¹Sébastien Charnoz,²Paolo A.Sossi,³Yueh-Ning Lee,¹Julien Siebert,⁴Ryuki Hyodo,¹Laetitia Allibert,¹Francesco C.Pignatale,¹Maylis Landeau,⁵Apurva V.Oza,¹Frédéric Moynier
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114451]
¹Université de Paris, Institut de Physique du Globe de Paris, CNRS, F-75005 Paris, France
²Institute of Geochemistry and Petrology, ETH Zürich, CH-8092 Zürich, Switzerland
³Department of Earth Sciences, National Taiwan Normal University, 88, Sec. 4, Ting-Chou Road, Taipei City 11677, Taiwan
⁴ISAS, JAXA, Sagamihara, Japan
⁵Physikalisches Institut, Universität Bern, Bern, Switzerland
Copyright Elsevier

Prevailing models for the formation of the Moon invoke a giant impact between a planetary embryo and the proto-Earth (Canup, 2004; Ćuk et al., 2016). Despite similarities in the isotopic and chemical abundances of refractory elements compared to Earth’s mantle, the Moon is depleted in volatiles (Wolf and Anders, 1980). Current models favour devolatilisation via incomplete condensation of the proto-Moon in an Earth-Moon debris-disk (Charnoz and Michaut, 2015; Canup et al., 2015; Lock et al., 2018). However the physics of this protolunar disk is poorly understood and thermal escape of gas is inhibited by the Earth’s strong gravitational field (Nakajima and Stevenson, 2014). Here we investigate a simple process, wherein the Earth’s tidal pull promotes intense hydrodynamic escape from the liquid surface of a molten proto-Moon assembling at 3–6 Earth radii. Such tidally-driven atmospheric escape persisting for less than 1 Kyr at temperatures ∼1600 − 1700 K reproduces the measured lunar depletion in K and Na, assuming the escape starts just above the liquid surface. These results are also in accord with timescales for the rapid solidification of a plagioclase lid at the surface of a lunar magma ocean (Elkins-Tanton et al., 2011). We find that hydrodynamic escape, both in an adiabatic or isothermal regime, with or without condensation, induces advective transport of gas away from the lunar surface, causing a decrease in the partial pressures of gas species (Ps) with respect to their equilibrium values (Psat). The observed enrichment in heavy stable isotopes of Zn and K (Paniello et al., 2012; Wang and Jacobsen, 2016) constrain Ps/Psat > 0.99, favouring a scenario in which volatile loss occurred at low hydrodynamic wind velocities (<1% of the sound velocity) and thus low temperatures. We conclude that tidally-driven atmospheric escape is an unavoidable consequence of the Moon’s assembly under the gravitational influence of the Earth, and provides new pathways toward understanding lunar formation.

¹Mathilde Faure et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114462]
¹Université Grenoble Alpes, CNRS, Institut de Planétologie et Astrophysique de Grenoble (IPAG), UMR 5274, Grenoble F-38041, France
Copyright Elsevier

We question here the radiolytic origin of (i) polyaromatic insoluble organic matter (IOM) recovered from primitive chondrites, and (ii) organics at the surface of reddish Trans-Neptunian Objects (TNOs), some minor planets and icy satellites. Organic synthesis by ion irradiation was investigated through experiments on a variety of targets: Polyethylene glycol 1450, lignin, cellulose and sucrose, exposed to low (C 40 keV and Ne 170 keV) and high energy (C 12 MeV, Ni 17 MeV, 78Kr 59 MeV) ions. These experiments show that all carbonaceous precursors evolve towards a sp2-rich amorphous carbon (a-C) above a critical nuclear dose of 10−7+10 eV.atom−1. A thorough review of the literature shows that this value applies for a large range of carbonaceous materials, including C-rich simple ices. Below this critical dose, irradiated targets are carbonized and transformed into cross-linked polymeric disordered solids, with abundant olefinic and acetylenic bonds, but devoid of aromatic or polyaromatic species. Ion irradiation of simple compounds, e.g. ices, is thereby not a viable process to synthesize IOM. However, in the case of aromatic-rich precursors, swift heavy ions irradiation leads to polyaromatic materials, by bridging existing aromatic or polyaromatic units. In the context of Early Solar System, i.e. Galactic Cosmic Rays (GCR) irradiation during 10–20 Myr, the formation of chondritic IOM from simple ices mixed with interstellar Polycyclic Aromatic Hydrocarbons (PAHs) appears as a plausible mechanism. This scenario, based on the recycling of existing carbonaceous interstellar grains under low-temperature conditions, would account for the heterogeneity of the D, 15N and 13C isotopic fractionations at the molecular scale, and the preservation of deuterium hot spots that are highly sensitive to high-temperature conditions (> 300 °C). At the surface of TNOs, sp2-rich amorphous carbons are formed by the implantation of GCRs and Solar wind ions. The electronic dose is also very high for an irradiation time of several Gyr (> 100 eV.atom−1), leading to the formation of reddish disordered solids, provided that the surface contains a minimum abundance of carbonaceous species. Finally, sp2-rich amorphous carbons produced in the laboratory (e.g. the ACAR compound from Zubko et al., 1996) are fair analogues of the darkening agent produced by radiolysis.

