records of an ancient martian magnetic field in alh84001 although mars does not presently appear to have a global dynamo magnetic field strong crustal fields have recently been detected by the mars global surveyor above surfaces formed 3 or more ga we present magnetic and textural studies of martian meteorite alh84001 demonstrating that 4 ga carbonates containing magnetite and pyrrhotite carry a stable natural remanent magnetization because 40 ar 39 ar thermochronology demonstrates that most alh84001 carbonates have probably been well below the curie point of magnetite since near the time of their formation weiss et al earth planet sci lett 2002 this issue their magnetization originated at 3 9 4 1 ga on mars this magnetization is at least 500 million years myr older than that known in any other planetary rock and its strong intensity suggests that mars had generated a geodynamo and global magnetic field within 450 650 myr of its formation the intensity of this field was roughly within an order of magnitude of that at the surface of the present day earth sufficient for magnetotaxis by the bacteria whose magnetofossils have been reported in alh84001 and possibly for the production of the strong crustal anomalies chromite in alh84001 may retain even older records of martian magnetic fields possibly extending back to near the time of planetary formation

magnetic remanence in the murchison meteorite the murchison meteorite is a carbonaceous chondrite containing a small amount of chondrules various inclusions and matrix with occasional porphyroblasts of olivine and or pyroxene it also contains amino acids that may have served as the necessary components for the origin of life magnetic analyses of murchison identify an ultrasoft magnetic component due to superparamagnetism as a significant part of the magnetic remanence the rest of the remanence may be due to electric discharge in the form of lightning bolts that may have formed the amino acids the level of magnetic remanence does not support this possibility and points to a minimum ambient field of the remanence acquisition we support our observation by showing that normalized mineral magnetic acquisition properties establish a calibration curve suitable for rough paleofield determination when using this approach 1 2 of the natural remanence left in terrestrial rocks with trm and or crm determines the geomagnetic field intensity irrespective of grain size or type of magnetic mineral with the exception of hematite the same method is applied to the murchison meteorite where the measured meteorite remanence determines the paleofield minimum intensity of 200 2000 nt during and or after the formation of the parent body

thermoremanent magnetization in monodomain monoclinic pyrrhotite fe7s8 the intensity and stability of thermoremanent magnetization trm acquired by fine particles 1 m of synthetic monoclinic pyrrhotite were determined a specific trm magnetization per unit inducing field of 32 with a median destructive field of 36 ka m 1 was observed the median destructive field of saturation isothermal remanence is 42 ka m 1 and the coercive force is 62 ka m 1 the intensity of trm has been compared to that predicted by the classical monodomain model adapted to the magnetic symmetry of pyrrhotite if the optically determined particle size is incorporated into the model the agreement between model and observation is poor a factor of 100 difference if a grain size 0 05 m inferred from the measurement of coercive force is adopted model and observation are in agreement within experimental error

a 4 gyr shock age for a martian meteorite and implications for the cratering history of mars analyses of meteorites that originated on mars provide important insights into the geological and atmospheric evolution of the planet such analyses have hitherto been restricted to relatively young martian rocks1 the oldest martian meteorites have an age of approximately 1 3 billion years but the recently recognized2 martian meteorite allan hills 84001 which is distinct from the other martian meteorites2 4 shows evidence for a much older age5 6 here we report an analysis of the shock alteration history of this meteorite based on argon isotope dating from which we derive a shock age of 4 0 0 1 billion years the age and geological history of this meteorite suggest that it came from the heavily cratered noachian age terrains of mars s southern hemisphere and it may thus provide an absolute chronology for this region of the planet independent of that inferred from the cratering record the shock age of the meteorite also coincides with that of the so called lunar cataclysm a relatively short period during which many of the craters on the moon are believed to have formed supporting the idea7 that intense bombardment was widespread throughout the inner solar system between 3 9 and 4 1 billion years ago

