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I) Continental Margin Volcanism
The thermal evolution of the earth is manifested in many ways, one of which is the melting of deep-seated rock, followed by upward transport of magma. The land masses on which we live are products of this mass transport, having formed in large part by magmatism where dense oceanic plates dive beneath continental margins. During the past few decades, much has been learned about the origin and causes of diversity among continental-margin magmas. Still, many questions remain
. There is much yet to be learned, for example, about the physical processes taking place in crustal magma chambers where ascending melt is often stored. We also know little about the depth and shape of magma reservoirs, and about what causes them to "leak" during volcanic eruption. Finally, we seek to better understand why style of volcanism varies in different convergent margin settings, and even with time at any one place.
Much of my attention has been focused on unravelling the volcanic history and origin of mid-Tertiary magmas in the Sierra Madre Occidental of western Mexico, where enormous volumes of silicic ash-flow tuff and lavas were erupted from large caldera complexes above the subducting Farallon plate. Although still trying to wrap up some of that work, my attention is now directed to understanding the world's largest active caldera system: the Toba caldera complex, in northern Sumatra, Indonesia. My petrologic work there is being done in conjunction with a seismic tomography study of magma distribution by Professor Rob McCaffrey and graduate student Masturyono.
In addition to these field based studies, I dabble now and then with experiments 
designed to simulate conditions in magma chambers. The experiments are designed to understand the origin of disequilibrium mineral textures (such as rapakivi feldspar, embayed quartz, and "patchy" plagioclase) common to many volcanic rocks. Such textures record, and thus provide valuable information about, processes taking place in magma reservoirs at depth.
II) Fluids in the Mantle and Deep Crust
Aqueous and CO2-rich fluids play a very important - yet poorly understood - role in the chemical evolution of our planet. In fact, it is probably safe to say that most geochemical processes involve fluids to varying degrees. As geochemical transport agents, the important role of fluids stems from their extreme mobility (compared to melts or rocks), the high diffusivities of their solutes (deep-seated fluids are excellent solvents for certain chemical components), and even from their strong effect on the melting relations of rocks. Unfortunately, although most geochemists and petrologists acknowledge the important role played by fluids, few data exist to quantify their effects.
In the past few years, I've been working with Professor E. Bruce Watson and Jonathan Price to characterize the transport properties of fluids in the deep earth. This work, sponsored by the Department of Energy, is taking place in Watson's Experimental Geochemistry Laboratory. There, we are directly measuring:
- permeabilities of texturally-equilibrated ("annealed") fluid-bearing rocks
- diffusivities of components in supercritical fluids
Recently, we've shown that the grain-scale distribution of fluids is a function of grain size. Fluids and melts will concentrate in domains of finer grain size to the extent that permeability of these domains will exceed the permeability of nearby, coarser-grained regions. This may have important ramifications for many crustal processes that involve fluids.