¹Craig R. Walton,²Ioannis Baziotis,³Ana Černok,⁴Ludovic Ferrière,⁵Paul D. Asimow,^1,6Oliver Shorttle,^3,7Mahesh Anand
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13648]
¹Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ UK
²Department of Natural Resources Management and Agricultural Engineering, Agricultural University of Athens, IeraOdos 75, 11855 Athens, Greece
³Department of Physical Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA UK
⁴Natural History Museum, Burgring 7, A‐1010 Vienna, Austria
⁵Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E California Blvd, Pasadena, California, 91125 USA
⁶Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 OHA UK
⁷Department of Earth Sciences, The Natural History Museum, London, SW7 5BD UK
Published by arrangement with John Wiley & Sons

The geochemistry and textures of phosphate minerals can provide insights into the geological histories of parental asteroids, but the processes governing their formation and deformation remain poorly constrained. We assessed phosphorus‐bearing minerals in the three lithologies (light, dark, and melt) of the Chelyabinsk (LL5) ordinary chondrite using scanning electron microscope, electron microprobe, cathodoluminescence, and electron backscatter diffraction techniques. The majority of studied phosphate grains appear intergrown with olivine. However, microtextures of phosphates (apatite [Ca5(PO4)3(OH,Cl,F)] and merrillite [Ca9NaMg(PO4)7]) are extremely variable within and between the differently shocked lithologies investigated. We observe continuously strained as well as recrystallized strain‐free merrillite populations. Grains with strain‐free subdomains are present only in the more intensely shocked dark lithology, indicating that phosphate growth predates the development of primary shock‐metamorphic features. Complete melting of portions of the meteorite is recorded by the shock‐melt lithology, which contains a population of phosphorus‐rich olivine grains. The response of phosphorus‐bearing minerals to shock is therefore hugely variable throughout this monomict impact breccia. We propose a paragenetic history for P‐bearing phases in Chelyabinsk involving initial phosphate growth via P‐rich olivine replacement, followed by phosphate deformation during an early impact event. This event was also responsible for the local development of shock melt that lacks phosphate grains and instead contains P‐enriched olivine. We generalize our findings to propose a new classification scheme for Phosphorus‐Olivine‐Assemblages (Type I–III POAs). We highlight how POAs can be used to trace radiogenic metamorphism and shock metamorphic events that together span the entire geological history of chondritic asteroids.

^1,2Caroline E. Caplan,²Gary R. Huss,²Hope A. Ishii,²John P. Bradley,^2,3Birger Schmitz,¹Kazuhide Nagashima
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13649]
¹Department of Earth Sciences, University of Hawai‘i at Mānoa, 1680 East‐West Road, Honolulu, Hawai‘i, 96822 USA
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, 1680 East‐West Road, Honolulu, Hawai‘i, 96822 USA
³Astrogeobiology Laboratory, Department of Physics, Lund University, Lund, Sweden
Published by arrangement with John Wiley & Sons