magnetic properties of howardite eucrite and diogenite hed meteorites ancient magnetizing fields and meteorite evolution abstract the remanent magnetism and other magnetic properties of four howardites kapoeta petersburg le teilleul and eet 87503 two eucrites sioux county and pca 82502 and three diogenites shalka johnstown and eet 79002 have been investigated the dominant carrier of magnetic properties is low nickel kamacite present to the extent of 3 5 by weight in petersburg and 0 1 1 in the others a wide range of remanent magnetization nrm intensities characteristics and behaviour during alternating field and thermal demagnetization is observed two common features are scattered directions of nrm between meteorite subsamples and anomalous behaviour of nrm direction and intensity during demagnetization all the meteorites show evidence of a primary nrm acquired during an early evolutionary and some acquired secondary nrm the heterogeneous nrm with which anomalous demagnetization appears to be associated implies magnetization of constituent fragments of the meteorite prior to and no remagnetization after final meteorite assembly it is tentatively proposed that the primary magnetizing field was of internal origin in the parent body and that thermoremanent magnetization occurred when the body cooled after differentiation the coherent magnetic fabric observed in all but two of the heds was most probably imparted at the time of final assembly of the meteorites

magnetic properties of a freshly fallen ll ordinary chondrite the bensour meteorite abstract a comprehensive rock magnetic magnetic anisotropy and paleomagnetic study has been undertaken in the brecciated ll6 bensour ordinary chondrite a few months only after its fall on earth microscopic observations and electronic microprobe analyses indicate the presence of ni rich taenite tetrataenite and rare co rich kamacite tetrataenite is the main carrier of remanence magnetization and anisotropy measurements were performed on mutually oriented 125 mm 3 sub samples a very strong coherent susceptibility and remanence anisotropy is evidenced and interpreted as due to the large impact responsible for the post metamorphic compaction of this brecciated material and disruption of the parent body we show that the acquisition of remanent magnetization postdates metamorphism on the parent body and predates the entering of the meteorite in earth s atmosphere three components of magnetization could be isolated a soft coherent component is closely related to the anisotropy of the meteorite and is interpreted as a shock remanent magnetization acquired during the same large impact on the parent body two harder components show random directions at a few mm scale this randomness is attributed either to the formation mechanism of tetrataenite or to post metamorphic brecciation all components are likely acquired in very low t to null ambient magnetic field as demonstrated by comparison with demagnetization behavior of isothermal remanent magnetization two other ll6 meteorites kilabo and st mesmin have also been studied for comparison with bensour

toward a robust normalized magnetic paleointensity method applied to meteorites abstract we propose a new paleointensity method based on normalization by isothermal remanent magnetization derivative vs alternating field rem method it provides an estimate of the absolute paleointensity with an uncertainty of about a factor two contrary to methods using normalization by total isothermal remanent magnetization or anhysteretic magnetization it is applicable to multicomponent magnetizations artificial or natural isothermal remanent magnetizations can be recognized and relaxation of natural remanent magnetization can be taken into account it is applicable for magnetizations carried by magnetite feni alloys and pyrrhotite the rem method is of particular interest to estimate paleointensities in meteorites as these materials are often characterized by complex multicomponent magnetization of various origins impact magnetization thermoremanence isothermal remanent magnetization and metastable magnetic minerals not suitable for thellier experiments however like other paleointensity methods the rem method underestimates the true paleointensity for meteorite samples with magnetizations that are heterogeneous on spatial scales below the sampling scale for l ordinary chondrites an upper limit of 1 t is proposed for the paleofield tentative paleofield estimates in the 0 05 0 5 t range are proposed for ll ordinary chondrites which is much lower than previous results the first paleofield estimates for rumuruti chondrites indicate the presence of magnetic fields around 6 t during the last major impact on their parent body aubrites and hed achondrites paleofields of at least 10 t may suggest the existence of a dynamo field during the cooling of their parent body results from martian meteorites are scattered between 1 and 24 t and may represent the crustal magnetic field of the planet after dynamo shutdown carbonaceous chondrites provide contrasting results with possible evidence of strong field mt processes that clearly require additional studies

