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• Ca-siderite comprises between trace amounts and 4% of Lafayette sections. • Electron Microscopy and synchrotron analysis shows variable carbonate dissolution. • Ferric-(hydr)oxide nanoparticles are present on saponite and corroded siderite. • Serpentine-berthierine is re-described as odinite, a ferric low-temperature mineral. • Dissolution of carbonate can lead to methane on Mars via a Sabatier type reaction. New mineralogical studies of Lafayette reveal that it contains a notably variable abundance of martian carbonate. Four percent was identified in mesostasis (3.2%) and olivine-hosted (0.8%) fractures in one polished section, but only 0.2% of both textural types in another. The Lafayette carbonates are Mg 0.0-2.0 Cc 13.2-38.6 Sd 17.7-81.9 Rh 3.1-42.9. They have undergone variable but extensive amounts of dissolution and replacement as the nakhlite secondary fluid evolved, associated with the precipitation of ferric saponite in olivine fractures and a serpentine-like phyllosilicate in the mesostasis. The mesostasis carbonate has undergone the highest degree of corrosion and replacement. TEM analysis has shown the presence of Fe-(hydr)oxide (likely ferrihydrite) nanoparticles on olivine-hosted carbonates which can be linked to the cessation of more extensive carbonate dissolution at those sites. The mesostasis serpentine-like mineral has been described here on the basis of WDS and EDX analyses, HRTEM and Fe-K XANES, as odinite, a ferric, 0.7 nm d 001 -spacings phyllosilicate mineral with a characteristic 1:1 serpentine-like structure. The carbonate dissolution stage and then formation of Fe-(hydr)oxide nanoparticles occurred under circumneutral-alkaline conditions (7) < pH < 10. This range of pH is also where the general dissolution mechanism switched from a proton-promoted, to a water hydrolysis reaction associated with a reduction in the dissolution rates. As dissolution rates were reduced and the fluid had cooled to ≤50 °C, the precipitation of the ferric saponite and odinite, a phyllosilicate associated with temperatures of ∼25 °C, dominated over the carbonate dissolution. The extensive dissolution of such crustal carbonate across the upper martian crust, producing bicarbonate and carbon dioxide, and the coupled formation of ferric phyllosilicates, would lead to the formation of CH 4 in substantial amounts via a Fischer-Tropsch type reaction. The results of our study illustrate a process to explain the relatively low abundance of detected carbonate on Mars and a likely source for some of the methane on Mars. [ABSTRACT FROM AUTHOR]

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 (~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 ~ 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. [ABSTRACT FROM AUTHOR]

Introduction: The nakhlite secondary alteration assemblage consists mainly of Fe-rich carbonate partially corroded and replaced by ferric saponite and an odinite-like serpentine phase [1,2]. This mineral assemblage and its alteration stages can illustrate key aspects of the Amazonian carbon cycle. For instance, the origin of methane in the Martian atmosphere is one of the key unresolved questions in Mars' atmospheric evolution and current near-surface processes [3,4]. A possible route for methane production was described in [5] which demonstrated that CH4 is expected to dominate over carbon oxides under the redox conditions associated with martian hydrothermal fluids and the nakhlite secondary minerals. Here we use mineralogical studies of the Amazonian [6] nakhlite secondary assemblage to explore links between the carbonate-bearing crust and routes for organic formation. Methods and Samples: Mineralogical data on the nakhlite secondary minerals have been gathered with FE-SEM, EDX, WDS, FIB-(S)TEM at University of Leicester's Advanced Microscopy Facility and University of Nottingham, and Fe-K XANES at Diamond Light Source e.g. [1,2]. The 12 nakhlites carry varying abundances and parts of the overall secondary assemblage depending on depth of the parent rock within the nakhlite pile and extent of exposure to the secondary fluid [1,2,7]. We have studied many of the nakhlites as part of this project, but the near pristine nakhlite Lafayette has particular importance due to the abundance of its secondary assemblage and the textural evidence for an evolving fluid. We have anlysed Lafayette sections USNM 7849 and a section originally from the CFM. Results: Carbonate is up to 4% of one Lafayette thin section, and Mg0.0-2.0Cc13.2-38.6Sd17.7-81.9Rh3.1-42.9 [1]. A key feature of the nakhlite phylloslicate-carbonate assemblage in the nakhlites is that carbonate is variably replaced by ferric saponite in olivine fractures and odinite-like serpentine in mesostasis textural sites (Fig. 1). For instance, another thin section of the same Lafayette meteorite contained only minute traces of unaltered carbonate, most having been replaced. The odinite-like phase has d001-spacings of 0.7 nm and (Fetot+Mg)/Al at. of 0.8-0.9 [1]. Fe-K XANES show Fe3+/SFe ratios from 0.86-1.0 within the odinite and 0.59-0.96 within the saponite. Small traces of ferrihydrite were identified near the margins of corroded carbonate and saponite. Discussion: The nakhlite secondary fluid was initially a HCO3-charged brine associated with the precipitation of carbonate within olivine and mesostasis sites. Once the fluid's HCO3 was exhausted, the evolving, neutral-mildly alkaline, =50 oC brine corroded the carbonate and precipitated the phyllosilicate as a replacement of carbonate and directly within fractures. As the fluid was no longer buffered by Fe2+-carbonate, the iron oxidation state increased, as recorded in the Fe3+-rich phyllosilicates and precipitation of small traces of ferrihydrite near the margins of corroded carbonate and saponite. Extensive dissolution of such carbonate across the upper martian crust, producing bicarbonate and carbon dioxide, and the coupled formation of ferric phyllosilicates, would lead to the formation of CH4 in substantial amounts via a Fischer-Tropsch type reaction [8-10]. Such reactions could be substantially inhibited by kinetic factors but the likely relatively high abundance of carbonate and clay-bearing assemblages in much of the Mars upper crust [11] suggest that substantial quantities of CH4 and PAHs would still have been created. Seasonal release of some trapped CH4 reservoirs formed from this process are, for instance, of sufficient likely magnitude to explain localised atmospheric CH4 enrichments detected by Mars Science Laboratory [3]. As more of the crust is being studied via meteorites, MSL [12,13], Mars2020-MSR and orbiters, the role and significance of carbonate-phyllosilicate abundances within the ancient carbon cycle and organic reservoirs of Mars will continue to be revealed [14]. [ABSTRACT FROM AUTHOR]

