Ar‐Ar ages and thermal histories of enstatite meteorites

Abstract– Compared with ordinary chondrites, there is a relative paucity of chronological and other data to define the early thermal histories of enstatite parent bodies. In this study, we report ³⁹Ar‐⁴⁰Ar dating results for five EL chondrites: Khairpur, Pillistfer, Hvittis, Blithfield, and Forrest; five EH chondrites: Parsa, Saint Marks, Indarch, Bethune, and Reckling Peak 80259; three igneous‐textured enstatite meteorites that represent impact melts on enstatite chondrite parent bodies: Zaklodzie, Queen Alexandra Range 97348, and Queen Alexandra Range 97289; and three aubrites, Norton County, Bishopville, and Cumberland Falls Several Ar‐Ar age spectra show unusual ³⁹Ar recoil effects, possibly the result of some of the K residing in unusual sulfide minerals, such as djerfisherite and rodderite, and other age spectra show ⁴⁰Ar diffusion loss. Few additional Ar‐Ar ages for enstatite meteorites are available in the literature. When all available Ar‐Ar data on enstatite meteorites are considered, preferred ages of nine chondrites and one aubrite show a range of 4.50–4.54 Ga, whereas five other meteorites show only lower age limits over 4.35–4.46 Ga. Ar‐Ar ages of several enstatite chondrites are as old or older as the oldest Ar‐Ar ages of ordinary chondrites, which suggests that enstatite chondrites may have derived from somewhat smaller parent bodies, or were metamorphosed to lower temperatures compared to other chondrite types. Many enstatite meteorites are brecciated and/or shocked, and some of the younger Ar‐Ar ages may record these impact events. Although impact heating of ordinary chondrites within the last 1 Ga is relatively common for ordinary chondrites, only Bethune gives any significant evidence for such a young event.     

The early martian evolution—Constraints from basin formation ages

Impact basin formation ages give insight into the early evolution of a planet. The martian basins Hellas, Isidis and Argyre provide an important time-marker for the cessation of the magnetic dynamo and the crustal thickness distribution, both established before 4 Ga ago. No martian surfaces are older than 4.15 Ga based on crater count statistics, and all are younger than the oldest lunar ones. I show that the heavy bombardment period on the Moon and Mars evolved similarly, but endogenic processes have removed the oldest martian basin record. The basin-forming projectile population appears to be different from the impactor population observed today in the inner Solar System. It is yet uncertain whether the heavy bombardment period is cataclysmic or characterized by the decaying flux of planetary formation.     

The prospect of high‐precision Pb isotopic dating of meteorites

Abstract— The radiogenic ²⁰⁷Pb/²⁰⁶Pb ratio is the only extant nuclide chronometer with sufficient time resolution for studies of the solar nebula accretion and early asteroidal differentiation and metamorphism. Pb isotopic dates can be used to link the dates obtained from extinct nuclide chronometers to the absolute time scale. The factors that control precision and accuracy of Pb isotopic dates of meteorites: instrumental mass fractionation in isotopic analysis, mass spectrometer sensitivity, removal of common Pb, multi‐stage evolution of U‐Pb systems, disturbances caused by diffusion, alteration, and shock metamorphism, and uncertainties in decay constants and the natural ratio of the U isotopes are reviewed. The precision of Pb isotopic dates of meteorites attained with currently available techniques and methodology is ±0.5–1.0 Myr in favorable cases. The accuracy of time interval measurements is approximately the same. The most serious limitation on precision and accuracy of Pb isotopic dates is placed by the presence of common Pb of uncertain and/or variable isotopic composition. Improvement in precision and accuracy of Pb isotopic dates would be possible through combined advancement of techniques of isotopic analysis (most importantly, better control over instrumental mass fractionation) and more effective techniques for the removal of common Pb, together with a better understanding of the effects of thermal metamorphism, shock metamorphism, and aqueous alteration on the U‐Pb system in meteorites.     

