¹Marina Martínez,^1,2Charles K. Shearer,¹Adrian J. Brearley
American Mineralogist 108, 2024-2042 Link to Article [http://www.minsocam.org/msa/ammin/toc/2023/Abstracts/AM108P2024.pdf]
¹Department of Earth & Planetary Sciences, MSC03-2040
²University of New Mexico, Albuquerque, New Mexico 87131, U.
Copyright: The Mineralogical Society of America

The microstructures of selected F-, Cl-, and OH-bearing martian apatite grains, two in Northwest
Africa (NWA) 998 (cumulus apatites, embedded in pyroxene) and a set of four in Nakhla (intercumulus
apatites), were studied by focused ion beam–transmission electron microscopy (FIB-TEM) techniques.
Our results show that the nanostructure of martian apatite is characterized by a domain structure at
the 5–10 nm scale defined by undulous lattice fringes and slight differences in contrast, indicative
of localized elastic strain within the lattices and misorientations in the crystal. The domain structure
records a primary post-magmatic signature formed during initial subsolidus cooling (T <800 °C), in which halogens clustered by phase separation (exsolution), but overall preserved continuity in the crystalline structure. Northwest Africa 998 apatites, with average Cl/F ratios of 1.26 and 2.11, show higher undulosity of the lattice fringes and more differences in contrast than Nakhla apatites (average Cl/F = 4.23), suggesting that when Cl/F is close to 1, there is more strain in the structure. Vacancies likely played a key role stabilizing these ternary apatites that otherwise would be immiscible. Apatites in Nakhla show larger variations in halogen and rare-earth element (REE) contents within and between grains that are only a few micrometers apart, consistent with growth under disequilibrium conditions and crystallization in open systems. Nakhla apatite preserves chemical zonation, where F, REEs, Si, and Fe are higher in the core and Cl increases toward the outer layers of the crystal. There is no evidence of subsolidus ionic diffusion or post-magmatic fluid interactions that affected bulk apatite compositions in NWA 998 or Nakhla. The observed zonation is consistent with crystallization from a late-stage melt that became Cl-enriched, and assimilation of volatile-rich crustal sediments is the most plausible mechanism for the observed zonation. This work has broader implications for interpreting the chemistry of apatite in other planetary systems.

¹Kaushik Mitra¹,^2,3Jeffrey G. Catalano,¹Yatharth Bahl,¹Joel A. Hurowitz
Earth and Planetary Science 624, 118464 Link to Article [https://doi.org/10.1016/j.epsl.2023.118464]
¹Department of Geosciences, Stony Brook University, Stony Brook, NY 11727 USA
²Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130 USA
³McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130 USA
Copyright Elsevier

Sulfate salts are abundant, widespread, and temporally distributed on the surface of Mars. Several processes, including sulfide weathering, have been proposed to explain the formation and accumulation of oxidized iron(III) hydroxysulfate (e.g., jarosite) in Martian sediments. Oxidative weathering of iron sulfide minerals (like pyrite and pyrrhotite) could explain the presence of sulfate as well as the relative absence of sulfide minerals in Martian sediments. However, the effectiveness of oxyhalogen compounds, plausible oxidants present on Mars, to weather iron sulfides remain unknown. Here we investigate the oxidative weathering of iron sulfide minerals, pyrite and pyrrhotite, by chlorate and bromate in Mars-relevant fluids. Our results demonstrate that both oxyhalogen species readily oxidize iron sulfides and produce an alteration assemblage comprised of elemental sulfur, Fe(III) (oxyhydr)oxide (magnetite, goethite, and lepidocrocite), and Fe(III) hydroxysulfates (jarosite and schwertmannite). The mineral products depend strongly on the type of sulfide, oxidant, and initial solution pH. Owing to their abundance and highly reactive nature, oxyhalogen brines could be important Fe(III) hydroxysulfate-forming reagents on early and modern Mars and substantially impact the survivability of sulfide minerals at the Martian surface. Additionally, sulfide bearing units might serve as indicators of minimal post-depositional alteration. Oxyhalogens may be responsible for the loss of magmatic sulfides in surface materials given the prevalence of oxyhalogen brines and the reactivity of the sulfides.

