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We present here analyses of mass-independent effects in the Ti isotope ratios of 17 coarse-grained (compact Type A and Type B) calcium–aluminum-rich inclusions (CAIs) from the Allende CV3 chondrite utilizing a method developed for rapid measurement of Ti isotopes by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS). Based on the analyses of the synthetic CMAS glasses doped with varying amounts of Ca, Cr and V, an empirical scheme for correction of potential isobaric interferences and matrix effects was developed. Using this scheme, mass-independent variations of the Ti isotope ratios 46Ti/47Ti, 48Ti/47Ti and 50Ti/47Ti were measured with external reproducibilities (2SD) of ± 0.4, ± 0.5, and ± 1.8 for e46Ti, e48Ti, and e50Ti, respectively, based on repeat measurements of standard glasses. The Ti isotope compositions of the 17 Allende refractory inclusions analyzed here show that most of these CAIs are “normal” with limited variation in their mass-independent Ti isotope composition. One CAI (designated as CMS-1) has a significantly larger mass-independent e50Ti anomaly and is identified as having Fractionation and Unidentified Nuclear (FUN) effects. The limited range in the Ti isotope compositions of normal CAIs (e50Ti excesses ranging from 2.9 to 11.4) suggests that they originated from a nebular reservoir that was relatively well-mixed, although not completely homogenized (at the level of precision of the analyses reported here), in its isotopic composition. The distinctive isotopic composition of the FUN CAI CMS-1 indicates that it formed from a reservoir in the protoplanetary disk that was spatially or temporally distinct from that from which the normal CAIs were formed.

The hydrogen isotopic composition of planetary reservoirs can provide key constraints on the origin and history of water on planets. The sources of water and the hydrological evolution of Mars may be inferred from the hydrogen isotopic compositions of mineral phases in Martian meteorites, which are currently the only samples of Mars available for Earth-based laboratory investigations. Previous studies have shown that δD values in minerals in the Martian meteorites span a large range of −250 to +6000‰. The highest hydrogen isotope ratios likely represent a Martian atmospheric component: either interaction with a reservoir in equilibrium with the Martian atmosphere (such as crustal water), or direct incorporation of the Martian atmosphere due to shock processes. The lowest δD values may represent those of the Martian mantle, but it has also been suggested that these values may represent terrestrial contamination in Martian meteorites. Here we report the hydrogen isotopic compositions and water contents of a variety of phases (merrillites, maskelynites, olivines, and an olivine-hosted melt inclusion) in Tissint, the latest Martian meteorite fall that was minimally exposed to the terrestrial environment. We compared traditional sample preparation techniques with anhydrous sample preparation methods, to evaluate their effects on hydrogen isotopes, and find that for severely shocked meteorites like Tissint, the traditional sample preparation techniques increase water content and alter the D/H ratios toward more terrestrial-like values. In the anhydrously prepared Tissint sample, we see a large range of δD values, most likely resulting from a combination of processes including magmatic degassing, secondary alteration by crustal fluids, shock-related fractionation, and implantation of Martian atmosphere. Based on these data, our best estimate of the δD value for the Martian depleted mantle is −116 ± 94‰, which is the lowest value measured in a phase in the anhydrously prepared section of Tissint. This value is similar to that of the terrestrial upper mantle, suggesting that water on Mars and Earth was derived from similar sources. The water contents of phases in Tissint are highly variable, and have been affected by secondary processes. Considering the H2O abundances reported here in the driest phases (most likely representing primary igneous compositions) and appropriate partition coefficients, we estimate the H2O content of the Tissint parent magma to be ≤0.2 wt%.

Angrites are a group of basaltic achondrites with distinctive mineralogic and geochemical characteristics that have the potential to provide insights into processes occurring on planetesimals in the early Solar System. These achondrites have been used as anchors linking the relative age information obtained from short-lived, extinct chronometers (e.g., Al–Mg, Hf–W, and Mn–Cr) with absolute chronometers (e.g., U–Pb). Angrites provide excellent examples of early differentiation processes, such as core formation and silicate differentiation, on protoplanetary bodies. The significant increase in the number of known angrite samples in recent years has offered the opportunity to compare several short- and long-lived isotopic systems in samples with different petrogenetic histories that formed on the same parent body. To this end, the 147Sm–143Nd, 146Sm–142Nd, 176Lu–176Hf, and 87Rb–87Sr isotope systematics have been investigated in a suite of plutonic, coarse-grained (NWA 4590, NWA 4801, and NWA 2999) and quenched, fine-grained (D’Orbigny) angrites. The coupled 147,146Sm–143,142Nd systematics indicate possible isotopic disturbances in two angrites (D’Orbigny and NWA 2999) resulting from post-crystallization processes. The internal 146Sm–142Nd isochrons of two coarse-grained angrites (NWA 4590 and NWA 4801) provide an updated best estimate of the initial Solar System 146Sm/144Sm ratio (i.e., at 4568 Ma) of 0.0084 ± 0.0003. The 176Lu–176Hf isotope systematics in these angrites do not provide evidence of a previously proposed intense irradiation event in the early Solar System. The internal 176Lu–176Hf isochrons for the NWA 4590 and D’Orbigny angrites provide an estimate for the Solar System initial 176Hf/177Hf ratio of 0.279775 ± 0.000031 (2σ) that agrees within uncertainty with the value of average chondrites reported by Bouvier et al. (2008). Finally, the calculated initial 87Sr/86Sr ratios based on the measured Sr-isotopic composition of plagioclase in these angrites yield an estimated initial 87Sr/86Sr ratio of 0.698980 ± 0.000011 for the angrite parent body. This is indistinguishable from a recently determined value for the Solar System initial 87Sr/86Sr based on values measured in calcium–aluminum-rich inclusions (CAIs) after correcting for nucleosynthetic effects in the CAIs. The low initial 87Sr/86Sr of the angrite parent body implies that it acquired its volatile element depleted characteristic within 1.8 Ma of Solar System formation, likely because it accreted from volatile depleted planetesimals that formed in the hot inner nebula. These integrated isotopic systematics suggest a complex history for the angrite parent body not previously inferred from short-lived chronometers and provide new estimates for the initial isotopic composition of the early Solar System.

Magnesium isotope ratios are known to vary in solar system objects due to the effects of 26Al decay to 26Mg and mass-dependent fractionation, but anomalies of nucleosynthetic origin must also be considered. In order to infer the amount of enhancement of 26Mg/24Mg due to 26Al decay or to resolve small nucleogenetic anomalies, the exact relationship between 26Mg/24Mg and 25Mg/24Mg ratios due to mass-dependent fractionation, the mass-fractionation “law”, must be accurately known so that the 25Mg/24Mg ratio can be used to correct the 26Mg/24Mg ratio for mass fractionation. Mass-dependent fractionation in mass spectrometers is reasonably well characterized, but not necessarily fully understood. It follows a simple power fractionation law, sometimes referred to as the “exponential law”. In contrast, mass fractionation in nature, in particular that due to high temperature evaporation that likely caused the relatively large effects observed in calcium-, aluminum-rich inclusions (CAIs), is reasonably well understood, but mass-fractionation laws for magnesium have not been explored in detail. The magnesium isotopic compositions of CAI-like evaporation residues produced in a vacuum furnace indicate that the slope on a log 25Mg/24Mg vs. log 26Mg/24Mg plot is ∼0.5128, and different from those predicted by any of the commonly used mass-fractionation laws. Evaporation experiments on forsterite-rich bulk compositions give exactly the same slope, indicating that the measured mass-fractionation law for evaporation of magnesium is applicable to a wide range of bulk compositions. We discuss mass-fractionation laws and the implications of the measured fractionation behavior of magnesium isotopes for 26Al–26Mg chronology.

