Raman spectroscopic and geochemical studies of primary and secondary minerals in Martian meteorite Northwest Africa 10720

Petrological and mineralogical analyses of Northwest Africa (NWA) 10720 indicate that this meteorite is one Martian nakhlite with an unbrecciated augitic igneous cumulate texture, consisting of clinopyroxene (65.4 vol.%) and olivine (19.4 vol.%) set in a fine-grained intercumulus matrix. Texturally and compositionally, complex primary zoning in cumulus augite and the presence of oscillatory zoning in olivine phenocrysts indicate that NWA 10720 has experienced at least two-stage magmatic activity before the eruption. The first primary parental magmas of nakhlites generated magnesian augite cores (Mg#(62)). A subsequent more evolved magma partly melted and resorbed augite cumulus, facilitating that olivine crystallized and augite growth resumed simultaneously and nearly at a final stage. Secondary mineral assemblages are composed of iddingsite-like veins (similar to 0.2 vol.%), threadlike maghemite, hematite veins, and altered pyrrhotite. Olivine alteration veins are likely to contain a higher proportion of saponite and low abundant serpentine in terms of major elements. Their compositions (Mg# = 13.5 and Fe/Si wt = 0.83) indicate that NWA 10720 appears to have encountered a weak acidic fluid (pH similar to 3.5-4.5). The lack of siderite also supports the acidic nature of the fluid. The absence of sulfates demonstrates the following: (1) The fluid permeating downwards through this nakhlite could have depleted SO42- when it reached the parental rocks; (2) the hydrothermal fluid leaching NWA 10720 parental rocks was no longer saturated in SO42- at the extremely hydrothermal system initiated by melting of buried water ice; and (3) sulfates might have been lost during terrestrial residence. These observations reveal that NWA 10720 could have experienced separate evolutionary pathways in the shallow Martian surface with extensive liquid fractionation activities, different from secondary alteration processes of the Lafayette subgroup in an open-system environment. Various Fe-oxide species and high abundant hematite veins demonstrate that the oxidization and dehydration might dominate late-stage alteration once the exhaustion of parental fluid.