¹Demidova S.I.,¹Badyukov D.D.
Geochemistry International 61, 453-467 Lonk to Article [DOI 10.1134/S0016702923050038]
¹Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, 119991, Russian Federation

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^1,2Lidia Pittarello,^1,2Ludovic Ferrière,³Stepan M. Chernonozhkin,³Frank Vanhaecke,⁴Steven Goderis
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14091]
¹Naturhistorisches Museum Wien, Mineralogisch-Petrographische Abteilung, Vienna, Austria
²Department of Lithospheric Research, University of Vienna, Vienna, Austria
³Atomic & Mass Spectrometry—A&MS Research Unit, Department of Chemistry, Ghent University, Ghent, Belgium
⁴Archaeology, Environmental Changes and Geo-Chemistry, Vrije Universiteit Brussel, Brussels, Belgium
Published by arrangement with John Wiley & Sons

Diogenites, which are part of the howardite–eucrite–diogenite (HED) group, are considered to represent rocks from the lower crust and mantle of a differentiated planetary body, likely the asteroid 4 Vesta. The Northwest Africa 12973 (NWA 12973) meteorite was classified as an anomalous diogenite due to the occurrence of a vesiculated layer. This work reports on the petrographic and geochemical study of two fragments of this meteorite, aiming to better constrain the origin of the vesiculated layer. Whereas the interior of NWA 12973 (here called host) presents the typical characteristics of an olivine diogenite, that is, coarse-grained pyroxene, olivine ribbons, chromite, and accessory phases, the vesiculated layer presents a fine-grained pyroxene groundmass with local rounded relics of olivine and interstitial chromite and metal, and is characterized by abundant large vesicles. The contact between the vesiculated layer and the host is sharply defined. The composition of individual minerals does not show any significant differences between the host and the vesiculated layer, suggesting in situ melting. Geothermobarometry indicates a slightly higher crystallization temperature at lower pressure for the vesiculated layer, consistent with melting and crystallization under lower crustal conditions upon exhumation. The trigger for the local melting was likely a large impact event, which was responsible for adiabatic decompression in the central area or deep faulting and frictional melting.

^1,2Alan E. Rubin,³Brent D. Turrin
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14088]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, USA
²Maine Mineral & Gem Museum, Bethel, Maine, USA
³Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
Published by arrangement with John Wiley & Sons

About 17% of L6 chondrites (15/87) show significant reduction features in BSE images in thin section. Because some thin sections of these meteorites do not show reduction features, this percentage is a lower limit. Reduction features include: (1) 4–5-μm-thick BSE-dark reduction rims on olivine and orthopyroxene grains and along fracture boundaries in these grains, (2) 4–12-μm-thick dark bands (probably poorly crystalline pyrrhotite) at the margins and along fractures in troilite grains, and (3) 2–5-μm-thick dark rinds of kamacite around some taenite grains. Only one of 70 L-group chondrites (1.4%) of lower petrologic type exhibits minor reduction. The L6 chondrites showing major reduction have 40Ar/39Ar plateau ages ranging from 156 ± 1 Ma for Guangnan to 4543 ± 3 Ma for Thamaniyat Ajras. Reduction occurred after silicate, sulfide, and metal grains had attained their present sizes during parent-body thermal metamorphism (and had been fractured by parent-body collisions). The precise plateau age of Thamaniyat Ajras probably marks the timing of the L6 reduction event. It seems likely the reductant was a low-viscosity fluid, plausibly CO, derived from oxidation of poorly graphitized and amorphous carbon within fine-grained matrix. Water-ice that had accreted to the L-chondrite asteroid was heated and mobilized during metamorphism, causing oxidation. After peak metamorphism, ~75% of the water had been used up or lost; the remaining water facilitated continuing graphite oxidation so that, after this point, overall reduction effects exceeded those of oxidation. L chondrites of lower petrologic type were less affected by reduction due to their lower metamorphic temperatures.