Bunburra Rockhole is a unique basaltic achondrite that has many mineralogical and petrographic characteristics in common with the noncumulate eucrites, but differs in its oxygen isotope composition. Here, we report a study of the mineralogy, petrology, geochemistry, and chronology of Bunburra Rockhole to better understand the petrogenesis of this meteorite and compare it to the eucrites. The geochemistry of bulk samples and of pyroxene, plagioclase, and Ca-phosphate in Bunburra Rockhole is similar to that of typical noncumulate eucrites. Chronological data for Bunburra Rockhole indicate early formation, followed by slow cooling and perhaps multiple subsequent heating events, which is also similar to some noncumulate eucrites. The 26 Al- 26 Mg extinct radionuclide chronometer was reset in Bunburra Rockhole after the complete decay of 26 Al, but a slight excess in the radiogenic 26 Mg in a bulk sample allows the determination of a model 26 Al- 26 Mg age that suggests formation of the parent melt for this meteorite from its source magma within the first ~3 Ma of the beginning of the solar system. The 207 Pb- 206 Pb absolute chronometer is also disturbed in Bunburra Rockhole minerals, but a whole-rock isochron provides a re-equilibration age of ~4.1 Ga, most likely caused by impact heating. The mineralogy, geochemistry, and chronology of Bunburra Rockhole demonstrate the similarities of this achondrite to the eucrites, and suggest that it formed from a parent melt with a composition similar to that for noncumulate eucrites and subsequently experienced a thermal history and evolution comparable to that of eucritic basalts. This implies the formation of multiple differentiated parent bodies in the early solar system that had nearly identical bulk elemental compositions and petrogenetic histories, but different oxygen isotope compositions inherited from the solar nebula.

Sulfur storage and transport between different reservoirs such as core, mantle, crust and atmosphere of Mars are tied to igneous processes. Martian meteorites carry a record of mantle melting and subsequent differentiation history of Martian magmas. Investigation of S geochemistry of Martian meteorites can thus provide an understanding of how S is transferred from the Martian interior to the exosphere. In this study we measured bulk S concentration of 7 Martian meteorites and modeled the behavior of S during both isobaric crystallization of primary Martian magmas and isentropic partial melting of Martian mantle. Comparisons between measured data and modeled results suggest that (1) sulfides may become exhausted at the source during decompression melting of the mantle and mantle-derived basalts may only become sulfide-saturated after cooling and crystallization at shallow depths and (2) in addition to degassing induced S loss, mixing between these differentiated sulfide-saturated basaltic melts and cumulus minerals with/without cumulate sulfides could also be responsible for the bulk sulfur contents in some Martian meteorites. In this case, a significant quantity of S could remain in Martian crust as cumulate sulfides or in trapped interstitial liquid varying from 2 to 95 percent by weight. Our modeling also suggests that generation of sulfide-undersaturated parental magmas requires that the mantle source of Martian meteorites contain

The fall and recovery of the Tissint meteorite in 2011 created a rare opportunity to examine a Martian sample with a known, short residence time on Earth. Tissint is an olivine-phyric shergottite that accumulated olivine antecrysts within a single magmatic system. Coarse olivine grains with nearly homogeneous cores of Mg# >80 suggest slow re-equilibration. Many macroscopic features of this sample resemble those of LAR 06319, including the olivine crystal size distribution and the presence of evolved oxide and olivine compositions. Unlike LAR 06319, however, no magmatic hydrous phases were found in the analyzed samples of Tissint. Minor and trace element compositions indicate that the meteorite is the product of closed-system crystallization from a parent melt derived from a depleted source, with no obvious addition of a LREE-rich (crustal?) component prior to or during crystallization. The whole-rock REE pattern is similar to that of intermediate olivine-phyric shergottite EETA 79001 lithology A, and could also be approximated by a more olivine-rich version of depleted basaltic shergottite QUE 94201. Magmatic oxygen fugacities are at the low end of the shergottite range, with log fO2 of QFM-3.5 to -4.0 estimated based on early-crystallized minerals and QFM-2.4 estimated based on the Eu in pyroxene oxybarometer. These values are similarly comparable to other depleted shergottites, including SaU 005 and QUE 94201. Tissint occupies a previously unsampled niche in shergottite chemistry: containing olivines with Mg# >80, resembling the enriched olivine-phyric shergottite LAR 06319 in its crystallization path, and comparable to intermediate olivine-phyric shergottite EETA 79001A, depleted olivine-phyric shergottite DaG 476, and depleted basaltic shergottite QUE 94201 in its trace element abundances and oxygen fugacity. The apparent absence of evidence for terrestrial alteration in Tissint (particularly in trace element abundances in the whole-rock and individual minerals) confirms that exposure to the arid desert environment results in only minimal weathering of samples, provided the exposure times are brief.

The recent witnessed fall of the meteorite Tissint represents the delivery of a pristine new sample from the surface of Mars. This meteorite provides an unprecedented opportunity to study a variety of aspects about the planet's evolution. Using the Rb-Sr and Sm-Nd isotopic systems, we determined that Tissint, a depleted shergottite, has a crystallization age of 574 20 Ma, an initial e 143 Nd =+ 42.2 0.5, and an initial 87 Sr/ 86 Sr = 0.700760 11. These initial Nd and Sr isotopic compositions suggest that Tissint originated from a mantle source on Mars that is distinct from the source reservoirs of the other Martian meteorites. The known crystallization ages, geochemical characteristics, ejection ages, and ejection dynamics of Tissint and other similarly grouped Martian meteorites suggest that they are likely derived from a source crater up to approximately 90 km in diameter with an age of approximately 1 Ma that is located on terrain that is approximately 600 million years old.

The isotopic composition of our Solar System reflects the blending of materials derived from numerous past nucleosynthetic events, each characterized by a distinct isotopic signature. We show that the isotopic compositions of elements spanning a large mass range in the earliest formed solids in our Solar System, calcium-aluminum-rich inclusions (CAIs), are uniform, and yet distinct from the average Solar System composition. Relative to younger objects in the Solar System, CAIs contain positive r-process anomalies in isotopes A140 and negative r-process anomalies in isotopes A140. This fundamental difference in the isotopic character of CAIs around mass 140 necessitates (i) the existence of multiple sources for r-process nucleosynthesis and (ii) the injection of supernova material into a reservoir untapped by CAIs. A scenario of late supernova injection into the protoplanetary disk is consistent with formation of our Solar System in an active star-forming region of the galaxy.

