Chlorine isotopic composition in seafloor serpentinites and high-pressure metaperidotites. Insights into oceanic serpentinization and subduction processes

Bulk-rock chlorine content and isotopic composition (delta Cl-37) were determined in oceanic serpentinites, high-pressure metaperidotites and metasediments in order to gain constraints on the global chlorine cycle associated with hydrothermal alteration and subduction of oceanic lithosphere. The distribution of insoluble chlorine in oceanic serpentinites was also investigated by electron microprobe. The hydrothermally-altered ultramafic samples were dredged along the South West Indian Ridge and the Mid-Atlantic Ridge. The high-pressure metamorphic samples were collected in the Western Alps: metaperidotites in the Erro-Tobbio unit and metasediments in the Schistes Lustres nappe. Oceanic serpentinites show relatively large variations of bulk-rock Cl contents and 637C1 values with mean values of 1105 +/- 596 ppm and -0.7 +/- 0.4 parts per thousand, respectively (n = 8; l sigma). Serpentines formed after olivine (meshes) show lower CI content than those formed after orthopyroxene (bastites). In bastites of two different samples, Cl is positively correlated with Al2O3 and negatively correlated with SiO2. These relationships are interpreted as reflecting preferential Cl-incorporation into the bastite structure distorted by A1 (substituted for Si) rather than different alteration conditions between olivine and orthopyroxene minerals. High-pressure metaperidotites display relatively homogeneous Cl contents and 637C1 values with mean values of 467 88 ppm and -1.4 +/- 0.1 parts per thousand, respectively (n = 7; 1 sigma). A macroscopic high-pressure olivine-bearing vein, formed from partial devolatilization of serpentinites at similar to 2.5 GPa and 500-600 degrees C, shows a Cl content and a delta Cl-37 value of 603 ppm and -1.6 parts per thousand, respectively. Metasediments (n = 2) show low whole-rock Cl contents (<15 ppm Cl) that did not allow Cl isotope analyses to be obtained. The range of negative delta Cl-37 values observed in oceanic serpentinites is likely to result from water-rock interaction with fluids that have negative delta Cl-37 values. The homogeneity of delta Cl-37 values from the high-pressure olivine-bearing vein and the metaperidotite samples implies that progressive loss of Cl inherited from oceanic alteration throughout subduction did not significantly fractionate Cl isotopes. Chlorine recycled in subduction zones via metaperidotites should thus show a range of delta Cl-37 values similar to the range found in oceanic serpentinized peridotites. (c) 2007 Elsevier Ltd. All rights reserved.