Ar‐Ar and I‐Xe ages and the thermal history of IAB meteorites

Abstract— Studies of several samples of the large Caddo County IAB iron meteorite reveal andesitic material enriched in Si, Na, Al, and Ca, which is essentially unique among meteorites. This material is believed to have formed from a chondritic source by partial melting and to have further segregated by grain coarsening. Such an origin implies extended metamorphism of the IAB parent body. New ³⁹Ar‐⁴⁰Ar ages for silicate from three different Caddo samples are consistent with a common age of 4.50‐4.51 Gyr. Less well‐defined Ar‐Ar degassing ages for inclusions from two other IABs, EET (Elephant Moraine) 83333 and Udei Station, are ?4.32 Gyr, whereas the age for Campo del Cielo varies considerably over about 3.23‐4.56 Gyr. New ¹²⁹I‐¹²⁹Xe ages for Caddo County and EET 83333 are 4557.9 ± 0.1 Myr and 4557–4560 Myr, respectively, relative to an age of 4562.3 Myr for Shallowater. Considering all reported Ar‐Ar degassing ages for IABs and related winonaites, the range is ?4.32‐4.53 Gyr, but several IABs give similar Ar ages of 4.50‐4.52 Gyr. We interpret these older Ar ages to represent cooling after the time of last significant metamorphism on the parent body and the younger ages to represent later ⁴⁰Ar diffusion loss. The older Ar‐Ar ages for IABs are similar to Sm‐Nd and Rb‐Sr isochron ages reported in the literature for Caddo County. Considering the possibility that IAB parent body formation was followed by impact disruption, reassembly, and metamorphism (e.g., Benedix et al. 2000), the Ar‐Ar ages and IAB cooling rates deduced from Ni concentration profiles in IAB metal (Herpfer et al. 1994) are consistent if the time of the postassembly metamorphism was as late as about 4.53 Gyr ago. However, I‐Xe ages reported for some IABs define much older ages of about 4558–4566 Myr, which cannot easily be reconciled with the much younger Ar‐Ar and Sm‐Nd ages. An explanation for the difference in radiometric ages of IABs may reside in combinations of the following: a) I‐Xe ages have very high closure temperatures and were not reset during metamorphism about 4.53 Gyr ago; b) a bias exists in the ⁴⁰K decay constants which makes these Ar‐Ar ages approximately 30 Myr too young; c) the reported Sm‐Nd and Rb‐Sr ages for Caddo are in error by amounts equal to or exceeding their reported 2‐sigma uncertainties; and d) about 30 Myr after the initial heating that produced differentiation of Caddo silicate and mixing of silicate and metal, a mild metamorphism of the IAB parent body reset the Ar‐Ar ages.     

81Kr‐Kr cosmic ray exposure ages of individual chondrules from Allegan

⁸¹Kr‐Kr cosmic ray exposure (CRE) ages of individual chondrules (6–10 mg) and adjacent matrix samples (5–10 mg) from the Allegan H5 chondrite have been measured using a new highly sensitive resonance ionization mass spectrometer. No conclusive evidence of variations among the CRE ages of individual chondrules or between chondrules and matrix has been observed—average CRE ages of 5.90 ± 0.42 Ma (⁸¹Kr‐⁷⁸Kr) and 5.04 ± 0.37 Ma (⁸¹Kr‐⁸⁰⁺⁸²Kr) are identical within error to those determined for the matrix (7.42 ± 1.27 Myr, ⁸¹Kr‐⁸⁰⁺⁸²Kr) and agree well with the literature value for bulk Allegan. If any accumulation of cosmogenic krypton in the early solar system took place, either it was below our detection limit in these samples (<100 atoms), or any such gas was lost during parent body metamorphism. However, this demonstration that useful ⁸¹Kr‐Kr ages can be obtained from few milligram samples of chondritic material has clear relevance to the analysis of samples returned by planned missions to asteroids and to the search for a signature of pre‐exposure in other, less processed meteorites.     