Several olivine-phyric shergottites contain enough olivine that they could conceivably represent the products of closed-system crystallization of primary melts derived from partial melting of the Martian mantle. Larkman Nunatak (LAR) 06319 has been suggested to represent a close approach to a Martian primary liquid composition based on approximate equilibrium between its olivine and groundmass. To better understand the olivine–melt relationship and the evolution of this meteorite, we report the results of new petrographic and chemical analyses. We find that olivine megacryst cores are generally not in equilibrium with the groundmass, but rather have been homogenized by diffusion to Mg# 72. We have identified two unique grain types: an olivine glomerocryst and an olivine grain preserving a primary magmatic boundary that constrains the time scale of eruption to be on the order of hours. We also report the presence of trace oxide phases and phosphate compositions that suggest that the melt contained approximately 1.1% H2O and lost volatiles during cooling, also associated with an increase in oxygen fugacity upon degassing. We additionally report in situ rare earth element measurements of the various mineral phases in LAR 06319. Based on these reported trace element abundances, we estimate the oxygen fugacity in the LAR 06319 parent melt early in its crystallization sequence (i.e., at the time of crystallization of the low-Ca and high-Ca pyroxenes), the rare earth element composition of the parent melt, and those of melts in equilibrium with later formed phases. We suggest that LAR 06319 represents the product of closed-system crystallization within a shallow magma chamber, with additional olivine accumulated from a cumulate pile. We infer that the olivine megacrysts are antecrysts, derived from a single magma chamber, but not directly related to the host magma, and suggest that mixing of antecrysts within magma chambers may be a common process in Martian magmatic systems.

We present high-precision measurements of the Mg isotopic compositions of a suite of types I and II chondrules separated from the Murchison and Murray CM2 carbonaceous chondrites. These chondrules are olivine- and pyroxene-rich and have low 27 Al/ 24 Mg ratios (0.012-0.316). The Mg isotopic compositions of Murray chondrules are on average lighter (d 26 Mg ranging from 0.95& to 0.15& relative to the DSM-3 standard) than those of Murchison (d 26 Mg ranging from 1.27& to +0.77&). Taken together, the CM2 chondrules exhibit a narrower range of Mg isotopic compositions than those from CV and CB chondrites studied previously. The least-altered CM2 chondrules are on average lighter (average d 26 Mg = 0.39 0.30&, 2SE) than the moderately to heavily altered CM2 chondrules (average d 26 Mg = 0.11 0.21&, 2SE). The compositions of CM2 chondrules are consistent with isotopic fractionation toward heavy Mg being associated with the formation of secondary silicate phases on the CM2 parent body, but were also probably affected by volatilization and recondensation processes involved in their original formation. The low-Al CM2 chondrules analyzed here do not exhibit any mass-independent variations in 26 Mg from the decay of 26 Al, with the exception of two chondrules that show only small variations just outside of the analytical error. In the case of the chondrule with the highest Al/Mg ratio (a type IAB chondrule from Murchison), the lack of resolvable 26 Mg excess suggests that it either formed >1 Ma after calcium-aluminum-rich inclusions, or that its Al-Mg isotope systematics were reset by secondary alteration processes on the CM2 chondrite parent body after the decay of 26 Al.

We use VSWIR microimaging spectroscopy to survey the spectral diversity of HED meteorites at 80-μm/pixel spatial scale. Our goal in this work is both to explore the emerging capabilities of microimaging VSWIR spectroscopy and to contribute to understanding the petrologic diversity of the HED suite and the evolution of Vesta. Using a combination of manual and automated hyperspectral classification techniques, we identify four major classes of materials based on VSWIR absorptions that include pyroxene, olivine, Fe-bearing feldspars, and glass-bearing/featureless materials. Results show microimaging spectroscopy is an effective method for rapidly and non-destructively characterizing small compositional variations of meteorite samples and for locating rare phases for possible follow-up investigation. Future work will include incorporating SEM/EDS results to quantify sources of spectral variability and placing observations within a broader geologic framework of the differentiation and evolution of Vesta.

Events occurring within the first 10 million years of the Solar System’s approximately 4.5 billion-year history, such as formation of the first solids, accretion, and differentiation of protoplanetary bodies, have determined the evolutionary course of our Solar System and the planetary bodies within it. The application of high-resolution chronometers based on short-lived radionuclides is critical to our understanding of the temporal sequence of these critical events. However, to map the relative ages from such chronometers onto the absolute time scale, they must be “anchored” to absolute ages of appropriate meteoritic materials using the high-precision lead–lead (Pb–Pb) chronometer. Previously reported Pb–Pb dates of the basaltic angrite meteorites, some of which have been used extensively as time anchors, assumed a constant 238 U/ 235 U ratio (= 137.88). In this work, we report measurements of 238 U/ 235 U ratios in several angrites that are distinct from the previously assumed value, resulting in corrections to the Pb–Pb ages of ≥1 million years. There is no resolvable variation in the 238 U/ 235 U ratio among the angrite bulk samples or mineral separates, suggesting homogeneity in the U isotopic composition of the angrite parent body. Based on these measurements, we recalculated the Pb–Pb age for the commonly used anchor, the D’Orbigny angrite, to be 4563.37 ± 0.25 Ma. An adjustment to the Pb–Pb age of a time anchor (such as D’Orbigny) requires a corresponding correction to the “model ages” of all materials dated using that anchor and a short-lived chronometer. This, in turn, has consequences for accurately defining the absolute timeline of early Solar System events.

We report elemental abundances and the isotopic systematics of the short-lived 26Al–26Mg (half-life of ∼0.73 Ma) and long-lived U–Pb radiochronometers in the ungrouped basaltic meteorite Northwest Africa (NWA) 2976. The bulk geochemical composition of NWA 2976 is clearly distinct from that of the eucrites and angrites, but shows broad similarities to that of the paired NWA 001 and 2400 ungrouped achondrites indicating that it is likely to also be paired with these two samples. The major and trace element abundances in NWA 2976 further indicate that it formed by extensive melting and magmatic fractionation processes on its parent body. The Al–Mg and Pb–Pb isotope systematics indicate that this meteorite represents the earliest stages of crust formation on a differentiated parent body in the early Solar System. The absolute Pb–Pb internal isochron age of NWA 2976, obtained from acid leaching residues of three whole-rock samples and two pyroxene separates, is 4562.89 ± 0.59 Ma (MSWD = 0.02). This Pb–Pb age is calculated using the measured 238U/235U ratio of a NWA 2976 whole-rock of 137.751 ± 0.018 (2σ) which was determined relative to the recently revised value of 137.840 ± 0.008 for the SRM 950a U isotope standard. The Al–Mg systematics reveal the presence of 26Mg isotopic anomalies produced by the decay of 26Al with an (26Al/27Al)0 of (3.94 ± 0.16) × 10−7, and indicate a time of formation of 0.26 ± 0.18 Ma after the D’Orbigny angrite. Using the revised Pb–Pb age of 4563.36 ± 0.34 Ma for the D’Orbigny anchor (corrected for its U isotopic composition), we deduce an Al–Mg model age of 4563.10 ± 0.38 Ma for NWA 2976, which is consistent with its Pb–Pb internal isochron age. The concordance of the Pb–Pb and Al–Mg chronometers, when taking into account the differences in the U isotopic compositions of the D’Orbigny and NWA 2976 achondrites (whose parent bodies likely formed in distinct regions of early Solar System as indicated by their different oxygen isotopic compositions), implies that 26Al was homogeneously distributed in the early Solar System. It also suggests that igneous processes on planetesimals, as represented by the formation of various basaltic meteorite groups that likely originated on distinct parent bodies (e.g., eucrites and angrites, as well as ungrouped achondrites), were widespread throughout the protoplanetary disk within the first ∼5 Ma of the history of the Solar System.

