Mineralogical indicators of magmatic and hydrothermal processes in continental arc crust

This dissertation explores several important consequences of H2 O-rich fluids in magmatic and ore-forming systems within continental arc crust.North Sister, a stratovolcano in the Oregon High Cascades, provides a window into magma generation processes in the deep crust. Eruption of a remarkably limited basaltic andesite composition over the lifespan of this volcano may reflect last equilibration of mantle derived magma within a deep crustal hot zone. High pressure, water-undersaturated phase equilibrium experiments show that an anhydrous, augite-rich gabbro at &sim12 kbar (40 km depth) and &sim1175°C is the most probable lithology with which North Sister basaltic andesite with &sim3.5 wt% H2O last equilibrated within the deep crust before erupting.While magma often erupts at the planets surface as at North Sister, a greater volume never reaches the surface and solidifies within the upper crust. Exsolution of magmatic fluids is an inevitable consequence of crystallization of hydrous crustal magmas. The fate of these fluids is the focus of the remainder of this dissertation.Modeling of CO2 and H2O variations during crystallization of granitic magma reveals that exsolution of a large mass of fluid occurs only after CO2 is largely degassed, creating ideal conditions for hydrofracturing and formation of porphyry copper deposits. CO2 and H2O solubility relations suggest that H2O-rich magma was required to produce the porphyry-Cu-Mo deposit at Butte, Montana, which may explain its distinctively deep generation. Electron microprobe analyses of Ti in quartz and Zr in rutile in samples from Butte yield porphyry magma temperatures (630-770°C) that overlap substantially with hydrothermal vein temperatures (<430-750°C). Veins display large temperature ranges (50-250°C) that signify variable degrees of cooling of hot magmatic fluids upon contact with cooler wall rock during vein growth. Modeling of Ti diffusion in quartz suggests that individual dikes and veins likely cooled over short timescales (10s-1000s years), indicating that porphyry systems may evolve by episodic magmatic fluid injections with discrete thermal spikes. Modeling of Ti diffusion in quartz combined with electron backscatter diffraction maps show that small hydrothermal quartz veins likely formed by epitaxial growth.This dissertation includes co-authored material both previously published and in preparation for submission.