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 δ37Cl 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 ~30 km depth and have δ37Cl values of structurally bound chloride of +0.4‰ ± 0.4‰ (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 (~115–130 km depth), volcanic gases and ashes have δ37Cl values averaging −1.1‰ ± 1.0‰ (n = 29), precisely overlapping the range measured in sediments from ODP cores (−1.1‰ ± +0.7‰, 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 δ37Cl value of +0.2‰ ± 0.2‰ (n = 3), suggesting a second serpentine-derived source, in this case from antigorite breakdown at ~200 km depth.