article i nfo We report composition-dependent matrix effects in the in-situ measurement of Mg and Si isotope ratios by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) in natural and synthetic silicate glasses. These were determined using two 193 nm wavelength excimer laser ablation-multicollector ICPMS systems, one employing a double-focusing mass spectrometer (ThermoFinnigan Neptune) and one employing a single-focusing mass spectrometer with a hexapole/collision cell (GV Instruments IsoProbe). Observed matrix effects in isotopic measurements by LA-ICPMS range from subtle (i.e., less than ≈0.7‰ per amu for Si isotope measurements in natural and synthetic glasses in all instrumental configurations) to significant but mass-dependent (i.e., less than 1.2‰ per amu for Mg isotope measurements performed using the Neptune) to large and significantly non-mass-dependent (i.e., up to 6‰ mass dependent/3‰ non-mass dependent for Mg isotope measurements of natural glasses made with the IsoProbe). Composition-dependent differences in the magnitude and direction of within-run isotopic fractionation (particularly for Mg isotope measurements) suggest that isotopic fractionation at the site of ablation is the main source of matrix effects in measurements employing the double-focusing mass spectrometer. However, the large and significantly non- mass dependent (but systematic and reproducible) matrix effects affecting LA-ICPMS Mg isotope data obtained using the single-focusing IsoProbe appear to be largely due to non-linear scattering processes occurring in the hexapole/collision cell as Mg becomes increasingly diluted by matrix elements.

Shergottites sampled two distinct geochemical reservoirs on Mars. Basaltic and olivine-phyric shergottites individually sampled both geochemically enriched and depleted reservoirs, whereas lherzolitic shergottites are previously known only to exhibit a relatively limited intermediate geochemical signature that may have resulted from the mixing of the two geochemical end-member reservoirs. Here we show that recently discovered shergottites Robert Massif (RBT) 04261 and RBT 04262 are the first examples of lherzolitic shergottites originating from the enriched reservoir. RBT 04261 and RBT 04262, initially identified as olivine-phyric shergottites, are actually lherzolitic shergottites. Both meteorites exhibit nearly identical textures and mineral compositions, suggesting that they should be paired. Each consists of two distinct textures: poikilitic and non-poikilitic. The poikilitic areas are composed of pyroxene oikocrysts enclosing olivine grains; all pyroxene oikocrysts have pigeonite cores mantled by augite. The non-poikilitic areas are composed of olivine, pyroxene, maskelynite and minor amounts of merrillite, chromite and ilmenite. Olivine and pyroxene show the lowest Mg-number, and maskelynite has the lowest anorthite component among the lherzolitic shergottites. Moreover, the modal abundances of maskelynite in these two meteorites are distinctly higher than the other lherzolitic shergottites. The rare earth element (REE) budgets of RBT 04261 and RBT 04262 are dominated by merrillite. The slightly light rare earth element (LREE)-enriched pattern of this mineral is similar to that of merrillite in the geochemically enriched basaltic shergottites Shergotty and Zagami, and unlike the LREE-depleted pattern of merrillite in the other lherzolitic shergottites. The REE patterns of both high- and low-Ca pyroxenes are also similar to those in Shergotty and Zagami. The REE pattern of a melt calculated to be in equilibrium with the core of a pyroxene oikocryst is parallel to that of the RBT 04262 whole-rock as well as whole-rock compositions of other geochemically enriched basaltic shergottites. These observations imply that RBT 04262 sampled an enriched and oxidized reservoir similar to that sampled by some of the basaltic shergottites and are consistent with an oxidizing condition for the formation of RBT 04262 (log f O 2 = QFM-1.6). The petrographic and geochemical observations presented here suggest that RBT 04261 and RBT 04262 represent the most evolved magma among the lherzolitic shergottites and that this magma originated from a geochemically enriched reservoir on Mars. Based on an evaluation of the relationship between petrographic, geochemical and chronological signatures for shergottites including RBT 04261 and RBT 04262, we propose that both geochemically enriched and depleted shergottites were ejected from the same launch site on Mars.

The age of the Solar System is defined by the formation of the first solid grains in the solar nebula. Pb–Pb age dating of these solids, which were later trapped in a meteorite, indicates that the Solar System is 0.34–1.91 million years older than previously thought. The age of the Solar System can be defined as the time of formation of the first solid grains in the nebular disc surrounding the proto-Sun. This age is estimated by dating calcium–aluminium-rich inclusions in meteorites. These inclusions are considered as the earliest formed solids in the solar nebula. Their formation marks the beginning for several long- and short-lived radiogenic clocks that are used to precisely define the timescales of Solar System events, such as the formation and evolution of planetary bodies1,2,3. Here we present the 207Pb–206Pb isotope systematics in a calcium–aluminium-rich inclusion from the Northwest Africa 2364 CV3-group chondritic meteorite, which indicate that the inclusion formed 4,568.2 million years ago. This age is between 0.3 (refs 4, 5) and 1.9 (refs 1, 6) million years older than previous estimates and is the oldest age obtained for any Solar System object so far. We also determined the 26Al–26Mg model age of this inclusion, and find that it is identical to its absolute Pb–Pb age, implying that the short-lived radionuclide 26Al was homogeneously distributed in the nebular disc surrounding the proto-Sun. From the consistently old ages in the studied inclusion, we conclude that the proto-Sun and the nebular disc formed earlier than previously thought.

The short-lived radionuclide 26 Al existed throughout the solar nebula 4.57 Ga ago, and the initial abundance ratio ( 26 Al/ 27 Al)0, as inferred from magnesium isotopic compositions of calcium–aluminum-rich inclusions (CAIs) in chondritic meteorites, has become a benchmark for understanding early solar system chronology. Internal mineral isochrons in most CAIs measured by secondary ion mass spectrometry (SIMS) give ( 26 Al/ 27 Al)0 ∼ (4–5) × 10 −5 , called “canonical.” Some recent high-precision analyses of (1) bulk CAIs measured by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS), (2) individual CAI minerals and their mixtures measured by laserablation MC-ICPMS, and (3) internal isochrons measured by multicollector (MC)-SIMS indicated a somewhat higher “supracanonical” ( 26 Al/ 27 Al)0 ranging from (5.85 ± 0.05) × 10 −5 to >7 × 10 −5 . These measurements were done on coarse-grained Type B and Type A CAIs that probably formed by recrystallization and/or melting of fine-grained condensate precursors. Thus the supracanonical ratios might record an earlier event, the actual nebular condensation of the CAI precursors. We tested this idea by performing in situ high-precision magnesium isotope measurements of individual minerals in a fine-grained CAI whose structures and volatility-fractionated trace element abundances mark it as a primary solar nebula condensate. Such CAIs are ideal candidates for the fine-grained precursors to the coarse-grained CAIs, and thus should best preserve a supracanonical ratio. Yet, our measured internal isochron yields ( 26 Al/ 27 Al)0 = (5.27 ± 0.17) × 10 −5 . Thus our data do not support the existence of supracanonical ( 26 Al/ 27 Al)0 = (5.85–7) × 10 −5 . There may not have been a significant time interval between condensation of the CAI precursors and their subsequent melting into coarse-grained CAIs.

