Water in martian magmas: Clues from light lithophile elements in shergottite and nakhlite pyroxenes

摘要

There is abundant geomorphic evidence that Mars once had potentially significant amounts of water on its surface. Bulk martian meteorites are curiously dry, nd hydrated minerals found in some of these rocks are also surprisingly low in water content. We look for evidence of pre-eruptive magmatic water by analyzing the abundances of Li, Be, and B, light lithophile elements that have proven useful in tracking water-magma interactions in terrestrial studies because of their solubility differences. We performed secondary ionization mass spectrometer (SIMS) analysis of these incompatible elements in pyroxenes of two nakhlites and two basaltic shergottites, with quite different results. In Nakhla and Lafayette, all three elements behave as incompatible elements, with increasing abundance with magma evolution from pyroxene cores to rims. In Shergotty and Zagami, Be increases, but both B and Li decrease from pyroxene cores to rims. From terrestrial studies, it is known that Be is virtually insoluble in aqueous hydrothermal fluids, whereas B and Li are quite soluble. We suggest, therefore, that the elemental decreases in the shergottite pyroxenes reflect dissolution and loss of B and Li in a hot, aqueous fluid exsolved from the magma. Consistent with our results, recent experimental work proposes that the shergottite parent magmas contained 1.8 wt% water (Dann et al., 2001). We suggest that the pyroxene cores grew at depth (>4 km) where the water would remain dissolved in the magma. Once the magma began to ascend, the volatile component could gradually exsolve, removing the soluble species from the melt in the process. upon eruption, the volatile component might then be lost through degassing, leaving a B- and Li-depleted magma to crystallize pyroxene rims and plagioclase. This magmatic water might have derived from the martian mantle or resulted from deep crustal contamination. If the water contents proposed for the shergottite magmas, and implied by our results, are typical of basaltic magmas on Mars, this mechanism could provide an efficient method of delivering substantial amounts of water to the martian surface at later times in martian history.