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 analogue s of crustal material which may be transferred back into the mantle du ring subduction. Thus they can be used to evaluate the extent of the r ecycling of volatile elements, such as chlorine. Fire report new oxyge n isotope ratios of omphacite, and fluid inclusion data determined fro m eclogitic metagabbros, of the Rocciavre massif (Italian Alps). The d ata are compared with those obtained for the Monviso, Cyclades and the Franciscan Complex high-pressure rocks. In all localities, relies of early dehydration fluids are preserved as primary fluid inclusions in the cores of omphacite megacrysts (Rocciavre, Monviso and Franciscan C omplex) or garnet (Cyclades). Salinity estimates of the inclusion flui ds range from 32 to 45 wt% NaCl in Rocciavre, 17 to 21 wt% NaCl in Mon viso, and are similar to seawater in other areas. Omphacite and bulk-r ock delta(18)O values of Rocciavre (5.1-6.8 parts per thousand) and Mo nviso (3.0-5.3 parts per thousand) metagabbros are markedly lower than those of Cyclades (6.8-14.3 parts per thousand) and the Franciscan (6 .7-13.1 parts per thousand) metabasites. The fluid salinity-delta(18)O systematics of eclogitic rocks is similar to that documented along a typical section of the altered oceanic crust and unmetamorphosed ophio lites. This suggests that high-pressure metamorphism, and associated p rocesses, did not modify significantly the variability in chlorine con centrations and oxygen isotope ratios, inherited from a stage of sea-f loor hydrothermal alteration under low-(basaltic layer) and high-tempe rature (gabbroic layer) conditions, respectively. Extrapolating the es timated H2O and Cl contents of eclogitic rocks to a representative sec tion 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 x 10(-2) for the subducted oceanic crust, which is similar to E-MORB. On the basis of available CI isoto pic data, we infer that a large proportion (70%) of the Cl stored in t he altered crust should be recycled to the mantle to generate an isoto pic composition of the subducted crust equivalent to the source of una ltered mid-ocean ridge basalt (delta(37)Cl = 4.7 parts per thousand). (C) 1998 Elsevier Science B.V. All rights reserved.