Hydrous melt generation in the Earth's mantle

Abstract

This thesis focuses on quantifying the role of H₂O in the generation and modification of mantle melts at shallow pressures in subduction zones. In the first and second chapters, two experimental studies are presented that investigate direct mantle melting and then subsequent reaction of deeper mantle melts with overlying, cooler depleted mantle. In Chapter 1, the melting behavior of an olivine + orthopyroxene +/- spinel - bearing fertile mantle composition is investigated as a function of variable pressure and water content. The experimental results are used to calibrate a model that can predict the pressure and temperature or the temperature and H₂O content of last equilibration for mantle melts that were in equilibrium with olivine orthopyroxene +/- spinel. In Chapter 2, reaction experiments were conducted to explore the role of melt - rock reaction in the shallow part of the mantle wedge. Results demonstrate the importance of both the temperature of the overlying mantle and the amount of infiltrating melt on the mantle lithology that remains after reaction. Reaction coefficients are calculated to quantify the experimental results. In Chapter 3, H₂O solubility was experimentally determined at upper mantle pressures. The 1.0 GPa result is the first H₂O solubility determination in basalt at any pressure above 0.6 GPa. The final chapter is a modeling study that shows how and when to correct for low pressure fractional crystallization to get lavas back to equilibrium with the mantle. Terms are calibrated (in part, on experiments presented in Chapter 1) that add H₂O to spinel lherzolite multiple saturation point models as well as to thermometers and barometers. The H₂O correction provides the quantitative and qualitative basis for making low pressure fractional crystallization corrections to near-primitive hydrous lavas. All chapters contribute to the understanding of subduction zone magmatism, with a particular emphasis on processes in the shallowest region of the mantle wedge.