Introduction: Nakhlites are a group of basaltic meteorites originating from the mid-Amazonian igneous crust of Mars [1]. They represent an interesting set of samples to study the aqueous alteration at the Martian subsurface, as they contain aqueous alteration phases in olivine called "iddingsite" (e.g. [2]). Studying iddingsite in nakhlites can reveal the water-crust interactions and the hydrothermal physico-chemical conditions that prevailed at the Martian subsurface, with implications for the past habitability of Mars, and in particular for microbial life during the Amazonian period. We studied iddingsite in Caleta el Cobre (CeC) 022, a nakhlite that contains a high amount of alteration products, and compared this meteorite to 7 other nakhlites. Methods: A serie of analyses were conducted on eight nakhlites polished sections: CeC 022, Governador Valadares, Lafayette, Miller Range (MIL) 03346, Nakhla, North-West Africa (NWA) 817, NWA 998 and Yamato (Y) 000953 to assess the mineralogy, the chemical and the isotopic composition of the alteration products in nakhlites: electronic microscopy (FEG-SEM and TEM), EDX microanalyses and chemical maps, LA-ICPMS major elements analyses, Raman spectra, and finally, hydrogen isotopic measurements by SIMS. Results: In all nakhlites, the alteration veins show at least two types of iddingsite (Fig. 1A): (i) a coarse iddingsite in contact with olivine at the border of the alteration veins, showing 50-200 nm crystals, composed of a mixture of 1:1 phyllosilicates of greenalite-cronstedtite composition, and Fe-oxyhydroxides; (ii) a fine iddingsite, in the inner part of the alteration veins, with <10 nm crystals, with a composition close to saponite (2:1 phyllosilicate). Complex chemical zoning of Mg, Ca, Mn, S, P and Al, has been observed on EDS-TEM mappings in NWA 10153. In most nakhlites, olivine grains also display planes of secondary inclusions, composed of pyroxene, magnetite and a void potentially filled by a fluid (Fig. 1B). In most nakhlites, magnetite-pyroxene symplectites are found in olivine grains, often at the border of the secondary inclusion planes. Those secondary inclusions and symplectites can also be observed at the center of the iddingsite veins (Fig. 1A). Also, sulfide-magnetite veinlets are observed at the center of the iddingsite veins, and also crosscutting olivine and pyroxene grains and mesostasis. Finally, organic matter is observed on Raman spectra in the iddingsite of many nakhlites, and is located in coarse iddingsite, as shown by TEM observations of NWA 10153. Discussion and conclusion: Our favored scenario is that the secondary inclusions and the symplectites are formed by a first fluid alteration event triggered by late magmatic fluids, prior to the iddingsite formation event,. These secondary inclusions represent weakness planes that facilitate the circulation of the alteration fluid forming the iddingsite inside the olivine grains. The composition and texture of both types of iddingsite is suggestive of a crystallization by filling of existing fractures, with an alteration fluid enriched in elements from basaltic glass and host olivine dissolution, and changes in the alteration conditions or fluid composition. The sulfide-magnetite veinlets represent either the late stage of the same alteration event as the iddingsite formation, or a different later fluid injection. With the two types of iddingsite, the complex centers of veins and the chemical zoning in the fine iddingsite, we suggest that fluid alteration in the nakhlites has a complex multistage history of fluid injections. [ABSTRACT FROM AUTHOR]