows no relationship whatsoever with terrestrial weathering rims. These petrographic observations indicate that the opal is (a) extraterrestrial in origin, and (b) native to the ureilite parent body. Suessite and schreibersite are considered to be the products of reduction during shock metamorphism. Therefore formation of the opal occurred after the formation of suessite but before the formation of the regolith represented by polymict ureilites.

**Opal Composition:** Opal is hydrated silica containing 10-30 wt% H₂O. XRD analysis suggests that the opal in EET 83309 is of the amorphous opal-A variety, although it appears to have undergone some recrystallization to a SiO₂ phase. The largest opal clast has a composition of approximately 65.5 wt% SiO₂, 6 wt% FeO and <1wt% MgO. In contrast, terrestrial opals only contain up to 1.0 wt% FeO and 0.06 wt% MgO [2]. The origin of the water or ice which reacted with Si-rich minerals to form the opal is unknown but must be extra-terrestrial in origin, possibly due to impact of an icy body such as a comet on the surface of the ureilite parent body.

The identification of hydrated silica (opal) by the Mars Exploration Rover (MER) Spirit in the Gusev crater, Mars [3] and presence of opal in ureilite EET 83309 has implications for the presence of water and its delivery by cometary bodies during the early formation of the Inner solar system.

**References:** [1] Beard A. D. et al. Abstract #5027, 72nd Annual Meteoritic Society Meeting. [2] Gaillou E. et al. 2008. *Ore Geology Reviews* 34:113-126. [3] Rice et al. *Icarus* 205:375-395.
## PETROLOGIC AND TEXTURAL DIVERSITY IN ONE OF THE LARGEST SAMPLES OF THE VESTAN REGOLITH

A. W. Beck, H. Y. McSween and C. E. Viviano. Planetary Geosciences Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996. E-mail: abeck3@utk.edu.

**Introduction:** With the upcoming arrival of Dawn, it will be beneficial to understand the scale of heterogeneity expected in the Vestan regolith. This will help in data interpretation from Dawn, and also in understanding regolith maturity. The best-suited samples for investigating regolith diversity are howardites [1]. Here, we examine the PCA02' paired howardite group, collectively among the largest samples from the Vestan megaregolith (~1 m diameter, pre-atmospheric size) [2]. Along with reporting mineralogical diversity, we examine the scale of textural diversity using a new method where 8 x-ray maps are combined into a single image, allowing us to characterize distributions of specific subtypes of HEDs.

**Methods:** We have selected 6 PCA02' howardites for this study: PCA 02009(P9), PCA 02013(P13), PCA 02014(P14), PCA 02015(P15), PCA 02018(P18) and PCA 02019(P19). Mineral chemistries and the 8-element x-ray maps were acquired using EMP. X-ray maps were merged and classified using image-processing software [3].

**Results and Discussion:** Pyroxene compositions range from $WO_1$ $En_{90}$ $Fs_{90}$ to $WO_8$ $En_{28}$ $Fs_{64}$, spanning the entire range of HEDs, including subtypes of eucrite and diogenite [4,5]. The majority of olivines fall within the diogenite range ($Fo_{75-65}$) [1]. However, groups of $Fo_{80-90}$ and $Fo_{44-60}$ olivine also occur. Plagioclase compositions are like those in basaltic and cumulate eucrites [1]. All samples contain metal, some of which is likely exogenic based on $Co/Ni$ ratios [6].

X-ray maps reveal distinctive olivine-phyric (0.2–1 mm) basaltic clasts, with fine-grained groundmass (~10 µm) of $Ol + Pyx + Plag$. These are not typical eucrite basalts, as they contain olivine. Groundmass olivine is Fe-rich (~$Fo_{50}$). Olivine xenocrysts are ~$Fo_{70}$ and ~$Fo_{90}$, both of which are zoned, the rims being in equilibrium with the groundmass ($Fo_{50}$). Several xenoliths of harzburgitic diogenite occur in these basaltic clasts. These clasts also contain ~80 µm pieces of metal, some of which have exogenic $Co/Ni$ [6].

We propose that these clasts are impact melts, not a new Vestan basalt. We hypothesize that they were generated from the melting of a polymict howardite, rich in a harzburgitic component, or the melting of a yet undiscovered Vestan norite ($Opx + Ol + Plag$). We favor the melting of a howardite rich in harzburgitic material given the xenocrysts and xenoliths observed in the melt clasts. The large metal grains in these melts suggest that the impactor was metal-rich (e.g. OC or Iron).

Modes among the samples vary considerably. Diogenite orthopyroxene ($En_{90-66}$) is the most dominant phase, ranging from 34–89%. Cumulate eucrite pyroxene (~$En_{64-45}$) ranges from 2 to 12%, and basaltic eucrite pyroxene ($En_{44-30}$) ranges from 3 to 22%. Olivine ranges from 1 to 10% and is correlated to the abundance of melt clasts. Melt clasts are ≤15%, though they comprise 33% of P15. All samples are exceptionally rich in metal, typically containing 1%, with P18 containing an impressive 20% metal.

The grain size of diogenitic material is primarily ≥300 µm, whereas cumulate eucrite material is ~100 µm and basaltic eucrite material is 80 –3 µm matrix material. This grain size distribution is important to Dawn VIR data interpretation, as spectral absorptions in the near infrared occur due to volumetric scattering, and fine-grained textures promote multiple scattering and would relatively increase reflectance [7]. From our textural observations, we anticipate the basaltic matrix material will disproportionately dominate the Dawn reflectance spectra of the regolith.

**References:** [1] McSween et al. 2011. *Space Science Reviews.* doi:10.1007/s11214-010-9637-z. [2] Welten et al. 2009 *Meteoritics & Planetary Science* 44:A216. [3] ENVI 4.2, ITT Visual Information Solutions 2006. Boulder, Colorado. [4] Mayne et al. 2009. *Geochimica et Cosmochimica Acta* 73:794–819. [5] Beck and McSween. 2010. *Meteoritics & Planetary Science* 45:850–872. [6] Moore et al. 1969. In *Meteorological Research* 738–748, Springer-Verlag, NY, USA. [7] Hapke. 1993. *Introduction to the theory of reflectance and emittance spectroscopy*, Cambridge University Press, NY, USA.

## CO-EVOLUTION OF CHONDRITIC ORGANICS AND MINERALS DURING IMPACT METAMORPHISM

P. Beck¹, H. Yabuta², T. Atou³, E. Quirico¹, Z. Yoldi-Martinez¹, A. Guillot¹, L. Bonal¹, G. Montagnac⁴, B. Schmitt¹ and G. Montes-Hernandez⁵. ¹UJF-Grenoble 1/CNRS-INSU, Institut de Planetologie et d'Astrophysique de Grenoble (IPAG), France, E-mail: pierre.beck@obs.ujf-grenoble.fr. ²Department of Earth and Space Science, Osaka University, Japan. ³Secure Material Center Materials and Structures Laboratory, Tokyo Institute of Technology, Japan. ⁴Laboratoire de Sciences de la Terre. ENS Lyon, France. ⁵Institut des Sciences de la Terre (ISTERRE), UJF-Grenoble 1, France.

**Introduction:** Chondritic organic matter might have evolved on its parent body where subjected to shock metamorphism [1,2]. If the effects of shock waves on minerals have been studied in details from the experimental and observational approaches [3,4] little is known on their effect on the carbonaceous component. Here, we experimentally exposed fragments of matrix of the Murchison chondrite to shock loading up to 40 GPa. We describe the effects on the mineralogy as well as on the organic component.

**Methods:** Samples of Murchison meteorite (CM) were obtained through loan by the Museum of Natural History (Chicago). The sample was powdered and pressed to obtain 1 cm in diameter and 1 mm thick pellets. Pellets were then incorporated in a steel container and shocked to pressure of approximately 5–10–20 and 40 GPa, using a single-stage propellant gun (5–10–20 GPa) and a two-stage light gas gun (40 GPa) at the Tokyo Institute of Technology. Samples were then recovered and powdered. IR spectra were measured using the Hyperion microscope at IPAG, and Raman spectra were obtained using the Labram HR at ENS Lyon.

**Mineralogical Effect:** Infra red spectra were obtained in the 2–15 micron region, and provide insights into the nature of the silicate phase present. As shock pressure increases the 3 µm band (-OH and $H_2O$) appears to decrease in intensity with regard to the 10 µm band, indicative of a progressive dehydration of the phyllosilicate phase. For the sample shocked at pressure of 5–20 GPa, minor effects are observed on the 10 µm band, which remains broad and structureless, which reveals the presence of a poorly crystalline phyllosilicate [5]. However, in the case of the sample shocked at a pressure of 40 GPa, the 10 µm band clearly shows structure, which we interpret by the presence of well crystalline phyllosilicates.

**Impact on the Structure of the IOM:** Raman spectra were directly measured on the powdered sample, and analyzed following [6]. When plotted in a FWHM vs Band G diagram, the samples shocked at 5, 10 and 20 GPa do not appear distinct from the initial samples. However, in the case of the sample shocked at 40 GPa, a clear distinction is seen with regard to the other samples. A metamorphic trend is observed, which is distinguishable from long duration thermal metamorphism. The origin of this trend and its relation with those observed in chondritic samples will be discussed.

**References:** [1] Quirico et al. 2009. Meteoritical Society Meeting, [2] Yabuta et al. 2010. *Meteoritics & Planetary Science* 1446–1460. [3] Ahrens T. J. 1980. *Science* 207:1035–1041. [4] Gillet P. et al. 2007. Mineralogy of shocked meteorites. *GSA Monograph*, edited by E. Ohtani. [5] Beck et al. 2010. *Geochimica et Cosmochimica Acta* 74:4881–4892. [6] Bonal et al. *Geochimica et Cosmochimica Acta* 70:1849–1863.
## HOW DO ACCRETIONARY RIMS AROUND CHONDRULES INFLUENCE THE FORMATION OF CHONDRITES?

E. Beitz¹, C. Güttler¹, R. Weidling¹, J. Blum¹, D. C. Hezel² and A. Pack³. ¹Institut für Geophysik und extraterrestrische Physik, TU Braunschweig. ²National History Museum, London. ³Göttingen University, Germany, E-mail: e.beitz@tu-bs.de.

Up to 80% of the volume of chondric meteorites consists of chondrules. These mm-sized spherules are high-temperature inclusions and most of these are surrounded by a dusty layer. These rims are hypothesized to have formed by accretion of dust grains in a dense region of the solar nebula [1]. It is likely that these rims would influence collisional interaction between the chondrules [2]. We will present new experiments to study the dynamic behavior of dust-coated chondrule analogues.

To produce chondrule rims we used two experimental methods forming rims with different morphologies and porosities around artificial chondrules. One method uses the aerodynamic levitation of millimeter-sized glass beads (our chondrule analogues) in a gas flow, enriched with µm-sized, monodisperse, spherical SiO₂ dust. Here, the artificial chondrules accrete very even, highly porous rims with a porosity of 82%. The second method aims at producing dust rims, formed in multiple low-velocity collisions of chondrules with dust aggregates of the same size. Here, dust from the dust aggregates is transferred to the surface of the solid chondrule. This is realized by shaking a glass bead for a defined time in a dust filled container. These collisions also tend to form rims but less even and with a significantly lower porosity of 60%. To constrain the influence of the different rim structures on the collision behavior, and thus on the formation of larger bodies in the solar nebula, we set up a multiple collision experiment for studying free particle-particle collisions under microgravity conditions [3].

**Results:** In November 2010 we performed four experiments under microgravity conditions at the Bremen drop tower. Here, we varied the rim porosity (see above) and the particle size (2 and 3 mm glass beads). It turned out that dust coated particles are much more cohesive in collisions compared with non-coated particles, and even dust aggregates of the same size are less likely to stick to each other at the same velocities. For the conducted experiments, we found a correlation between the sticking probability, the rim morphology and the chondrule size. Additionally, we observed the formation of clusters of several tens of chondrules as a result of subsequent sticking collisions [4].

**Further Steps:** Based on the hypothesis that the chondrules could be still very hot when they start accreting their dust rims, sintering effects and changes in the rim morphology should be taken into account. Therefore we improved our aerodynamical levitation experiment by attaching a laser to heat the chondrules while accreting their rims. For these experiments we exchanged the glass beads by artificial chondrules of realistic compositions, and powdered San Carlos olivine was used to simulate the dust of the early solar nebula. We will present the resultant influence of the chondrule composition, the rim material, and the temperature on the morphology and porosity of the rims based on nano-CT and micro-CT analysis of the produced samples.

**References:** [1] Metzler K. et al. 1992. *Geochimica et Cosmochimica* 56:2873-2897. [2] Ormel C. et al. 2007. *Astronomy & Astrophysics* 679:1588-1610. [3] Weidling R. et al. 2011. *arXiv eprint* 1105:3909. [4] Beitz E. et al. 2011. *arXiv eprint* 1105:3897.

## ELEMENTAL MAPPING OF A SULFIDE NODULE

G. K. Benedix¹,², I. C. Lyon³, M. Schönbächler³, T. Henkel³ and D. Rost³. ¹Impacts & Astromaterials Research Centre, (IARC) Dept. of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK. E-mail: g.benedix@nhm.ac.uk. ²IARC/Dept. of Earth Science and Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK. ³School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL.

