PETROGENESIS OF THE MARS CRUST FROM THERMAL EMISSION SPECTROSCOPY

Analysis of silica-rich rocks at the Mars Pathfinder site and orbital spectral interpretations that such rocks are widespread on the martian surface may imply that Earth is not unique in having a crust containing highly differentiated materials. Remote sensing observations bearing on the composition of the ancient martian crust (thermal emission spectra of major surface units, analyzed chemistry of fines) allow basalt plus andesite, but are also consistent a crust containing only mafic rocks that are partly weathered. Calculated chemistries have been determined for global surface units, using Mars Global Surveyor TES spectra with an extended spectral range and deconvolved using a variety of spectral end members. These data indicate that surface type 1 materials dominating the southern highlands have basaltic andesite composition and surface type 2 sediments covering the northern lowlands have andesite composition. If these surface types represent volcanic materials, their calc-alkaline compositions on a FeO*/MgO versus silica diagram suggest production by extensive melting of hydrated mantle sources. On Earth, this petrogenesis requires subduction, and may suggest an early period of plate tectonics on Mars. However, anorogenic formation might have been possible if the primitive martian mantle were significantly wetter than generally supposed. Alternatively, chemical weathering diagrams suggest that surface type 2 and possibly surface type 1 materials could have formed by weathering of basalt, leading to depletion in soluble cations and mobility of silica. A weathered crust model is consistent with the concentration of surface type 2 materials within a global depocenter and with new APXS analyses of ~2 wt. % water in weathering rinds on Pathfinder rocks, and may obviate the need for a wet Mars mantle. Planetary remote sensing thus provides a very different view of Mars crustal petrogenesis, relative to that afforded by studies of martian meteorites.