¹Yunhua Wu et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.10.031]
¹Planetary Environmental and Astrobiological Research Laboratory, School of Atmospheric Sciences, Sun Yat-sen University, Zhuhai 519082, China
Copyright Elsevier

Impact events on the Moon have been recognized as modifying the composition of surface materials through processes such as shock metamorphism and mixing with exotic components. Previous studies have indicated that the regolith sampled by the Chinese Chang’E-5 mission was primarily evolved from local mare basalts, likely originating from a single episode of effusive volcanism. The relatively simple and monotonic protolith of the Chang’E-5 regolith presents a unique opportunity to investigate the chemical modifications induced by impact events. In this study, we conducted detailed petrographic and mineralogical analyses on a diverse set of lunar regolith samples, including thirty-three small impact glass particles (39–227 μm), one large agglutinate (∼1.6 mm), and sixteen basaltic clasts (0.1–1.6 mm). Numerical modeling was also conducted to quantitatively assess the melting behavior of basaltic clasts under different impact conditions. Our primary objective was to evaluate the chemical variations of impact glass in relation to lithic clasts. While the majority of homogeneous impact glass spherules and larger basaltic clasts (≥1 mm) exhibit similar bulk compositions (e.g., Al2O3, CaO and FeO) to the local regolith, we recognize additional effects of impact processes, including impact comminution, impact melting and crystallization, differential volatilization, and potentially selective melting. These processes have modified the texture and geochemistry of lunar regolith components. The chemical signatures of small clasts in the Chang’E-5 regolith indicate that the fine size fractions (e.g., those with diameters of ≤ 300 μm) are predominantly composed of lithic and monomineralic fragments with a substantial proportion being dominated by mesostasis. Melting of these sub-millimeter fractions may lead to the formation of impact glass with chemical compositions deviating from the average local regolith. These results have implications for understanding the compositional evolution of the Chang’E-5 regolith. Notably, our study suggests that impact glasses spherules with different major (e.g., TiO2, MgO) and minor (e.g., REEs, Zr, Th and U) element compositions could be derived from the same protolith of local mare basalts, instead of being exclusively attributed to exotic impact ejecta. In this case, small-scale local impacts may have played a crucial role in the impact history of the Chang’E-5 landing site.

¹Jun Korenaga,²Simone Marchi
The Proceedings of the National Academy of Sciences (PNAS)120 (43) e2309181120 Link to Article [https://doi.org/10.1073/pnas.23091811]
¹Department of Earth and Planetary Sciences, Yale University, New Haven, CT 06520
²Department of Space Studies, Southwest Research Institute, Boulder, CO 80302

Highly siderophile elements (HSEs; namely Ru, Rh, Pd, Re, Os, Ir, Pt, and Au) in Earth’s mantle require the addition of metals after the formation of Earth’s core. Early, large collisions have the potential to deliver metals, but the details of their mixing with Earth’s mantle remain unresolved. As a large projectile disrupts and penetrates Earth’s mantle, a fraction of its metallic core may directly merge with Earth’s core. Ensuing gravitational instabilities remove the remaining projectile’s core stranded in Earth’s mantle, leaving the latter deprived of HSEs. Here, we propose a framework that can efficiently retain the metallic components during large impacts. The mechanism is based on the ubiquitous presence of a partially molten region in the mantle beneath an impact-generated magma ocean, and it involves rapid three-phase flow with solid silicate, molten silicate, and liquid metal as well as long-term mixing by mantle convection. In addition, large low-shear-velocity provinces in the lower mantle may originate from compositional heterogeneities resulting from the proposed three-phase flow during high-energy collisions.

¹Jose Daniel Castro-Cisneros,²Renu Malhotra,³Aaron J. Rosengren
Communications Earth & Environment 4, 372 Open Access Link to Article [DOI https://doi.org/10.1038/s43247-023-01031-w]
¹Department of Physics, The University of Arizona, Tucson, 85721, AZ, USA
²Lunar and Planetary Laboratory, The University of Arizona, Tucson, 85721, AZ, USA
³Mechanical and Aerospace Engineering, UC San Diego, La Jolla, 92093, CA, USA

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¹Chi Ma,¹Jinping Hu,²Martin D. Suttle,¹Yunbin Guan,³Thomas G. Sharp,¹Paul D. Asimow,⁴Paul J. Steinhardt,⁵Luca Bindi
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14089]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
²School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, UK
³School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
⁴Department of Physics, Princeton University, Princeton, New Jersey, USA
⁵Dipartimento di Scienze della Terra, Università di Firenze, Florence, Italy
Published by arrangement with John Wiley & Sons