Remnant extraterrestrial chrome spinels from terrestrial sediments provide information on how the mixture of meteoritic materials falling to Earth has changed over Earth’s history. The parent meteorite type of each grain can be identified by characteristic elemental and oxygen‐isotope abundances. Some meteorite types can be difficult to classify because their chrome‐spinel compositional ranges overlap. Silicate inclusions within chrome spinels of modern ordinary chondrites have been shown to have discriminating power among meteorite subclasses. We employed energy‐dispersive X‐ray spectroscopy in a scanning electron microscope (SEM) and in a (scanning) transmission electron microscope (S/TEM) to investigate inclusions in chrome‐spinel grains from Ordovician and Jurassic sediments. Unaltered Ordovician inclusions allowed us to establish the size limits for reliable SEM analysis of inclusions. The Jurassic grains were more altered, but the use of STEM techniques on small inclusions (<3 μm diameter at their polished surfaces) allowed us to determine chemical compositions and mineral structures of inclusions in three chrome spinels. The parent meteorite type was determined for one Jurassic grain based on its inclusion compositions. Our study confirms that silicate inclusions can be used to classify parent meteorite types of chrome‐spinel grains, but the size of the inclusions and the complex effects of terrestrial alteration must be taken into account. During our study, we also found some interesting exsolution phenomena in the host chrome‐spinel grains.

¹Akimasa Suzumura,²Noriyuki Kawasaki,³Yusuke Seto,²Hisayoshi Yurimoto,¹Shoichi Itoh
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.03.030]
¹Department of Earth and Planetary Sciences, Kyoto University, Kyoto 606-8502, Japan
²Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
³Department of Planetology, Kobe University, Kobe 657-8501, Japan
Copyright Elsevier

We report the in-situ oxygen isotopic distributions corresponding to the petrographic-mineralogical observation on a compact Type A (CTA) Ca-Al-rich inclusion (CAI), KU-N-02, from a reduced CV3 chondrite, Northwest Africa 7865. The CTA has an igneous texture and mainly consists of spinel, melilite, and Al-Ti-rich clinopyroxene (fassaite). Oxygen isotopic compositions of the constituent minerals plot along the carbonaceous chondrite anhydrous mineral line. The spinel grains are poikilitically enclosed in the melilite and fassaite and are uniformly 16O-rich (Δ17O = approximately −23‰). The fassaite is texturally classified into two types: blocky fassaite and intergranular fassaite. The blocky fassaite crystals exhibit growth zoning as they change from Ti-rich to Ti-poor along the inferred directions of crystal growth from core to rim, while the oxygen isotopic compositions change from 16O-poor (Δ17O = approximately −6‰) to 16O-rich (Δ17O = approximately −23‰) with crystal growth. The intergranular fassaite crystals exist between the melilite crystals and exhibit variable Ti abundance and oxygen isotopic compositions. Additionally, their relationships between Ti contents and oxygen isotopic composition are similar to those of the blocky fassaite. The melilite grains are homogeneously 16O-poor (Δ17O = approximately −2‰), irrespective of their åkermanite (Åk) content. Each melilite grain generally exhibits growth zoning with increasing Åk contents from core to rim, although the melilite contains Åk-rich patches within single crystal. Åk-rich patches often include two types of fassaite: small blebby crystals attached to spinel crystals and round crystals. The oxygen isotopic compositions of the Åk-rich patch and blebby fassaite are 16O-poor (Δ17O = approximately −2‰), similar to that of the host melilite. On the other hand, the round fassaite exhibits significant variation in oxygen isotopic compositions ranging from Δ17O = −23‰ to −4‰, which are different from those of the host melilite. These petrographic textures and oxygen isotopic variations indicate the presence of a solid precursor with variable oxygen isotopic compositions for the CTA. The spinel and round fassaite grains are relicts of the precursor that melted in the 16O-poor nebular gas, resulting in the crystallization of the host melilite from the 16O-poor melt. The Åk-rich patches and blebby fassaite crystallized from melts trapped by the growing host melilite crystals. The blocky and intergranular fassaite crystallized after the melilite did, and the oxygen isotopic composition of the melt changed to 16O-rich during the crystallization process, suggesting that the oxygen isotopic composition of the surrounding nebular gas could be varied. The inferred oxygen isotopic evolution for CTA is consistent with those inferred for Type B CAIs, suggesting that coarse-grained igneous CAIs formed in a similar nebular environment regardless of precursor chemistry.