**Introduction:** Sulfide occurs predominantly as troilite (FeS) nodules, bars or dumbbells in iron meteorites [1]. These nodules form when the immiscible sulfide liquid is entrained in crystals of Fe, Ni metal as they solidify. Therefore the crystallization timing of the Fe, Ni metal could be recorded in these nodules. Recent studies [2, 3] determined the timing of crystallization of IAB iron meteorites using the ²⁰⁵Pb-²⁰⁵Tl and ¹⁰⁷Ag-¹⁰⁷Pd decay systems. In the studies, both metal and sulfide were analyzed. The elements in the metal produced a correlation that was interpreted as an isochron, whilst the sulfide data were erratic and no correlation was found. A study of the trace elements in sulfide was undertaken last year [4] to elucidate why the sulfide phase exhibit disturbed Pd-Ag and Pb-Tl isotope systematics. Elemental variation did not deviate from a mean for a traverse across a nodule, with the exception of a few elements: Ni, Pb, Ag, and Cu. In this study we extend those results to elemental mapping of the sulfide to ascertain the nature of these element variations.

**Samples and Analytical Techniques:** The sulfide (FeS) nodule from [4] was analyzed using the UKCAN TOF-SIMS facility at the University of Manchester. The section was analyzed with a pulsed Au⁺ beam focused to a spot size of ~1 micrometer rastered over selected ~200 µm areas of the sample. Positive secondary ions were detected with a mass resolution of ~3500. High-resolution, back-scattered electron images were acquired using the Zeiss EVO 15LS variable pressure SEM at The Natural History Museum. Mineral phases were identified by EDS analysis on the same instrument.

**Results and Discussion:** The FeS nodule is rimmed almost continuously by schreibersite. TOF analyses were obtained from areas within the sulfide, at the sulfide-schreibersite and the sulfide-metal boundaries. Initial results show Cr streaks within the nodule itself and unrelated to the boundaries. These are attributable to microscopic inclusions of daubreelite, consistent with [1]. Further analysis reveals hotspots of Ni, Mn, and Cu in the FeS. These hotspots tend to occur close to the contacts with schreibersite or metal. Analysis by SEM reveal that MnS and (Fe,Ni)S may be the source of these hotspots. The Cu is associated with the MnS. The boundaries between FeS/schreibersite and FeS/metal also contain crushed (most likely sample preparation-derived) and weathered material. The erratic behavior of Ni, Pb and Ag through the FeS nodule may be attributable to the presence of microscopic inclusions of sulfides of those cations. Further analysis will be undertaken to determine the distribution and abundance of these inclusions. The presence of these microscopic sulfides may offer clues to the crystallization history of the FeS nodule during cooling of the parent body as a whole.

**References:** [1] Buchwald V. F. 1975. *Handbook of iron meteorites*. [2] Nielsen S. G. et al. 2006. *Geochimica et Cosmochimica Acta* 70:2643-2657. [3] Woodland et al. 2005. *Geochimica et Cosmochimica Acta* 69:2153-2163. [4] Benedix G. K. et al. 2010. *Meteoritics & Planetary Science* 45:A13.
## HYDROTHERMAL SYNTHESIS OF CUBANITE: IMPLICATIONS FOR FLUIDS ON COMET 81P/WILD 2 AND THE CI-CHONDRITE PARENT BODY

E. L. Berger¹, D. S. Lauretta¹ and L. P. Keller². ¹Lunar & Planetary Laboratory, Tucson, AZ 85721, USA, E-mail: elberger@lpl.arizona.edu. ²NASA Johnson Space Center, Houston, TX 77573, USA.

**Introduction:** The low-temperature form of CuFe₂S₃, cubanite, has been identified in the CI-chondrite and Stardust collections [1]. We report the first synthesis of cubanite under hydrothermal conditions relevant to the CI-chondrite parent body.

An aqueous system can be described by the interplay between pH, temperature, oxygen and sulfur fugacities ($fO_2$, $fS_2$). These four variables are interrelated; the specification of three of them de facto determines the fourth [2]. Temperatures of 50150 °C, pH of 7–10 and log $fO_2$ > 10⁻⁵⁵–10⁻⁷⁰ on the CI-chondrite parent body have been predicted from oxygen isotope measurements [3] and modeling of hydrous asteroids [4]. The presence of cubanite confers a temperature maximum, as it undergoes an irreversible phase transition at 210 °C [5,6].

**Methods:** We form Cu-Fe sulfides by controlling temperature, pH and $fO_2$. Experiments are run at 100, 150, and 200 °C over the pH range 7–10. Oxygen fugacity is held at the iron-magnetite and quartz-fayalite-magnetite buffers.

Compositions of the experimental products are obtained via EMPA on the Cameca SX-50 at LPL. Crystal structure is determined using FIB-TEM techniques at JSC on the FEI Quanta 3D 600 dual-beam FIB and the JEOL 2500SE 200kV FE-STEM.

**Results and Discussion:** Cubanite forms under aqueous conditions, in the presence of iron and magnetite, at a pH of 9 and temperatures of 150 and 200 °C. While only a minor sulfide constituent at 150 °C, it is the chief product at 200 °C with an average composition (at.%) of Cu₁₅.₇Fe₃₃.₆S₅₀.₇.

Two FIB sections were prepared, each containing numerous cubanite grains. Measurements of SAED patterns obtained at multiple orientations from individual grains are consistent with cubanite [5]. TEM analyses reveal extensive twinning. The experimentally produced cubanite, like the CI-chondrite cubanite [1], has a superstructure, likely based on cation ordering. There are superstructure reflections present along both a* and c*.

The successful synthesis of cubanite, under aqueous conditions, aids in refining the parameters (T, pH, $fO_2$ and $fS_2$) of the time-varying low-temperature fluids which existed on the CI-chondrite parent body.

**Acknowledgments:** This work was supported by NASA grants: NNX08AW48H (ELB) and NNX09AC60G (DSL PI).

**References:** [1] Berger E. L. et al. 2011. *Geochimica et Cosmochimica Acta* 75:3501–3513. [2] Barnes H. L and Kullerud G. 1961. *Economic Geology* 56:648–688. [3] Clayton R. N and Mayeda T. K. 1999. *Geochimica et Cosmochimica Acta* 63:2089–2104. [4] Zolensky M. E. et al. 1989. *Icarus* 78:411–425. [5] Fleet M. E. 1971. *Zeitschrift fur Kristallographie* 132:276–287. [6] Cabri L. J. et al. 1973. *Canadian Mineralogist* 12:33–38.

## 3D CHEMICAL ANALYSIS OF THE GUJBA CB CHONDRITE: COMBINING µ-XRF, EDS, EBSD, RAMAN, LA-ICP-MS, AND AFM/MFM

J. Berlin¹, A. Käppel¹, B. K. Hansen¹, T. Salge¹, S. Scheller¹, M. Falke¹, D. Goran¹, I. Nemeth¹, R. Chemnitzer¹, R. Tagle¹, U. Waldschläger¹, H. M. Chappell², R. H. Jones² and J. I. Goldstein³. ¹Bruker Nano GmbH, Schwarzschildstrasse 12, 12489 Berlin, Germany. ²Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA. ³Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA 01003, USA. E-mail: jana.berlin@bruker-nano.de.

Recent technological developments have significantly improved the possibilities to display the 3D structure of meteorites [e.g., 1]. Here, we present a 3D chemical study of the CB chondrite Gujba combining micro-X-ray fluorescence (μ-XRF) and energy dispersive X-ray spectrometry (EDS)/SEM with serial sectioning (manual grinding/polishing) to display not only the shape of 3D structures in the µm to mm range, but also to reveal their chemical composition. The time needed for data acquisition is minimized by using modern SDD technology with high input count rates. Furthermore, element maps are acquired as hyperspectral images (also called PTS: position-tagged spectroscopy)—a format that allows later offline-evaluation and quantification.

The Figure below shows some of our first results, emphasizing the different kind of information obtained with µ-XRF and EDS/SEM. The µ-XRF map of a complete 2D section of the sample was obtained within 1 h using an M4 Tornado. The green rectangle indicates the EDS acquisition site shown beneath. For this image, nine layers were mapped with a Quantax EDS system and reconstructed in 3D with Amira® software (section depth: 5 ± 2 µm; total time spent: 20 h).

Using the information from µ-XRF and EDS as a basis, selected regions of interest are examined with additional techniques, such as electron backscatter diffraction (EBSD), laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), Raman spectroscopy and atomic/magnetic force microscopy (AFM/MFM). The main goal of this project is to examine physical and chemical interactions between large metal and silicate clasts and impact melt matrix in CB meteorites. We aim to increase the quality of the analytical work by combining the information obtained with different techniques.

**Reference:** [1] Ebel D. S. et al. 2008. *Meteoritics & Planetary Science* 43:1725–1740.

<figure><img src="image_3.png" /></figure>
5147

EFFECTS OF METEOROID SHAPE ON THE PRODUCTION RATES OF COSMOGENIC NUCLIDES

J. Beňo¹ and J. Masarik¹. ¹Faculty of Mathematics, Physics and Informatics, Comenius University, Mlynska dolina F1, Sk-84248, Bratislava, Slovakia, E-mail: beno.juraj@gmail.com.

**Introduction:** The shape of the meteoroid irradiated in the space can be one of the factors influencing the production of the cosmogenic nuclides. Numerical simulations were done for spherical, cylindrical and elliptical geometries of irradiated object. Simulations show that statistically important differences in particle fluxes dependent on the shape of irradiated object and position of sample within it can be found. Results of simulations were compared with experimental data for St. Severin and other objects that had non-spherical shapes in space during their exposure times.

**Simulations:** The accurate modeling of the production processes is prerequisite for the physical interpretation of measured depth profiles of cosmogenic nuclides. Productions rates of cosmogenic nuclides are dependent on incident particle fluxes, the meteoroid's preatmospheric shape and size, and its bulk chemical composition. Size effects [1] and bulk chemical composition [2] effects were studied in details and therefore we concentrate our attention in this paper on effects of the parent body shape.

In this work the nucleon spectra are calculated by using the LCS/ MCNP [3]. Having calculated particle fluxes the production rates of cosmogenic nuclides are calculated by folding these spectra with experimental and theoretical cross sections of nuclear reactions leading to the production of particular nuclide.

The geometries investigated in this work were sphere with radius 27 and 45 cm which is that of a sphere with volume equal to the real St. Severin [5] and Knyahinya, respectively. Further ellipsoid with various ratios between semiaxes and with equal mass were also simulated. Model objects were divided into concentric shells with thickness 2.5 cm. In all shells proton and neutron fluxes were calculated. For the sphere particle fluxes were averaged over the whole volume of the shell. In the case of ellipsoid fluxes were calculated for cylinders with radius 1 cm and their axis identical with coordinate system axis. The last division of ellipsoid was motivated by the actual location of measured samples [4].

**Conclusions:** Production rates for the ellipsoids were lower than those for spheres. The differences between the ellipsoids and sphere varied with the nuclide. The shape of irradiated body influences the number of particles escaping from the investigated body without participating in particle cascade development. This can lead to the change of absolute value and the shape of differential particle fluxes. As in the case of ellipsoid there are some regions with larger and smaller curvature than is the curvature of sphere, there are also different numbers of escaping particles in this regions.

**References:** [1] Bhandari N. et al. 1993. *Geochimica et Cosmochimica Acta* 57:2361–2375. [2] Masarik J. and Reedy R. C. 1994. *Geochimica et Cosmochimica Acta* 58:5307–5317. [3] Prael R. E. and Lichtenstein H. 1989. *Los Alamos Report LA-UR-89-3014*, [4] Englert P. and Herr W. 1980. *Earth and Planetary Science Letters* 47:361–372. [5] Graf Th., Baur H. and Signer P. 1990. *Geochimica et Cosmochimica Acta* 54:2511–2520.

5053

COSMOGENIC NOBLE GASES IN CHONDRULES FROM CV AND CR CHONDRITES

U. Beyersdorf¹, M. Trieloff², U. Ott¹ and J. Bennett³. ¹Max-Planck-Institut für Chemie, Joh.-J.-Becherweg 27, D-55128 Mainz, Germany. E-Mail: uta.beyersdorf@mpic.de. ²University of Heidelberg, Germany. ³ANSTO, Lucas Heights, NSW 2234 Australia.

**Introduction:** Previous work [1–4] has provided hints for pre-irradiation of chondrules. While the evidence for pre-irradiation in parent body regoliths seems strong [4], significant uncertainties exist concerning a solar nebula setting. We have started a project to search for such effects in primitive meteorites. Here, we present data of two chondrules from Allende (CV3) and one chondrule from El Djouf 001 (CR2).

**Experimental:** Chondrules were separated using the freeze-thaw technique similar to [5]. INAA was used to determine abundances of important target elements. Noble gas measurements were performed on the same material analyzed by INAA thus eliminating problems of sample inhomogeneity. Data for bulk/matrix samples at identical shielding will be obtained later. Here, we compare our chondrule results with literature data for the bulk meteorites. Production rates were derived following [6].

**Results and Discussion:** Data for neon are summarized in Table 1. Concentration of ²¹Ne in chondrules is higher compared to the matrix because of the higher magnesium contents. Allende chondrules yielded cosmic-ray exposure (CRE) ages of 4.9 Ma, which agree with previous analyses [3, 4]. Since this is similar to the CRE age of Allende matrix [1, 3, 4] there is no evidence for pre-irradiation. The same is true for El Djouf 001, where the 7.2 Ma CRE age of our chondrule is in the range of ages previously reported for El Djouf 001 [7, 8].

