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We used Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) multispectral mapping data to assess heterogeneity of primary and secondary mineral compositions of Noachian‐aged regions of Martian crust. Multispectral data corroborate interpretations from CRISM targeted observations and Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activité global mapping of large horizontal differences in vertical crustal structures, and degree and grade of alteration to secondary minerals. CRISM multispectral data analysis conducted at a combination of high spatial resolution and coverage not available in other data sets also reveals previously unrecognized exposures. At one extreme, basaltic crustal material is minimally altered, mostly to smectite clay; at the other, Al‐enriched alteration products are present, alteration is widespread, and superposed surface materials rich in salts and precipitates, suggesting multiple episodes of alteration. The revealed vertical structure of primary mineralogy is consistent with that inferred in previous studies. Controls on the extent and metamorphic grade of secondary mineral assemblages are proximity to heat sources including large impact basins and inferred magmatic bodies. Plain Language Summary: We used the CRISM mapping spectral database with ∼86% coverage of the surface of Mars to confirm previously observed trends in the composition of crust exposed during different eras. The exposures of minerals that are formed through primary crust‐forming processes (e.g., volcanism) reveal distinct changes to mineral compositions through time. Secondary minerals formed through alteration of the primary crust also have a distinct signature with time and proximity to sources of heat: such as large basins that formed through giant impacts, and heat sources from magma reservoirs deep in the crust. Key Points: Previous data suggest heterogeneous primary and secondary mineralogy exposures in Noachian crustCompact Reconnaissance Imaging Spectrometer for Mars multispectral mapping data were used to study regions reported to exhibit differing degrees of alterationHeterogeneous alteration is confirmed and likely results from differences in heat availability [ABSTRACT FROM AUTHOR]

The mechanisms of fluid penetration through the gabbroic lower crust are important for the hydration of oceanic lithosphere. In the Oman ophiolite, which preserves an entire sequence of oceanic lithosphere formed at a fast‐spreading ridge, the layered gabbros and dunites are extensively serpentinized. In this paper, we describe the characteristic textures of serpentinized troctolite and olivine gabbros recovered from the CM1A site of the Oman Drilling Project. In the troctolite, an olivine mesh texture is pervasively developed and is characterized by two types of veins: early lizardite + brucite + magnetite and late Al‐rich lizardite + magnetite. These veins suggest the initiation of serpentinization at <350°C and a supply of Si and Al from plagioclase during the later stages of serpentinization. Plagioclase surrounding serpentinized olivine grains commonly shows radial fracturing. Numerical simulations using the discrete element method applied to coupled fluid flow, reaction, and fracturing reveal that volume expansion of olivine grains during serpentinization results in the simultaneous fracturing of olivine and surrounding plagioclase, and that the thermal stress during cooling of oceanic lithosphere might also cause preferential olivine fracturing prior to serpentinization. The simulations also predict a self‐organizing fracture network that connects the olivine grains and passes through both olivine‐rich and olivine‐poor layers, resulting in permeability enhancement during serpentinization. Our results suggest that reaction‐induced fracturing plays an essential role in the infiltration of seawater through the lower crust and into the mantle within oceanic lithosphere. Plain Language Summary: The Oman ophiolite preserves evidence of pervasive serpentinization of the lower crust, Moho transition zone, and uppermost mantle. In this study, we investigated a troctolite and olivine gabbros from the CM1A site of the Oman Drilling Project, and compared fracture patterns in these rocks with those predicted by discrete element method (DEM) numerical simulations. In the troctolite, olivine grains are serpentinized to form a mesh texture, and radial fractures are developed in plagioclase and clinopyroxene adjacent to olivine. The olivine mesh texture consists of two types of veins: early serpentine + brucite + magnetite and late Al‐rich serpentine + magnetite. Numerical simulations of coupled fracturing, reaction, and fluid flow reproduced the observed fracture patterns and indicate that reaction‐induced fracturing plays a key role in permeability enhancement within the lower oceanic crust during low‐temperature alteration. Key Points: Olivine mesh textures and radial fractures in plagioclase developed simultaneously during troctolite serpentinization in the Oman ophioliteNumerical simulations reproduce fracture patterns of olivine and plagioclase, and predict enhanced permeability during olivine hydrationReaction‐induced fracturing plays a key role in enabling fluid flow through the lower crust to uppermost mantle [ABSTRACT FROM AUTHOR]

Abstract: Surface conditions on early Mars were likely anoxic, similar to early Earth, but the timing of the evolution to oxic conditions characteristic of contemporary Mars is unresolved. Ferrous trioctahedral smectites are the thermodynamically predicted products of anoxic basalt weathering, but orbital analyses of Noachian‐aged terrains find primarily Fe3+‐bearing clay minerals. Rover‐based detection of Fe2+‐bearing trioctahedral smectites at Gale Crater suggests that ferrous smectites are the unoxidized progenitors of orbitally detected ferric smectites. To assess this pathway, we conducted ambient‐temperature oxidative alteration experiments on four synthetic ferrous smectites having molar Fe/(Mg + Fe) from 1.00 to 0.33. Smectite suspension in air‐saturated solutions produced incomplete oxidation (24–38% Fe3+/ΣFe). Additional smectite oxidation occurred upon reexposure to air‐saturated solutions after anoxic hydrothermal recrystallization, which accelerated cation and charge redistribution in the octahedral sheet. Oxidation was accompanied by contraction of the octahedral sheet (d(060) decreased from 1.53–1.56 Å to 1.52 Å), consistent with a shift toward dioctahedral structure. Ferrous smectite oxidation by aqueous hydrogen peroxide solutions resulted in nearly complete Fe2+ oxidation but also led to partial Fe3+ ejection from the structure, producing nanoparticulate hematite. Reflectance spectra of oxidized smectites were characterized by (Fe3+,Mg)2‐OH bands at 2.28–2.30 μm, consistent with oxidative formation of dioctahedral nontronite. Accordingly, ferrous smectites are plausible precursors to observed ferric smectites on Mars, and their presence in late‐Noachian sedimentary units suggests that anoxic conditions may have persisted on Mars beyond the Noachian. [ABSTRACT FROM AUTHOR]