A recently described micrometeorite from the Nubian desert (Sudan) contains an exotic Al-Cu-Fe assemblage closely resembling that observed in the Khatyrka chondrite (Suttle et al., 2019; Science Reports 9:12426). We here extend previous investigations of the geochemical, mineralogical, and petrographic characteristics of the Sudan spherule by measuring oxygen isotope ratios in the silicate components and by nano-scale transmission electron microscopy study of a focused ion beam foil that samples the contact between Al-Cu alloys and silicates. O-isotope work indicates an affinity to either OC or CR chondrites, while ruling out a CO or CM precursor. When combined with petrographic evidence we conclude that a CR chondrite parentage is the most likely origin for this micrometeorite. SEM and TEM studies reveal that the Al-Cu alloys mainly consist of Al metal, stolperite (CuAl), and khatyrkite (CuAl2) together with inclusions in stolperite of a new nanometric, still unknown Al-Cu phase with a likely nominal Cu3Al2 stoichiometry. At the interface between the alloy assemblage and the surrounding silicate, there is a thin layer (200 nm) of almost pure MgAl2O4 spinel along with well-defined and almost perfectly spherical metallic droplets, predominantly iron in composition. The study yields additional evidence that Al-Cu alloys, the likely precursors to quasicrystals in Khatyrka, occur naturally. Moreover, it implies the existence of multiple pathways leading to the association in reduced form of these two elements, one highly lithophile and the other strongly chalcophile.

¹Bruce Fegley Jr,¹Katharina Lodders,²Nathan S. Jacobson
Geochemistry (Chemie der Erde) (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2023.125961]
¹Planetary Chemistry Laboratory, Dept. of Earth & Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St Louis, MO 63130, USA
²Materials and Structures Division, NASA Glenn Research Center/HX5, 21000 Brookpark Road, Cleveland, OH 44135, USA
Copyright Elsevier

The chemical equilibrium distribution of 69 elements between gas and melt is modeled for bulk silicate Earth (BSE) material over a wide P – T range (1000–4500 K, 10−6–102 bar). The upper pressure end of this range may occur during lunar formation in the aftermath of a Giant Impact on the proto-Earth. The lower pressures may occur during evaporation from molten silicates on achondritic parent bodies. The virial equation of state shows silicate vapor behaves ideally in the P – T range studied. The BSE melt is modeled as a non-ideal solution and the effects of different activity coefficients and ideal solution are studied. The results presented are 50% condensation temperatures, major gas species of each element, and the pressure and temperature dependent oxygen fugacity (fO2) of dry and wet BSE material. The dry BSE model has no water because it excludes hydrogen; it also excludes the volatile elements (C, N, F, Cl, Br, I, S, Se, Te). The wet BSE model has water because it includes hydrogen; it also includes the other volatiles. Some key conclusions include the following: (1) much higher condensation temperatures in silicate vapor than in solar composition gas at the same total pressure due to the higher metallicity and higher oxygen fugacity of silicate vapor (cf. Fegley et al. 2020), (2) a different condensation sequence in silicate vapor than in solar composition gas, (3) good agreement between different activity coefficient models except for the alkali elements, which show the largest differences between models, (4) agreement, where overlap exists, with prior published silicate vapor condensation calculations (e.g., Canup et al. 2015, Lock et al. 2018, Wang et al. 2019), (5) condensation of Re, Mo, W, Ru, Os oxides instead of metals over the entire P – T range, (6) a stability field for Ni-rich metal as reported by Lock et al. (2018), (7) agreement between ideal solution (from this work and from Lock et al. 2018) and real solution condensation temperatures for elements with minor deviations from ideality in the oxide melt, (8) similar 50% condensation temperatures, within a few degrees, in the dry and wet BSE models for the major elements Al, Ca, Fe, Mg, Si, and the minor elements Co, Cr, Li, Mn, Ti, V, and (9) much lower 50% condensation temperatures for elements such as B, Cu, K, Na, Pb, Rb, which form halide, hydroxide, sulfide, selenide, telluride and oxyhalide gases. The latter results are preliminary because the solubilities and activities of volatile elements in silicate melts are not well known, but must be considered for the correct equilibrium distribution, 50% condensation temperatures and mass balance of halide (F, Cl, Br, I), hydrogen, sulfur, selenium and tellurium bearing species between silicate melt and vapor.