The effect of oxygen fugacity and volatile content on Martian and terrestrial magmatic evolution

This dissertation addresses the role of water and oxygen fugacity in magmatic systems on Mars and the earth by correlating analyses of eruptive products with laboratory studies that are designed to reproduce the conditions necessary to replicate the observed features. The first two chapters of this dissertation focus on the oxygen fugacity (fO2) variations of Martian magmas as recorded by elemental fractionations in the Martian meteorite pyroxenes. The multivalent cations europium and iron were calibrated for use as oxybarometers in the meteoritic pyroxenes. The results indicate that variations on the order of three log units oxygen fugacity are recorded in the meteorites. Several processes have been proposed to explain the origin of this fO2 range, the majority of which involve assimilation of an oxidized source. A potential correlation between new pyroxene data (an early crystallizing phase) from this study and recent data from late-crystallizing Fe-Ti oxides, however, is consistent with the idea that intrinsic fO2 differences in the magma source region are responsible for the measured f O2 variation. This implies that the Martian meteorite source region, the mantle or lithosphere, may be heterogeneous in nature. It should be noted, however, that early assimilation events that took place before crystallization commenced cannot be ruled out.; The third chapter explores the effects of water on phase equilibria and ascent rate of a terrestrial dacitic magma from Black Butte, a flank dome of Mount Shasta, California. The pre-eruption storage conditions were determined through phase equilibria experiments and analytical petrography. The phenocryst assemblage was reproduced experimentally at 900°C, 300 MPa, X H2O = 1, and fO2 = NNO+1. Magma ascent rate was quantified using amphibole breakdown rim and plagioclase microlite crystal size distribution (CSD) measurements in both the natural and experimentally decompressed samples. Ascent rates calculated from both the amphibole and plagioclase methods indicate similar ascent rates (0.002 m/s) for the Black Butte magma, similar to effusive extrusion rates calculated for other subduction zone volcanic systems.