Abstract
Bulk chlorine concentrations and chlorine stable isotope compositions were determined for hydrothermally altered basalt (extrusive lavas and sheeted dikes) and gabbro samples (n=50) from seven DSDP/ODP/IODP drill sites. These altered oceanic crust (AOC) samples span a range of crustal ages, tectonic settings, alteration type, and crustal depth. Bulk chlorine concentrations range from <0.01wt.% to 0.09wt.%. In general, higher chlorine concentrations coincide with an increase in temperature of alteration and amphibole content. δ37Cl values of whole rock AOC samples range from −1.4 to +1.8‰. High δ37Cl values (≥ ~0.5‰) are associated with areas of higher amphibole content. This observation is consistent with theoretical calculations that estimate amphibole should be enriched in 37Cl compared to co-existing fluid. Negative to near zero δ37Cl values are found in areas dominated by clay minerals. Chlorine geochemistry is a rough indicator of metamorphic grade and mineralogy. AOC is a major Cl host in the subducting oceanic lithospheric slab. Here we show that bulk chlorine concentrations are ~3 times higher than previous estimates resulting in a greater contribution of Cl to the mantle.
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