Chlorine isotope variations across the Izu-Bonin-Mariana arc

Chlorine isotope ratios were determined for volcanic gas, geothermal well, ash, and lava samples along the Izu-Bonin-Mariana volcanic front, serpentinite clasts and muds from serpentine seamounts (Conical, South Chamorro, Torishima), basalts from the Guguan cross-chain, and sediments from Ocean Drilling Program (ODP) Sites 800, 801, 802, and 1149. There is no systematic variation in delta Cl-37 values along the volcanic front in either gas or ash samples. In contrast, distinct variations occur across the arc, implying variations in the fluid source at different depths within the subduction zone. Serpentinite clasts and serpentine muds from the seamounts tap a source of similar to 30 km depth and have delta Cl-37 values of structurally bound chloride of +0.4 parts per thousand +/- 0.4 parts per thousand (n = 24), identical to most seafloor serpentinites, suggesting a serpentinite (chrysotile and/or lizardite to antigorite transition) fluid source. Tapping deeper levels of the subduction zone (similar to 115-130 km depth), volcanic gases and ashes have delta Cl-37 values averaging -1.1 parts per thousand +/- 1.0 parts per thousand (n = 29), precisely overlapping the range measured in sediments from ODP cores (-1.1 parts per thousand +/- +0.7 parts per thousand, n = 11) and limited altered oceanic crust (AOC). Both sediments and AOC are possible Cl sources in the volcanic front. The Guguan cross-chain basalts come from the greatest depths and have an average delta Cl-37 value of +0.2 parts per thousand +/- 0.2 parts per thousand (n = 3), suggesting a second serpentine-derived source, in this case from antigorite breakdown at similar to 200 km depth.