paleomagnetic record of martian meteorite alh84001 1 the natural remanent magnetization nrm of the martian meteorite alh84001 is predominantly carried by fine magnetite which is found in association with carbonate the magnetite is in epitaxial and topotactic relation with the carbonate and formed from the carbonate in the major impact event at 4 0 ga the nrm will therefore record this field the local preferential crystallographic and shape alignment of the magnetite defines local easy directions of magnetization may account for the observed inhomogeneity of the nrm on a microscopic scale normalizing the intensity of the nrm by the saturation isothermal remanence irms then gives an estimate for the 4 0 ga martian field one order smaller than the present geomagnetic field such a field is unlikely to be strong enough to generate the high intensity martian magnetic anomalies alh 84001 in its pristine state as an orthopyroxenite is not a plausible source rock for the martian anomalies because its magnetite was not formed until the 4 0 ga event

influence of seabed topography on the distribution of manganese nodules and associated features in the central indian basin a study based on photographic observations abstract the distribution of manganese nodules and associated rock outcrops and sediment cover in the central indian basin 10 16 s 73 82 e were studied from seabed photographs in relation to topographic features categorised as the crests of seamounts and abyssal hills their slopes valleys and abyssal plains the nodule coverages are high on the crests and slopes mean coverage 12 3 and 15 3 and low in the valleys and plains mean coverage 0 3 and 1 4 the occurrence of nodules is related to the rock outcrops and sediment cover which are in turn controlled by variations in seabed topography rock outcrops are exposed on crests and slopes sediment thickness increases from nil on the crests to very thin on the slopes and from thick to very thick in the plains and valleys high nodule concentration along the slopes and near the crests of the seamounts and abyssal hills is a result of the availability of rock fragments as nuclei which are derived from the rock outcrops however areas where nodules are absent are common in all the topographic settings

matching martian crustal magnetization and magnetic properties of martian meteorites magnetic properties of 26 of 32 unpaired martian meteorites sncs are synthesized to further constrain the lithology carrying martian magnetic crustal sources magnetic properties of ultramafic cumulates i e chassigny allan hills alh 84001 and lherzolitic shergottites alh 77005 lewis cliff lew 88516 are one or two orders of magnitude too weak to account for the crustal magnetizations assuming magnetization in an earth like field nakhlites and some basaltic shergottites which are the most magnetic sncs show the right intensity titanomagnetite is the magnetic carrier in the nakhlites 7 meteorites whereas in most basaltic shergottites 11 meteorites it is pyrrhotite dhofar dho 378 los angeles and nwa 480 1460 and 2046 are anomalous basaltic shergottites as their magnetism is mainly due to titanomagnetite pyrrhotite should be among the candidate minerals for the magnetized noachian crust

postshock annealing and postannealing shock in equilibrated ordinary chondrites implications for the thermal and shock histories of chondritic asteroids abstract in addition to shock effects in olivine plagioclase orthopyroxene and ca pyroxene petrographic shock indicators in equilibrated ordinary chondrites oc include chromite veinlets chromite plagioclase assemblages polycrystalline troilite metallic cu irregularly shaped troilite grains within metallic fe ni rapidly solidified metal sulfide intergrowths martensite and various types of plessite metal sulfide veins large metal and or sulfide nodules silicate melt veins silicate darkening low ca clinopyroxene silicate melt pockets and large regions of silicate melt the presence of some of these indicators in every petrologic type 4 to 6 ordinary chondrite demonstrates that collisional events caused all equilibrated oc to reach shock stages s3 s6 those type 4 to 6 oc that are classified as shock stage s1 on the basis of sharp optical extinction in olivine underwent postshock annealing due to burial beneath materials heated by the impact event those type 4 to 6 oc that are classified s2 on the basis of undulose extinction and lack of planar fractures in olivine were shocked to stage s3 s6 annealed to stage s1 and then shocked again to stage s2 some oc were probably shocked to stage s3 after annealing it seems likely that many oc experienced multiple episodes of shock and annealing because 40ar 39ar chronological data indicate that mil 99301 ll6 s1 was annealed 4 26 ga ago presumably as a consequence of a major impact it seems reasonable to suggest that other equilibrated s1 and s2 oc which contain relict shock features were also annealed by impacts because some type 6 s1 oc e g guarena kernouve portales valley all of which contain relict shock features were annealed 4 44 4 45 ga ago during a period when impacts were prevalent and most oc were thermally metamorphosed it follows that impact induced annealing could have contributed significantly to oc thermal metamorphism

