Chlorine Isotope Geochemistry of Altered Oceanic Crust: Empirical and Experimental Observations

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Intellectual Merit. Because of its strong partitioning into aqueous fluids, Cl and its isotopes (37Cl and 35Cl) are potentially powerful tracers of volatile and fluid migration in the crust and subduction zones, as well as the interaction between oceanic lithosphere and seawater derived hydrothermal fluids. To advance the utility of these tracers knowledge of its distribution and isotopic fractionation within various reservoirs is required. Recent work has begun to define the d37Cl values of various global Cl reservoirs (sediments, pore fluids, serpentinites), but isotopic data is very limited for one of the largest reservoirs, altered oceanic crust (AOC). AOC hosts ~2.5-3 x 10^12 g of Cl subducted worldwide, yet only three d37Cl values are published. Also, calculations and laboratory experiments imply minor fractionation between Cl-bearing phases, so it is commonly assumed that Cl faithfully records the isotopic composition of the hydration source. However, theoretical calculations use partition function ratios for divalent chlorides as analogues for silicates and fractionation experiments are limited to coexisting vapor, liquid, and precipitated salts in the H2O-NaCl and HCl systems. To properly address isotopic fractionation in systems for which Cl is a fluid tracer, experiments are needed for the common Cl-bearing silicate minerals in AOC (e.g., hornblende, actinolite, chlorite). Such information will enable better assessment of isotopic fractionation during fluid-rock and fluid-melt interactions and aid in interpreting Cl isotope data in general. The proposed research includes [1] a survey of d37Cl values for altered basalt and gabbro samples from DSDP/ODP/IODP drill cores, and [2] experimental determination of Cl isotope fractionation factors between the aforementioned hydrous silicate minerals and aqueous chloride solutions (seawater and brine). These complementary efforts are needed to fingerprint chlorine sources and accurately interpret data from natural samples. Ultimately, Cl isotope data will be incorporated into mass balance calculations to better understand material recycling at subduction zones.Broader impacts. PI Barnes is a first year assistant professor at UT-Austin. This proposal will partially fund one Ph.D. and one M.S. student. The proposed research will provide a foundation as Barnes begins her faculty career and will be integrated into the development of her new stable isotope lab. Two undergraduates will be partnered with the graduate students involved in this project, resulting in two honors theses. Co-PI Gardner is an associate professor at UT-Austin and will serve as a mentor to the newly hired Barnes. Both PIs will oversee and mentor students with laboratory work and data interpretation.