In future work we will concentrate on CR3 chondrites of highly primitive nature and only incipient alteration [9].

**Acknowledgment:** We thank the DFG (OT171/6-1) for financial support.

**References:** [1] Das J. P. and Murty S. V. S. 2009. *Meteoritics & Planetary Science* 44:1797–1818. [2] Polnau E. et al. 2001. *Geochimica et Cosmochimica Acta* 65:1849–1866. [3] Eugster O. et al. 2007. *Meteoritics & Planetary Science* 42:1351–1371. [4] Roth A. S. G. et al. *Meteoritics & Planetary Science*. In press. [5] Grossman L. 2010. *Meteoritics & Planetary Science* 45:7–20. [6] Eugster O. 1988. *Geochimica et Cosmochimica Acta* 52:1649–1662. [7] Bischoff A. et al. 1993. *Geochimica et Cosmochimica Acta* 57:1587–1603. [8] Weber H. W. and Schultz L. 1991. *Meteoritics & Planetary Science* 26:406. [9] Abreu N. M. and Brearley A. J. 2008. Abstract #2013. 31st Lunar and Planetary Science Conference.

<table><caption>Table 1. Production rate and concentration of cosmogenic neon, T21 CRE ages, shielding parameter (²²Ne/²¹Ne)ₐ, Mg and Fe contents of chondrules from Allende (A) and El Djouf 001 (E). Uncertainties in the last digits are given in parentheses. P21 and ²¹Neₐ are given in 10⁻⁸ cc (g⁻¹Ma) and 10⁻⁸ cc g⁻¹, respectively.</caption><thead><tr><th></th><th>Weight<br>mg</th><th>Mg<br>wt%</th><th>Fe<br>wt%</th><th>P21</th><th>²¹Ne<sub>c</sub></th><th>(²²Ne/<br>²¹Ne)<sub>c</sub></th><th>T21<br>Ma</th></tr></thead><tbody><tr><td>A-Ch3</td><td>5.26</td><td>19.3</td><td>15.9</td><td>0.530</td><td>2.62 (10)</td><td>1.07 (1)</td><td>4.9</td></tr><tr><td>A-Ch6</td><td>1.17</td><td>20.6</td><td>16.0</td><td>0.499</td><td>2.45 (11)</td><td>1.07 (2)</td><td>4.9</td></tr><tr><td>E-Ch4</td><td>7.83</td><td>19.6</td><td>12.8</td><td>0.335</td><td>2.12 (9)</td><td>1.20 (2)</td><td>7.2</td></tr></tbody></table>
IMPACT OF METEOR SCIENCE OUTREACH IN NEPAL

S. Bhattarai. St. Xavier's College, Kathmandu, Nepal. E-mail: chikursharma@gmail.com.

The Chinese record of meteor shower dates back thousands of years from now, but the modern meteor science is still in its infancy in Asia. The South Asian countries, especially Nepal, lack the documentation of the shower details and meteorite strikes despite the fact that there might have been a number of potential meteorite strikes in these nations. Meteor outreach that has been carried out in Nepal since 2007 will be explained and its impact upon the societies will be analyzed. Major meteor shower analysis done so far from this part in optical domain will also be discussed in short. This paper also tries to present some of the evidence for meteorite falls in Nepal so far. Possible collaborations for the tracking of meteorites/fireballs will be discussed in detail.

SPECTRAL SIGNATURES OF MARTIAN METEORITES AND
WHAT THEY CAN TELL US ABOUT ROCKS ON MARS

J. L. Bishop¹, M. Parente² and V. E. Hamilton³. ¹SETI Institute, Mountain View, CA, USA. E-mail: jbishop@seti.org. ²Brown University, Providence, RI. ³Southwest Research Institute, Boulder, CO, USA.

**Introduction:** Martian meteorites provide important compositional information about crustal rocks and enable ground truthing of orbital data. Coordinated VNIR and mid-IR spectral analyses of the meteorites and mineralogy of Mars are important in order to fully understand the composition of the surface. Hamilton et al. [1] used TES data to identify several olivine and orthopyroxene-bearing regions near Nili Fossae, Ganges, Argyre and Hellas that are consistent with ALHA77005, Chassigny and ALH 84001 meteorites, as well as clinopyroxene-rich locations in parts of the Valles Marineris region and Syrtis Major resembling Nakhla-type meteorites. More recently, VNIR spectra collected by OMEGA and CRISM have been used to document mafic outcrops on Mars [2, 3]. Here we present a summary of the VNIR and mid-IR spectral properties of several Martian meteorites correlated with their compositions. These data are compared to regions on Mars where mafic outcrops with similar spectral characteristics are observed.

**Spectral Properties of Martian Meteorites:** We have acquired VNIR reflectance and thermal emissivity spectra of several Martian meteorites including: ALH 84001, EETA79001, Los Angeles, Dar al Gani 670, and NWA 1068. These samples contain pyroxenes ranging from orthopyroxene to pigeonite to augite, feldspar (and maskelynite), fayalitic and fosteritic olivine, silica, and glass. We are comparing the mineral modeling results of the two spectral regions with each other and with the meteorite petrology from other studies. Gaussian modeling is used for the electronic absorptions in the VNIR region spectra based on code developed by Sunshine et al. [4, 5] and adapted by Parente et al. [6]. Gaussian modeling of Martian meteorite spectra has revealed the presence of pyroxene, olivine and glass. Modeling of the thermal emission spectra is based on vibrational absorptions [7] and modeling of Martian meteorite spectra has shown the presence of feldspar, pyroxene, olivine and glass [1, 8].

**Coordinating Spectral Properties of Meteorites with the Surface of Mars:** Mafic outcrops are observed across the surface of Mars in non-dusty areas where the caprock has been eroded. Spectral analyses of Martian meteorites are assisting in interpreting these mafic outcrops. Many of these surface mafic outcrops are small; thus, the better spatial resolution of THEMIS and CRISM has enabled improved resolution of these mafic regions. CRISM can detect features due to olivine and pyroxene as they have Fe excitation bands. TES (and sometimes THEMIS) can detect these minerals as well as feldspar.

**References:** [1] Hamilton V. E. et al. 2003. *Meteoritics & Planetary Science* 38:871-885. [2] Mustard J. F. et al. 2005. *Science* 307:1594-1597. [3] Mustard J. F. et al. 2009. *Journal of Geophysical Research* 114, doi:10.1029/2009JE003349. [4] Sunshine J. M. et al. 1990. *Journal of Geophysical Research* 95:6955-6966. [5] Sunshine J. M. et al. 1993. *Icarus* 105:79-91. [6] Parente M. et al. 2011. *Planetary and Space Science* 59:423-442. [7] Ramsey M. S. and Christensen P. R. 1998. *Journal of Geophysical Research* 103:577-596. [8] Hamilton V. E. et al. 1997. *Journal of Geophysical Research* 102:25593-25603.
WHEN A CM GROUP METEORITE DECOMPOSES AT THE SEA FLOOR, WHAT WILL REMAIN?

K. Bjärnborg and B. Schmitz. Dept. of Geology, Lund University, Lund, Sweden. E-mail: karolina.bjarnborg@geol.lu.se.

**Introduction:** Recently our group has successfully recovered relict chromite grains (ca. 63–150 µm) from unmelted meteorites and micrometeorites that have fallen and decomposed on ancient sea floors, up to 470 Myr old, e.g. [1]. We dissolve 30–100 kg of slowly formed (ca. 2 mm kyr⁻¹) marine limestone in HCl and HF acid. This can yield an almost clean assemblage of resistant chromite from ordinary chondrites. Noble gas analyses of the chromite grains indicate that they mainly originate from micrometeorites [2]. Because carbonaceous chondritic material, primarily CM-group material, makes up a significant fraction of the micrometeorite flux today, we are testing if there are resistant minerals in this type of meteorite that can be recovered from ancient sediments. For different chondrite groups different oxide minerals have been reported. Chromite dominates the oxide fraction in ordinary chondrites, in enstatite chondrites oxides are rare, chromian spinel dominates in R chondrites, whereas in carbonaceous chondrites a wide range of oxide minerals occur, e.g. magnetite, chromite and different spinel group varieties [3, 4]. In CM2 meteorite Murchison spinel is primarily found in CAIs and as inclusions in olivine, chromite grains are found in matrix and in CAIs [5]. Here we further evaluate what is the case for CM meteorites, with focus on large spinel grains.

**Methods:** Two four-gram samples of CM2 chondrite Acfer 331 were treated in strong HF acid for 24 h. All spinel grains >63 µm were recovered from the residue under an optical microscope. The grains were mounted in epoxy, polished flat and analyzed with SEM-EDS, see [1].

**Results:** The samples contained 17 and 20 spinel grains >63 µm, respectively. Most grains are 63–150 µm, a few grains reach ca. 250 µm. Pink and colorless grains dominate, but red and blue grains also exist. The grains are all rich in Al and Mg whereas Cr and Fe contents are low: Mg 13.2–14.6 at%, Al 24.8–28.3 at%, Cr up to 3.4 at% and Fe up to 1.2 at%. Ti and V contents are mostly low, but Ti occurs up to 0.1 at% and V up to 0.2 at%. The grains richer in Cr are mostly pink or red, whereas colorless grains generally are poorer in Cr. The grains are often heterogeneous in composition; in backscatter mode they often show patchy or gradational variations, a few grains show zoning. The within-grain variations reach maximum 1.5 at% for Cr and 0.3 at% for Fe.

**Conclusions:** A CM meteorite of one gram being dissolved on the sea floor may thus leave behind ca. four to five relict spinel grains >63 µm. These grains can be recovered from sediment samples with the same methods we use for recovering ordinary chondritic chromite. As a comparison, chromite makes up 0.25 wt% of ordinary chondrites [6], but this includes chromite grains in all size fractions. Apparently our approach of recovering ancient spinels can be extended to reveal the flux variations through Earth history of several of the meteorite types that fall on Earth today.

**References:** [1] Cronholm A. and Schmitz B. 2010. *Icarus* 208:36–48. [2] Heck P. R. et al. 2008. *Meteoritics & Planetary Science* 43:517–528. [3] Rubin A. E. 1997. *Meteoritics & Planetary Science* 32:231–247. [4] Simon S. B. et al. 1994. *Geochimica et Cosmochimica Acta* 58:1313–1334. [5] Fuchs L. H. et al. 1973. *Smithsonian Contributions to Earth Sciences* 10:1–39. [6] Keil K. 1962. *Journal of Geophysical Research* 67:4055–4061.

SECONDARY PROCESSES WITHIN THE PARIS CM CHONDRITE

I. Blanchard¹,², M. Gounelle¹, M. Bourot-Denise¹ and A. Kearsley³. ¹LMCM, UMR 7202, CNRS & MNHN, CP 52, 57 rue Cuvier, 75005 Paris, France. E-mail: gounelle@mnhn.fr. ²IPGP, PRES SPC, Université Paris-Diderot, 4 Place Jussieu, 75005 Paris. ³IARC, Department of Mineralogy, Natural History Museum, London SW7 5BD, UK.

**Introduction:** The Paris meteorite is a pristine rock whose petrography, oxygen isotopic and bulk chemical compositions are compatible with a classification as a CM chondrite [1, 2]. Based on petrography and mineral compositions it was initially classified as a 3.0 chondrite [2, 3]. This classification is however ambiguous as it has been known for a while that hydrothermalism and thermal metamorphism might be unrelated within CM chondrites [4]. The goal of the present work is to place Paris on two *distinct* alteration scales (hydrothermalism and thermal metamorphism) proposed for CM chondrites by [5–7].

**Methods:** We investigated the Paris polished blocks 2010-05 and 2010-07 prepared at MNHN. X-ray mapping was done at NHM London [8]. Standard SEM and EMPA techniques were performed in Paris itself.

**Results:** As previously noted, Paris is a breccia whose alteration properties vary at the cm-scale [1, 2]. Matrix is made of a mixture of abundant phyllosilicates, tochilinite and other minerals that cannot be resolved individually by the techniques we have used so far. Chondrule mesostasis is usually turned into phyllosilicates. Paris has a Fe-Ni metal abundance of about 1%. Metal is predominantly kamacite with Ni <10 wt% and Co <0.5 wt%. Metal grains often have an oxidized rim. Iron sulfides are pyrrhotite and pentlandite. The latter appears as rims around or lamellae within the former. PCPs are abundant as clumps, and have elevated S/SiO₂ (0.7) and FeO/SiO₂ (4.1) ratios. Olivine mean Cr₂O₃ content is 0.37 wt% (σ = 0.08). Lacking matrix analyses with S <2 wt%, we were not able to calculate Paris Mineralogical Alteration Index [9].

**Discussion:** All these properties place Paris among one of the least altered CM chondrite on the *hydrothermal* scale designed by [7]. On that scale, it is at least a 2.6 rock, possibly a 2.7 or 2.8 given the chemical composition of PCPs. Paris cannot be considered as a (hydrothermal) type 3.0 given the large abundance of secondary minerals such as phyllosilicates or tochilinite. Metal and sulfide compositions, as well as their petrographic relationships, place Paris in between the groups A and B defined by [6] on the basis of the thermal metamorphism experienced. It shows features indicative of both an absence of heating and a moderate amount of heating. Thermal metamorphism temperatures of group B are below 700°C [5]. We strongly recommend that two different scales are used for hydrothermal alteration and thermal metamorphism experienced by CM parent-bodies, be they cometary or asteroidal [10]. We propose that Paris is a CM 2.7/2.8 A/B chondrite, the number referring to hydrothermal alteration and the letter to thermal metamorphism.