Ion microprobe U‐Th‐Pb dating of phosphates in martian meteorite ALH 84001

Abstract— Phosphates in martian meteorites are important carriers of trace elements, although, they are volumetrically minor minerals. PO₄ also has potential as a biomarker for life on Mars. Here, we report measurements of the U‐Th‐Pb systematics of phosphates in the martian meteorite ALH 84001 using the Sensitive High Resolution Ion MicroProbe (SHRIMP) installed at Hiroshima University, Japan. Eleven analyses of whitlockites and 1 analysis of apatite resulted in a total Pb/U isochron age of 4018 ± 81 Ma in the ²³⁸U/²⁰⁶Pb‐²⁰⁷Pb/²⁰⁶Pb‐²⁰⁴Pb/²⁰⁶ Pb 3‐D space, and a ²³²Th‐²⁰⁸Pb age of 3971 ± 860 Ma. These ages are consistent within a 95% confidence limit. This result is in agreement with the previously published Ar‐Ar shock age of 4.0 ± 0.1 Ga from maskelynite and other results of 3.8–4.3 Ga but are significantly different from the Sm‐Nd age of 4.50 ± 0.13 Ga based on the whole rock and pyroxene. Taking into account recent studies on textural and chemical evidence of phosphate, our result suggests that the shock metamorphic event defines the phosphate formation age of 4018 ± 81 Ma, and that since then, ALH 84001 has not experienced a long duration thermal metamorphism, which would reset the U‐Pb system in phosphates.     

Complex exposure histories for meteorites with “short” exposure ages

Abstract— We report measurements of ²⁶Al and ¹⁰Be activities in nine ordinary chondrites and of the light noble gas concentrations and ³⁶Cl and ⁴¹Ca activities in subsets of those meteorites. All but Murray have low ²¹Ne concentrations (<1.0 × 10^?8cm³STP/g) and have previously been used to estimate ²¹Ne production rates. Ladder Creek, Murchison, Sena, and Timochin have inventories of cosmogenic radionuclides that are compatible with a single stage of irradiation and give ²¹Ne production rates that are consistent with the standard L-chondrite value of 0.33 × 10^?8cm³STP/g/Ma. In contrast, Cullison, Guenie, Shaw, and Tsarev experienced complex irradiation histories. They and several other meteorites with low nominal exposure ages also have lower ³He/²¹Ne ratios than expected based on their ²²Ne/²¹Ne ratios. A general association between low ²¹Ne contents and ³He losses suggests that meteorites with short lifetimes often occupy orbits with small perihelia. However, meteorites with low ²¹Ne contents, one-stage exposure histories, and losses of cosmogenic ³He are rare. Possible explanations for the scarcity are (1) statistical, (2) that it is harder for more deeply buried protometeoroids to lose gas in a liberating collision, and (3) that it is harder to insert more deeply buried protometeoroids directly into orbits with small perihelia.     

Calibration of cosmogenic noble gas production based on 36Cl‐36Ar ages. Part 2. The 81Kr‐Kr dating technique

We calibrated the ⁸¹Kr‐Kr dating system for ordinary chondrites of different sizes using independent shielding‐corrected ³⁶Cl‐³⁶Ar ages. Krypton concentrations and isotopic compositions were measured in bulk samples from 14 ordinary chondrites of high petrologic type and the cosmogenic Kr component was obtained by subtracting trapped Kr from phase Q. The thus‐determined average cosmogenic ⁷⁸Kr/⁸³Kr, ⁸⁰Kr/⁸³Kr, ⁸²Kr/⁸³Kr, and ⁸⁴Kr/⁸³Kr ratiC(Lavielle and Marti 1988; Wieler 2002). The cosmogenic ⁷⁸Kr/⁸³Kr ratio is correlated with the cosmogenic ²²Ne/²¹Ne ratio, confirming that ⁷⁸Kr/⁸³Kr is a reliable shielding indicator. Previously, ⁸¹Kr‐Kr ages have been determined by assuming the cosmogenic production rate of ⁸¹Kr, P(⁸¹Kr)_c, to be 0.95 times the average of the cosmogenic production rates of ⁸⁰Kr and ⁸²Kr; the factor Y = 0.95 therefore accounts for the unequal production of the various Kr isotopes (Marti 1967a). However, Y should be regarded as an empirical adjustment. For samples whose ⁸⁰Kr and ⁸²Kr concentrations may be affected by neutron‐capture reactions, the shielding‐dependent cosmogenic (⁷⁸Kr/⁸³Kr)_c ratio has been used instead to calculate P(⁸¹Kr)/P(⁸³Kr), as for some lunar samples, this ratio has been shown to linearly increase with (⁷⁸Kr/⁸³Kr)_c (Marti and Lugmair 1971). However, the ⁸¹Kr‐Kr ages of our samples calculated with these methods are on average ~30% higher than their ³⁶Cl‐³⁶Ar ages, indicating that most if not all the ⁸¹Kr‐Kr ages determined so far are significantly too high. We therefore re‐evaluated both methods to determine P(⁸¹Kr)_c/P(⁸³Kr)_c. Our new Y value of 0.70 ± 0.04 is more than 25% lower than the value of 0.95 used so far. Furthermore, together with literature data, our data indicate that for chondrites, P(⁸¹Kr)_c/P(⁸³Kr)_c is rather constant at 0.43 ± 0.02, at least for the shielding range covered by our samples ([⁷⁸Kr/⁸³Kr]_c = 0.119–0.185; [²²Ne/²¹Ne]_c = 1.083–1.144), in contrast to the observations on lunar samples. As expected considering the method used, ⁸¹Kr‐Kr ages calculated either directly with this new P(⁸¹Kr)_c/P(⁸³Kr)_c value or with our new Y value both agree with the corresponding ³⁶Cl‐³⁶Ar ages. However, the average deviation of 2% indicates the accuracy of both new ⁸¹Kr‐Kr dating methods and the precision of the new dating systems of ~10% is demonstrated by the low scatter in the data. Consequently, this study indicates that the ⁸¹Kr‐Kr ages published so far are up to 30% too high.     

