Chlorine cycling during subduction of altered oceanic crust

Eclogitic rocks that have experienced devolatilization, with little or no interaction with external fluid sources, can be viewed as analogues of crustal material which may be transferred back into the mantle during subduction. Thus they can be used to evaluate the extent of the recycling of volatile elements, such as chlorine. We report new oxygen isotope ratios of omphacite, and fluid inclusion data determined from eclogitic metagabbros, of the Rocciavré massif (Italian Alps). The data are compared with those obtained for the Monviso, Cyclades and the Franciscan Complex high-pressure rocks. In all localities, relics of early dehydration fluids are preserved as primary fluid inclusions in the cores of omphacite megacrysts (Rocciavré, Monviso and Franciscan Complex) or garnet (Cyclades). Salinity estimates of the inclusion fluids range from 32 to 45 wt% NaCl in Rocciavré, 17 to 21 wt% NaCl in Monviso, and are similar to seawater in other areas. Omphacite and bulk-rock δ18O values of Rocciavré (5.1–6.8‰) and Monviso (3.0–5.3‰) metagabbros are markedly lower than those of Cyclades (6.8–14.3‰) and the Franciscan (6.7–13.1‰) metabasites. The fluid salinity–δ18O systematics of eclogitic rocks is similar to that documented along a typical section of the altered oceanic crust and unmetamorphosed ophiolites. This suggests that high-pressure metamorphism, and associated processes, did not modify significantly the variability in chlorine concentrations and oxygen isotope ratios, inherited from a stage of sea-floor hydrothermal alteration under low- (basaltic layer) and high-temperature (gabbroic layer) conditions, respectively. olating the estimated H2O and Cl contents of eclogitic rocks to a representative section of the subducted oceanic crust indicates that a minimum of 100 to 200 ppm Cl could be recycled into the mantle during subduction. This yields a Cl/H2O ratio of 3.6 to 7.5×10−2 for the subducted oceanic crust, which is similar to E-MORB. On the basis of available Cl isotopic data, we infer that a large proportion (70%) of the Cl stored in the altered crust should be recycled to the mantle to generate an isotopic composition of the subducted crust equivalent to the source of unaltered mid-ocean ridge basalt (δ37Cl = 4.7‰).