Experimental Crystallization of Dry and Wet Humphrey at 9.3 kbar: Implications for Compositional Diversity of the Martian Crust

In regions of thick crust on Earth, igneous compositional diversity arises readily by fractionation of mantle-derived magmas at depth and ascent of residual liquids into the upper crust at various stages of fractionation. This process appears to have also occurred on Mars as evidenced by the relationship between the composition of the surface rock Backstay (analyzed by the MER Rover Spirit in the Columbia Hills of Gusev Crater) and the mineralogy of the Chassigny dunite (Nekvasil et al., this conference). In addition to the likely diversity caused by such a process, additional compositional diversity can arise from differing amounts of bulk water in the parental magma. The goal of this work is to understand the effect of water on derivative melt compositions when crystallization occurs at the base of the martian crust. The liquid composition chosen was that of Humphrey of the Adirondack class basalts analyzed by the MER rover Spirit in Gusev Crater, Mars. The Adirondack class basalts are picro-basalts with some evidence of olivine accumulation, however, they are considered by many to represent primitive basalt compositions. The Humphrey composition was created by mixing powdered oxides, Fe sponge, CaF2, and NaCl. Experiments on the powdered mix were conducted in piston-cylinder presses at 9.3 kbar using graphite capsules and BaCO3 cells. Nominally dry experiments were conducted by drying the powder in a pot furnace at 800°C for 20 minutes to drive off structural and absorbed H2O before being loaded into the piston- cylinder press. Wet experiments were conducted at 1 wt% H2O, and were created by synthesizing a hydrous glass of Humphrey composition with ~4 wt% H2O and mixing this hydrous glass with dry Humphrey mix proportionally such that 1 wt% bulk H2O would be achieved. All water contents were verified by micro- FTIR, and all run products were analyzed by electron microprobe. The liquid line-of-descent for nominally dry experiments follows a silica-depletion trend as is observed for similar terrestrial compositions. Residual liquids are enriched in Fe, Ti, and P, which is also observed terrestrially in fine- grained FTP rocks of anorthosite complexes (e.g., the Laramie anorthosite complex) and in lavas of the Snake River Plain. The liquid line-of-descent for experiments with a starting bulk H2O content of 1 wt% follows a silica-enrichment trend consistent with being on a path towards more rhyolitic compositions. Residual liquids are enriched in Si, Al, Fe, and alkalis. This is in keeping with experiments on a terrestrial tholeiite with starting bulk water contents greater than 0.4 wt%. Ascent of these residual liquids into the upper crust would result in very different bulk compositions depending upon bulk water content. If the compositions of martian soils and bulk surface reflect primarily an igneous substrate without major sedimentary re-working, then the compositions suggest that magmas added to the martian upper crust were likely low in bulk water content.