¹F. N. Lindsay et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13637]
¹Department of Chemistry & Chemical Biology, Rutgers University, New Brunswick, New Jersey, 08854 USA
Published by arrangement with John Wiley & Sons

The Martian breccias NWA 7034, NWA 7533, and paired meteorites record events ranging in age from 4.47 Ga to <200 Ma. Published ages indicate a period of major disturbance at ~1.4 Ga, examined in detail here through 40Ar/39Ar dating of handpicked grains and two small chips. Argon diffusion parameters were obtained for six samples. Also presented are He, Ne, Ar, Kr, and Xe contents of two small (<100 µg), handpicked mineral separates, a felsic “Light” sample and a mafic/pyroxene‐rich “Dark” sample. The 40Ar/39Ar ages of five samples, four containing >1 wt% K and thought to be rich in feldspar and one containing <~1 wt% K, cluster near 1.4 Ga. The 40Ar/39Ar ages of nine grains with low K contents have a wide range of apparent ages from 0.3 ± 0.1 Ga to 2.9 ± 0.1 Ga for individual temperature steps, and from 0.74 ± 0.06 Ga to ~2.1 Ga for plateau ages. Isochron ages are less precise, but generally agree with plateau ages. Only two isochrons have the significantly positive intercepts expected in the presence of terrestrial or Martian atmospheric argon. At higher release temperatures, activation energies for diffusion obtained from 39Ar data for six samples are generally 160–200 kJ mol−1, consistent with published values for feldspathic minerals. For three of these samples, lower temperature data on Arrhenius plots are best fit with a much lower activation energy of <100 kJ mol−1. We attribute the low values to the effects of varying degrees of shock on feldspathic minerals and/or the presence of phases in vitrophyric spherules produced by hydrothermal alteration. The low activation energies place an upper limit of ~14 ka on the terrestrial age of NWA 7034. Much lower concentrations of cosmogenic (c) 3He and 21Ne in the Light than in the Dark separate indicate substantial losses concurrent with or postdating cosmic ray irradiation. A one‐stage, cosmic ray exposure (CRE) age for the Dark separate from NWA 7034 is estimated to be between 7 and 10 Ma from the concentrations of 3Hec and 38Arc, and of close to 15 Ma from the concentration of 21Nec. Most of the 40Ar/39Ar and noble gas data are compatible with (1) a heating and alteration event ~1.40 Ga caused by contact metamorphism, an impact, and/or the infiltration of hydrothermal fluids; and (2) at least one later event at lower temperatures that led to either loss of He and Ar from phases with low activation energies, or to gain of K. Most of the 40Ar/39Ar ages are consistent with the assembly of NWA 7034 1.4 Ga ago or perhaps earlier followed more recently by selective alteration. A more recent time of assembly is also consistent with these ages provided that the temperature stayed low. The five most precise 40Ar/39Ar ages of the samples analyzed are all ~1.4 Ga, a value seen frequently in other NWA 7034 chronometers and very similar to crystallization ages of nakhlites and chassignites (NC). Some CRE ages based on noble gases in NWA 7034 agree within their considerable uncertainties with those of NC. These two chronometric coincidences suggest that the NWA 7034 clan and the NC share a launch date on Mars. We propose that K‐rich fluids derived from the nakhlite source area interacted with proto‐NWA 7034 and modified the K/Ar ratios and ages of previously shocked feldspar grains, with the degree of modification depending on the degree of shock. The NWA 7034 clan may therefore be considered components from a metamorphic aureole around a nakhlite massif.

^1,2F. Jourdan,^2,3L. Forman,^1,2T. Kennedy,¹G. K. Benedix,⁴E. Eroglu,¹C. Mayers
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13640]
¹Western Australian Argon Isotope Facility, John de Laeter Centre, TIGeR, Curtin University, Perth, 6845 Western Australia, Australia
²Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, Perth, 6845 Western Australia, Australia
³Department of Earth & Planetary Sciences, Western Australian Museum, Locked Bag 49, Welshpool DC, Perth, 6986 Western Australia, Australia
⁴School of Molecular and Life Sciences, Curtin University, Perth, 6845 Western Australia, Australia
Published by arrangement with John Wiley & Sons