Precursors of Mars: Constraints from nitrogen and oxygen isotopic compositions of martian meteorites

Abstract— We present an approach to assess the nature of materials involved in the accretion of Mars by the planet's nitrogen (δ¹⁵N) and oxygen (Δ¹⁷O) isotopic compositions as derived from data on martian meteorites. δ¹⁵N for Mars has been derived from nitrogen and xenon systematics, while Δ¹⁷O has been taken from the literature data. These signatures indicate that Mars has most probably accreted from enstatite and ordinary chondritic materials in a ratio of 74:26 and may not have a significant contribution from the carbonaceous (CI, CM, or CV) chondrites. This is consistent with the chromium isotopic (?⁵³Cr) signatures of martian meteorites and the bulk planet Fe/Si ratio for Mars as suggested by the moment of inertia factor (I/MR²) obtained from the Mars Pathfinder data. Further, a simple homogeneous accretion from the above two types of materials is found to be consistent with the planet's moment of inertia factor and the bulk composition of the mantle. But, it requires a core with 6.7 wt% Si, which is consistent with the new results from the high pressure and temperature melting experiments and chemical data on the opaque minerals in enstatite chondrites.     

FTIR 2–16 micron spectroscopy of micron‐sized olivines from primitive meteorites

Abstract— Infrared spectra of mineral grains from primitive meteorites could be useful for comparison with astronomical infrared spectra since some of their grains might be similar to those formed in the planet‐forming disks around young stars or in the envelopes surrounding late‐type stars. To assess the usefulness of meteorite spectra, olivine grains separated from primitive meteorites have been analyzed using FTIR microscope techniques in the 2–16 μm wavelength range. The sub‐micron sizes of the grains made a complex preparation process necessary. Five characteristic bands were measured near 11.9, 11.2, 10.4, 10.1, and 10.0 μm. The results of 59 analyses allow the calculation of band positions for meteoritic olivines as a function of their iron and magnesium contents. Comparison of the meteoritic results with astronomical data for comets and dust around young and old stars, which exhibit bands similar to the strongest infrared bands observed in the grains (at 11.2 μm), show that the spectral resolution of the astronomical observations is too low to ascertain the exact iron and magnesium (Mg: Fe) ratio of the dust in the 8–13 μm wavelength range.     

Ferric iron in SNC meteorites as determined by Mössbauer spectroscopy: Implications for martian landers and martian oxygen fugacity

Abstract— Mössbauer spectra of martian meteorites are currently of great interest due to the Mössbauer spectrometers on the Athena mission MER rovers as well as the European Space Agency Mars Express mission, with its Beagle 2 payload. Also, considerable current effort is being made to understand the oxygen fugacity of martian magmas because of the effect of fO₂ on mineral chemistry and crystallization processes. For these 2 reasons, the present study was conceived to acquire room temperature Mössbauer spectra of mineral separates and whole rock samples of 10 SNC meteorites. The results suggest that mineral identification using remote application of this technique will be most useful when the phases present have distinctive parameters arising from Fe in very different coordination polyhedra; for example, pyroxene coexisting with olivine can be discriminated easily, but opx versus cpx cannot. The MER goal of using Mössbauer spectroscopy to quantify the relative amounts of individual mineral species present will be difficult to satisfy if silicates are present because the lack of constraints on wt% FeO contents of individual silicate phases present will make modal calculations impossible. The remote Mössbauer spectroscopy will be most advantageous if the rocks analyzed are predominantly oxides with known stoichiometries, though these phases are not present in the SNCs. As for the detection of martian oxygen fugacity, no evidence exists in the SNC samples studied of a relationship between Fe³⁺ content and fO₂ as calculated by independent methods. Possibly, all of the Fe³⁺ observed in olivine is the result of dehydrogenation rather than oxidation, and this process may also be the source of all the Fe³⁺ observed in pyroxene. The observed Fe³⁺ in pyroxene also likely records an equilibrium between pyroxene and melt at such low fO₂ that little or no Fe³⁺ would be expected.     

