Alteration assemblages in the nakhlites: Variation with depth on Mars

Abstract

Abstract-Secondary mineral assemblages in the nakhlite meteorites, Lafayette, Governador Valadares (GV), Nakhla, Yamato (Y)-000593 ⁄ Y-000749 have been studied using scanning electron microscopy, transmission electron microscopy, and electron probe micro analysis. The different nakhlites have distinctive secondary assemblages in their olivine grains and mesostases, showing compositional fractionation correlated with their relative depths below the Martian surface. Fracture-filled veins in Lafayette at the bottom of the pile consist of a siderite-phyllosilicate-Fe oxide-hydrated silicate gel assemblage. Corresponding veins in Nakhla and GV further up the pile are predominantly a siderite-gel assemblage, with additional evaporites including gypsum. Y-000593 ⁄ Y-000749 veins are dominated by gel. The gel's Mg ⁄ (Mg + Fe) ratio decreases from Lafayette (0.37) to GV (0.32), Nakhla (0.24), and Y-000593 (0.15). We suggest that hydrothermal fluid flowed up this depth profile, initiated by melting of buried H 2 O-CO 2 ice. Our results show a complex mix of Fe-rich phyllosilicate within the veins and mesostasis of Lafayette with d-spacings of 0.7-1.1 nm suggesting a mixture of smectite and serpentine. The phyllosilicate formed at close to neutral pH, £150°C. We also suggest that water rock ratios (W ⁄ R) of 1-10 occurred in Lafayette with smaller values for the other nakhlites. This is reflected in the volume of alteration minerals: 10% of olivine in Lafayette to 3% in Nakhla. Textural evidence of rapid cooling, together with the W ⁄ R and likely fluid velocities, suggest that the secondary assemblages formed quickly, e.g., within months. A model is proposed in which the secondary assemblages formed in an impact-induced hydrothermal system terminated by precipitation of the gel and evaporation of soluble salts.

Geochemistry of Carbonates on Mars: Implications for Climate History and Nature of Aqueous Environments

“…They all represent mixtures of carbonates with igneous minerals, suggesting formation via rock alteration by subsurface fluids rather than sedimentary precipitation. The carbonates in ALH 84001 and the nakhlites (Changela and Bridges 2011;Valley et al 1997) were also likely formed during brief aqueous events in the subsurface. Taken together, these carbonates are evidence for the presence of subsurface water as opposed to surficial bodies of water, and they likely pre-date the formation of the valley networks and the Noachian-Hesperian transition when the dense CO 2 atmosphere was supposed to have been lost (Carr and Head 2010;Head 2008, 2011).…”

“…The scatter in the data likely reflects the variety of different carbonate phases within the martian meteorites. These multiple episodes vary strongly in carbon isotopes and may reflect evolution of the martian atmosphere through time that suggests that the carbonate-forming aqueous systems were short lived and subject to dynamic environmental changes (Changela and Bridges 2011;Niles et al 2005;Valley et al 1997). Experimental work performed to date has succeeded in forming carbonates with a similar chemical composition as the ALH 84001 carbonates (Golden et al 2000(Golden et al , 2001, but not with the correct variation in isotopic composition.…”

“…Particularly, Yamato 00593 shows evidence for faster cooling and fewer cumulus phases (Mikouchi et al 2003) while Lafayette is considered to have cooled the slowest. Furthermore the secondary phases in the nakhlites are not evenly distributed, perhaps also reflecting different source depths in the original cumulate pile (Changela and Bridges 2011).…”

“…More recently a hydrothermal model has been proposed that suggests that fluids of moderate temperatures (<150°C) were mobilized from subsurface ice deposits, progressively altering the rocks in the cumulate pile during ascent and evaporation at the surface (Changela and Bridges 2011). Under this model, the elevated δ 13 C values in the nakhlite carbonates were likely obtained through a remobilization of more ancient carbonate deposits ) rather than through exposure to the atmosphere during the time of their formation, and the 17 O anomaly was inherited from the water source being atmospherically derived.…”

A Conspicuous Clay Ovoid in Nakhla: Evidence for Subsurface Hydrothermal Alteration on Mars with Implications for Astrobiology

“…This alteration probably occurred rapidly, due to circulation through the Nakhla parent rock of low-temperature hydrothermal fluids, which probably originated from the melted permafrost. Hydrothermal circulation was initiated by the impact of a meteor that opened a crater at least *2 km in diameter (Changela and Bridges, 2011). Nakhla is thought to have been situated at a very shallow depth of about 10-20 m from the martian surface (Lentz et al, 1999) and, therefore, more likely to have been exposed to surface alteration.…”