**References:** [1] Zanda B. et al. 2011. Abstract #2040. 42nd Lunar and Planetary Science Conference. [2] Bourot-Denise M. et al. 2010. Abstract #1683. 41st Lunar and Planetary Science Conference. [3] Zanda B. et al. 2010. Abstract #5312. *Meteoritics & Planetary Science* 45 (Suppl). [4] Akai J. 1990. *Proceedings of the NIPR Symposium on Antarctic Meteorites*. 3rd 55–68. [5] Nakamura T. 2005. *Journal of Mineralogical and Petrological Sciences* 100 260–272. [6] Kimura M. et al. 2011. *Meteoritics & Planetary Science* 46:431–442. [7] Rubin A. E. et al. 2007. *Geochimica et Cosmochimica Acta* 71:2367–2382. [8] Kearsley A. T. et al. 2011. *Meteoritics & Planetary Science*, This conference. [9] Browning L. B. et al. 1996. *Geochimica et Cosmochimica Acta* 60:2621–2633. [10] Gounelle M. 2011. *Elements* 7:29–34.
## HETEROGENEOUS SHOCK IN POROUS CHONDRITES: IMPLICATIONS FOR ALLENDE MAGNETIZATION

P. A. Bland¹, A. R. Muxworthy¹, G. S. Collins¹, J. Moore¹, T. M. Davison² and F. J. Ciesla². ¹IARC, Dept. Earth Science & Engineering, Imperial College London, SW7 2AZ, UK. E-mail: p.a.bland@imperial.ac.uk. ²Dept. Geophysical Science, University of Chicago, 5734 South Ellis Av., Chicago, IL 60430, USA.

**Introduction:** Paleomagnetic data from Allende has been interpreted as suggesting an internally generated field within the parent asteroid, and a liquid metallic core [1–3] i.e., the CV parent body is a differentiated object [3,4]. Impact-generated fields were considered as a possible mechanism but excluded [3] based on the absence of macroscopic shock textures in Allende (S1) [5].

**Results and Discussion:** Our paleomagnetic analysis of Allende is consistent with previous studies [3], but we also find evidence to suggest that the strong intrinsic magnetic anisotropy may be coincident with a pervasive uniaxial crystallographic fabric delineated by oriented matrix grains that was plausibly impact-induced [6]. Based on EBSD analysis we can quantify pre-compaction porosity, and by modeling impact-induced porosity reduction [7] we can define the continuum shock pressure and temperature that Allende must have experienced. Assuming a (conservative) initial temperature of 300 K, the fabric/porosity data would be consistent with P (shock) of 1.7 GPa and T (final) of 553 K (below the Curie T for pyrrhotite, the principle magnetic carrier). But note that these are continuum P&T estimates—valid at large scales. At sub-µm scales in porous materials—relevant to matrix—shock is highly heterogeneous, with temperature excursions of 2–3 × above background commonly observed [8–10], i.e., pyrrhotites should have experienced temperature excursions of 180–430 K above their Curie T. Our own 2D meso-scale numerical simulations of shock waves propagating through chondrule/matrix mixtures support this: matrix experiences much higher peak shock P&T than chondrules, and shows significant heterogeneity in shock effects. There is evidence for this in Allende: TEM studies of matrix indicate high shock P [11] and T [12]. High continuum T [13] is not effective in recording an impact-induced field: cooling is too slow. High transient T is, as matrix pyrrhotites would equilibrate rapidly with the lower continuum T.

Our P (shock) estimates allow us to constrain the position of Allende within the parent body with respect to various impact scenarios: 1.7 GPa pressures are encountered in the breccia lens. The magnitude of an impact generated field can be estimated based on scaling relations derived from experimental data [14]. Allende would have been proximal to a significant impact generated field. In summary, heterogeneous shock effects in sub-µm porous matrix may be an explanation for some aspects of chondrite magnetization. This process has implications for lithification of pristine chondrites [15] and for the survival of pre-solar grains.

**References:** [1] Butler R. F. 1972. *Earth and Planetary Science Letters* 17:120–128. [2] Funaki M. and Wasilewski P. 1999. *Meteoritics & Planetary Science* 34:A39. [3] Weiss B. P. et al. 2010. *Space Science Reviews* 152:341–390. [4] Elkins-Tanton L. T. et al. 2011. *Earth and Planetary Science Letters* 305:1–10. [5] Scott E. R. D. et al. 1992. *Geochimica et Cosmochimica Acta* 56:4281–4293. [6] Bland P. A. et al. 2011. *Nature Geoscience* 4:244–247. [7] Davison T. M. et al. 2010. *Icarus* 208:468–481. [8] Kieffer S. W. 1971. *JGR* 76:5449–5473. [9] Baer M. R. and Trott W. M. 2002. AIP Conference Proceeding 620:517–520. [10] Sharp T. G. and DeCarli P. S. 2006. *MESS II:pp.* 653–677. [11] Müller W. F. 1978. 9th Lunar and Planetary Science Conference pp. 609–610. [12] Brenker F. E. and Krot A. N. 2004. *American Mineralogist* 89:1280–1289. [13] Folco L. et al. 2004. *Geochimica et Cosmochimica Acta* 68:2379–2397. [14] Crawford D. A. and Schultz P. H. 1999. *International Journal of Impact Engineering* 23:169–180. [15] Bland P. A. et al. 2007. *Meteoritics & Planetary Science* 42:1417–1427.

## IN SITU SIMS OXYGEN ISOTOPE MEASUREMENTS OF ZONED OLIVINES IN THE TAGISH LAKE METEORITE CHONDRULES

A. Blinova¹, R. Stern¹,² and C. D. K. Herd¹. ¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada. ²Canadian Centre for Isotopic Microanalysis (CCIM), University of Alberta, Edmonton, Canada. E-mail: blinova@ualberta.ca.

**Introduction:** The Tagish Lake (TL) meteorite is a brecciated carbonaceous Type 2 chondrite consisting of several lithological variations [e.g., 1, 2] with different chondrule abundances [3]. CM-like specimen TL5b contains the most abundant number of chondrule-like objects up to 500 µm diameter set in a phyllosilicate-rich matrix. A large (2000 µm diameter), porphyritic type I (Fe-poor) chondrule dominated by pyroxene and olivine (Fo₉₅–₉₆) set in a slightly altered, Al-rich mesostasis was found in 5b during processing [1]. The more altered specimen TL11i contains less chondrule-like objects and is dominated by altered matrix. This specimen contains a porphyritic chondule with zoned olivines [1]. The largest olivine has a euhedral shape and exhibits oscillatory zoning (Fo₇₀–₇₉). Other olivine grains within this chondrule exhibit normal zoning from Mg-rich cores (Fo₇₉–₉₉) to Fe-rich rims (Fo₆₁–₇₄).

**Methods:** Oxygen analyses were obtained using a Cameca IMS1280 ion microprobe at the CCIM, utilizing a approximately 12 µm diameter ion Cs ion probe, and olivine reference materials for IMF correction (Fo₇₅ and Fo₉₃). A Zeiss EVO 15 scanning electron microscope was utilized for microstructural characterization.

**Discussion of Results:** Here we present first three-oxygen isotope results on olivines obtained from the CCIM facility.

**Chondrule in TL5b:** We have analyzed a total of 14 distinct olivine grains within this chondrule, with one to four analyses on each olivine. All data form a cluster just to the left of CCAM line on a three-O isotopic plot, with the average $\Delta^{17}O = -2.6 (\pm 0.5)$ and mean in $\delta^{17}O$ and $\delta^{18}O$ of -1.7 and 1.7‰, respectively.

**Chondrule in TL11i:** We analyzed seven zoned olivine grains (total 14 analyses). The average of eleven analyses on zoned grains (Fo₆₀–₇₉) is $\Delta^{17}O = -1.73 (\pm 0.69)$. In contrast to the TL5b data, these analyses are more ¹⁷,¹⁸O-enriched and make a scattered cluster (mean $\delta^{17}O = 1.0 \pm 0.6$ ‰ and $\delta^{18}O = 5.3 \pm 0.2$ ‰) that fall on the CCAM line. Three other grains have ¹⁶O-enriched relict cores. One of these olivine grains contains a relict core with diffuse boundaries and shows the most extreme Fo and $\Delta^{17}O$ differences in the core and rim amongst these olivine grains: core (Fo₉₈) has $\Delta^{17}O = -8.6 (\pm 0.68)$, and the rim (Fo₇₁) has $\Delta^{17}O = -1.9 (\pm 0.62)$. This core is more ¹⁶O-enriched than data for TL from [4, 5], but not as extreme as the data on “enriched” grains by [6]. It appears that the relict cores from these three olivine grains condensed in the ¹⁶O-enriched nebular environment, whereas the rims formed later in a ¹⁶O-depleted environment, perhaps concurrently with other zoned olivine grains during melting and formation of this chondrule.

**References:** [1] Blinova A. et al. 2009. Abstract #2039. 40th Lunar Prospector Spacecraft. [2] Izawa M. R. M. et al. 2010. *Planetary and Space Science* 58:1347–1364. [3] Blinova et al. 2010. *Meteoritics & Planetary Science* 45:A17. [4] Leshin et al. 2001. Abstract #1843. 32nd Lunar Prospector Spacecraft. [5] Russell et al. 2010 *Geochimica et Cosmochimica Acta*, 74:2484–2499. [6] Engrand et al. 2001. Abstract #1568. 32nd Lunar Prospector Spacecraft.
## CHROMITE AND ILMENITE FRACTIONATION TRENDS IN HOWARDITES

J. S. Boesenberg. Earth and Planetary Sciences, American Museum of Natural History, Central Park West, New York, NY 10024, USA. E-mail: bosenbrg@amnh.org.

**Introduction:** At the 42nd Lunar and Planetary Science Conference, [1] presented pyroxene Ti-Cr and Fe-Mg data from 11 howardites that suggests many howardites do not consist of unrelated rock fragments that were subsequently lithified, but rather contain diogenitic and eucritic fragments that form continuous and extensive fractional crystallization trends. Up to six types of patterns in pyroxene have been found so far. Here, we report on the chromite and ilmenite phases within most of those howardites to determine whether they are in agreement with this fractionation relationship. These minor phases are important because the timing of chromite and ilmenite crystallization prior to, during or after pyroxene formation is the primary cause of any change in the Ti-Cr versus Fe-Mg pattern observed in the pyroxene fractionation trends.

**Results and Discussion:** Oxides from eight of the 11 howardites used in [1] were analyzed by electron microprobe. These include grains from lithic clasts and mineral fragments from Winterhaven, Bialystok, Le Teilleul, Petersburg, Bholghati, Chaves, Luotolax and Zmenj. The Fe-Mg-Ti-Cr-Al systematics within the chromites vary moderately between samples in modal abundance and compositional range when plotted in a Fe₂TiO₄-FeCr₂O₄-FeAl₂O₄ ternary. Chromite Ti concentrations are the most variable and increase with Fe/(Fe + Mg). Typically Al decreases with FFM and Cr is either somewhat steady or decreases. Bholghati, Luotolax, Zmenj and Le Teilleul all contain chromites with wide compositional ranges that include very Mg-rich (FFM approximately 64-72) grains. These Mg-rich grains are consistent with derivation from the diogenitic component of the howardites based on the high proportion of diogenitic pyroxene previously measured in those samples [1-3]. Very Fe-rich chromites (FFM > 0.9) found in many samples contain large variations in their concentrations of Al, Cr as well as Ti, possibly reflecting metamorphic alteration towards ulvospinel [4].

Compared to the compositional variations found in the chromites, the ilmenites are remarkably uniform. The only significant variation between the different howardite ilmenites is their range of Fe/(Fe + Mg) ratios, with Winterhaven having the most Mg-rich (FFM approximately 86-95) and Bialystok the most Fe-rich (FFM approximately 95-97). All eight howardites have very low minor/trace concentrations in elements such as Al, Cr and Ca, with only Bialystok having mildly variable Al and Cr.

**Conclusions:** The distribution of chromite compositions is consistent with a fractional crystallization model to produce the full array of lithologies measured in the pyroxene. The ilmenites, though containing little trace element evidence measurable by electron microprobe, are also consistent with fractionation based on their range of FFM ratios.

The one unusual sample in this study is Zmenj, which shows a more convincing continuous compositional fractionation trend in chromite than pyroxene, possibly indicating the section analyzed is unrepresentative of this howardite.

**References:** [1] Boesenberg J. S. and Erb I. R. 2011. Abstract #1017. 42nd Lunar and Planetary Science Conference. [2] Mittlefehldt D. W. et al. 1998. *Planetary Materials*, Reviews in Mineralogy edited by J. J. Papike, pp. 4-1-4-195. [4] Mittlefehldt D. W. 1994. *Geochimica et Cosmochimica Acta* 58:1537-1552. [3] Arai T. et al. 1998. *Antarctic Meteorite Research* 11:71-91.

