Unit Affiliation: Geochemistry, Lamont-Doherty Earth Observatory (LDEO)
This project involves an interdisciplinary study of hydration, carbonation and oxidation of mantle peridotite interacting with aqueous fluids at temperatures below ~ 300C. The PIs will combine observations of outcrops and boreholes, geochemical analyses, structural measurements, geomechanical experiments and numerical modeling to investigate feedback between alteration and fluid transport, and to quantify the resulting geochemical fluxes. Field observations and sampling will take place mainly in the Samail ophiolite of Oman, where peridotite has undergone spreading-ridge-related hydrothermal alteration, hydration and carbonation in the hanging-wall of the subduction zone that emplaced the ophiolite over metasediments, and subaerial weathering. The PIs project will provide matching funds and results that dovetail with the 2015-2018 International Continental Scientific Drilling Program (ICDP) Oman Drilling Project, and the many other related efforts just getting underway. The PIs will continue their independently supported research on subduction zone alteration of mantle wedge peridotites at a range of pressures and temperatures, and work closely with other groups investigating seafloor and subduction-related peridotite alteration, in order to quantify the similarities and differences in alteration processes in these different tectonic environments. They will generalize their results to global alteration processes and geochemical cycles. Alteration of peridotite is an essential process in Earth dynamics. Hydration of oceanic crust and mantle, followed by subduction, supplies water to drive arc volcanism, and modulates the hydrogen content of the mantle over time. Carbonate formation during alteration of peridotite, near the surface and in the hanging wall in subduction zones, is an important but poorly characterized link in the carbon cycle. Oxidation of minerals and concomitant reduction of fluids produces H2 and hydrocarbons, and a niche for chemosynthetic microbes. Chemical weathering is as important as magmatism and plate tectonics in shaping the Earths surface. The interplay of chemical and physical mechanisms of peridotite alteration is not well understood, but will be transformed as a result of emerging understanding of equilibria and kinetics in peridotite alteration, and reaction-driven cracking that has left us poised on the brink of a breakthrough at this little-studied frontier. The PIs will take advantage of low temperature, near surface, active peridotite alteration in Oman to study inputs, outputs, and the reaction zone in situ. Such a study is more difficult in smaller peridotite exposures with limited outcrop and more rainfall, nearly impossible in submarine hydrothermal systems, and completely impossible in studies of ancient systems. Such a comprehensive approach via 250 to 600 meter boreholes is very rare, if not unprecedented.