Description: Chlorine stable isotopes (delta37Cl) and halogen concentrations (e.g. Br/Cl) in 168 pore Fluids and 23 serpentines and other solids from three subduction zones, the Nankai Trough, Costa Rica, and Mariana Forearc, provide critical information on fluid sources, flow paths, and reaction conditions. The delta37Cl values of pore fluids at the Nankai and Costa Rica subduction zones, are significantly more negative (minimum -7.8‰, 2 sigma +/- 0.3‰) than seawater value (0‰). At Nankai Trough, the minimum delta37Cl value is situated below the décollement and evolves laterally from -7.8‰ at the most arcward ODP Site 808, to -7.1‰ at Site 1174, not, vert, similar 2 km seaward from Site 808, and to -5.8‰ at the reference Site 1173. At Costa Rica, along the décollement the minimum delta37Cl value evolves from -5.5‰ at the most arcward ODP Site 1040/1254, to -3.2‰ at Site 1043/1255, ~1 km seaward, and to 0‰ at the reference Site 1039/1253. At both subduction zones, the Br/Cl ratios are higher than the seawater value (1.5 * 10**-3) and also show seaward evolutions. These pore fluids originate from greater depth arcward, at 〉= 250 °C, from hydrous mineral formation that preferentially incorporates 37Cl and excludes Br. In contrast, the delta37Cl values in the pore fluids at the Mariana serpentine mud volcanoes are higher than the seawater value (+ 0.3‰ to + 1.8‰); and the Br/Cl ratios are lower. These pore fluid values and the high Cl concentrations with positive delta37Cl values (+ 1.2 to + 6.0‰) in the serpentines, support that the upwelling pore fluid originates from dehydration of the subducting slab that releases water enriched in 37Cl, into the fluid phase. The constancy of the ocean delta37Cl over the past 200 Ma suggests that the isotopically fractionated chlorine in serpentinites and the Cl exchanged in subduction zones are efficiently recycled back into seawater. If the efficiency is 〈 100%, the residual would be transferred to the mantle, with a maximum Cl flux between 2 to 3 * 10**17 moles/Ma that would lead to an isotopic difference between the mantle and seawater over the age of the earth on the order of a few per mil.