## POSSIBLE DEBRIS OF HIGHLY EVOLVED LITHOLOGIES IN HOWARDITES: ADDITIONNAL EVIDENCE OF K-RICH AREAS ON 4 VESTA?

M. Bohn¹, A. Yamaguchi², A. Jambon³, J. A. Barrat¹, C. Bollinger¹ and O. Boudouma³. ¹Université Européenne de Bretagne, UBO-IUEM, CNRS UMR 6538, Place Nicolas Copernic, 29280 Plouzané Cedex, France. E-mail: barrat@univ-brest.fr. ²NIPR and Graduate University for Advanced Sciences, Tachikawa, Tokyo 190-8518, Japan. ³Université Pierre et Marie Curie- Paris 6, 4 place Jussieu, 75252 Paris cedex 05, France.

4 Vesta will be soon studied by the Dawn space probe, and is generally believed to be the parent body of the HED meteorites [1]. During the last forty years, these rocks have been used to constrain the composition and the history of this large asteroid. It is generally assumed that its surface exposes, in addition to a variety of breccias (e.g., howardites), basaltic rocks (eucrites) and ultramafic cumulates. The rocks exposed on 4 Vesta are certainly not restricted to the known HED lithologies. Recently, the studies undertaken on impact glasses found in howardites have shown that K-rich areas, with possibly felsites or granites, are probably exposed on 4 Vesta [2, 3]. We have found unusual clasts in the howardites where K-rich glasses were found (NWA 1664 and NWA 1769). These low-Mg clasts display a fine grained-subophitic texture. They are generally made of assemblages of silica-fayalitic olivine-hedenbergite (pyroxferroite breakdown materials (PBM) ?), partly destabilized low-Mg pyroxenes, ilmenites, silica, troilites, merrillites, Cl-apatites, barian K-feldspars (hyalophane), with accessory baddeleyites and zircons. In two cases, high Ca pyroxenes displaying fine exsolution lamellae of low-Ca pyroxenes have been observed. REE abundances of PBMs are very high. They range from 10 × CI to about 100 × CI for HREEs. Merrillites too (> 10,000 × CI for LREEs, > 4000 × CI for the HREEs) and apatites, which are frequent, are unsurprisingly rich in REEs too. Thus, these clasts are undeniably REE-rich, P-rich, and K-rich. One may suggest these “KREEPy” clasts are debris of areas of mesostasis from eucritic lithologies. Such an explanation is not satisfactory because: (1) these clasts and mesostases from equilibrated or unequilibrated eucrites are texturally distinct; (2) ilmenites in the clasts display lower Mg abundances than those found in eucritic mesostasis; (3) hyalophane has never been described in eucrites; K-feldspars are exceptional in eucrites, and are Ba poor. Thus, these clasts are more likely remnants of not-previously-described highly-evolved lithologies.

**References:** [1] McCord T. B. et al. 1970. *Science* 168:1445-1447. [2] Barrat J. A. et al. 2009. *Meteoritics & Planetary Science* 44:359-374. [3] Barrat J. A. et al. 2009. *Geochimica et Cosmochimica Acta* 73:5944-5958.
5491

## HOW HETEROGENEOUS IS ORGUEIL REALLY?

C. Bollinger¹, B. Zanda², F. Moynier³, J. A. Barrat¹ and C. Liorzou¹. ¹Université Européenne de Bretagne, UBO-IUEM, CNRS UMR 6538, Place Nicolas Copernic, 29280 Plouzané Cedex, France. E-mail: barrat@univ-brest.fr. ²MNHN & CNRS UMR7202, 61, rue Buffon, 75005 Paris, France. ³Dept. of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, USA.

We have obtained six chips of Orgueil prepared from five different stones, and weighing between 600 mg to more than 1 g. This sampling has been complemented by a sample of Ivuna and Alais. The samples have been analyzed by ICP-AES (Ti, Al, Fe, Mn, Mg, Ca, Na, P, Cr, Co, Ni) and ICP-MS (40 minor and trace elements) using procedures adapted from [1]. They allows the determination of abundances and ratios of trace elements in chondrites with a good reproducibility (< 3% for abundances, < 1.5% for ratios such as La/Sm, Eu/Eu*, Zr/Hf or Nb/Ta). At first glance, our samples of Orgueil display only small range of abundances. Among the 46 elements determined, only 4 show RSDs > 10% (Na, K, Rb, U), and 10 show RSDs between 5 and 10% (Mn, Ca, P, Sr, Nb, Cs, La, Ta, W, and Pb). In the case of W, the RSD is close to the expected analytical reproducibility. In the other cases, the RSDs reflect real small heterogeneities of the meteorite: for example, the results obtained for Mn by ICP-AES are confirmed by ICP-MS (0.17–0.21 wt%); Na (ICP-AES, 0.29–0.59 wt%) is strongly correlated with K (ICP-MS, 365–677 ppm). The range of concentrations is reduced when one of the samples is removed. The later seems abnormal. It displays the lowest alkali abundances and is slightly more light REE enriched than the other chips. The average composition calculated using the five other chips is very close to the previously compiled values for CI chondrites or high quality ID analyses (e.g., [2–5]), and can be used as a CI normalization reference. Possible interlaboratory biases can be eliminated if the analyses are adjusted to the same international standard values. Unsurprisingly, Ivuna displays major and trace element abundances alike the Orgueil average. To the opposite, our Alias chips is unusual and exhibits a convex REE pattern when normalized to our Orgueil average.

The Cu and Zn isotopic compositions have been determined on the same powders at Saint Louis. TM66Zn and TM65Cu values range respectively from 0.42 to 0.54 per ml, and from 0.02 to 0.24 per ml, in agreement with previous studies [6,7].

**References:** [1] Barrat J. A. et al. 2008. *Meteoritics & Planetary Science* 43:1759–1775. [2] Anders E. and Grevesse N. 1989. *Geochimica et Cosmochimica Acta* 53:197–214. [3] Lodders K. et al. 2009. In Landolt-Börnstein, new series, vol. VI/4B. [4] Beer H. et al. 1984. *Physical Review* 30:464–478. [5] Münker et al. 2003. *Science* 301:67–84. [6] Luck J. M. et al. 2003. *Geochimica et Cosmochimica Acta* 67:143–151. [7] Luck J. M. et al. 2005. *Geochimica et Cosmochimica Acta* 69:5351–5363.

5125

## STRUCTURE OF PRIMITIVE POLYAROMATIC CARBONACEOUS MATTER: IDPs VERSUS CHONDRITES.

L. Bonal¹*, E. Quirico¹ and G. Montagnac². ¹Institute of Planetology and Astrophysics of Grenoble (IPAG), UJF-Grenoble 1 / CNRS-INSU, France. ²Laboratoire de Sciences de la Terre—ENS Lyon, France.
*E-mail: lydie.bonal@obs.ujf-grenoble.fr.

Resolving the origin(s) and initial characteristics of the organic components, and determining whether primitive cosomaterials accreted similar organic precursors are key points in cosmochemistry. As the structure of polyaromatic carbonaceous matter (PCM) is controlled by (i) the alteration episode of the hosting rock and (ii) the organic precursor [e.g., 1,2], it is a powerful tracer to distinguish pre-accretion heterogeneities from post-accretion modifications. Interplanetary Dust Particles (IDPs) collected in the Earth’s stratosphere are fragments of primitive asteroids and comets that are potentially unsampled by meteorites. By characterizing the structure of the PCM in IDPs and comparing it to primitive chondrites, we want to better constrain and understand (i) the heterogeneities within and among IDPs; (ii) the asteroid-comet continuum in terms of organic precursor.

Seventeen clustered IDPs were received in small drops of silicon oil on glass plates. Preliminary Raman spectra ($\lambda_{laser} = 514$ nm) were directly acquired with no further sample preparation (i.e., no hexane rinsing). Extreme care was taken to avoid any laser damage. Further Raman characterizations correlated to chemical and isotopic maps are planned after the IDP extractions and specific sample preparations. The preliminary Raman examination already reveals clear differences among our IDPs. Three main types of Raman spectra were obtained: (i) Group I: presence of the typical D and G carbon bands superimposed onto a large fluorescence background (3/17); (ii) Group II:D and G carbon bands with no significant fluorescence background (9/17); (iii) Group III: absence of carbon bands but presence of mineral bands (olivine and hematite, 2/17). No identifying Raman features were obtained on 3 IDPs. Spectral parameters of the carbon bands were obtained through our usual analytical treatment [e.g., 2]. Fluorescence spectra of Group I IDPs were obtained.

IDPs from Groups I and II are all characterized by the presence of PCM. The raw spectra exhibit variable spectral valleys, relative intensities and widths of the D and G bands. This reflects a variable structural order of the PCM. The broad carbon bands and shallow spectral valleys are typical of poorly structurally ordered PCM. Raman spectra of the PCM in type 1 and 2 chondrites (CI, CM, CR) are typically characterized by a high fluorescence background, in contrast to type 3 chondrites. Thus the absence of significant fluorescence background in the Raman spectra of Group II IDPs clearly differentiates them from CI, CM, and CR. The Raman spectra of the IDPs do not match either with type 3 chondrites. Indeed, the D and G bands spectral parameters do not follow the trends as defined by the type 3 chondrites. These differences can be interpreted in terms of variable organic precursors and/or thermal events (nebular, parent body, atmospheric entry) between IDPs and chondrites.

This preliminary study reveals a real structural variability of the PCM in our IDPs collection, similarly to previous observation [3,4]. Further Raman characterization (mapping, multi-wavelength analysis) on the extracted dusts will allow us to refine the differences compared to chondrites.

**References:** [1] Wopenka and Pasteris 1993. *American Mineralogist* 78:553. [2] Bonal et al. 2006. *Geochimica et Cosmochimica Acta* 70:1849. [3] Wopenka 1988. *Earth and Planetary Science Letters* 88:221. [4] Quirico et al. 2005. *Planetary and Space Science* 53:1443.
THERMAL DEGRADATION OF N-RICH POLYMERIC SOLIDS: CONSTRAINTS ON THE FORMATION OF INSOLUBLE ORGANIC MATTER

J-Y. Bonnet¹, E. Quirico¹, A. Buch², C. Szopa³, N. Fray⁴, H. Cottin⁴ and R. Thissen¹. ¹Institut de Planetologie et d'Astrophysique de Grenoble, France. E-mail: bonnetjy@obs.ujf-grenoble.fr. ²Ecole Centrale Paris, France. ³LATMOS, Guyancourt, France. ⁴LISA Univ. Paris XII France.

**Introduction:** Insoluble organic matter (IOM) in carbonaceous chondrites and stratospheric IDPs is the end-product of a complex history that took place in the presolar molecular cloud and/or in the solar nebula, followed by the action of various post-accretional processes [1,2]. Nitrogen is a minor element in IOM, with abundance ranging from ~1 up to 20 wt% in some IDPs [1,3]. N-bearing heterocycles and cyanide have been identified, while amino groups were not [4,5]. The polyaromatic structure of IOM suggests a formation process through thermal degradation of one or several organic precursors, which were possibly nitrogen rich. To examine this scenario, N-rich polymeric solids were thermally degraded at various temperatures, simulating short duration (~ hour) thermal processes in the solar nebula.

**Experiments:** Analogs selected for this study were tholins (SA90) and HCN polymers that are N-rich polymeric organic solids [6]. They were heated at 300, 500 700 and 1000 °C under an argon atmosphere. Combustion analysis, infrared and Raman micro-spectroscopy (244 and 514 nm excitations) were subsequently performed to measure the N/C ratio, chemical composition (including nitrogen speciation) and the degree of organization of the carbonaceous skeleton of the thermally processed samples.

**Results:** The N/C ratio was found to dramatically decrease upon heating. However, at 500 °C, the N/C ratio and the abundance of amine functional groups were still large with respect to chondritic IOM, while alkyl groups and almost all cyanides had disappeared. This suggests that the bulk chondritic IOM could not be formed from an N-rich compound like a HCN polymer. 514 nm Raman spectra exhibit the first order carbon bands G and D and point to very disordered carbon materials. The width of the G band (FWHM-G) was found to decrease upon heating and its position ωG to increase, and both parameters linearly correlate. This evidences the presence of structural changes during the carbonization process, i.e., aromatization and growth of the polyaromatic units. This also supports the idea that IOM experienced thermal processing in the solar nebula, as this linear correlation between FWHM-G vs ωG was previously reported for OM in Antarctic micrometeorites [7]. The presence of materials similar to thermally processed tholins or HCN polymer in the N-rich fraction of stratospheric IDPs OM cannot be excluded. Hence, Raman spectra, using 514 and 244 nm excitations, are similar for both types of materials. A better understanding of nitrogen speciation in stratospheric IDPs is necessary to go further. A study of kinetic effects on day/month time scale is under way.

**References:** [1] Alexander CMOD et al. 2007. *Geochimica et Cosmochimica Acta* 71:4380-4404. [2] Huss G. et al. 2003. *Geochimica et Cosmochimica Acta* 67:4823-4848. [3] Aléon J. 2003. *Geochimica et Cosmochimica Acta* 67:3773-3783. [4] Remusat L. et al. 2005. *Geochimica et Cosmochimica Acta* 69:3919-3932. [5] Quirico et al. 2008. *IAU proceedings 251* 409–416. [6] Quirico et al. 2008. *Icarus* 198:218–231. [7] Dobrica et al. *Meteoritics & Planetary Science*, in press.

LONG, STRANGE TRIPS: PARTICLE TRAJECTORIES IN A MARGINALLY GRAVITATIONALLY UNSTABLE SOLAR NEBULA

A. P. Boss¹, C. M. O'D. Alexander¹ and M. Podolak². ¹DTM, Carnegie Institution, 5241 Broad Branch Road NW, Washington, DC 20015, USA. ²Tel Aviv University, Israel. E-mail: boss@dtm.ciw.edu.