The recently recovered paired Antarctic achondrites Graves Nunatak 06128 and 06129 (GRA) are meteorites that represent unique high-temperature asteroidal processes that are identified in only a few other meteorites. The GRA meteorites contain high abundances of sodic plagioclase, relatively Fe-rich pyroxenes and olivine, abundant phosphates, and low temperature alteration. They represent products of very early planetesimal melting (4565.9 +/- 0.3 Ma) of an unsampled geochemical reservoir from an asteroid that has characteristics similar to the brachinite parent body. The magmatism represented by these meteorites is contrary to the commonly held belief that the earliest stages of melting on all planetary bodies during the first 2-30 Ma of solar system history were fundamentally basaltic in nature. These sodic plagioclase-rich rocks represent a series of early asteroidal high-temperature processes: (stage 1) melting and partial extraction of a low-temperature Fe-Ni-S melt, (stage 2) small degrees of disequilibrium partial melting of a sodium- or alkali-rich chondritic parent body with additional incorporation of Fe-Ni-S melt that was not fully extracted during stage 1, (stage 3) volatile-enhanced rapid extraction and emplacement of the Na-rich, high-normative plagioclase melt, (stage 4) final emplacement and accumulation of plagioclase and phosphates, (stage 5) subsolidus reequilibration of lithology between 962 and 600 degrees C at an fO(2) Of IW to IW + 1.1, and (stage 6) replacement of merrillite and pyroxene by Cl-apatite resulting from the interaction between magmatic minerals and a Cl-rich fluid/residuum melt. The subsolidus events started as early as 4561.1 Ma and may have continued for upwards of 144 million years. The existence of assemblages similar to GRA on several other planetary bodies with different geochemical characteristics (ureilite, winonaites, IAB irons) implies that this type of early asteroidal melting was not rare. Whereas, eucrites and angrites represent extensive melting of a parent body with low concentrations of moderately-volatile elements, GRA represents low-degrees of melting of a parent body with chondritic abundances of moderately volatile elements. The interpretation of the low-temperature mineral assemblage is somewhat ambiguous. Textural features suggest multiple episodes of alteration. The earliest stage follows the interaction of magmatic assemblages with a Cl-rich fluid. The last episode of alteration appears to cross-cut the fusion crust and earlier stages or alteration. Stable isotopic measurements of the alteration can be interpreted as indicating that an extraterrestrial volatile component was preserved in GRA. (C) 2009 Elsevier Ltd. All rights reserved.

We report on an investigation of the 26Al–26Mg isotope systematics in the D’Orbigny and Sahara 99555 angrites. High precision Mg isotope compositions and Al/Mg ratios were measured in mineral separates and whole rock samples from D’Orbigny and Sahara 99555 using multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Plagioclase separates from both angrites have resolvable excesses in 26Mg (Δ26Mg) that correlate with their respective Al/Mg ratios. 26Al–26Mg systematics in the mineral separates and whole rocks define precise isochrons that correspond to 26Al/27Al ratios of (5.06 ± 0.92) × 10−7 and (5.13 ± 1.90) × 10−7 and initial Δ26Mg values of −0.006 ± 0.040‰ and −0.016 ± 0.047‰ for D’Orbigny and Sahara 99555, respectively. The slopes and initial Δ26Mg values are identical for these two meteorites within errors and the data for both angrites considered together define an isochron corresponding to a 26Al/27Al ratio of (5.10 ± 0.55) × 10−7 and initial Δ26Mg value of −0.012 ± 0.019. Relative to the Efremovka E60 CAI, the 26Al/27Al values reported here for these angrites imply 26Al–26Mg ages of 4562.42 ± 0.29 Ma and 4562.43 ± 0.53 Ma for D’Orbigny and Sahara 99555, respectively. These 26Al–26Mg ages are concordant with model ages determined using other extinct radionuclide chronometers (e.g., 53Mn–53Cr and 182Hf–182W), but are ∼2 Myr younger than the absolute 207Pb–206Pb ages that have been reported recently for these angrites. The reason for this discrepancy is not presently known, but may imply disturbance of one or more of the isotope systems under consideration or a possible bias in the 207Pb–206Pb ages of the angrites resulting from natural or analytical causes.

Asuka 881394 is a unique basaltic meteorite that originated in the crust of a differentiated planetesimal in the early Solar System. We present high precision Pb, Mg, and Cr isotopic compositions of bulk samples and mineral separates from this achondrite. A 207Pb–206Pb internal isochron obtained from the radiogenic pyroxene and whole-rock fractions of Asuka 881394 yields an absolute age of 4566.5 ± 0.2 Ma, which we consider to be the best estimate for the crystallization age of this basaltic achondrite. The 26Al–26Mg systematics show some evidence of disturbance, but 5 of the 6 analyzed whole-rock and mineral fractions define an isochron corresponding to a 27Al/26Al ratio of (1.28 ± 0.07) × 10−6. Comparison with the 26Al–26Mg and Pb–Pb systematics in the D’Orbigny achondrite translates to a 26Al–26Mg age of 4565.4 ± 0.2 Ma for Asuka 881394. The 53Mn–53Cr systematics in whole-rock, silicate and chromite fractions correspond to a 53Mn/55Mn ratio of (3.85 ± 0.23) × 10−6. Compared to the most precise 53Mn–53Cr and Pb–Pb systematics available for the D’Orbigny angrite, this translates to a 53Mn–53Cr age of 4565.3 ± 0.4 Ma; similarly, a comparison with the NWA 4801 angrite yields a 53Mn–53Cr age of 4565.5 ± 0.4 Ma, in agreement with the age obtained relative to D’Orbigny. While the 26Al–26Mg and 53Mn–53Cr ages appear to be concordant in Asuka 881394, these ages are ∼1 Ma younger than its 207Pb–206Pb age. This discordance might have been caused by one or more of several reasons, including differences in the closure temperatures for Pb versus Cr and Mg diffusion in their host minerals combined with slow cooling of the parent body as well as differential resetting of isotopic systems by a process other than volume diffusion, e.g., shock metamorphism. The ancient age of Asuka 881394 suggests that basaltic volcanism on its parent planetesimal occurred within ∼3 Ma of the formation of earliest solids in the Solar System, essentially contemporaneously with chondrule formation. This requires that the Asuka 881394 parent body was fully accreted within ∼500,000 yrs of Solar System formation.

The recently recovered Antarctic achondrites Graves Nunatak 06128 and 06129 are unique meteorites that represent high-temperature asteroidal processes in the early solar system never before identified in any other meteorite. They represent products of early planetesimal melting (4564.25 ± 0.21 Ma) and subsequent metamorphism of an unsampled geochemical reservoir from an asteroid that has characteristics similar to the brachinite parent body. This melting event is unlike those predicted by previous experimental or geochemical studies, and indicates either disequilibrium melting of chondritic material or melting of chondritic material under volatile-rich conditions.