aspects of the validation of magnetic remanence in meteorites meteorite magnetic records constitute physical evidence of processes acting during early solar system evolution consequently the validation of these records is important in meteorite research the first step in the validation process should be the rem value the rem value is the ratio of natural remanence nrm to saturation remanent magnetization imparted by a 1 t magnetic field sirm the rem values range over 3 to 4 orders of magnitude for stony meteorites and for chondrules from allende c3v s1 bjurbole l4 s1 and chainpur ll3 s1 meteorites the rem values computed from published nrm and sirm data identify many orders of magnitude range in the rem values including rem values 100 10 3 these data suggest a dependence for the nrm intensity on the curatorial location from which the sample was obtained any earth rock acquiring thermoremanent magnetization trm in the geomagnetic field has a restricted range in rem mostly between 5 and 50 10 3 the exception being the mineral hematite in the multidomain size range the only terrestrial samples with rem much greater than 100 10 3 are those struck by lightning the rem value provides a physical basis for recognition between valid records and those that might be contaminated the isothermal remanence acquisition ra curve is presented as a contamination curve that allows an indication of the level of magnetic field contamination required to give the computed rem rm sirm value in the case of the bjurbole and chainpur chondrules with rem values 100 10 3 the ra curve indicates that unrealistically large contamination magnetic fields would be required to give rem values greater than 100 10 3 this would suggest contamination other than by a hand magnet that is normally available to an experimenter this would require an explanation that would involve large magnetic fields during chondrule formation or some extraordinary remanence acquisition mechanism that remains to be described magnetic contamination experiments using 80 and 40 mt magnets demonstrate that the rem values and extent of modification of the magnetic vector record are mineralogy dependent and this is mostly related to the amount and characteristics of the mineral tetrataenite the complexity of the meteorite records suggest validation of the record as a first step the rem value is the first physical statement that can be made in this validation

paleomagnetic evidence of a low temperature origin of carbonate in the martian meteorite alh84001 indirect evidence for life on mars has been reported from the study of meteorite alh84001 the formation temperature of the carbonates is controversial some estimates suggest 20 to 80 c whereas others exceed 650 c paleomagnetism can be used to distinguish between these possibilities because heating can remagnetize ferrimagnetic minerals study of two adjacent pyroxene grains from the crushed zone of alh84001 shows that each possesses a stable natural remanent magnetization nrm implying that mars had a substantial magnetic field when the grains cooled however nrm directions from these particles differ implying that the meteorite has not been heated significantly since the formation of the internal crushed zone about 4 billion years ago the carbonate globules postdate this brecciation and thus formed at low temperatures

433 eros problems with the meteorite magnetism record in attempting an asteroid match the magnetometer experiment mag onboard the near shoemaker spacecraft detected no global scale magnetization and established a maximum magnetization of 2 1x10 6 am 2 kg 1 for asteroid 433 eros this is in sharp contrast with the estimated magnetization of other s class asteroids gaspra 2 4 x 10 2 am 2 kg 1 braille 2 8 x 10 2 am 2 kg 1 and is below published values for all types of ordinary chondrites this includes the l ll types considered to most closely match 433 eros based on preliminary interpretations of near remote geochemical experiments the ordinary chondrite meteorite magnetization intensity data was reviewed in order to assess the reasonableness of an asteroid meteorite match based on magnetic property measurements nrm natural remanent magnetization intensities for the ordinary chondrite meteorites show at least a 2 order of magnitude range within each of the h l ll groups all well above the 2 1 x10 6 am 2 kg 1 level for 433 eros the rem values ratio of the nrm to the sirm saturation remanent magnetization range over 3 orders of magnitude for all chondrite groups indicating no clear relationship between nrm and the amount of magnetic material levels of magnetic noise in chondrite meteorites can be as much as 70 or more of the nrm consequently published values of the nrm should be considered suspect unless careful evaluation of the noise sources is done goddard studies of per unit mass intensities in large 10 000gm and small down to 1 gm samples from the same meteorite demonstrate magnetic intensity decreases as size increases this would appear to be explained by demagnetization due to magnetic vector randomness at unknown scale sizes in the larger samples this would then argue for some level of demagnetization of large objects such as an asteroid the possibility that 433 eros is an ll chondrite cannot be discounted

