Fluids and Sulfate Vein Formation in Gale Crater, Mars.

Thursday, 18 December 2014: 12:05 PM
Susanne P Schwenzer, Open University, Milton Keynes, MK7, United Kingdom, John Bridges, University of Leicester, Leicester, LE1, United Kingdom, Richard J Leveille, McGill University, Montreal, QC, Canada, Frances Westall, CNRS, Paris Cedex 16, France, Roger C Wiens, Space Science and Applications, Los Alamos, NM, United States, Nicolas Mangold, LPGN Laboratoire de Planétologie et Géodynamique de Nantes, Nantes Cedex 03, France, Amy McAdam, NASA Goddard Space Flight Center, Planetary Environments Laboratory, Greenbelt, MD, United States, Pamela Gales Conrad, NASA Goddard Space Flight Center, Greenbelt, MD, United States, Javier Martín-Torres, Instituto Andaluz de Ciencias de la Tierra, Granada, Spain and Maria-Paz Zorzano, INTA-CSIC, Madrid, Spain
Curiosity detected sulfate veins crosscutting mudstones in the Sheepbed member of Gale Crater (Grotzinger et al., DOI: 10.1126/science.1242777; Manchon et al., DOI:10.1002/2013JE004588). We have used this information to evaluate the clay formation conditions in detail through thermochemical modeling (Bridges et al. submitted to JGR; this conference, session 2128) and compare the calculated fluid to those modeled for the nakhlite alteration mineralogy (Hicks et al. DOI: 10.1016/j.gca.2014.04.010, Bridges and Schwenzer, DOI: 10.1016/j.epsl.2012.09.044) and other Martian fluids. Concentrating the modeled Gale fluid though evaporation (or freezing) leads to a complex set of precipitates, which include silica, sulfate and halite. For example, 1 kg of brine produced by the alteration of a mixture of 70 % amorphous component, 20 % olivine and 10 % host rock (W/R 1000, T= 10 °C) evaporated to dryness (less that 1 % water left) will precipitate 70 mg of anhydrite, 46 mg of silica, 6.5 mg of halite and traces of pyrite, sulfur, calcite, and apatite. A fluid from this host rock with more mature alteration (W/R 100, T = 10 °C) precipitates the same minerals, but in very different abundances: the most abundant phase at dryness is halite (330 mg), followed by silica (88 mg) and anhydrite (30 mg). The calculated pH varies between 8 and 7.3 in both cases. If the evaporating brine is allowed to interact with the precipitate, a ‘dirty’ sulfate layer or vein filling would result, which could subsequently be refined through dissolution and re-precipitation, a mechanism that, for example, is proposed for the gypsum veins at the UK Triassic coast near Watchet (Philipp doi:10.1017/S0016756808005451). Factors that influence the nature of the precipitate include alteration stage of the host rock during clay formation, and pH and degree of fractionation of the early formed minerals from the evaporating fluid. Using REMS data we also consider desiccation of sulfates in the near surface.