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Geochemistry of Igneous, Sedimentary, and Metamorphic Rocks on Mars


May 5, 2015, noon - 1 p.m.
3843 Slichter

Presented By:
Hap McSween
U Tennesee

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Geochemical investigations of martian rocks have been accomplished through analyses by instruments on Mars rovers (with geologic context provided by orbiter measurements), and by laboratory analyses of martian meteorites. Igneous rocks are primarily lavas and volcaniclastic rocks of basaltic composition, and ultramafic cumulates; alkaline rocks are common in ancient terranes and tholeiitic rocks occur in younger terranes, suggesting global magmatic evolution. Relatively uncommon feldspathic rocks represent the ultimate fractionation products, and calc-alkaline/granitic rocks are unknown. Sedimentary rocks are of both clastic (mudstone, sandstone, conglomerate, all containing significant igneous detritus) and chemical (evaporitic sulfate and less common carbonate) origin, and high-silica sediments formed by hydrothermal activity. Sediments on Mars formed from different protoliths and were weathered under different environmental conditions from terrestrial sediments, and illustrate different chemical weathering trajectories. Metamorphic rocks have only been inferred from orbital remote-sensing measurements. Metabasalts and serpentinites have mineral assemblages consistent with those predicted from low-pressure phase equilibria and likely formed in geothermal systems. The martian rock cycle during early time periods was similar in many respects to that of Earth. However, without plate tectonics Mars did not experience the thermal metamorphism and flux melting associated with subduction, nor deposition in subsided basins and rapid erosion resulting from tectonic uplift. The rock cycle during more recent time has been truncated by desiccation of the planet’s surface and a lower geothermal gradient in its interior. Mars rocks are intriguingly different from Earth, but the tried-and-true methods of geochemistry are clearly translatable to another world.