The release of volatiles during subduction is one of the most important processes to have shaped the Earth and it continues to mediate processes that sustain the planet’s long-term habitability. Water and gases released from subducting tectonic plates initiate magmatism, leading to explosive volcanic activity, metallogenesis and the formation of new continental crust. Volatile release from subducting rock is also considered to be a controlling factor in the generation of deep earthquakes. Furthermore, the presence of even small amounts of volatiles, particularly water, in the mantle is thought to strongly enhance mantle creep rate, thereby directly influencing plate tectonics and, ultimately, the long-term chemical and thermal evolution of Earth. Remarkably, despite these important implications, the physical and chemical mechanisms responsible for processing volatiles during subduction are not understood.
Owing to their unreactive nature, the noble gases have the potential to trace the pathways of volatile transport during subduction. Toward this aim, I am studying the noble gas systematics of metamorphic rocks exhumed from subduction zones. Such rocks provide unique insight into the physical and chemical processes that control the release and movement of fluids from the downgoing slab.
Much of the Earth's continental crust is formed from metamorphic rocks. Metamorphic minerals preserve chemical records of the thermal signatures imparted to them by a range of geophysical processes, from mountain-building to crustal extension. In the past, direct quantification of the rates of these processes has eluded community efforts; however, recent advances in microanalytical technologies now position metamorphic petrologists and geochemists to address these issues. Specifically, LA-ICPMS and SIMS methods permit measurement of intragrain elemental and isotopic concentration gradients of both major and trace species, including U, Th and Pb. Future research will focus on combining high spatial resolution analyses of major and trace elements with kinetic models for diffusion and element partitioning. Applied to regional and high-pressure metamorphic rocks, this approach will help resolve current uncertainties over the relative contributions of kinetic factors, P and T in driving metamorphic reactions. Furthermore, coupled to numerical models, these data will form the basis for a better-informed understanding of how heat and mass are transported through the Earth’s crust.
The (U-Th)/Pb system is a remarkably powerful tool to investigate the thermal history of the lithosphere. Radiogenic Pb is diffusively mobile in the minerals apatite, rutile and titanite between ~400°C and 650°C—a temperature range that has long been inaccessible to other thermochronometers. The competing effects of radiogenic Pb production and thermally activated volume diffusion generate U-Pb age distributions within accessory minerals that are extremely sensitive to thermal history. Combined with numerical models of Pb diffusion, this information can be used to identify the thermal signature of key geodynamic process that shape the chemical and physical evolution of the crust and lithospheric mantle.