LIQUID EVOLUTION DURING EXPERIMENTAL CRYSTALLIZATION OF THE GUSEV BASALT, HUMPHREY, AT 10 KBAR

The basaltic rocks, Humphrey, Adirondack and Mazatzal were analyzed by the MER rover Spirit in Gusev Crater, Mars. These rocks are believed to be largely representative of a liquid composition with possible minor olivine or pyroxene accumulation. Additional rocks analyzed from Gusev crater include the alkalic rocks Wishstone and Champagne from the Columbia Hills. These rocks are strongly alkalic and characterized by low silica contents, high phosphorus and titanium contents, and are in many respects similar to the jotunites of the anorthosite complexes on Earth. Previous experimental work has shown that such rocks can arise from crystallization of tholeiite at about 10 kbar with low dissolved water contents. Can similarly Wishstone-class rocks be derived from Humphrey basalt?

Experiments are being conducted to assess this question using a synthetic mix of the composition of Humphrey (RAT 1) from Gellert et al. [(2006) JGR-Planets. V. 111]. All experiments were run in piston-cylinder apparatus using graphite capsules and BaCO₃ cells. Experimentally produced minerals and glass were analyzed by EMP and FTIR for major elements and water content. Preliminary results indicate that the liquid evolution follows a silica-depletion trend at 10 kbar increasing in FeO, TiO₂, and P₂O₅ contents with increasing degrees of crystallization. This is in keeping with pMELTS calculations using the same starting bulk composition, and is suggestive that Humphrey liquid compositions can evolve towards the Wishstone class rocks seen in the Colombia Hills.

Work is ongoing to assess whether the liquid compositions derived from crystallization of Gusev basalt can produce titanium and phosphorous contents as high as those observed in the Wishstone-class rocks from Columbia Hills.