New high-precision W isotope measurements are presented for 33 iron meteorites from 8 magmatic groups (IC, IIAB, IID, IIIAB, IIIE, IIIF, I VA and IVB), 2 non-magmatic groups (IAB–IIICD and IIE), and one ungrouped iron (Deep Springs). All magmatic irons have e182W values that are, within errors, equal to, or less radiogenic than, the Solar System initial of − 3.47 ± 0.20. A method was developed to correct the measured e182W values of magmatic iron meteorites for the presence of cosmogenic effects produced during space exposure to galactic cosmic rays. The corrected data provide new constraints on the timing of metal-silicate differentiation in iron meteorite parent bodies, which must have taken place within a few million years (

Use of the extinct 182 Hf- 182 W chronometer to constrain the timing of planetary accretion and differentiation rests ontheassumptionthatthesolarnebulahadhomogeneoustungstenisotopiccomposition.Here,wereportdeficiencies of � 0.1 part in 10,000 in the abundance of 184 Win group IVB iron meteorites relative to the silicate Earth. These are mostlikelyduetoincompletemixingattheplanetesimalscale(2Y4kmradiusbodies)oftheproductsof slow(s-)and rapid (r-) neutron-capture nucleosynthesis in the solar nebula. The correction that must be applied to the 182 Hf- 182 W model age of core formation in IVB irons due to the presence of these nuclear anomalies is � 0.5 Myr. Subject headingg minor planets, asteroids — nuclear reactions, nucleosynthesis, abundances — solar system: formation — stars: abundances

The current state of knowledge regarding redox conditions of rocks from asteroidal bodies (as represented by various classes of meteorites), the Moon and Mars is discussed here. In the case of the differentiated meteorite parent bodies, the redox conditions range over at least six orders of magnitude (from ~5 log units below the Iron-Wustite buffer to slightly above it) and are determined in large part by the compositions of the undifferentiated precursor materials that accreted to form these parent bodies. Lunar basalts record oxygen fugacities ranging from close to the Iron-Wustite (IW) buffer to ~2 log units below it. The current best estimate of the oxygen fugacity of the lunar mantle is ~1 log unit below IW. Martian crustal rocks represented by the Shergottite-Nakhlite-Chassignite group of meteorites record a wide range of oxygen fugacities. The basaltic shergottites range from slightly below the IW buffer to ~2 log units above it, whereas the cumulate nakhlites (and chassignites) are relatively oxidized (~3–4 log units above the IW buffer). Following early metal-silicate and crust-mantle differentiation on Mars, the depleted martian mantle is likely to have been reduced (close to the IW buffer or slightly lower). Metasomatism and secondary (hydrous) alteration are likely to have produced silicate reservoirs on Mars that are relatively more oxidized (most likely ≥3 log units above the IW buffer). The redox conditions on the other terrestrial planets (Mercury and Venus) are not well constrained. Based on the limited information from remote spacecraft and telescopic observations of surface rocks on these planets, it is inferred that silicate reservoirs on Mercury are highly reduced; those on Venus are likely to be somewhat more oxidized than on Mercury, possibly similar to the lower mantle of Earth.

We measured nickel isotopes via multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) in the bulk metal from 36 meteorites, including chondrites, pallasites, and irons (magmatic and non-magmatic). The Ni isotopes in these meteorites are mass fractionated; the fractionation spans an overall range of ≈0.4 amu^(-1). The ranges of Ni isotopic compositions (relative to the SRM 986 Ni isotopic standard) in metal from iron meteorites (≈0.0 to ≈0.3 amu^(-1)) and chondrites (≈0.0 to ≈0.2 amu^(-1)) are similar, whereas the range in pallasite metal (≈-0.1 to 0.0 amu^(-1)) appears distinct. The fractionation of Ni isotopes within a suite of fourteen IIIAB irons (≈0.0 to ≈0.3 amu^(-1)) spans the entire range measured in all magmatic irons. However, the degree of Ni isotopic fractionation in these samples does not correlate with their Ni content, suggesting that core crystallization did not fractionate Ni isotopes in a systematic way. We also measured the Ni and Fe isotopes in adjacent kamacite and taenite from the Toluca IAB iron meteorite. Nickel isotopes show clearly resolvable fractionation between these two phases; kamacite is heavier relative to taenite by ≈0.4 amu^(-1). In contrast, the Fe isotopes do not show a resolvable fractionation between kamacite and taenite. The observed isotopic compositions of kamacite and taenite can be understood in terms of kinetic fractionation due to diffusion of Ni during cooling of the Fe-Ni alloy and the development of the Widmansttten pattern.

Vacuum evaporation experiments with Type B CAI-like starting compositions were carried out at temperatures of 1600, 1700, 1800, and 1900 � C to determine the evaporation kinetics and evaporation coefficients of silicon and magnesium as a function of temperature as well as the kinetic isotope fractionation factor for magnesium. The vacuum evaporation kinetics of silicon and magnesium are well characterized by a relation of the form J = JoeE/RT with Jo = 4.17 · 10 7 mol cm � 2 s � 1 , E = 576 ± 36 kJ mol � 1 for magnesium, Jo = 3.81 · 10 6 mol cm � 2 s � 1 , E = 551 ± 63 kJ mol � 1 for silicon. These rates only apply to evaporation into vacuum whereas the actual Type B CAIs were almost certainly surrounded by a finite pressure of a hydrogen-dominated gas. A more general formulation for the evaporation kinetics of silicon and magnesium from a Type B CAI-like liquid that applies equally to vacuum and conditions of finite hydrogen pressure involves combining our determi- nations of the evaporation coefficients for these elements as a function of temperature (c = c0eE/RT with c0 = 25.3, E = 92 ± 37 kJ mol � 1 for cSi; c0 = 143, E = 121 ± 53 kJ mol � 1 for cMg) with a thermodynamic model for the saturation vapor pressures of Mg and SiO over the condensed phase. High-precision determinations of the magnesium isotopic composition of the evaporation residues from samples of different size and different evaporation temperature were made using a multicollec- tor inductively coupled plasma mass spectrometer. The kinetic isotopic fractionation factors derived from this data set show that there is a distinct temperature effect, such that the isotopic fractionation for a given amount of magnesium evaporated is smaller at lower temperature. We did not find any significant change in the isotope fractionation factor related to sample size, which we interpret to mean that recondensation and finite chemical diffusion in the melt did not affect the isotopic fractiona- tions. Extrapolating the magnesium kinetic isotope fractionations factors from the temperature range of our experiments to temperatures corresponding to partially molten Type B CAI compositions (1250-1400 � C) results in a value of aMg � 0.991, which is significantly different from the commonly used value of aMg ¼ 0:97977 ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 23:98504=24:98584 p .

To investigate whether magnesium isotopes are fractionated during basalt differentiation, we have performed high-precision Mg isotopic analyses by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) on a set of well-characterized samples from Kilauea Iki lava lake, Hawaii, USA. Samples from the Kilauea Iki lava lake, produced by closed-system crystal-melt fractionation, range from olivine-rich cumulates to highly differentiated basalts with MgO content ranging from 2.37 to 26.87 wt. %. Our results demonstrate that although these basalts have diverse chemical compositions, mineralogies, crystallization temperatures and degrees of differentiation, their Mg isotopic compositions display no measurable variation within the limits of our external precision (average δ 26 Mg= −0.36±0.10 and δ 25 Mg= −0.20±0.07; uncertainties are 2SD). This indicates that Mg isotopic fractionation during crystal-melt fractionation at temperatures of ≥1055 °C is undetectable at the level of precision of the current investigation. Calculations based on our data suggest that at near-magmatic temperatures the maximum fractionation in the 26 Mg/ 24 Mg ratio between olivine and melt is 0.07‰. Two additional oceanic basalts, two continental basalts (BCR-1 and BCR-2), and two primitive carbonaceous chondrites (Allende and Murchison) analyzed in this study have Mg isotopic compositions similar to the Kilauea Iki lava lake samples. In contrast to a recent report [U. Wiechert, A.N. Halliday, Non-chondritic magnesium and the origins of the inner terrestrial planets, Earth and Planetary Science Letters 256 (2007) 360–371], the results presented here suggest that the Bulk Silicate Earth has a chondritic Mg isotopic composition. © 2007 Published by Elsevier B.V.

