Phase equilibria of the Shergotty meteorite: Constraints on pre-eruptive water contents of martian magmas and fractional crystallization under hydrous conditions

An experimental investigation of the Shergotty meteorite was performed at 0.1 MPa under anhydrous conditions at the quartz-fayalite-magnetite buffer and at 100 and 200 MPa under H2O-saturated conditions at the nickel-nickel oxide buffer. The results of these experiments are used to infer magmatic conditions recorded by co-crystallization of augite and pigeonite phenocrysts found in Shergotty and to investigate the effect of H2O on fractional crystallization paths followed by shergottite magmas. The phase relations and compositions of the homogeneous magnesian pyroxene cores in Shergotty are most closely approximated by crystallization under H2O-saturated conditions at 1120 degreesC (+/- 10 degreesC) and 56 MPa (+/- 18 MPa), corresponding to dissolved H2O contents of 1.8 wt% (+/-0.6 wt%) and a depth of 5 km (+/-1.5 km) in the martian crust (uncertainties are 2 sigma values). The Shergotty magma then lost this water during ascent and eruption. Fractional crystallization of the Shergotty magma under anhydrous conditions produces liquids that follow a strong Fe-enrichment trend at nearly constant SiO2. Crystallization under H2O-saturated conditions generates derivative liquids, depleted in FeO and Al2O3 and enriched in SiO2, that are compositionally similar to the Mars Pathfinder andesite rock composition. The presence of similar to1.8 wt% water in Shergotty parental magmas could result from assimilation of hydrated crustal materials or from dehydration of hydrous phases in the mantle source region.