Smectite Formation in Gale Crater, Mars and in the Nakhlite Martian Meteorites
Abstract: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.).