Chlorine stable isotopes and halogen concentrations in convergent margins with implications for the C1 isotopes cycle in the ocean

Chlorine stable isotopes (delta(37) Cl) 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 delta Cl-37 Values of pore fluids at the Nankai and Costa Rica subduction zones, are significantly more negative (minimum -7.8 parts per thousand, 2 sigma +/- 0.3 parts per thousand) than seawater value (O parts per thousand). At Nankai Trough, the minimum delta Cl-37 value is situated below the decollement and evolves laterally from -7.8 parts per thousand at the most arcward ODP Site 808, to -7.1 parts per thousand at Site 1174, -2 km seaward from Site 808, and to -5.8 parts per thousand at the reference Site 1173. At Costa Rica, along the decollement the minimum delta Cl-37 value evolves from -5.5%. at the most arcward ODP Site 1040/1254, to -3.2 parts per thousand at Site 1043/1255, similar to 1 km seaward, and to O%o at the reference Site 1039/1253. At both subduction zones, the Br/Cl ratios are higher than the seawater value (1.5 x 10(-3)) and also show seaward evolutions. These pore fluids originate from greater depth arcward, at >= 250 degrees C, from hydrous mineral formation that preferentially incorporates Cl-37 and excludes Br. In contrast, the delta Cl-37 values in the pore fluids at the Mariana serpentine mud volcanoes are higher than the seawater value (+0.3 parts per thousand to + 1.8 parts per thousand); and the Br/Cl ratios are lower. These pore fluid values and the high Cl concentrations with positive delta Cl-37 values (+ 1.2 to + 6.0 parts per thousand) in the serpentines, support that the upwelling pore fluid originates from dehydration of the subducting slab that releases water enriched in Cl-37, into the fluid phase. The constancy of the ocean delta Cl-37 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 x 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. (c) 2007 Published by Elsevier B.V.