magnetic transition at 30 34 kelvin in pyrrhotite insight into a widespread occurrence of this mineral in rocks abstract a characteristic magnetic transition at 30 34 k is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low temperature techniques as applied to magnetite and hematite extended down to liquid helium temperature a review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals unblocking temperature up to 350 c and extreme resistance against af may be encountered in fine grained pyrrhotite

the anomalous demagnetization behaviour of chondritic meteorites abstract alternating field af demagnetization of chondritic samples often shows anomalous results such as large directional and intensity changes saw tooth intensity vs demagnetizing field curves are also prevalent an attempt to explain this behaviour is presented using a computer model in which individual mineral grains can be magnetized in a variety of different ways a simulated demagnetization can then be carried out to examine the results it was found that the experimental behaviour of chondrites can be successfully mimicked by loading the computer model with a series of randomly orientated and sized vectors the parameters of the model can be changed to reflect different trends seen in experimental data many published results can be modelled using this method a known magnetic mineralogy can be modelled and an unknown mineralogy deduced from af demagnetization curves only by comparing data from mutually orientated samples can true stable regions for palaeointensity measurements be identified calling into question some previous estimates of field strength from meteorites

nrm irm s demagnetization plots an aid to the interpretation of natural remanent magnetization af demagnetization of thermoremanent magnetization trm and saturation isothermal remanent magnetization irm s are used to establish diagnostic criteria for primary trm and for secondary magnetization in igneous rocks in applying the approach to the interpretation of natural remanent magnetization nrm the demagnetization of irm s documents the nature of the magnetic phases in the rock while the demagnetization of the nrm gives information about the process of magnetization the ratio of nrm irm s measures the efficiency of the magnetization process

optimized deghosting of over under towed streamer data in the presence of noise seismic exploration is widely used to locate geologic formations for hydrocarbon accumulations in a typical marine seismic survey one or more marine seismic streamers are towed behind a survey vessel as the streamers are towed acoustic signals commonly referred to as shots are produced by the seismic source these travel down through the water column into strata beneath the water bottom surface where they are reflected from the various geologic formations and travel back to the sea surface one well known marine seismic problem is that these upgoing waves are then reflected with inverted polarity at the sea surface because of the air water interface hence the sensors in the seismic streamer cable record not only the desired wavefield i e the upgoing wave reflected signal from various subterranean geologic formations but also their reflections from the sea surface the downgoing wave the undesired downgoing reflected signal is known as the receiver ghost depending on the incidence angl

shock metamorphism of carbonaceous chondrites abstract we have studied shock effects in carbonaceous chondrites using optical microscopy of thin sections and find that our petrographic classification of progressive shock metamorphism in ordinary chondrites can also be applied to carbonaceous chondrites we find that sixty nine carbonaceous chondrites can be assigned to four shock stages si to s4 largely on the basis of shock effects in olivine the least shocked chondrite groups are cm2 and co3 thirty six out of thirty eight members are classified as shock stage s1 differences between the mean shock levels of certain groups of chondrites e g co3 and cv3 are probably due to stochastic differences in the sampling of their parent bodies however there is an overall tendency for the mean shock level of carbonaceous chondrites and ordinary chondrites to increase with increasing petrologic type that may reflect real differences in the shock level of near surface materials on their parent bodies caused by intrinsic variations in the chemical and physical properties of these materials petrologic type 2 and 3 chondrites are more porous and richer in volatiles than types 4 to 6 the greater porosity of types 2 3 causes higher post shock temperatures and melting at lower pressures and the higher volatile contents ensure that strongly shocked material is more readily dispersed on release from high pressure we suggest that type 2 3 material shocked above 20 30 gpa normally escapes from the parent asteroids and forms particles that are too small to survive as meteorites in the cv3 group there is a correlation between the degree of chondrule flattening and the intensity of shock metamorphism analogous to that discovered in ordinary chondrites by sneyd et al 1988 suggesting that shock rather than static overburden pressure is responsible for chondrule flattening we infer that the correlations are due to collapse of pores under shock pressures of at least 5 10 gpa and that shock is an important process affecting many physical properties of type 2 4 chondrites

