Mineralogical control on the chlorine isotope composition of altered oceanic crust

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 < 0.01 wt.% to 0.09 wt.%. 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.