**Introduction:** The formation of the solar system's gas giant planets appears to require that the solar nebula was massive enough at some phase to become at least marginally gravitationally unstable (MGU), whether the giant planets formed by core accretion [1] or disk instability [2]. Previous models have shown that such a MGU disk is capable of the rapid radial transport of gas and small particles both inward and outward [3, 4], as well of the rapid homogenization of isotopic spatial heterogeneity associated with isotopic injection from, e.g., supernova shock waves [5, 6]. The previous models used a color field equation to follow the trajectories of gas particles and dust grains small enough to remain coupled to the gas. Here we present the first results of MGU disk models where the particles are represented as finite-size bodies that interact with the disk gas by gas drag [7], while moving on Keplerian orbits, subject to the gravity of the protosun and of the simultaneously evolving MGU protoplanetary disk.

**Models:** Three new sets of 3D MGU disk models with particle sizes ranging from 1 mm to 1 m have been computed, or are currently underway, for disks with sizes of either 1 to 10 AU or 10 to 40 AU. The particles orbit in the disk midplanes. In order to speed the disk evolutions, the initial disk temperature distributions are held fixed, while the disk gas evolves temporally in three spatial dimensions, allowing the 10 AU disk models to be followed approximately 1000 yr in time and the 40 AU disk models for approximately 10,000 yr.

**Results:** For the 10 AU disks, particles with sizes of 1 mm, 1 cm, or 10 cm that are initially close together can remain close together for 100s of years, exhibiting "closed system" behavior, while clumps of 1 m-sized bodies tend to disperse on these time scales. For all sizes, a large fraction of the trajectories move either outward to the outer grid boundary, or inward to the inner grid boundary, on the same time scales. Individual particles can move inward and outward in the disk repeatedly on eccentric orbits, sampling a wide range of nebular densities and temperatures. For the 40 AU disks, particles initially distributed throughout the disk's radial extent tend to either migrate to the outer disk boundary, or hover on orbits around 25 AU, with the smaller particles often migrating to the inner disk boundary at 10 AU, over time scales of 1000s of years.

**Conclusions:** These models imply that MGU disks may explain the cosmochemical facts that small, refractory grains can be transported outward to great distances [8], and can even make long, strange trips back and forth throughout the nebula [9].

**References:** [1] Chambers J. E. 2008. *Icarus* 198:655. [2] Boss A. P. 2010. *The Astrophysical Journal* 725:L145. [3] Boss A. P. 2007. *The Astrophysical Journal* 660:1707. [4] Boss A. P. 2008. *Earth and Planetary Science Letters* 268:102. [5] Boss A. P. et al. 2010. *The Astrophysical Journal* 708:1268. [6] Boss A. P. and Keiser S. A. 2010. *The Astrophysical Journal* 717:L1. [7] Haghighipour N. and Boss A. P. et al. 2003. *The Astrophysical Journal* 583:996. [8] Brownlee D. E. et al. 2006. *Science* 314:1711. [9] Simon J. I. et al. 2011. *Science* 331:1157.
5156

THE USE OF AUGER SPECTROSCOPY FOR IN SITU
ELEMENTAL CHARACTERIZATION WITH SUBMICROMETER
SPATIAL RESOLUTION: APPLICATIONS AND OUTLOOK

M. Bose. Laboratory for Space Sciences and Physics Department,
Washington University, St. Louis, MO 63130, USA. E-mail: mbose@
physics.wustl.edu.

**Introduction:** Auger spectroscopy is well established in the field of material science and in the semiconductor industry [1], but its use for geological applications has been limited [2], largely because of sample charging issues. Recently, however, the technique has found widespread application in the elemental characterization of presolar grains [3–6] and in the analysis of residues in impact craters from the Stardust mission to comet Wild 2 [7–8], where favorable analysis conditions mitigate most sample charging problems. I will review the development of this technique for the quantitative elemental analysis of submicrometer presolar silicate grains and outline future prospects for characterizing the small volumes of residue possibly remaining in impact craters from the Al foils of the Stardust interstellar dust collector [9–10].

**Principles of Auger Spectroscopy:** This technique has the capability to perform elemental characterizations at a high spatial resolution (approximately 15 nm). Such a high spatial resolution can be achieved because of the small primary electron beam size that is attained, and the small analytical volume from which characteristic Auger electrons are measured.

Other advantages include routine nondestructive measurements, detection of all elements in the periodic table except H and He, and no need for special sample preparation.

**Quantitative Silicate Grain Analysis:** Elemental sensitivity factors and their associated uncertainties were obtained by measuring pyroxene and olivine silicate grain standards with varying mg#s [11]. These elemental factors have been used to determine the elemental composition of hundreds of presolar silicate grains identified in primitive meteorites [e.g., 3–6]. Characterization of numerous silicate grains has shown that the majority exhibit nonstoichiometric elemental compositions. In addition, new presolar phases namely magnesiowüstite [12] and SiO₂ [e.g., 6] grains could be identified.

**Measurement of Residues from ISPE Impact Craters:** In addition to cometary samples, the Stardust mission also sampled dust from the contemporary interstellar medium. The Al foils from this collector are being searched to identify impact craters [e.g., 9] that may contain residue corresponding to interstellar dust. Auger spectroscopy can be useful for the analysis of such residue, which is expected to present in only minor amounts. The first such analyses have recently been carried out [13, 14] and results will be discussed at the meeting.

**References:** [1] Watts J. F. and Wolstenhome J. 2003. *An Introduction to Surface Analysis* by XPS & AES Chichester: Wiley & Sons. [2] Hochella M. F., Jr. et al. 1986. *American Mineralogist* 71:1247-1257. [3] Floss C. and Stadermann F. J. 2009. *Geochimica et Cosmochimica Acta* 73:2415-2440. [4] Vollmer C. et al. 2009. *Geochimica et Cosmochimica Acta* 73:7127-7149. [5] Bose M. et al. 2010. *The Astrophysical Journal* 714:1624-1636. [6] Nguyen A. et al. 2010. *The Astrophysical Journal* 719:166-189. [7] Stadermann F. J. et al. 2007. Abstract #1334. 38th Lunar and Planetary Science Conference. [8] Stadermann F. J. and Floss C. 2008. *Meteoritics & Planetary Science* 43:A147. [9] Stroud R. M. et al. 2011. Abstract #1753. 42nd Lunar and Planetary Science Conference. [10] Floss C. et al. 2011. Abstract #1576. 42nd Lunar and Planetary Science Conference. [11] Stadermann F. J. et al. 2009. *Meteoritics & Planetary Science* 44:1033-1049. [12] Floss C. et al. 2008. *The Astrophysical Journal* 672:1266-1271. [13] Stroud R. M. et al. 2011. This conference. [14] Floss C. et al. 2011. This conference volume.

5071

NON-EQUILIBRIUM PRESOLAR CONDENSATES IN PRIMITIVE
METEORITES

M. Bose, C. Floss, and F. J. Stadermann. Laboratory for Space Sciences and Physics Department, Washington University, St. Louis, MO 63130. E-mail: mbose@physics.wustl.edu.

**Introduction:** The compositions of dust grains forming in O-rich environments under equilibrium conditions are well understood, predicting the formation of abundant (Mg-rich) forsterite and enstatite as well as some Fe metal [1]. However, elemental characterization of presolar silicate grains identified in primitive meteorites has shown that the majority of the grains are Fe-rich [e.g., 2, 3]. This is most likely due to their formation in dynamic stellar environments, where equilibrium conditions are not maintained. In addition, presolar SiO₂ grains have recently been identified in meteorites [2, 4, 5]; SiO₂ grains have been dubbed “mythical condensates” [1], and can only condense under non-equilibrium conditions [6].

Auger characterization has shown that many presolar silicates have non-stoichiometric elemental compositions [2, 3] and TEM work suggests that at least some presolar silicates are amorphous aggregates with variable compositions [e.g., 7]. Here we present data for additional presolar silicates identified in the ALHA77307 CO3.0 chondrite and explore the possible origins of these grains.

**Results and Discussion:** Seventy-eight O-anomalous grains were identified in the ALHA77307 matrix. Elemental characterization of 46 of these grains shows that 43 exhibit non-stoichiometric silicate grain compositions. More than half of the grains have compositions intermediate between olivine and pyroxene; more than half are also Fe-rich, although a significant fraction do have Mg-rich compositions.

We also found two composite grains in ALHA77307. One of these consists of two silicate sub-grains with different elemental compositions; the oxygen isotopic composition is uniformly ¹⁷O-rich over both sub-grains. The other one consists of a number of ~50–70 nm sized sub-grains that co-exist as a loose aggregate; elemental spectra indicate that some of the sub-grains are Ca-rich. Some heterogeneity appears to be present in the O isotopic composition of this composite grain, but it is not clear if the variations are statistically significant. Finally, we have also found a Mg-oxide grain in ALHA77307. Comparison to a periclase standard indicates that the Mg/O ratio of the grain is 1.0 ± 0.1. This is the first reported occurrence of this grain type in the presolar grain inventory.

Extensive study of presolar graphite and SiC has shown that equilibrium condensation models adequately explain the formation of these grains [e.g., 8]. In contrast, the presence of large numbers of non-stoichiometric grains in the presolar silicate inventory suggests that condensation of presolar silicates more often occurred under non-equilibrium conditions. We will discuss possible scenarios for the formation of these grains. In addition, we will evaluate the conditions under which the composite grains found in ALHA77307 may have formed.

**References:** [1] Lodders K. and Fegley B. 1999. In: AGB stars, IAU Symposium No. 191. [2] Floss C. and Stadermann F. J. 2009. *Geochimica et Cosmochimica Acta* 73, 2415. [3] Bose M. et al. 2009. *The Astrophysical Journal* 714, 1624. [4] Bose M. et al. 2010. Abstract #1812. 41st Lunar and Planetary Science Conference. [5] Nguyen A. N. et al. 2007. *The Astrophysical Journal* 656, 1223. [6] Ferrarotti A. S. and Gail H. -P. 2001. *Astronomy & Astrophysics* 371, 133. [7] Stroud R. M. et al. 2009. Abstract #1063. 40th Lunar and Planetary Science Conference. [8] Bernatowicz T. et al. 2006. In: Meteorites and the Early solar system II, pp. 109–126.
5160

C AND N ION IMAGING IN THE EH3 CHONDRITE SAH 97159

M. Bose¹, X. Zhao², C. Floss¹, F. J. Stadermann¹ and Y. Lin².
¹Laboratory for Space Sciences and Physics Department,
Washington University, St. Louis, MO 63130, USA. E-mail: mbose@physics.wustl.edu.
²Laboratory of Earth's Deep Interior, Institute
of Geology and Geophysics, Chinese Academy of Sciences, Beijing
100029, China.

**Introduction:** The insoluble organic matter (IOM) in numerous primitive meteorites, notably the CR chondrites, is ¹⁵N-rich in bulk. Carbon and N isotopic imaging shows that discrete hotspots with large enhancements in ¹⁵N are often present in this matter; some of these also have associated anomalous C isotopic compositions [1–3]. Coordinated NanoSIMS and TEM analyses of C- and N-anomalous hotspots show that the isotopic hotspots are often associated with carbonaceous grains with variable morphologies [3, 4].

To date, C and N ion imaging measurements have been reported for a variety of carbonaceous chondrites [e.g., 5], but no enstatite chondrites have been explored as yet. We recently identified presolar silicate and C-anomalous grains in the paired EH3 chondrites SAH 97096 and SAH 97159 [6], indicating that high temperatures conditions in the nebular phase did not destroy stardust materials completely in these meteorites. Indeed, fifteen C-anomalous grains were identified in SAH 97159 on the basis of C and O imaging [6], of which six are most likely SiC grains. We, therefore, decided to explore whether N-anomalous material may have survived in this meteorite.

**Experimental:** We carried out NanoSIMS C and N ion imaging searches in the matrix of a thin-section of SAH 97159. Instrument tuning and standard measurements were carried out on terrestrial kerogen standards [7].

**Results and Discussion:** We identified five additional C-anomalous grains in the SAH 97159 matrix, with ¹²C/¹³C ratios from 52–110; the ¹⁴N/¹⁵N ratios range from 190–441. On the basis of the isotopic compositions and the ¹²C/²⁸Si elemental ratios two grains may be SiC, while the others may be carbonaceous material intermixed with silicates in varying proportions.

Nitrogen-anomalous hotspots were absent in the area analyzed. This is consistent with the N isotopic compositions of bulk IOM in enstatite chondrites [8]. In contrast, the most pristine CR chondrites, MET 00426 and QUE 99177 contain large isotopic anomalies both in bulk and in the form of localized hotspots [2]. The CO chondrite ALHA77307 shows only localized hotspots, at levels that are consistent with the lack of bulk ¹⁵N enrichment in the IOM of this meteorite [3]. It has been suggested that the material from which the enstatite chondrites formed experienced high temperature processing in the solar nebula [9]; the lack of N isotopic anomalies in SAH 97159 is consistent with this scenario.