Asteroid 4 Vesta is the only largely preserved differentiated asteroid and thus is an excellent proxy to study early magmatism occurring on planets and moons. In this study, we focus on eucrite Pecora Escarpment (PCA) 82502, a medium‐ to fine‐grained eucrite which chemical analyses suggest belongs to the main howardite–eucrite–diogenite clan, albeit with some peculiarities. We carried out backscattered electron and electron backscattered diffraction microscopy analyses of the meteorite along with step‐heating 40Ar/39Ar dating analyses of various types of groundmass aliquots. We show that Pecora Escarpment 82502 is composed of medium‐grained igneous crystalline clasts and smaller fractured satellite clasts surrounded by approximately 50 µm wide impact melt veins of the same composition. Our results show that the large crystalline clasts and fine‐grained veins both display little evidence of shock processes. Six groundmass aliquots from large crystalline clasts returned concordant plateau (>70% of 39Ar) or mini‐plateau (50–70% of 39Ar) 40Ar/39Ar ages with a weighted mean of 4531 ± 6 Ma (P = 0.67). Thermodynamic cooling and 40Ar diffusion models suggest that the K/Ar system recorded and preserved the igneous age despite subsequent infiltration of hot and quickly quenched melt veins. Our new igneous age, combined with evidence for four other young volcanic and plutonic eucrites of similar age, shows that Vesta was still magmatically active around 4531 Ma. The lack of younger ages suggests that this age might well represent the end of the magmatic activity in the upper crust of Vesta. When combined with existing paleomagnetic constraints, our data suggest that 4 Vesta had an active dynamo that was still active ~35 Ma after accretion.

¹Robert W.Nicklas,¹James M.D.Day,²Zoltan Vaci,³Arya Udry,⁴Yang Liu,⁵Kimberly T.Tait
Earth and Planetary Science Letters 564, 116876 Link to Article [https://doi.org/10.1016/j.epsl.2021.116876]
¹Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, 92093, USA
²Institute of Meteoritics, Department of Earth and Planetary Science, University of New Mexico, Albuquerque, NM, 87131, USA
³Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, NV 89154, USA
⁴Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁵Department of Natural History, Royal Ontario Museum, Toronto, ON, M5S 2C6, Canada
Copyright Elsevier

Martian meteorites are the only available samples that can be directly measured to constrain the geological evolution of Mars. It has been suggested that the oxygen fugacity (fO2) of martian shergottite meteorites, which have low (∼7 wt.%) to high-MgO (∼30 wt.%) compositions, correlates with incompatible trace element enrichment (i.e., La/Yb), and 87Sr/86Sr, 143Nd/144Nd, 187Os/188Os and 176Hf/177Hf at the time of crystallization. These relationships have been interpreted to result from early magmatic processes segregating enriched and more oxidized from depleted and more reduced reservoirs in Mars. Here we use the V-in-olivine oxybarometer to constrain the fO2 of shergottites and the dunitic chassignites. These data, utilizing early crystallizing silicate phases, constrain the shergottite fO2 range to between −3.72 ± 0.07 and −0.21 ± 0.55 ΔFMQ (log units relative to the fayalite-magnetite-quartz buffer), with no correlation with trace element enrichment or Nd isotope systematics. Previously employed oxybarometers that use later-formed or multiple mineral phases, and that show such correlations, likely differ from the V-in-olivine oxybarometer in that they record effects from late-stage magmatic processes. In contrast to shergottites, chassignites are relatively oxidized, at +2.1 ± 0.4 to +2.2 ± 0.5 ΔFMQ. The chassignites, along with the nakhlites, have been proposed to be sourced from metasomatized lithospheric mantle, and their high fO2 strengthens this model. The new data implies that the martian mantle sources of shergottites have fO2 of −2.1 ± 1.8 ΔFMQ. This estimate indicates that the mantle and core of Mars are not in redox equilibrium and therefore that oxidation of the martian mantle following core formation is required.

¹Johan Vandenborre,³Laurent Truche,¹Amaury Costagliola,¹Emeline Craff,¹Guillaume Blain,¹Véronique Baty,²Ferid Haddad,²Massoud Fattahi
Earth and Planetary Science Letters 564, 116892 Link to Article [https://doi.org/10.1016/j.epsl.2021.116892]
¹Subatech, UMR 6457, Institut Mines-Télécom Atlantique, CNRS/IN2P3, Université de Nantes, 4, Rue Alfred Kastler, La chantrerie BP 20722, 44307 Nantes cedex 3, France
²GIP ARRONAX, 1 rue ARRONAX, CS 10112, 44817 Saint-Herblain Cedex, France
³University Grenoble Alpes, CNRS, ISTerre, CS 40700, 38058 Grenoble, France
Copyright Elsevier

Low molecular weight carboxylate anions such as formate (HCOO−), acetate (CH3COO−) and oxalate (C2O) have been shown to play an important role in supporting deep subsurface microbial ecosystems. Their origin whether biological or abiotic is currently highly debated, but surprisingly radiolytic production has rarely been considered, as it is the case for H2. Here, we address this question through dedicated irradiation experiments. Aqueous solutions containing carbonate, formate, acetate or oxalate have been irradiated using both the 60.7 MeV α-beam of the ARRONAX cyclotron (Nantes, France) and 661.7 keV γ-Ray in order to reveal the mechanism and chemical yield of radiation-induced dissolved carbonate degradation.

