Geochemical and Petrological Investigations into Mantle Minerals from Experiments and Natural Samples

This dissertation consists of a series of studies investigating geochemical and petrological aspects of mantle minerals, namely, olivine, spinel, orthopyroxene, and clinopyroxene. The first study is an experimental look at the solubility of the clinopyroxene mineral diopside (CaMgSi2O6) in natural fluids. Results allow insight into the chemistry of high P-T fluids and the behavior of a major mantle mineral in equilibrium with such fluids. The second study focuses on iron isotopic compositions of mantle minerals as powerful tracers for geochemical processes in the mantle, such as partial melting, metasomatism, and oxidation. To address this, we studied inter-mineral iron isotopic fractionation of minerals from five distinct mantle-xenolith lithologies from San Carlos, Arizona. We compared the fractionations with opposing calculations predicting equilibrium iron fractionation at high temperatures, and applied the results to implications of petrogenesis of the xenoliths. The last study is an experimental determination of equilibrium magnesium isotope fractionation between spinel, forsterite, and magnesite from 600°C to 800°C. In these experiments we implemented the three-isotope method with forsterite and magnesite, and with spinel and magnesite, at three different temperatures in high-pressure piston cylinder apparatus for varying lengths of time, using carbonate as the exchange medium. The result is the first rigorous high temperature experimental calibration of magnesium isotope fractionation of mantle minerals, and is generally consistent with expectations based on crystal chemical environment of Mg in these phases. The combination of experimental petrology with isotope geochemistry is a powerful approach for understanding mantle processes. Comparisons of these types studies with natural samples and theoretical predictions provide new insights into Earth's mantle