Lafayette, a case study for quantitative determination of P, T and X of a Martian subsurface fluid – and application to orbiter and lander data

Schwenzer, Susanne P. and Bridges, John (2012). Lafayette, a case study for quantitative determination of P, T and X of a Martian subsurface fluid – and application to orbiter and lander data. In: American Geophysical Union’s 45th Annual Fall Meeting, 3-7 Dec 2012, San Francisco, CA, USA.


The nakhlite meteorites provide excellent access to the alteration mineralogy formed in short-term, high-temperature fluid pulses in Martian rocks (Changela and Bridges, MAPS, 45, 1847-1867; Hicks et al. LPSC 2012, #2253). Veins within the Lafayette nakhlite contain a succession of Fe-rich carbonate, Fe-saponite and gel of the same composition as the smectite. We apply thermochemical modelling to the observed alteration assemblages to gain insights into the formation conditions. Carbonate formed from a CO2-rich hydrothermal fluid at 150 ≤ T ≤ 200 °C, pH of 6–8 with a water to rock ratio (W/R) ≤300. This was followed by phyllosilicate formation at 50 °C, at pH 9 and W/R of 6 (Bridges and Schwenzer, EPSL in review). Subsequent rapid cooling led to gel formation. The fluid forming the phyllosilicate was enriched in the most soluble elements. When evaporating, a succession of salts, such as gypsum (or hydrated sulphates), halite and eventually sylvite can precipitate. Some of these salts are present in the nakhlites (Bridges et al. 2001).

Given the high-temperatures required for carbonate precipitation, the much lower temperatures at which phyllosilicate forms, and the small size of the veins in the nakhlites, we assume alteration of the nakhlite pile (Mikouchi et al. 2003, Antarctic Met. Res. 16: 34–57) happened in a sharp, short thermal pulse. Since typical volcanic volatiles such as SO2, HCl or HF would cause alteration minerals indicative for them (e.g., fluorite, Filiberto and Schwenzer, this conference), but those are not observed in the mineral assemblage, we assume an impact heating of ground ice as heat and water source for the hydrothermal fluid pulse. The observed alteration assemblage thereby represents the crustal clays ubiquitously observed on Mars from orbit (Ehlmann et al. 2011, Nature, 479: 53–60). Orbital observations are single mineral detections or give insights into a small number of minerals found in one place. They cannot alone provide the geologic context and assemblage information needed to determine formation conditions. Meteorite work therefore provides the basis needed to more accurately use orbiter observations to assess Martian subsurface conditions and habitability.

The case study of mineralogical observations combined with thermochemical modeling of Lafayette also demonstrates the insights to be gained from data returned from landed missions, which are capable of observing detailed geologic context in combination with mineral species and chemistry. In particular, the Mars Science Laboratory mission will obtain an accurate set of chemical and mineralogical data from ChemCam, CheMin, APXS, and SAM; the geologic context will be provided by a set of cameras, including close up views from MAHLI (Grotzinger et al. 2012, Space Sci. Rev., DOI 10.1007/s11214-012-9892-2). The likely data are comparable to that being gained from meteorites. Thermochemical modeling routines as presented here are thus expected to give insights into P, T, and X of the observed sites, aiding the assessment of habitability.

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