Current Research in EEPS Seminar: Dr. Harold McSween, University of Tennessee, Knoxville
Rocks on Mars: Igneous, Sedimentary, and Metamorphic
The rock record on Mars has been studied using a combination of orbital remote sensing, in situ analyses by rovers, and martian meteorites. These studies reveal that Mars is a world covered by basaltic rocks, with associated ultramafic cumulates. Felsic rocks are virtually absent; the few examples known formed by limited fractionation of basalt, rather than flux melting as in terrestrial subduction zones. The basaltic magmas have evolved over billions of years, from alkalic to tholeiitic. Martian magmas formed by decompression melting, likely under a progressively thickening lithosphere resulting from global cooling. Compositionally heterogeneous mantle source regions, probably produced by an early magma ocean, are indicated by differences in radiogenic isotope ratios, trace elements, and redox state.
Clastic sedimentary rocks formed from basaltic protoliths and were altered under acidic, low-water conditions, resulting in geochemical trajectories that differed from Earth. Clays vary in composition, depending on whether they formed by surface chemical weathering or subsurface hydrothermal processes, and evaporites are mostly sulfates and chlorides, rather than carbonates. Sedimentary environments also evolved over time, as Mars progressively desiccated and aeolian processes became more important.
Metamorphism on Mars is represented primarily by rocks altered by shock, or by thermally metamorphosed rocks formed in low-pressure (sub-greenschist facies) hydrothermal systems, likely associated with large impact craters. Predicted mineralogies for martian rock compositions are recognized from orbital spectroscopy.
Without plate tectonics, Mars did not have hydrous melting or thermal metamorphism associated with subduction, nor deposition in subsided basins and rapid erosion from tectonic uplift. Thus, compared to Earth, the martian rock cycle is incomplete. In more recent times, the rock cycle was further truncated by global desiccation and a lower geothermal gradient.