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Observations of martian surface morphology have been used to argue that an ancient ocean once existed on Mars. It has been thought that significant quantities of such water could have been supplied to the martian surface through volcanic outgassing, but this suggestion is contradicted by the low magmatic water content that is generally inferred from chemical analyses of igneous martian meteorites. Here, however, we report the distributions of trace elements within pyroxenes of the Shergotty meteorite--a basalt body ejected 175 million years ago from Mars--as well as hydrous and anhydrous crystallization experiments that, together, imply that water contents of pre-eruptive magma on Mars could have been up to 1.8%. We found that in the Shergotty meteorite, the inner cores of pyroxene minerals (which formed at depth in the martian crust) are enriched in soluble trace elements when compared to the outer rims (which crystallized on or near to the martian surface). This implies that water was present in pyroxenes at depth but was largely lost as pyroxenes were carried to the surface during magma ascent. We conclude that ascending magmas possibly delivered significant quantities of water to the martian surface in recent times, reconciling geologic and petrologic constraints on the outgassing history of Mars.

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Abstract-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 H20-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 H20 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 1 120 "C (k 10 "C) and 56 MPa (2 18 MPa), corresponding to dissolved H20 contents of 1.8 wt% (k0.6 wt%) and a depth of 5 km (2 1.5 km) in the martian crust (uncertainties are 20 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 Si02. Crystallization under H20-saturated conditions generates derivative liquids, depleted in FeO and A1203 and enriched in SiO2, that are compositionally similar to the Mars Pathfinder andesite rock composition. The presence of -1.8 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.

Abstract. Based upon their geochemical similarity, we propose that the 3.5 Ga Barberton basaltic komatiites (BK) are the Arcbean equivalents of modern boninites, and were produced by the same melting processes (i.e. hydrous melting in a subduction zone). The Barberton komatiites also share some geochemical characteristics with boninites, including petrologic evidence for high magmatic H20 contents. Experimental data indicates that the Arcbean sub-arc mantle need only be 1500-1600øC to produce hydrous komatiitic melts. This is considerably cooler than estimates of mantle temperatures assuming an anhydrous, plume origin for komatrites (up to 1900 ø C). The depleted mantle residue that generates the Barberton komatiites and BK will be cooled and metasomatised as it resides beneath the fore-arc, and may represent part of the material that formed the Kaapvaal cratonic keel.