The yields (G-values) of carboxylate anions production/degradation in low-concentration carbonate solution (0.01 to 1 mmol L−1) are measured. Carbonate degradation occurs through three consecutive steps (Carbonate
Formate Acetate Oxalate) involving formate radical (CO2−•), dihydrogen (H2), and carbon dioxide (CO2) generation. Dissolved carbonate radiolysis provides a consistent pathway for both enhancing two-fold the radiolytic H2 production compared to pure water and generating carboxylic species, chiefly oxalate, readily available for microbes. Radiation-induced carbonate degradation may produce substantial amount (millimolar concentration) of carboxylate anions in ancient groundwaters from deep crystalline bedrocks. Subsurface lithoautotrophic microbial ecosystems may not only be supported by radiolytic H2 but also by carboxylate species from carbonate radiolysis. Carbonate radiolysis can be also an endogenous source of carboxylate species on Mars and other planetary bodies.

¹Caroline AVRIL,¹Valérie MALAVERGNE,²Eric D. VAN HULLEBUSCH,³Fabrice BRUNET,²Stephan BORENSZTAJN,⁴Jérôme LABANOWSKI,⁵Louis HENNET,⁶François GUYOT
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13641]
¹Laboratoire Géomatériaux et Environnement, Université Gustave Eiffel, EA 4508, UPEM, 5 boulevard Descartes, 77454 Marne‐la‐Vallée, Cedex 2, France
²Institut de physique du globe de Paris, CNRS, Université de Paris, F‐75005 Paris, France
³ISTerre, CNRS—Univ. Grenoble Alpes, Maison des Géosciences, BP 53, 38041 Grenoble Cedex 9, France
⁴Laboratoire de Chimie et Microbiologie de l’Eau, UMR CNRS 6008, Université de Poitiers, 40 avenue du recteur Pineau, 86022 Poitiers, France
⁵Conditions Extrêmes et Matériaux: Haute Température et Irradiation, UPR 3079 CNRS et université d’Orléans, 1d avenue de la recherche scientifique, 45071 Orléans Cedex 2, France
⁶Muséum National d’Histoire Naturelle, Sorbonne Universités, IMPMC UMR 7590 CNRS, 61 rue Buffon, 75005 Paris, France
Published by arrangement with John Wiley & Sons

Understanding the transformations of highly reduced enstatite chondrites (EC) in terrestrial environments, even on very short timescales, is important to make the best use of the cosmochemical and mineralogical information carried by these extraterrestrial rocks. Analogs of EC meteorites were synthesized at high pressure and high temperature. Then, their aqueous alterations, either abiotic or in the presence of the bacteria Acidithiobacillus ferrooxidans or Acidithiobacillus thiooxidans, were studied under air, at pH ~2, 20 °C, and atmospheric pressure. They stayed in shaken batch reactors for 15 days. Reference experiments were carried out separately by altering only one mineral phase among those composing the synthetic EC (i.e., sulfides: troilite or Mg‐Ca‐rich sulfides, enstatite, and Fe70Si30). Composition of the alteration aqueous media and microstructures of the weathered solids were monitored by inductively coupled plasma atomic emission spectroscopy and by scanning electron microscopy, respectively. Alteration sequence of the different mineral components of the synthetic EC was found to occur in the following order: magnesium‐calcium sulfides > troilite > iron‐silicon metallic phase > enstatite regardless of the presence or absence of the microorganisms. Such small biological effects might be due to the fact that the alteration conditions are far from biologically optimal, which is likely the case in most natural environments. The exposed surfaces of an EC meteorite falling on Earth in a wet and acidic environment could lose within a few hours their Ca‐ and Mg‐rich sulfides (oldhamite and niningerite). Then, in <1 week, troilite and kamacite could be altered. In a wet and acidic environment, only the enstatite would remain intact and would weather on a much slower geological timescale.