natural remanent magnetization thermoremanent magnetization and reliability of palaeointensity determinations on h chondrites abstract we have compared the nrm and artificial trm behaviour of several chondrites of type h the palaeointensity measurements do not give a straight line but a curve with a high slope for the low temperature steps the trm intensity depends on the highest temperature reached by the sample the magnetic viscosity is strongly dependent on the temperature with a drastic increase between 20 c and 95 c the fe ni alloys present in the meteorites probably show diffusion after effects these experiments cast a strong doubt on previous palaeointensity determinations by the double step heating method

the implications of the magnetism of ordinary chondrite meteorites abstract the magnetic properties of eleven ordinary chondrites eight ll chondrites and three l chondrites have been analysed the samples were repeatedly fragmented and oriented to a common reference direction natural remanent magnetisation nrm susceptibility and anisotropy of susceptibility measured at each stage the response of some fragments to alternating field a f demagnetisation thermal demagnetisation isothermal remanent magnetisation irm and magnetic hysteresis has been measured it was found that the orientation of the stable nrm was random down to a scale of 1 mm 3 the dominant magnetic carrier in types 4 6 was the ordered iron nickel mineral tetrataenite the type 3 s measured contained no tetrataenite and the nrm was carried by fine grained taenite a magnetic fabric analogous to a physical fabric was found in all samples in all but two the fabric was foliated tuxtuac and wold cottage showed lineation thermal demagnetisations of nrm showed a trend towards higher blocking temperatures for type 6 chondrites over types 3 5 because the fabric was continuous in the majority of samples and the nrm randomly orientated it was concluded that the magnetic carriers were magnetised before emplacement in the meteorite and that the meteorites are not finescale breccias the preservation of the random nrm leads to the preference of hot accretion as the mechanism for producing chondritic textures as opposed to metamorphic reheating as this would tend to erase the random magnetisation

remanent magnetic properties of unbrecciated eucrites abstract the remanent magnetic properties of five unbrecciated eucrites ranging from the coarse grained cumulate moore county to the quenched melt rock alh 81001 have been investigated in order to assess the strength of the magnetic field associated with their parent body during their formation two of the meteorites are judged as unlikely to have preserved their primary thermal remanence because of large variations in subsample remanence intensity and direction ibitira and lack of nrm resistance to af and thermal demagnetization pca 82502 furthermore moore county because it is dominated magnetically by coarser grained fe and because of its thermal instability is considered as a poor candidate for paleointensity estimates the low normalized nrm intensities for subsamples of alh 81001 and yamato 791195 may indicate the lack of a strong 0 01 mt magnetizing field during their cooling on the eucrite parent body this inference is supported by low paleofield values 0 001 0 005 mt associated with blocking temperatures 150 c for a second subsample of yamato 791195 these paleointensities were derived by a new version of the ktt experiment whereby remanence acquired and lost in discrete temperature intervals is measured at progressively higher temperatures although the magnetic data set for eucrites remains small the lack of evidence for a strong thermal remanence in these and previously measured achondrites does not provide direct evidence for the existence of a strong t tauri solar wind field which could be responsible for induction heating of the achondrite parent bodies

investigating the ancient martian magnetic field using microwaves abstract the new microwave palaeointensity technique has been used to investigate samples from the martian meteorite nakhla this technique is a promising new way to obtain absolute palaeointensity information regarding the ancient martian magnetic field as recorded by the martian meteorites assuming that a part of the magnetic remanence is of thermal origin and originating on mars the two samples studied yield estimates of 4 t for the martian magnetic field at 1 35 ga