A procedure was developed to accurately measure the W isotopic compositions of iron meteorites with a precision of better than +/-0.1 epsilon on epsilon182W and epsilon184W (normalized to 186W/183W). Purification of W was achieved through a two-step, ion-exchange procedure. In most cases, the yield is better than 80%, and purified W solutions are clear of matrix elements and direct isobars of W. The final W solutions were analyzed using a Micromass Isoprobe multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). Tests performed on mixtures of terrestrial standards and meteorite samples demonstrate that the method is accurate and that epsilon182W variations as small as approximately 0.1 epsilon can be detected. Analyses of three different aliquots of the Gibeon (IVA) iron meteorite obtained over a period of 6 months show identical epsilon182W values with a weighted mean of 3.38 +/- 0.05, consistent with literature data for IVA iron meteorites, and indicating that the metal-silicate differentiation event in its parent body was either contemporaneous with or slightly postdated (by up to approximately 2.5 My) the formation of refractory inclusions. We demonstrate our ability to measure epsilon184W accurately and precisely (within +/-0.1 epsilon), which is useful for characterizing cosmogenic and nucleosynthetic effects that may be present in iron meteorites. We also report for the first time measurements of epsilon180W, albeit with large error bars (+/-4 epsilon, in most cases).

Rare asteroidal vesicular basalts have previously been thought to form in surficial lava flows with CO as the vesicle-forming gas. However, vesicular lava flows are unlikely on small, airless bodies such as asteroids. To unravel the origin of these rocks, we analyzed vesicle sizes and abundances for two angrites and two eucrites using high-resolution X-ray computed tomography and conducted numerical modeling of bubble formation in a dike of ascending magma. Modeling results indicate that thin (< 30 cm wide) dikes are trapped at 5 km depth where 75 ppm of CO and CO2 contribute equally to vesicle formation. Vesicular eucrites were metamorphosed in this deep-seated environment, the gas was lost, and they were excavated by impacts.

We report on the petrology and geochemistry of the Northwest Africa 2737 (NWA 2737) meteorite that was recovered from the Morrocan Sahara in 2000. It is the second member of the chassignite subclass of the SNC (Shergotitte-Nakhlite-Chassignite) group of meteorites that are thought to have originated on Mars. It consists of black olivine- and spinel-cumulate crystals (89.7 and 4.6 wt%, respectively), with intercumulus pyroxenes (augite 3.1 wt% and pigeonite-orthopyroxene 1.0 wt%), analbite glass (1.6 wt%) and apatite (0.2 wt%). Unlike Chassigny, plagioclase has not been observed in NWA 2737. Olivine crystals are rich in Mg, and highly equilibrated (Fo = 78.7 +/- 0.5 mol%). The black color of olivine grains may be related to the strong shock experienced by the meteorite as revealed by the deformation features observed on the macroscopic to the atomic scale. Chromite is zoned from core to rim from Cr(83.4)Uv(3.6)Sp(13.0) to Cr(72.0)UV(6.9)Sp(21.1). Pyroxene compositional trends are similar to those described for Chassigny except that they are richer in Mg. Compositions range from En(78.5)Wo(2.7)Fs(18.8) to En(76.6)WO(3.2)Fs(20.2) for the orthopyroxene, from En(73.5)Wo(8.0)Fs(18.5) to En(64.0)Wo(22.1)Fs(13.9) for pigeonite, and from En(54.6)Wo(32.8)Fs(12.6) to En(46.7)Wo(44.1)Fs(9.2) for augite. Bulk rock oxygen isotope compositions confirm that NWA 2737 is a new member of the martian meteorite clan (Delta O-17 = 0.305 +/- 0.02 parts per thousand, n = 2). REE abundances measured in NWA 2737 mineral phases are similar to those in Chassigny and suggest a genetic relationship between these two rocks. However, the parent melt of NWA 2737 was less evolved and had a lower Al abundance. (c) 2006 Elsevier Inc. All rights reserved.

We report here the results of an investigation of W and Nd isotopes in the SNC (Shergottite-Nakhlite-Chassignite (martian)) meteorites. We have determined that e182W values in the nakhlites are uniform within analytical uncertainties and have an average value of ∼3. Also, while e182W values in the shergottites have a limited range (from 0.3–0.7), their e142Nd values vary considerably (from −0.2–0.9). There appears to be no correlation between e182W and e142Nd in the nakhlites and shergottites. These results shed new light on early differentiation processes on Mars, particularly on the timing and nature of fractionation in silicate reservoirs. Assuming a two-stage model, the metallic core is estimated to have formed at ∼12 Myr after the beginning of the solar system. Major silicate differentiation established the nakhlite source reservoir before ∼4542 Ma and the shergottite source reservoirs at 4525 [ + 19 − 21 ] Ma. These ages imply that, within the uncertainties afforded by the 182Hf-182W and 146Sm-142Nd chronometers, the silicate differentiation events that established the source reservoirs of the nakhlites and shergottites may have occurred contemporaneously, possibly during crystallization of a global magma ocean. The distinct 182W-142Nd isotope systematics in the nakhlites and the shergottites imply the presence of at least three isotopically distinct silicate reservoirs on Mars, two of which are depleted in incompatible lithophile elements relative to chondrites, and the third is enriched. The two depleted silicate reservoirs most likely reside in the Martian mantle, while the enriched reservoir could be either in the crust or the mantle. Therefore, the 182W-142Nd isotope systematics indicate that the nakhlites and the shergottites originated from distinct source reservoirs and cannot be petrogenetically related. A further implication is that the source reservoirs of the nakhlites and shergottites on Mars have been isolated since their establishment before ∼4.5 Ga. Therefore, there has been no giant impact or efficient global mantle convection to thoroughly homogenize the Martian mantle following the establishment of the SNC source reservoirs.

We report here results of ion microprobe analyses of rare earth element abundances in various phases in the new Antarctic (paired) nakhlites Yamato 000593 and Yamato 000749, as well as in two additional nakhlites recovered from the Saharan desert, NWA 817 and NWA 998. Although these nakhlites are all composed predominantly of augite and some olivine, they differ from each other, and from the three previously known nakhlites, in the abundance and degree of crystallinity of the interstitial mesostasis. Trace element abundances in various phases in these new nakhlites indicate that they are petrogenetically related to (and comagmatic with) each other and the previously known nakhlites. The calculated parent melt compositions (in equilibrium with augite core compositions) are LREE-enriched and have REE patterns parallel to those of their whole rocks. This suggests that subsequent to accumulation of the olivine and augite, the intercumulus trapped melt evolved in a closed system. The similarity in the estimated parent melt compositions and trace element zonation in the augites of the various nakhlites indicates that these rocks are likely to have formed within a single lithologic unit on Mars. In this scenario, the differences among these nakhlites may be explained in terms of differences in the depth of crystallization within the cumulus pile, represented by different horizons within the same lithologic unit. Based on the partitioning of Eu in their augite cores, the magmatic redox conditions for the nakhlites are estimated to be relatively oxidizing (QFM), implying an oxidized source reservoir in the martian mantle. Late metasomatism of their mantle source by LREE-enriched, oxidizing fluids is suggested to be responsible for the LREE-enrichment and oxidation condition of the nakhlite parent melts.