**References:** [1] Busemann H. 2006. *Science* 312:727. [2] Floss C. and Stadermann F. J. 2009. *The Astrophysical Journal* 697:1242. [3] Bose M. et al. 2010. *Meteoritics & Planetary Science* 45:A19. [4] Floss C. et al. 2011. Abstract #1455. 42nd Lunar and Planetary Science Conference. [5] Davidson J. et al. 2009. Abstract #1853. 40th Lunar and Planetary Science Conference. [6] Bose M. et al. 2010. POS-NIC XI 138. [7] Hayes J. M. et al. 1983. In Earth's earliest biosphere: Its origin and evolution. pp. 93–134. [8] Alexander C. M. O'D. et al. 2007. *Geochimica et Cosmochimica Acta* 71:4380. [9] Huss G. et al. 2003. *Geochimica et Cosmochimica Acta* 67:4823.

5185

U-PB CHRONOLOGY OF TWO LUNAR IMPACT MELT BRECCIAS

A. Bouvier¹, M. Wadhwa¹, R. L. Korotev², W. K. Hartmann³. ¹Arizona State University, Tempe, AZ, USA. ²Washington University in St. Louis, St. Louis, MO, USA. ³Planetary Science Institute, Tucson, AZ, USA. E-mail: audrey.bouvier@asu.edu.

**Introduction:** Current understanding of lunar impact history is largely based on Ar-Ar studies of impact breccias and glass spherules from the Apollo lunar samples, and more recently from lunar meteorites, which have been interpreted to show a dearth of >4 Ga ages (e.g., [1, 2]). This has led to the hypothesis of a late heavy bombardment (LHB) in the inner solar system at 3.9 ± 0.1 Ga. However, recent Ar-Ar and U-Pb studies of impact melts from Apollo samples and meteorites have yielded ages >4.0 Ga (e.g., [3–5]), calling into question the LHB hypothesis. Large-scale impacts such as the Imbrium event could be responsible for resetting the isotope systematics of various radiometric chronometers to different degrees in samples from different locations on the Moon. Since Ar has a lower (approximately 700 K) blocking temperature than Pb (approximately 1300 K) in silicates [6, 7], the U-Pb chronometer may be able to provide more robust “primary” impact melt ages. Therefore, we present here the U-Pb chronology of two feldspathic impact melt breccias: 65015,60 (from the Apollo 16 Descartes landing site) and Shišr 166 (a lunar meteorite).

**Results:** One bulk-rock sample (approximately 20 mg) for 65015 and four mineral separates (approximately 10–40 mg each) for 65015 and Shišr 166 were processed for U-Pb dating. Leaching (3 steps for 65015 and 5–7 steps for Shišr 166), dissolution, and Pb extraction methods were similar to [8]. Pb isotopic compositions and U/Pb of 65015 mineral residues were analyzed by MC-ICPMS and quadrupole ICPMS respectively at ASU. The 65015 residues have corrected 206Pb/204Pb≈9351–43,501. We obtain a U-Pb Concordia upper intercept age of 3,929 ± 11 Ma (MSWD = 3) and a consistent, but more precise, Pb-Pb isochron age of 3935 ± 2 Ma (MSWD = 12), using ²³⁸U/²³⁵U = 137.84. The Shišr 166 residues have ²⁰⁶Pb/²⁰⁴Pb ≈60–250 and give a Pb-Pb isochron age of 3567 ± 5 Ma (MSWD = 0.05).

**Discussion:** The 3935 Ma age for 65015 is consistent with, but more precise than, previous estimates using the Ar-Ar and Rb-Sr systems [9], but younger than a previously reported approximately 3.99 Ga Pb-Pb age [10]. Based on geochemistry and Ar-Ar chronometry, it was recently suggested that the Descartes terrains could be approximately 3.87 Ga ejecta from Imbrium instead of Nectaris basins [11]. The age of the 65015 breccia is older and may suggest that either it formed during a different event or the Imbrium impact event is older than previously thought (i.e., approximately 3.9 Ga based Ar-Ar chronometry of Apollo 15 and 16 breccias [1, 11]). The younger approximately 3.57 Ga age of Shišr 166 is recognized in other lunar meteorites by Ar-Ar chronometry [3] and suggests a protracted history for the formation of impact melt breccias on the surface of the Moon.

**References:** [1] Dalrymple G. B. and Ryder G. 1991. *Geophysical Research Letters* 18:1163–1166. [2] Cohen B. A. et al. 2000. *Science* 290:1754–1756. [3] Cohen B. A. 2008. LPI Contribution 1439:27–28. [4] Fernandes V. A. et al. 2008. LPI Contribution 1439:30–31. [5] Grange M. L. et al. 2011. *Geochimica et Cosmochimica Acta* 75:2213–2232. [6] Cherniak D. J. 2001. *Chemical Geology* 177:381–397. [7] Reiners P. W. 2009. *Geochimica et Cosmochimica Acta* 73:3612–3629. [8] Bouvier A. and Wadhwa M. 2010. *Nature Geoscience* 3:637–641. [9] Papanastassiou D. A. and Wasserburg G. J. 1972. *Earth and Planetary Science Letters* 17:52–63. [10] Nunes P. D. et al. 1973. Proceedings, 4th Lunar Science Conference 2:1797–1822. [11] Norman M. D. et al. 2010. *Geochimica et Cosmochimica Acta* 74:763–783.
A NEW METEORITE TYPE: FORSTERITE CHONDRITE WITH EH-3 AFFINITY: SAHARA 97158

M. Boyet¹, A. El Goresy², M. Miyahara³ and A. Gannoun¹. ¹Université Blaise Pascal, Lab. Magmas et Volcans, UMR CNRS 6524, BP 10448, F-63000 Clermont-Ferrand, France. E-mail: M.Boyet@opgc.univ-bpclermont.fr. ²Bayerisches Geoinstitut, Universitat Bayreuth, 95440 Bayreuth, Germany. ³Institute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University, Sendai, Japan.

**Introduction:** A large black clast was encountered inside a slab of the EH-3 chondrite Sahara 97158. The clast is poor in metals and sulfides. It has an unusual mineralogy and texture. It is barren of chondrules and depleted in metal-sulfide clasts that are characteristic of unequilibrated EH-3 or EL-3 chondrites. It consists of 40-60 vol% of large olivine crystals (up to 80 µm in diameter, (Fo0.99-Fa0.0025), up to 25 vol. % prismatic enstatite crystals (En0.99-Fs0.003Wo0.001), minor contents of glass and some 4-7 vol. % oldhamite, FeNi-metal and other sulfides. The silicate assemblage depicts igneous-like texture and feigns an unusual olivine-rich achondrite but is entirely different from any aubricitic meteorite. The sulfide and metal alloy assemblage consists of oldhamite, ninningerite, djerfisherite, sphalerite, schreibersite, troilite, caswellsilverite and metallic FeNi. The sulfide-metal inventory is very similar to that of EH-3 chondrites. Djerfisherite is depleted in K (3.36 wt. %) compared to other EH-3s: (8.40 wt. % in Qingzhen). Presence of the clast inside an EH-3 host indicates origin from a chondrule-free proto-asteroid that accreted and was destroyed prior to accretion of the host EH-3 Sahara 97158.

**Results and Discussion:** The clast was subjected to an endogenous heating event, partial melting and melt migration prior to its accretion with the other constituents of the EH-3 Sahara 97158 host. The pre-accretion heating event led to considerable melt migration of sulfides, specifically of troilite along with FeNi metal. There is no evidence of shock features like high-pressure phase transformation of olivine to ringwoodite or wadsleyite and enstatite to majorite or akimotoite so that a dynamic event is not responsible for the partial melting and sulfide-melt migration.

REE abundances were measured by LA-ICPMS in several oldhamite crystals in this clast in conjunction with the REE measurements in oldhamite grains in the Sahara 97158 EH-3 host and in an idiomorphic oldhamite in a chondrule in the same EH-3 host. REE patterns in oldhamite in the clast, in the host EH-3 chondrite and the one idiomorphic oldhamite in a chondrule in the EH-3 host display very similar patterns (e.g., Fig. 6 in [1]). All REE patterns display the mainstream type pattern of Sahara 97158 and Sahara 97072 with positive Eu and Yb anomalies [1].

We consider these findings as a strong hint that all the oldhamite crystals in the EH-3 host, chondrule and in the clast emerged from the same nebular reservoir. Oxygen isotopic investigations are planed and will be conducted after completion of the mineralogical investigations on olivine and enstatite in the clast and enstatite in the host EH-3 chondrite.

**Reference:** [1] Gannoun A. et al. 2011. *Geochimica et Cosmochimica Acta* 75:3269-3289.

ON THE OCCURRENCE OF CUBANITE IN THE KAIDUN METEORITE

F. Brandstätter¹ and A. Ivanov². ¹Naturhistorisches Museum, Burgring 7, 1010 Wien, Austria. E-mail: franz.brandstaetter@nhm-wien.ac.at.

²Vernadsky Institute of Geochemistry and Analytical Chemistry, Kosygin St. 19, Moscow 119991, Russia.

**Introduction:** Kaidun is a unique polymict microbreccia containing a large variety of oxidized and reduced chondritic and achondritic rocks. Its host was originally classified as CR-like material [1, 2]. However, it turned out that the carbonaceous chondrite (CC) lithologies dominating Kaidun contain numerous C1 and C2 materials which apparently are not known from other meteorites [3]. In addition, the isotopic and petrologic characteristics of most of these CC clasts are outside or intermediate between the established limits of CI, CM, and CR chondrites [4]. Here we report on the occurrence of a large individual cubanite grain found in a CC clast from section Kaidun #01.3.d1. Cubanite is a very rare mineral phase in meteorites. Up to date it has only been described from the CI chondrites Alais, Orgueil and Ivuna [5, 6], and from the Stardust collection [7].

**Results:** The host lithology of cubanite is a CI-like clast being chondrule-free and having a phyllosilicate- and magnetite-rich matrix. Magnetite typically occurs as framboids (up to 50 µm in size), plaquettes and spherulites. The second abundant opaque phase is pyrrhotite forming up to 30 µm large plates and laths. In places, subrounded isolated grains of dolomite and olivine (rarely exceeding 50 µm in size) are present. Cubanite occurs as a single subhedral grain, ~100 µm in diameter, terminated in part by crystal faces. Electron microprobe analyses performed along a line from rim to core revealed that the cubanite is chemically homogenous. Its average composition (15 analyses, in wt% ± 2σ) is near stoichiometric with 22.9 ± 0.4 Cu, 41.1 ± 0.4 Fe, 36.5 ± 0.6 S [formula Cu<sub>16.3</sub>Fe<sub>33.5</sub>S<sub>50.2</sub>] and agrees, within error, with those given for cubanite in Orgueil [8].

**Implications:** Upon heating above 210 °C, orthorhombic CuFe₂S₃ undergoes an irreversible phase transition to a high-temperature cubic form [9]. Cubanite found in CI chondrites is believed to be a hydrothermal alteration product [5] and it was shown that it is present in these meteorites as low-temperature orthorhombic CuFe₂S₃ [8]. Thus, we predict that the Kaidun cubanite is orthorhombic. This assumption implies that (i) cubanite in Kaidun formed at low temperatures by precipitating from an aqueous fluid, (ii) after the formation of cubanite its host clast did not experience temperatures above 210 °C, (iii) cubanite formed at a later stage than pyrrhotite, as suggested by [8].

**References:** [1] Ivanov A. V. et al. 1986. *Meteoritika* 45:3-19. In Russian. [2] Ivanov A. V. 1989. *Geochemistry International* 26:84-91. [3] Zolensky M. E. and Ivanov A. V. 2003. *Chemie der Erde* 63:185-246. 1997. [4] MacPherson G. J. et al. 2009. *Geochimica et Cosmochimica Acta* 73:5493-5511. [5] Bullock E. S. et al. 2005. *Geochimica et Cosmochimica Acta* 69:2687-2700. [6] Macdougall J. D. and Kerridge J. F. 1977. *Science* 197:561-562. [7] Berger E. L. et al. 2009. Abstract #1892. 40th Lunar and Planetary Science Conference. [8] Berger E. L. et al. 2010. Abstract #1160. 41st Lunar and Planetary Science Conference. [9] Caye R. et al. 1988. *Mineralogical Magazine* 52:509-514.
FORMATION OF Ca-PHOSPHATES DURING AQUEOUS
ALTERATION OF TYPE IIA CHONDRULES IN THE Y-791198 CM2
CARBONACEOUS CHONDRITE

A. J. Brearley. Department of Earth and Planetary Sciences, MSC03-2040, 1 University of New Mexico, Albuquerque, NM87131, USA. E-mail: brearley@unm.edu.

**Introduction:** The CM2 chondrites are a primitive group of meteorites that record a complex history of early solar system processes [1]. The CM2s show evidence of significant aqueous alteration that has resulted in the replacement of primary nebular phases by a variety of secondary alteration products [1]. An understanding the nature of the alteration reactions provides important insights into the composition of aqueous fluids and elemental mass transfer during alteration. To address the issue of elemental mobility during alteration, we have examined the behavior of Ca and P in type IIA chondrules [2, 3] that results in the formation of Ca-phosphates during aqueous alteration.

**Method:** Several type IIA chondrules and their associated fine-grained rims from the weakly altered CM2 chondrite, Y-791198 were characterized by SEM/BSE imaging and elemental X-ray mapping by EPMA. Selected regions from the interface between these chondrules and their fine-grained rims were extracted using FIB techniques and were studied in detail by TEM.