Cosmic ray exposure and pre‐atmospheric size of the Gebel Kamil iron meteorite

Cosmogenic He, Ne, and Ar as well as the radionuclides ¹⁰Be, ²⁶Al, ³⁶Cl, ⁴¹Ca, ⁵³Mn, and ⁶⁰Fe have been determined on samples from the Gebel Kamil ungrouped Ni‐rich iron meteorite by noble gas mass spectrometry and accelerator mass spectrometry (AMS), respectively. The meteorite is associated with the Kamil crater in southern Egypt, which is about 45 m in diameter. Samples originate from an individual large fragment (“Individual”) as well as from shrapnel. Concentrations of all cosmogenic nuclides—stable and radioactive—are lower by a factor 3–4 in the shrapnel samples than in the Individual. Assuming negligible ³⁶Cl decay during terrestrial residence (indicated by the young crater age <5000 years; Folco et al. 2011 ), data are consistent with a simple exposure history and a ³⁶Cl‐³⁶Ar cosmic ray exposure age (CRE) of approximately (366 ± 18) Ma (systematic errors not included). Both noble gases and radionuclides point to a pre‐atmospheric radius >85 cm, i.e., a pre‐atmospheric mass >20 tons, with a preferred radius of 115–120 cm (50–60 tons). The analyzed samples came from a depth of approximately 20 cm (Individual) and approximately 50–80 cm (shrapnel). The size of the Gebel Kamil meteoroid determined in this work is close to estimates based on impact cratering models combined with expectations for ablation during passage through the atmosphere (Folco et al. 2010 , 2011 ).     

Age‐dating a star‐burst with GEMINI/CIRPASS observations of the core of M83

We present preliminary results from a set of near‐IR integral field spectroscopic observations of the central, star‐burst, regions of the barred spiral galaxy M83, obtained with CIRPASS on Gemini‐S. We present maps in the Paβ and [FeII] 1.257 μm emission lines which appear surprisingly different. We outline the procedure in which we will use Paβ emission line strengths and measures of CO absorption to determine the relative and absolute ages of individual star‐forming knots in the central kpc region of M83. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A propensity for n‐ω‐amino acids in thermally altered Antarctic meteorites

Abstract– Asteroids and their fragments have impacted the Earth for the last 4.5 Gyr. Carbonaceous meteorites are known to contain a wealth of indigenous organic molecules, including amino acids, which suggests that these meteorites could have been an important source of prebiotic organic material during the origins of life on Earth and possibly elsewhere. We report the detection of extraterrestrial amino acids in thermally altered type 3 CV and CO carbonaceous chondrites and ureilites recovered from Antarctica. The amino acid concentrations of the thirteen Antarctic meteorites ranged from 300 to 3200 parts‐per‐billion (ppb), generally much less abundant than in amino acid‐rich CI, CM, and CR carbonaceous chondrites that experienced much lower temperature aqueous alteration on their parent bodies. In contrast to low‐temperature aqueously altered meteorites that show complete structural diversity in amino acids formed predominantly by Strecker–cyanohydrin synthesis, the thermally altered meteorites studied here are dominated by small, straight‐chain, amine terminal (n‐ω‐amino) amino acids that are not consistent with Strecker formation. The carbon isotopic ratios of two extraterrestrial n‐ω‐amino acids measured in one of the CV chondrites (δ¹³C approximately ?25‰) are consistent with ¹³C‐depletions observed previously in hydrocarbons produced by Fischer‐Tropsch type reactions. The predominance of n‐ω‐amino acid isomers in thermally altered meteorites hints at cosmochemical mechanisms for the preferential formation and preservation of a small subset of the possible amino acids.     

