My research is focused on the evolution of oceanic crust as it travels from the spreading ridge to the subduction zone. Specifically I am focused on determining the loci and degrees of hydration of the lower crust and upper mantle. Peridotite, the dominant rock in the mantle, is in disequilibrium with sea-water which leads to a sequence of reactions wherein primarily olivine and orthopyroxene (the dominant mineral phases) are altered to serpentine through the inclusion of volatiles. The subduction of serpentinite and the subsequent dehydration reactions at depth are an important parameter controlling the flux of volatiles into the subduction zone. The abundance of volatiles in the subduction zone has a significant effect on the character and extent of plate interface seismogenesis as well as influencing the degree of partial melting in the mantle wedge.
The first part of my research will be focused on a petrologic characterization of serpentinites from the north wall of the Puerto Rico Trench. The Puerto Rico Trench is the location of the deepest point in the Atlantic Ocean; it is also the only place where trench-proximal serpentinite has been sampled in situ. These samples will allow quantitative analysis of the hydration of the incoming plate in the Puerto Rico trench, a process that has been seismically inferred at other subduction zones. Using thin section petrography, electron microprobe analysis, and Raman spectroscopy as well as thermodynamic phase relations I will identify the volatile bearing phases present as well as generate a geochemical inventory of the samples. This will provide insight into the reaction pathways by which the peridotite was serpentinized and the dehydration reactions that can occur during subduction.
The focus of my following research will be on the hydration evolution of the Juan de Fuca plate from spreading ridge to trench. A combination of MCS (Multi-Channel Seismic) and OBS (Ocean Bottom Seismometer) data to be gathered by the R/V Langseth in 2012 will allow investigation of the state of hydration of a what is considered a young, warm end-member plate of subducting systems. The behavior of the Cascadia subduction zone including ETS (episodic tremor and slip) events, decreased mantle wedge velocities, and the composition of magmas produced in the volcanic arc indicates a relative abundance of volatiles. There is evidence for hydration of the upper crust but the level of hydration in the lower crust and upper mantle of the subducting lithosphere is not known. Using 2D and fan-shot wide-angle seismic profiles, the seismic velocity and anisotropy (directional dependence of velocity) of the crust and upper mantle can be evaluated. These seismic properties are proxies for hydration. Characterization of the fluid budget in the subduction zone is important because of the large influence volatiles play in seismicity, which has the capacity to affect the millions who live in Cascadia.
• Greg is being supported with funds from DOEI's Ocean Ridge Initiative