Quantifying the thermal structure and hydrothermal fluid fluxes in the lower oceanic crust

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New ocean crust is built along mid-ocean ridges by submarine volcanic activity. Seawater migrates down into this new permeable crust, and is progressively heated as it circulates due the presence of magma at depth. Seawater-derived fluids are chemically modified as they migrate through the crust because they react with the rocks through which they are passing. When these fluids exit the seafloor they are enriched in metals and H2S, leading to the formation of economically important volcanic hosted massive sulfide deposits and providing nutrients for unique biological ecosystems. The chemical exchange between the oceans and the ocean crust acts to modify the composition of both. Within the broader earth system this exchange plays an important role in regulating the composition of the ocean and the inputs into subduction zones, which influences the composition of arc volcanoes and contributes to heterogeneity in the mantle. A critical gap in our understanding of how the ocean crust is built and hydrothermally altered is related to the plutonic sequence that resides >2 km beneath the seafloor. At and near mid-ocean ridges, the interplay between magmatic intrusion and hydrothermal cooling and alteration controls the thermal and chemical evolution of the plutonic crust. To gain access to these critical rocks we take advantage of “tectonic windows,” where lower crustal exposures were unroofed by tectonic processes, thus bypassing the upper crust entirely. In the Pacific Ocean there are two such tectonic windows, Hess and Pito Deeps, and these are the only locations globally where these critical plutonic rocks have been mapped and sampled. Over the last 20+ years, I have participated in four submersible/ROV sampling cruises and led two international ocean drilling expeditions, leading to a globally unique sample suite for my proposed research. Over the next grant cycle, my research group will use this unique sample suite to test competing hypotheses for how crust is built at fast-spreading mid-ocean ridges by magmatic processes and subsequently cooled and altered by hydrothermal flow and reaction. We will also build a quantitative model for the magmatic and hydrothermal controls on the S and chalcophile element budget of the lower crust to expand our understanding of how the lower crust contributes to the global cycling of these elements.