Smectite Formation in Gale Crater, Mars and in the Nakhlite Martian Meteorites

Tuesday, 16 December 2014: 1:55 PM
John Bridges1, Susanne P Schwenzer2, Richard J Leveille3, Frances Westall4, Roger C Wiens5, Nicolas Mangold6, Thomas Bristow7 and Peter Edwards1, (1)University of Leicester, Leicester, LE1, United Kingdom, (2)Open University, Milton Keynes, MK7, United Kingdom, (3)McGill University, Montreal, QC, Canada, (4)CNRS, Paris Cedex 16, France, (5)Space Science and Applications, Los Alamos, NM, United States, (6)LPGN Laboratoire de Planétologie et Géodynamique de Nantes, Nantes Cedex 03, France, (7)NASA Ames Research Center, Moffett Field, CA, United States
Recent, detailed analyses of the nakhlite martian meteorites have enabled characterization of a ferric saponite and ferric serpentine (Hicks et al. 2014 10.1016/j.gca.2014.04.010). This nakhlite assemblage is part of a rapidly cooled, hydrothermal assemblage, cooled from ~150 oC, with the clay forming at ~50 oC (Bridges and Schwenzer 2012 10.1016/j.epsl.2012.09.044). Although there are differences between the overall secondary assemblage in the nakhlites and that identified in the Yellowknife Bay mudstones by CheMin (Vaniman et al. 2014 10.1126/science1243480), the trioctahedral iron-rich saponite is probably the closest analogue known to the smectite found in Gale Crater.

MSL analysed mudstones at the Yellowknife Bay, deposited in a fluvio-lacustrine setting followed by diagenesis ~50 oC (Grotzinger et al. 2014 10.1126/science.1242777). The mineralogical information provides allows us to constrain mineral reactions, W/R, pH, and redox associated using thermochemical modelling, and comparisons to the nakhlites. We use a 2 stage fluid model, with an initial Deccan-type brine composition (Minissale et al. 2000 10.1016/S0012-821X(00)00200-4) which reacts with the known rock compositions - using ChemCam and APXS data to produce a pore fluid. CHIM-XPT was used for the modelling.

Initial reaction of the early brine with olivine in the mudstone produces relatively Mg-rich phyllosilicate. This early diagenesis may correspond to the formation of Mg-Fe rich ridges (Leveille et al. 10.1002/2014JE004620). Subsequent reaction of the resultant fluid separated at a W/R of 100, produced a fluid that we then reacted with a range of different mineral and amorphous mixtures, T, W/R conditions. A mixture of 70% amorphous, 20% olivine, 10% host rock produced a clay-Fe oxide dominated assemblage, similar to that in Sheepbed. The clay has a similar composition to ferric saponite and gel in the nakhlites (Bridges et al. JGR, subm.).