On the night of March 26, 2003, a large meteorite broke up and fell upon the south suburbs of Chicago. The name Park Forest, for the village that is at the center of the strewnfield, has been approved by the nomenclature committee of the Meteoritical Society. Satellite data indicate that the bolide traveled from the southwest toward the northeast. The strewnfield has a southeast-northwest trend; however, this is probably due to the effects of strong westerly winds at high altitudes. Its very low 56Co and very high 60Co activities indicate that Park Forest had a preatmospheric mass that was at least ~900 kg and could have been as large as ~7 x 10^3 kg, of which only ~30 kg have been recovered. The average compositions of olivine and low-Ca pyroxene, Fa24.7 ± 1.1 and Fs20.8 ± 0.7, respectively, and its bulk oxygen isotopic composition, δ18O = +4.68‰, δ17O = +3.44‰, show that Park Forest is an L chondrite. The ferromagnesian minerals are well equilibrated, chondrules are easily recognized, and maskelynite is mostly ≤50 μm across. Based on these observations, we classify Park Forest as type 5. The meteorite has been strongly shocked, and based on the presence of maskelynite, mosaicism and planar deformation features in olivine, undulatory extinction in pyroxene, and glassy veins, the shock stage is S5. The meteorite is a monomict breccia, consisting of light-colored, angular to rounded clasts in a very dark host. The light and dark lithologies have essentially identical mineral and oxygen isotopic compositions. Their striking difference in appearance is due to the presence of a fine, pervasive network of sulfide veins in the dark lithology, resulting in very short optical path lengths. The dark lithology probably formed from the light lithology in an impact that formed a sulfide-rich melt and injected it into cracks.

We report here the results of a study of trace element microdistributions and 53Mn-53Cr systematics in several basaltic and orthopyroxenitic clasts from the Vaca Muerta mesosiderite. Ion microprobe analyses of selected trace and minor element abundances in minerals of the silicate clasts indicate that, following igneous crystallization, these clasts underwent extensive metamorphic equilibration that resulted in intra- and inter-grain redistribution of elements. There is also evidence in the elemental microdistributions that these clasts were subsequently affected to varying degrees by alteration resulting from redox reactions involving the indigenous silicates and externally derived reducing agents (such as phosphorus, derived from the mesosiderite metal) at the time of metal-silicate mixing. Furthermore, our results suggest that the varying degrees of alteration by redox reactions recorded in the different clasts were most likely facilitated by different degrees of remelting induced by heating during the metal-silicate mixing event. After taking into account the effects of these postmagmatic secondary processes, comparison of the trace and minor element concentrations and distributions in minerals of basaltic and orthopyroxenitic clasts with those of noncumulate eucrites and diogenites, respectively, suggests that the primary igneous petrogenesis, including parent magma and source compositions, of Vaca Muerta silicates were similar to those of achondritic meteorites of the Howardite-Eucrite-Diogenite (HED) association. Internal 53Mn-53Cr isochrons obtained for two basaltic (pebble 16 and 4679) and two orthopyroxenitic (4659 and 4670) clasts show that chromium isotopes are equilibrated within each clast. Nevertheless, just as for noncumulate eucrites and diogenites, 53Cr excesses in whole-rock samples of the basaltic clasts (∼1.01 e in pebble 16; ∼1.07 e in 4679) are significantly higher than in the orthopyroxene-rich clasts (∼0.62 e in 4659; ∼0.53 e in 4670). As in the case of the HED parent body, this suggests that Mn/Cr fractionation in the parent body of the Vaca Muerta silicate clasts occurred very early in the history of the solar system, when 53Mn was still extant. However, the slope of the 53Mn-53Cr isochron defined by the whole-rock samples of Vaca Muerta clasts (corresponding to a 53Mn/55Mn ratio of 3.3 ± 0.6 × 10−6) is distinctly lower than that defined by the HED whole-rock samples (corresponding to a 53Mn/55Mn ratio of 4.7 ± 0.5 × 10−6), indicating that the global Mn/Cr fractionation event that established mantle source reservoirs on the parent body of the Vaca Muerta silicate clasts occurred ∼2 Ma after a similar event on the HED parent body.

CK chondrites are the only group of carbonaceous chondrites with petrologic types ranging from 3 to 6. Although CKs are described as calcium-aluminum-rich inclusion (CAI)-poor objects, the abundance of CAIs in the 18 CK3–6 we analyzed ranges from zero to approximately 16.4%. During thermal metamorphism, some of the fine-grained CAIs recrystallized as irregular assemblages of plagioclase + Ca-rich pyroxene ! olivine ! Ca-poor pyroxene ! magnetite. Coarse-grained CAIs display zoned spinel, fassaite destabilization, and secondary grossular and spinel. Secondary anorthite, grossular, Ca-rich pyroxene, and spinel derive from the destabilization of melilite, which is lacking in all CAIs investigated. The Al-Mg isotopic systematics measured in fine- and coarsegrained CAIs from Tanezrouft (Tnz) 057 was affected by Mg redistribution. The partial equilibration of Al-Mg isotopic signatures obtained in the core of a coarse-grained CAI (CG1-CAI) in Tnz 057 may indicate a lower peak temperature for Mg diffusion of approximately 540–580 °C, while grossular present in the core of this CAI indicates a higher temperature of around 800 °C for the metamorphic event on the parent body of Tnz 057. Excluding metamorphic features, the similarity in nature and abundance of CAIs in CK and CV chondrites confirms that CVs and CKs form a continuous metamorphic series from type 3 to 6.

International audience; The Curiosity rover discovered fine-grained sedimentary rocks, which are inferred to represent an ancient lake and preserve evidence of an environment that would have been suited to support a martian biosphere founded on chemolithoautotrophy. This aqueous environment was characterized by neutral pH, low salinity, and variable redox states of both iron and sulfur species. Carbon, hydrogen, oxygen, sulfur, nitrogen, and phosphorus were measured directly as key biogenic elements; by inference, phosphorus is assumed to have been available. The environment probably had a minimum duration of hundreds to tens of thousands of years. These results highlight the biological viability of fluvial-lacustrine environments in the post-Noachian history of Mars.

H 2 O, CO 2 , SO 2 , O 2 , H 2 , H 2 S, HCl, chlorinated hydrocarbons, NO, and other trace gases were evolved during pyrolysis of two mudstone samples acquired by the Curiosity rover at Yellowknife Bay within Gale crater, Mars. H 2 O/OH-bearing phases included 2:1 phyllosilicate(s), bassanite, akaganeite, and amorphous materials. Thermal decomposition of carbonates and combustion of organic materials are candidate sources for the CO 2 . Concurrent evolution of O 2 and chlorinated hydrocarbons suggests the presence of oxychlorine phase(s). Sulfides are likely sources for sulfur-bearing species. Higher abundances of chlorinated hydrocarbons in the mudstone compared with Rocknest windblown materials previously analyzed by Curiosity suggest that indigenous martian or meteoritic organic carbon sources may be preserved in the mudstone; however, the carbon source for the chlorinated hydrocarbons is not definitively of martian origin.

International audience; The Radiation Assessment Detector (RAD) on the Mars Science Laboratory's Curiosity rover began making detailed measurements of the cosmic ray and energetic particle radiation environment on the surface of Mars on 7 August 2012. We report and discuss measurements of the absorbed dose and dose equivalent from galactic cosmic rays and solar energetic particles on the martian surface for similar to 300 days of observations during the current solar maximum. These measurements provide insight into the radiation hazards associated with a human mission to the surface of Mars and provide an anchor point with which to model the subsurface radiation environment, with implications for microbial survival times of any possible extant or past life, as well as for the preservation of potential organic biosignatures of the ancient martian environment.