An improved extraction system to measure carbon‐14 terrestrial ages of meteorites and pairing of the Antarctic Yamato‐75097 group chondrites

Abstract— We examined an improved system for extraction of carbon from meteorites, using a vacuum‐tight RF melting method. Meteorite samples mixed with an iron combustion accelerator, including a specific amount of carbon (0.052%), were combusted in a RF furnace (LECO HF‐10). ¹⁴CO₂ extracted from the meteorite was diluted with a known amount of nearly ¹⁴C‐free CO₂, evolved from the iron accelerator on combustion. The ¹⁴C activities of the recently fallen Holbrook (L6) and Mt. Tazerzait (L5) meteorites were measured by this method. The mean value was 56.5 ± 3.0 dpm/kg, which is similar to the values reported for recently fallen L6 chondrites. Furthermore, terrestrial ages were measured for four Antarctic meteorites: 1.8 ± 0.5 kyr for Yamato (Y‐) 75097 (L6), 1.8 ± 0.5 kyr for Y‐75108 (L6), and 0.1 ± 0.1 kyr for Y‐74192 (H5). For Y‐74190 (L6), an apparent age of 0.8 ± 0.5 kyr was calculated. After consideration of the shielding effect by using ²²Ne/²¹Ne values, we obtained about 1.8 kyr for the terrestrial age of this chondrite. The five samples Y‐74190, Y‐75097, and Y‐75108, together with Y‐75102 (L6) and Y‐75271 (L6), have been reported to be paired and fragments of an L‐chondrite shower (Honda 1981; Takaoka 1987). The result of this work and literature data for the latter two samples confirmed that they are paired. More discussion and experimental work are needed for other recently fallen meteorites, both for L and H chondrites, and a correction for the shielding effect should be done to determine a more reliable terrestrial age.     

Organic matter from comet 81P/Wild 2, IDPs,and carbonaceous meteorites; similarities and differences

Abstract— During preliminary examination of 81P/Wild 2 particles collected by the NASA Stardust spacecraft, we analyzed seven, sulfur embedded and ultramicrotomed particles extracted from five different tracks. Sections were analyzed using a scanning transmission X‐ray microscope (SXTM) and carbon X‐ray absorption near edge structure (XANES) spectra were collected. We compared the carbon XANES spectra of these Wild 2 samples with a database of spectra on thirty‐four interplanetary dust particles (IDPs) and with several meteorites. Two of the particles analyzed are iron sulfides and there is evidence that an aliphatic compound associated with these particles can survive high temperatures. An iron sulfide from an IDP demonstrates the same phenomenon. Another, mostly carbon free containing particle radiation damaged, something we have not observed in any IDPs we have analyzed or any indigenous organic matter from the carbonaceous meteorites, Tagish Lake, Orgueil, Bells and Murchison. The carbonaceous material associated with this particle showed no mass loss during the initial analysis but chemically changed over a period of two months. The carbon XANES spectra of the other four particles varied more than spectra from IDPs and indigenous organic matter from meteorites. Comparison of the carbon XANES spectra from these particles with 1. the carbon XANES spectra from thirty‐four IDPs (<15 micron in size) and 2. the carbon XANES spectra from carbonaceous material from the Tagish Lake, Orgueil, Bells, and Murchison meteorites show that 81P/Wild 2 carbon XANES spectra are more similar to IDP carbon XANES spectra then to the carbon XANES spectra of meteorites.     

A petrogenetic model for the origin and compositional variation of the martian basaltic meteorites

