METAMORPHISM IN THE CRUST OF MARS

The crust of Mars is composed of mafic and ultramafic igneous rocks and sediments derived from them, and metamorphism has traditionally had no obvious role in the planet’s geologic history. The chemical compositions of basaltic and ultramafic rocks analyzed by Mars rovers and occurring as martian meteorites permit predictions of metamorphic mineral assemblages that would likely form within the martian crust. We use phase diagrams to assess equilibrium assemblages and compare these with data from orbital spectroscopy of martian terrains thought to have formed by low-grade or hydrothermal metamorphism. This approach allows new insights into the compositions of protoliths and the conditions of metamorphism, and predicts candidate metamorphic minerals not yet identified on Mars using remote sensing techniques. ACF diagrams appropriate to martian conditions indicate that basaltic rocks should form chlorite + actinolite + albite + silica, accompanied by laumontite, prehnite, or pumpellyite. Ultramafic rocks should produce serpentine or talc with magnesite. The ancient (Noachian) Nili Fossae region on Mars contains prehnite + chlorite + albite + silica, although actinolite has not yet been identified. Serpentine and talc + magnesite were also found in this area. Metamorphic conditions are constrained to have been 250-400^oC and <2 kbar, with aqueous fluids having variable XCO₂ up to 0.3. Widespread occurrences of prehnite, chlorite, serpentine, and unspecified zeolites elsewhere on Mars suggest that these metamorphic conditions may have been common within the ancient martian crust. The Noachian geothermal gradient estimated from orbital measurement of radiogenic heat production was 12^oC/km. However, a gradient of >20^oC/km is required to produce prehnite. Under this gradient, prehnite would form at ~12 km depth, but the sizes of impact craters that exposed these rocks limit excavation depths to ~5 km. A hotter gradient requires an extra heat source, probably a regional hydrothermal system driven by magmatic activity.