DIVERSE IGNEOUS PROTOLITH CONTRIBUTIONS TO SEDIMENTS IN GALE CRATER: VARIABLE METASOMATISM OF THE MARS MANTLE

Igneous float rocks and least altered sedimentary bedrock examined in Gale Crater by the MSL rover Curiosity provide insight to the petrogenesis of the crystalline basement. Rocks observed early in the mission display elemental variations consistent with mixing between mafic and felsic lithologies and largely isochemical alteration (±Fe-ox cement). The element suite determined by Alpha Particle X-ray Spectrometer (APXS) includes major, minor (K, P, Ti) and trace elements (Cr, Ni) that range from very incompatible to very compatible in basaltic melts, allowing petrologic modelling to constrain igneous histories. Least altered basaltic sediments of the Bathurst class (n=13; includes Windjana drill sample) comprise dark-toned potassic siltstone sandstone, to matrix-supported conglomerate. Enrichments in K, Zn and Ni (up to 3.73 wt%, 4681 ppm, and 516 ppm, respectively) are thought to reflect enrichment in the sediment source. The Jake M class (n=13) includes igneous (phonotephritic to trachyandesitic) float blocks and cobbles that likely weathered out of conglomerate bedrock. Jake M rocks are rich in Na and Al (up to 16.9 wt% Al₂O₃) and low in Ni and Cr. Least altered, basaltic Stimson sandstone targets (sol 974 to present) are classified as the Ronan class (n=19) and are similar to local soils, but are more variable and range to lower K₂O and higher Ni.

Elevated K and compatible metals (Ni and Cr) observed in least altered basaltic sediments (Bathurst and Ronan classes) cannot be attributed to partial melting of typical Mars mantle (Wänke & Dreibus, 1988). Very low degree melting (≤0.5%) might yield melts with sufficient alkalis, but such low degree melts are unlikely to migrate and their Ni would be low by a factor of 4 to 11. The character of the Gale mantle is likely alkali and Ni-enriched by an oxidizing metasomatic event. Both alkali contents and oxidation state are therefore variable in the Mars interior. The Jake M class demonstrates that Mars generates diverse evolved alkaline magmas. Variable alkali contents over limited SiO₂ among Jake M rocks (4.1-7.1 wt% Na₂O, 49-53 wt% SiO₂) cannot result from fractional crystallization of a single parental magma. More likely, Jake M rocks are products of fractional crystallization of multiple parental magmas derived from variably alkali-enriched and metasomatised mantle.