Petrogenesis of high-phosphorous Wishstone Class rocks in Gusev Crater, Mars

The Spirit rover has analyzed a wide variety of alkaline volcanic rocks in Gusev Crater. Among them are Wishstone Class tephrites with unusually high P2O5 (>5 wt %) abundances. To obtain the chemical compositions of rock interiors, we extrapolated a trend defined by brushed and abraded surface compositions. The extrapolation analysis suggests the high-P2O5 tephrite signature is not attributable to secondary aqueous alteration but represents an igneous rock composition. Assessment of the compositional trend also suggests that merrillite is the phosphate mineral in Wishstone Class. Modified CIPW norm calculations using merrillite instead of apatite show that Wishstone Class rocks are silica saturated (hypersthene-olivine normative) and alkaline. The high-P2O5 whole rock compositions plot above solubility curves of merrillite in a diagram of P2O5 versus aluminosity, suggesting that mechanical admixture of merrillite is required. A source supplying merrillite cannot be a common silicate magma; instead, it could be a carbonatitic melt or a highly fractionated residual melt (jotunite) after anorthosite crystallization. Comparison with Earth analogs suggests that the Wishstone Class is akin to alkaline basalts in the eastern Snake River Plain, with regard to its chemistry, texture, and geologic context. These alkaline basalts are interpreted to originate from mantle metasomatized by CO2-fluids and highly alkaline melts in the wake of the Yellowstone hot spot. Thus, we propose that the Wishstone Class represents a silica-saturated alkaline suite that has mechanically mixed xenocrystic merrillites, probably during explosive volcanic eruption; the merrillites crystallized from carbonate-rich magma produced by melting of a CO2-bearing Martian mantle.