Lawsonite: an exceptional geochemical archive in HP/LT rocks

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Water and other chemical substances that are essential for life circulate from the Earth’s surface to the deep interior of the planet and back again in a cycle over many millions of years. Water is carried to great depths within mineral crystals in tectonic plates that dive (subduct) into the subsurface. One of the most important minerals in this cycle is lawsonite, which forms only under the characteristic high-pressure and relatively low-temperature conditions in the upper layer of subducting plates. Lawsonite is important because it contains a substantial amount of water and also contains trace amounts of other elements, such as uranium, lead, and rare earth elements. Although common in subducting plates, lawsonite rarely survives to be preserved in the rock record, particularly in the highest-pressure rocks of ancient subduction zones. This project uses the composition of rare, fresh lawsonite from a global suite of twelve subduction complexes as an archive of element-cycling processes in subduction zones. Lawsonite composition and zoning are exceptional recorders of fluid-rock interaction in subduction zones. Compositional and textural data illuminate the role of lawsonite and associated minerals in volatile and element cycling at the interface of the subducted oceanic crust and overlying mantle before major devolatilization at subarc depths. This research involves the first determination of oxygen isotope values in lawsonite and comprehensive characterization of the trace-element composition, zoning, and microstructures of lawsonite and associated minerals. Preliminary data indicate that oxygen isotope abundance in lawsonite is a sensitive indicator of the source(s) of fluids in subducted slabs, such as serpentinites (lower d18O) or oceanic sediments (higher d18O), and show that fluid sources in mélange vs. more structurally-coherent subduction complexes may be distinct. The sample suite consists of lawsonite eclogites and blueschists as well as lawsonite-bearing metasedimentary (carbonate, quartzite) and metasomatic rocks and veins, and includes prograde, peak, and retrograde generations of lawsonite. Owing to its high modal abundance and composition (12 wt% H2O, 1-2 wt% Fe2O3), lawsonite may be a significant factor in mantle redox conditions and H2O content since at least the Neoproterozoic.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.