^1,2C. S. Harrison,¹A. J. King,²R. H. Jones,³L. Piani
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70052]
¹Planetary Materials Group, Natural History Museum, London, UK
²Department of Earth and Environmental Sciences, The University of Manchester, Manchester, UK
³Centre de Recherches Pétrographiques et Géochemiques CNRS, Université de Lorraine, Metz, France
Published by arrangement with John Wiley & Sons

Phosphate minerals are significant carriers of volatiles (e.g., OH) and halogens in chondritic material; however, their origin in most groups of carbonaceous chondrites remains poorly characterized. We have determined the abundance, morphology, texture, and composition of phosphate grains in aqueously altered CI chondrites and in hydrated and thermally metamorphosed Antarctic CY chondrites using scanning electron microscopy and electron probe microanalysis. Phosphates include apatite (formula Ca5(PO4)3X, where X = F-, Cl-, OH- or other anions) and sodium-bearing magnesium phosphate, both of which formed during episodes of aqueous alteration on the CI and CY parent bodies. Apatite grains in the CI chondrites range up to 40 μm in size with a modal abundance of ~0.10 area%, while in the CYs, the largest grains are ~50 μm in size and the modal abundance is ≤0.70 area%. Analysis by secondary ion mass spectrometry (SIMS) indicates that apatite in the CYs contains ~1.0–1.8 wt% H2O, with δD values of −84‰ to 393‰ likely reflecting aqueous and thermal processing. Apatite in both the CI and CY chondrites is rich in fluorine, with fluorine abundances that range from 20 to 80 mole% of the X (anion) site. This contrasts with apatite in other chondrite groups, which is predominantly Cl-rich. Estimated bulk chondrite F abundances based on F abundance in apatite are 12–21 ppm F for the CI chondrites and 61 ppm F for the CY chondrites. This is comparable to bulk CI chondrite F abundances in the literature, suggesting that most fluorine is hosted in apatite. However, the chlorine content of CI chondrite apatite (<0.05 wt%) is too low to account for the bulk chondrite Cl abundance, indicating that Cl is hosted in other phases. Mg,Na-phosphate, a rare extraterrestrial mineral, has a modal abundance of ~0.02 area% in both the CI and CY chondrites. Mg,Na-phosphates in the CI and CY chondrites are halogen-poor (<0.15 wt%) and are typically hydrated in the CIs (analytical totals as low as 67 wt%) and dehydrated in the CYs (analytical totals >96.0 wt%). The occurrence of Mg,Na-phosphates in the CI and Antarctic CY chondrites is indicative of brines on their respective parent bodies. Similarities between the two groups, as well as with the phosphate mineral assemblage in asteroids Ryugu and Bennu, indicate that comparable fluid compositions and environmental conditions were prevalent on numerous parent bodies in the early Solar System.

¹Christopher D.K. Herd, ¹Sophie Benaroya
Geochimica et Cosmochimica Acta (in Press) (Open Access) Link to Article [https://doi.org/10.1016/j.gca.2025.10.001]
¹Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, AB T6G 2E3, Canada
Copyright Elsevier

We provide an updated compilation of oxygen fugacity (fO₂) estimates for martian meteorites, with a specific focus on the shergottites. The compilation includes estimates from over 70 distinct lithologies from the martian meteorite suite, calculated from olivine-pyroxene-spinel and Fe-Ti oxide oxybarometers. Olivine-pyroxene-spinel oxybarometry was recalculated from original data sources using an updated model. Results from V- in-olivine and Eu/Gd oxybarometry from the literature are provided for comparison. Oxygen fugacity data are plotted against chondrite-normalized La/Yb ratio to critically examine the correlation between fO₂ and incompatible trace element (ITE) enrichment previously postulated. We find that the correlation holds, when factors including differences in petrogenetic histories, distinctions between shergottite petrologic types, and early vs. late crystallizing assemblages, are taken into consideration. We model the degassing of H, C and S species from primitive martian magmas using the MAGEC model (Sun and Lee, 2022) and successfully reproduce the 2–3 log unit increase recorded within olivine-phyric shergottites between early and late crystallizing assemblages. We find that volatile degassing can account for most of the fO₂ increase in the olivine-phyric shergottites, without requiring extensive auto-oxidation, as long as their fO₂ remains at or below a value equivalent to the fayalite-magnetite-quartz (FMQ) equilibrium throughout their crystallization. With these considerations in mind, we propose a martian mantle redox-ITE trend defined by shergottite sources: a depleted source (La/Yb ∼ 0.1) with fO₂ = FMQ-4 ± 0.7, an intermediate source (La/Yb ∼ 0.5) at fO₂ = FMQ-3 ± 0.75 and an enriched source (Lab/Yb ∼ 1) at fO₂ = FMQ-2 ± 0.75. The depleted/reduced source is likely graphite saturated.

Comparisons with compilations of fO₂ from basaltic eruptives on Earth highlight fundamental differences between the two planets ultimately attributable to differences in degree of mantle convective mixing throughout their histories: terrestrial mantle sources produce basaltic eruptives with a relatively limited range of fO₂, within ±1 log unit of FMQ; any degassing from these magmas results in reduction, not oxidation. The mantle sources of the shergottites – while represented by a similarly limited range of fO₂, ∼FMQ-4 to FMQ-2 – produce basaltic eruptives with a range of low initial (magmatic) fO₂; the more reduced nature of these magmas make them more susceptible to overprinting by degassing of H-C-S species during eruption and emplacement. Whether the mantle sources inferred from the shergottites apply to other martian meteorites (or other martian igneous rocks) remains to be tested; however, post-magma ocean crystallization processes would have acted to oxidize and overprint initial mantle sources defined by the shergottite fO₂-ITE trend.