Chorine stable isotope variations in Izu Bonin tephra: Implications for serpentinite subduction

δ37Cl values were determined for Izu Bonin arc magmas erupted 0–44 Ma in order to better understand the time-dependent processing of volatiles in subduction zones. Pristine ash-sized particles (glass, pumice, scoria, and rock fragments) were handpicked from tephra drilled at ODP Site 782. δ37Cl values for these particles span a large range from − 2.1 to + 1.7‰ (error = ± 0.3‰) vs. SMOC (Standard Mean Ocean Chloride, defined as 0‰). The temporal data extend the previously reported range of δ37Cl values of − 2.6 to 0.4‰ (bulk ash) and − 5.4 to − 0.1‰ (volcanic gases) from the Quaternary Izu Bonin–Mariana volcanic front to more positive values. Overall, the temporal data indicate a time-progressive evolution, from isotopically negative Eocene and Oligocene magmas (− 0.7 ± 1.1‰, n = 10) to Neogene magmas that have higher δ37Cl values on average (+ 0.3 ± 1.1‰; n = 13). The increase is due to the emergence of positive δ37Cl values in the Neogene, while minimum δ37Cl values are similar through time. The range in δ37Cl values cannot be attributed to fractionation during melt formation and differentiation, and must reflect the diversity of Cl present in the arc magma sources. Cl clearly derives from the slab (> 96% Cl in arc magmas), but δ37Cl values do not correlate with isotope tracers (e.g. 207Pb/204Pb and 87Sr/86Sr) that are indicative of the flux from subducting sedimentary and igneous crust. Given the steady, high Cl flux since at least 42 Ma, the temporal variability of δ37Cl values is best explained by a flux from subducting isotopically positive and negative serpentinite formed in the ocean basins that mingles with and possibly overprints the isotopically negative flux from sediment and igneous crust at arc front depths. The change in the δ37Cl values before and after backarc spreading may reflect either a tectonically induced change in the mechanism of serpentinite formation on the oceanic plate, or possibly the integration of isotopically positive wedge serpentinite as arc fluid source during the Neogene. Our study suggests that serpentinites are important fluid sources at arc front depth, and implies the return of isotopically positive and negative Cl from the Earth surface to the mantle.