Abstract— The major element, trace element, and isotopic compositional ranges of the martian basaltic meteorite source regions have been modeled assuming that planetary differentiation resulted from crystallization of a magma ocean. The models are based on low to high pressure phase relationships estimated from experimental runs and estimates of the composition of silicate Mars from the literature. These models attempt to constrain the mechanisms by which the martian meteorites obtained their superchondritic CaO/Al₂O₃ ratios and their source regions obtained their parent/daughter (⁸⁷Rb/⁸⁶Sr, ¹⁴⁷Sm/¹⁴⁴Nd, and ¹⁷⁶Lu/¹⁷⁷Hf) ratios calculated from the initial Sr, Nd, and Hf isotopic compositions of the meteorites. High pressure experiments suggest that majoritic garnet is the liquidus phase for Mars relevant compositions at or above 12 GPa. Early crystallization of this phase from a martian magma ocean yields a liquid characterized by an elevated CaO/Al₂O₃ ratio and a high Mg#. Olivine‐pyroxene‐garnet‐dominated cumulates that crystallize subsequently will also be characterized by superchondritic CaO/Al₂O₃ ratios. Melting of these cumulates yields liquids with major element compositions that are similar to calculated parental melts of the martian meteorites. Furthermore, crystallization models demonstrate that some of these cumulates have parent/daughter ratios that are similar to those calculated for the most incompatible‐element‐depleted source region (i.e., that of the meteorite Queen Alexandra [QUE] 94201). The incompatible‐element abundances of the most depleted (QUE 94201‐like) source region have also been calculated and provide an estimate of the composition of depleted martian mantle. The incompatible‐element pattern of depleted martian mantle calculated here is very similar to the pattern estimated for depleted Earth's mantle. Melting the depleted martian mantle composition reproduces the abundances of many incompatible elements in the parental melt of QUE 94201 (e.g., Ba, Th, K, P, Hf, Zr, and heavy rare earth elements) fairly well but does not reproduce the abundances of Rb, U, Ta and light rare earth elements. The source regions for meteorites such as Shergotty are successfully modeled as mixtures of depleted martian mantle and a late stage liquid trapped in the magma ocean cumulate pile. Melting of this hybrid source yields liquids with major element abundances and incompatible‐element patterns that are very similar to the Shergotty bulk rock.     

Constraints on the structure of the martian interior determined from the chemical and isotopic systematics of SNC meteorites

Abstract— The crystallization ages of martian (SNC) meteorites give evidence that martian volcanism has continued until recent times‐perhaps until the present. These meteorites also indicate that the mantle source regions of this volcanism are modestly to extremely depleted by terrestrial standards. These 2 observations produce a conundrum. How is it that such depleted source regions have produced basaltic magma for such a long time? This contribution attempts to quantify the radiogenic heat production in 2 distinct martian mantle source regions: those of the shergottites and nakhlites. Compared to the depleted upper mantle of the Earth (MORB), the nakhlite source region is depleted by about a factor of 2, and the shergottite source region is depleted by a factor of 6. According to current geophysical models, the nakhlite source contains the minimum amount of radioactive heat production to sustain whole‐mantle convection and basalt generation over geologic time. A corollary of this conclusion is that the shergottite source contains much too little radioactivity to produce recent (<200 Ma) basalts. A model martian interior with a deep nakhlite mantle that is insulated by a shallow shergottite mantle may allow basalt production from both source regions if the divide between the nakhlite‐shergottite mantles acts as a thermal boundary layer. Similarities between lunar and martian isotopic reservoirs indicate that the Moon and Mars may have experienced similar styles of differentiation.     

The natural thermoluminescence of Antarctic meteorites and their terrestrial ages and orbits: A 2010 update

Abstract– We have examined the relationship between natural thermoluminescence (TL) and ²⁶Al in 120 Antarctic meteorites in order to explore the orbital history and terrestrial ages of these meteorites. Our results confirm the observations of Hasan et al. (1987) which were based on 23 meteorites. For most meteorites there was a positive correlation between natural TL and ²⁶Al, reflecting their similarity in decay rate under Antarctic conditions and thus in terrestrial age. For a small group with low TL and high ²⁶Al a small perihelion was proposed. Within this group, natural TL decreases with terrestrial age as determined by ³⁶Cl measurements, although the rate of TL decay is faster (half‐life approximately 10 ka) and the ages that can be determined are smaller (<200 ka) than for most meteorites. The faster decay rate and lower natural TL levels are a reflection of recent exposure to higher radiation doses and higher temperatures, since this history would populate less stable TL traps with smaller electron densities. We sort the 120 meteorites by perihelion and terrestrial age. The normal perihelion group range up to approximately 1000 ka and the small perihelion group range up to approximately 200 ka. An intermediate perihelion group tends to have short terrestrial ages (20–60 ka). There is acceptable agreement between most (34 out of 43) of our present terrestrial age estimates and those determined by isotopic means, the exceptions reflecting complex irradiation histories, long burial times in the Antarctic, or other issues.