**Results and Discussion:** X-ray maps of type IIA chondrules show that, in all cases, a narrow zone of Ca and P enrichment is present at the interface between altered mesostasis and the fine-grained rim [2]. The zone of Ca and P enrichment has been characterized in detail using TEM. These observations show that the Ca and P-bearing phase is well-crystallized apatite, but with a highly unusual morphology. The apatite occurs as continuous, elongate, but curved plates, that extend for several microns, but are clearly single crystals. In some locations, the apatite crystals resemble a wishbone with straight segments that may be 2–3 µm in length, linked by a short, curved segment. The apatites range from <0.1–0.8 µm in width. The TEM data show that the apatites are not located exactly at the interface between the chondrule and fine-grained rim, but are contained completely within the altered chondrule mesostasis, 2–3 µm from the edge of the chondrule. The altered mesostasis consists of fine-grained phyllosilicates and material that appears to be amorphous. EDS analysis of this material show that it is SiO₂- (approximately 40 wt%) and FeO-rich (40 wt%), but also contains significant concentrations of Al₂O₃, (approximately 7 wt%) MgO (approximately 8 wt%) and, notably, K₂O (approximately 4 wt%).

The occurrence of apatite in Y-791198 type IIA chondrules is remarkably similar to textures observed in partially altered glass in a type IIA chondrule in the CR2 chondrite EET 92015 [3]. We have argued that Ca and P leached from chondrule mesostasis during aqueous alteration are precipitated as apatite at a geochemical reaction front on the edge of the chondrule. Precipitation is promoted by an increase in pH, stabilizing apatite [4]. The Y-791198 phosphates appear to share a similar origin, except that the degree of alteration in Y-791198 is much more extensive. This suggests that early formed phosphates are stable to relatively advanced degrees of alteration, but do eventually undergo dissolution in more heavily altered CM2s [4].

**References:** [1] Brearley A. J. 2006. In 'MESS II' (Eds Lauretta, D. S. and McSween, H. Y. Jr). p. 587. [2] Brearley A. J. and Chizmadia, L. J. 2005. Abstract #2176. 36th Lunar Prospector Spacecraft. [3] Brearley A. J. and Burger P. V. 2009. *Meteoritics & Planetary Science* 72:5150. [4] Brearley A. J. 2006. Abstract #2074. 37th Lunar Prospector Spacecraft.

URANIUM ISOTOPE COMPOSITIONS OF MINERAL SEPARATES
FROM A SINGLE REFRACTORY INCLUSION

G. A. Brennecka and M. Wadhwa. School of Earth and Space Exploration, Arizona State University. E-mail: brennecka@asu.edu

**Introduction:** Recent work on bulk calcium-aluminum rich inclusions (CAIs) has shown variability in the ²³⁸U/²³⁵U ratio, which has major implications for the accuracy of Pb-Pb dating [1,2]. While ²³⁸U/²³⁵U variability among CAIs is clear, there have been no recent high-precision measurements of the uranium isotope compositions of minerals within individual CAIs. The ²³⁸U/²³⁵U ratios of various bulk fragments and residues of the coarse-grained Allende CAI, Egg-2 were previously reported by Chen and Wasserburg (1981), with an average value of 137.8 ± 0.5 [3]. This value is identical, within the reported uncertainty, to the previously accepted "terrestrial" ²³⁸U/²³⁵U ratio of 137.88. We revisit this sample with modern instrumentation and greatly improved precision to determine if uranium isotopic variability exists between mineral phases in a single CAI.

**Samples and Methods:** Mineral separates of the Allende CAI, Egg-2 were provided by G. J. Wasserburg. These mineral separates were obtained by a combination of heavy liquid and magnetic separation. The phases were divided into three groups: 3.3 g mL⁻¹ sink – magnetic (89 mg), 3.3 g mL⁻¹ sink – less magnetic (38 mg), and 3.1 g mL⁻¹ float (55 mg), representing high-Fe pyroxene, high-Mg pyroxene, and plagioclase-rich separates, respectively. These separates were fully digested using HF, HNO₃, and HCl. An aliquot (~1%) of each sample solution was reserved for trace element analyses by quadrupole ICPMS at Arizona State University (ASU). Uranium was separated from the remaining sample solutions for measurement of the ²³⁸U/²³⁵U ratio, using the procedure outlined in [4]. Uranium isotopes were measured on a Thermo Neptune MC-ICPMS at ASU following the ²³⁶U/²³⁵U double spike procedure discussed in [1].

**Results and Discussion:** The REE patterns of the mineral separates demonstrate that Egg-2 is a non-group II CAI. The Th/U ratios of all phases are indistinguishable from chondritic Th/U. The measured ²³⁸U/²³⁵U ratios of the high-Fe pyroxene (137.784 ± 0.034), high-Mg pyroxene (137.787 ± 0.034), and plagioclase-rich (137.793 ± 0.034) fractions are indistinguishable from one another. However, all mineral separates are distinct from their previously assumed ²³⁸U/²³⁵U value of 137.88 [3]. The reported uncertainty (2SD) is based on multiple runs of the SRM950a standard at the same concentrations as the samples, and includes the analytical uncertainty, and uncertainties in the compositions of the spike and the SRM950a standard [5].

This work confirms that no variability in the ²³⁸U/²³⁵U ratios exists between individual minerals in CAIs at the current level of analytical precision. However, if large Th/U fractionation is found between mineral phases in other CAIs, variability in ²³⁸U/²³⁵U could exist from the decay of ²⁴⁷Cm to ²³⁵U (t₁/₂ ~15.6 Ma) [1].

**References:** [1] Brennecka et al. 2010. *Science* 327:449–451. [2] Amelin et al. 2011. *Earth and Planetary Science Letters* 300:343–350. [3] Chen and Wasserburg 1981. *Analytical Chemistry* 53:2060–2067. [4] Weyer et al. 2008. *Geochimica et Cosmochimica Acta* 72:345–359. [5] Richter et al. 2010. *IJMS*. 295:94–97.
5070

INCORPORATION OF ⁷Be, ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl, ⁴¹Ca, And ⁵³Mn INTO
EARLY SOLAR SYSTEM MATERIALS IN THE SOLAR WIND
IMPLANTATION MODEL

G. E. Bricker¹ and M. W. Caffee². ¹Dept. of Physics, Purdue University
North Central, Westville, IN. 46391. ²Primelab, Dept. of Physics, Purdue
University, West Lafayette, IN. 47907.

**Introduction:** Numerous studies have indicated the incorporation of the short-lived radionuclides (SLRs) ($T_{1/2} < 5$Myr) ⁷Be, ¹⁰Be, ²⁶Al, ³⁶Cl, ⁴¹Ca, ⁵³Mn, and ⁶⁰Fe into early solar system materials, including calcium aluminum inclusions (CAIs), in amounts above background levels, although the mechanism for this incorporation is a matter of considerable debate (cf.[1]). Bricker & Caffee [2] proposed a solar wind implantation model for incorporation of ¹⁰Be in CAI precursor materials. It is known that ¹⁰Be is currently produced in the atmosphere of the Sun through the spallation of oxygen with energetic protons. This spallogenic ¹⁰Be in entrained in the solar wind and implanted in solar system materials exposed to the solar wind. Nishiizumi & Caffee [3] detected solar-wind-implanted ¹⁰Be in Apollo 17 trench samples. Bricker and Caffee proposed that the ¹⁰Be incorporated into CAIs was created in the proto-solar atmosphere, rather than in-situ in CAIs (cf. Lee et al. [4] & Gounelle et al. [5]) by the same mechanism we observe now, bombardment of O by solar energetic protons and He nuclei. In their solar wind implantation model, Bricker & Caffee proposed that some fraction of this outward flowing ¹⁰Be is incorporated into the inward flowing material which drops from the funnel flow accreting onto the proto-Sun and they were able to account for ¹⁰Be concentrations found in CAIs. Other SLRs: ⁷Be, ¹⁴C, ²⁶Al, ³⁶Cl, ⁴¹Ca, and ⁵³Mn can also be produced through solar energetic particle reactions. We now consider the incorporation these short lived radionuclides (⁷Be, ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl, ⁴¹Ca, and ⁵³Mn) into early solar system materials in accordance with the solar wind implantation model.

**Results:** We consider experimentally determined ¹⁰Be and ¹⁴C [6] contemporary spallation production rates scaled to x-ray luminosities scene in T-Tauri stars and ancient enhanced energetic particle fluxes [7] and theoretically determined ⁷Be, ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl, ⁴¹Ca, and ⁵³Mn production rates scaled to ancient energetic particle fluxes at the proto-Sun. Using this enhanced production value in conjunction with an accepted refractory mass inflow rates at .06 AU from the proto-Sun we model the concentrations of SLRs in CAI precursors. The model predictions are consistent with measurements of ⁷Be, ¹⁰Be, ³⁶Cl, ⁴¹Ca, and ⁵³Mn found in CAIs. The model fails to produce the ²⁶Al content by a factor of 15. We predict the content of ¹⁴C in CAIs will be 3.7 × 10¹² ¹⁴C g⁻¹ with an isotopic ratio of ¹⁴C/¹²C of 2.2 × 10⁻⁹.

**References:** [1] Gounelle M., Chaussidon M., and Montmerle T. 2007. C.R. Geoscience 339:885. [2] Bricker G. E. and Caffee M. W. 2010. *The Astrophysics Journal* 725:443. [3] Nishiizumi K. and Caffee M. W. 2001. *Science* 294:352. [4] Lee T., Shu F. H., Glassgold A. E., and Rehm K. E. 1998. *The Astrophysics Journal* 506:898. [5] Gounelle M., Shu F. H., Shang H., Glassgold A. E., Rehm K. E., and Lee T. 2006. *The Astrophysics Journal* 640:1163. [6] Jull A. J. T., Lal D., and Donahue D. J. 1995. *Earth and Planetary Science Letters* 136:693. [7] Feigelson E. D., Garmire G. P., and Pravdo S. H. 2002. *The Astrophysics Journal* 572:335.

5262

AMORPHOUS GEL IN THE NAKHLITES: PRODUCT OF A
RAPIDLY COOLED HYDROTHERMAL FLUID

J. C. Bridges and L. J. Hicks. Dept. of Physics & Astronomy, Space Research Centre, University of Leicester, LE1 7RH, UK. E-mail: j.bridges@le.ac.uk.

**Introduction:** It has been demonstrated that the nakhlites' alteration assemblage includes an hydrated silicate gel. This is within the central part of veins, surrounded by smectite-(serpentine) and siderite in some cases [1]. High resolution TEM has shown the non crystalline nature of the gel [1]. It formed during rapid cooling in the latter stages of the hydrothermal event. In related work we show that the fluid was derived through dissolution of host nakhlite by a dilute brine [2]. Here we use more nakhlites to investigate the variation of composition of secondary minerals with depth and constrain the nature of the hydrothermal event.

**Samples and Methods:** We have analyzed polished sections of Lafayette, NWA 998, Governador Valadares, Nakhla, NWA 817, Y000593, Y000749, Miller Range 03346, NWA 5790 by SEM-EDX, FIB-SEM and TEM. EDX totals are normalized to 100% anhydrous.

**Results:** Most of the nakhlite veins are found to include the amorphous gel. No pure gel analyses have yet been made in NWA 998 due to its small vein sizes or from NWA 5790 and Y000749 due to their terrestrial weathering, which is shown for instance, by the presence of pure Ca carbonate and jarosite.

The average Mg/(Mg + total Fe) numbers of the gel are: Lafayette 0.38, GV 0.33, Nakhla 0.25, NWA 817 0.22, Y000593 0.15, Miller Range 03346 0.14. Based on Fe K XANES analyses, the Fe³⁺/total Fe ratio of the gel is approximately 0.7–1.0 [3]. In Lafayette, the Fe/Si wt ratios of the gel 1.0–1.6 are in the same range as those of the phyllosilicate 1.0–1.6. Likewise the normalized Na₂O+K₂O contents of the Lafayette gel ≤0.8 wt% overlap those of the phyllosilicate ≤0.9 wt%. The highest gel Na₂O+K₂O contents seen so far ≤2.5 wt%, are in Nakhla. Al/Si wt ratios are similar in the phyllosilicate and gel ≤0.1.

**Discussion:** The variation of gel Mg# is not reflected in a parallel variation in the Mg# of the surrounding olivine. Nakhlite olivine has varying degrees of equilibration around ~Mg# 0.30. This suggests that the gel compositional fractionation is a function of variation in the composition of the fluid as it migrated up through the nakhlites rather than being controlled by the composition of the adjacent phases.

The gel Mg# compositions are mostly consistent with the predicted stratigraphy of the nakhlites which envisage NWA 998 then Lafayette at the base and Miller Range 03346 near the top [4]. However, the low abundance of gel in NWA 998 compared to Lafayette, which has a high abundance of secondary minerals and was closest to the fluid source, suggests that the nakhlites have preserved both lateral and vertical traces of the brine's passage and culmination. The presence of amorphous gel, with a composition similar to the Lafayette phyllosilicate, in most of the nakhlite veins demonstrates that the secondary assemblage formed in a short hydrothermal event, cooling too rapidly for phyllosilicate to continue crystallizing.

**References:** [1] Changela H. G. and Bridges J. C. 2010. *Meteoritics & Planetary Science* 45:1847–1867. [2] Schwenzer S. P. and Bridges J. C. 2011. *Meteoritics & Planetary Science*, this volume. [3] Hicks L. J. et al. 2011. *Meteoritics & Planetary Science*, this volume. [4] Mikouchi T. et al. Abstract #1865. 37th Lunar and Planetary Science Conference.