¹S.Arivazhagan, ¹A.Karthi
Planetary and Space Sciences (in Press) Link to Article [https://doi.org/10.1016/j.pss.2018.06.010]
Centre for Applied Geology, The Gandhigram Rural Institute-Deemed to be University, Gandhigram, Dindigul, 624302, Tamilnadu, India

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^1,2Edgar S.Steenstra, ¹Wim van Westrenen
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.06.023]
¹Faculty of Sciences, Vrije Universiteit, Amsterdam, The Netherlands
²The Geophysical Laboratory, Carnegie Institution of Science, Washington D.C., United States
Copyright Elsevier

Accurate constraints on the light element composition of the Martian core are required for models of the Martian core dynamo, the conditions under which the Martian core formed (i.e. the existence and extent of a magma ocean) and the overall volatile inventory of Mars. Here, we present a synthesis of geochemical constraints on the abundances of light elements S, C and O in the Martian core using mass balance calculations combined with published expressions that predict their high-pressure metal–silicate partitioning behaviour. We incorporate recently proposed bulk S Martian mantle abundances and find that the Martian core must be S-rich, virtually independent of the type of bulk composition considered and the P-T conditions during core-mantle differentiation of Mars. The core contains at least 7 wt % S and may be up to stoichiometric FeS in composition, depending on which P-T conditions (and bulk compositions) are assumed. If bulk Mars was formed from chondritic building blocks, the core S content is constrained to 13.5±3.5 wt.%, in good agreement with geophysical models of the Martian interior and with measured siderophile element depletions in SNC meteorites. Our calculations yield O contents for the Martian core of <4 wt.%, with the highest concentrations for the highest P-T conditions of Martian core formation. Carbon contents in the Martian core are expected to be low (<1.4 wt.%) given the abundance of C in chondritic meteorite groups. The calculated solubility limit for C in Fe-Ni-S alloys is higher than calculated core C contents in virtually all cases, suggesting the Martian primitive mantle is not graphite saturated if the bulk Mars C budget is (close to) chondritic.
The estimated depletions of volatile elements Se and Te in the Martian interior can be reconciled easily with formation of a Martian S-rich core. This implies that these volatile elements may not have been lost from Mars by degassing in Mars early history.
The calculated Martian core S contents cannot be used to distinguish between the two different proposed modes of core crystallization, but do suggest the Martian core may still be fully liquid today.

¹Elishevah M.M.E. van Kooten et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.06.021]
¹Centre for Star and Planet Formation and Natural History Museum of Denmark, University of Copenhagen, DK-1350 Copenhagen, Denmark
Copyright Elsevier

The recent fall of the relatively unaltered CM chondrite Maribo provides a unique opportunity to study the early stages of aqueous alteration on the CM chondrite parent body. We show using transmission electron microscopy of a matrix FIB-section from Maribo that this meteorite mainly appears to consist of tochilinite-cronstedtite intergrowths (TCIs), but also contains regions of amorphous or nanocrystalline silicates, anhydrous silicates and FeNi metal aggregates with thin iron oxide rims, suggesting that it experienced aqueous alteration to a relatively small degree. A comparison of Maribo with increasingly altered CM chondrites such as Jbilet Winselwan and Bells shows that during progressive aqueous alteration (1) the TCIs are replaced by coarser sulfides and increasingly Mg-rich serpentine, and (2) the abundance of 1515N-rich hotspots increases, whereas the magnitude of their 1515N enrichment decreases. We observe that the overall N isotope variability related to aqueous alteration is an order of magnitude lower than the variability observed between different chondrite groups. We suggest these high order differences are the result of heterogeneous accretion of insoluble or soluble organic carriers of 1515N to the different chondrite parent bodies. D/H ratios of matrices from Maribo, Jbilet Winselwan and Bells increase with progressive aqueous alteration, a trend that is opposite to expectations of mixing between D-poor water and D-rich organic matter. We argue that this behaviour cannot be related to Fe oxidation or serpentinization reactions and subsequent loss of D-poor H2 gas. We offer an alternative hypothesis and suggest that CM chondrites experienced two-stage aqueous alteration. During the first stage occurring at relatively low temperature, mixing of increasing amounts of D-poor water with D-rich organic matter results in a decrease of D/H ratio with increasing degree of alteration. During the second stage of alteration occurring at relatively high temperature (T << 300 °°C), decomposition of TCIs in CMs of petrologic type <<2.7 releases gaseous D-poor water that results in increase of the D/H ratio of the CM matrices. Finally, we report on changes in the organic structure of Maribo, Jbilet Winselwan and Bells using Carbon-K and Nitrogen-K edge electron energy loss spectroscopy. The organic matter initially has higher aromatic/aliphatic ratios (e.g., Maribo) and lower abundances of ketone and carboxyl functional groups, which we suggest are the result of chemical degradation of double bonded carbon from oxidation during hydrothermal alteration. Consequently, we propose that the organic matter of the CM chondrite Paris, for which lower aromatic/aliphatic ratios have been observed, may have been different from Maribo, perhaps reflecting the early accretion of Paris relative to Maribo.

¹David T. Vaniman et al. (>10)
American Mineralogist 103, 1011-1020 Link to Article [https://doi.org/10.2138/am-2018-6346]
¹Planetary Science Institute, Tucson, Arizona 85719, U.S.A.
Copyright: The Mineralogical Society of America

Analyses by the CheMin X-ray diffraction instrument on Mars Science Laboratory show that gypsum, bassanite, and anhydrite are common minerals at Gale crater. Warm conditions (∼6 to 30 °C) within CheMin drive gypsum dehydration to bassanite; measured surface temperatures and modeled temperature depth profiles indicate that near-equatorial warm-season surface heating can also cause gypsum dehydration to bassanite. By accounting for instrumental dehydration effects we are able to quantify the in situ abundances of Ca-sulfate phases in sedimentary rocks and in eolian sands at Gale crater. All three Ca-sulfate minerals occur together in some sedimentary rocks and their abundances and associations vary stratigraphically. Several Ca-sulfate diagenetic events are indicated. Salinity-driven anhydrite precipitation at temperatures below ∼50 °C may be supported by co-occurrence of more soluble salts. An alternative pathway to anhydrite via dehydration might be possible, but if so would likely be limited to warmer near-equatorial dark eolian sands that presently contain only anhydrite. The polyphase Ca-sulfate associations at Gale crater reflect limited opportunities for equilibration, and they presage mixed salt associations anticipated in higher strata that are more sulfate-rich and may mark local or global environmental change. Mineral transformations within CheMin also provide a better understanding of changes that might occur in samples returned from Mars.