FLUID COMPOSITION AND EVOLUTION IN COESITE-BEARING ROCKS (DORA-MAIRA MASSIF, WESTERN ALPS) - IMPLICATIONS FOR ELEMENT RECYCLING DURING SUBDUCTION

Fluid inclusions and F, Cl concentration of hydrous minerals were anal ysed in the coesite-pyrope quartzite, the interlayered jadeite quartzi te and their country-rock gneiss from the Dora-Maira massif using a co mbination of microthermometry, Raman spectrometry, synchrotron X-ray m icrofluorescence and electron microprobe analysis. Three populations o f fluid inclusions were recognized texturally and can be related to di stinct metamorphic stages. A low-salinity aqueous fluid occurs in the retrogressed country gneiss and as late secondary inclusions in jadeit e quartzite and chloritized pyrope. An earlier secondary population is found in matrix quartz of the jadeite- and pyrope-quartzites. This po pulation can be related to the early decompression and so to incipient breakdown of garnet into phlogopite-bearing assemblages. The inclusio n fluid is highly saline (up to 84 wt% equivalent NaCl) and contains N a, Ca, Fe, Cu and Zn as major cations. In pyrope quartzite, additional K was found in these brines, which locally coexist with CO2-rich incl usions. The oldest fluid inclusions are preserved in kyanite grains in cluded in fresh pyrope and in pyrope itself. In pyrope, all inclusions have decrepitated and contain magnesite, an Mg-phosphate, sheet-silic ate(s), a chloride and an opaque phase, with no fluid preserved. In co ntrast, the kyanite inclusions in pyrope preserve primary H2O-CO2 low- salinity fluid inclusions, probably owing to the low compressibility o f the kyanite inclusions and host garnet. In spite of in-situ re-equil ibration, these inclusions can be interpreted as relies of the dehydra tion fluid that attended pyrope growth. These correlations between tex tural and chemical fluid inclusion data and metamorphic stages are con sistent with the fluid composition calculated from the halogen content of different generations of phlogopite and biotite. The preservation of different fluid compositions, both in time and space, is evidence f or local control and possibly origin of the fluids, in agreement with isotopic data. These results, in particular the absence of CO2 in the jadeite quartzite, are best interpreted in terms of a fluid-melt syste m evolution. With increasing metamorphism, partitioning of H2O, Na, Ca , Fe and heavy metals into melt (jadeite quartzite) and Mg, Na/K, F, C O2 and P(?) into a residual aqueous fluid can account for depletion in Na, Ca and Fe of the pyrope quartzite. During the retrograde path, al pha(H2O) rose as melt crystallized, generating the two populations of hypersaline and water-rich fluids that were highly reactive to pyrope. The process of fluid-melt interaction envisioned here coupled with mo dels of melt extraction in subduction zones provides an attractive opp ortunity for the instantaneous (< 1 Ma) and selective transport of ele ments between a downgoing slab and the overlying mantle wedge.