In situ oxygen isotope analysis of garnet from high pressure metamorphic veins of the Italian Western Alps and New Caledonia: Resolving fluid flow regimes in subducted crust

Metamorphic veins cutting eclogite-facies rocks represent products of fluid-rock interaction under the high pressure (HP) conditions typical of suduction zones. We present new chemical and oxygen isotope data on minerals in HP veins and their mafic host rocks from paleo-subducted slabs of the Lago Superiore Unit (Monviso Massif, Italian Western Alps), Eclogitic Micaschist Complex (Sesia Zone, Italian Western Alps) and Pouébo Eclogite Melange (New Caledonia). X-ray mapping and analysis of garnets from these veins show complex zoning features indicative of complex history of vein evolution. These garnet zones which were targeted for δ18O measurements by ion microprobe help fingerprint the origin of the vein-forming fluids. Isotopic analyses were carried out on a Cameca IMS 1280 and corrected for instrumental mass fractionation due to varying cation chemistry in garnet by using a newly developed correction scheme (Page et al., 2010). The intragranular δ18O heterogeneities in all garnets from each sample are relatively small (≤1.5‰) which suggest that similar fluids were present during garnet formation. The oxygen isotope data for all zones of garnet from vein and metabasaltic host rock from the New Caledonian (6.1-6.5‰ relative to VSMOW) and Sesia Zone (3.8-4.9‰) samples are very homogeneous. This result does not support the involvement of external fluids during vein formation, despite previous suggestions that metasedimentary components may have been added to the New Caledonian veins (Spandler and Hermann, 2006). The mafic rocks from New Caledonia experienced low-temperature seafloor alteration prior to subduction, thus the veins must have formed by internal fluids produced during transition from blueschist to eclogite facies. The Monviso vein garnets have distinctly different oxygen isotope compositions (3.2-3.7‰) than garnets in their mylonitic Fe-Ti eclogite host rocks (2.1-2.3‰). The low δ18O values of garnet in these metagabbros are consistent with high-temperature hydrothermal alteration before subduction (Philippot et al., 1998). Low and high Cr zones in vein garnets, which exhibit either oscillatory zoning or fracture-like textures, have similar δ18O values. Based on all of these results, we suggest that externally-derived fluid played a significant role in the formation of these veins during prograde metamorphism. The source of the external fluids is likely to have been serpentinites and Mg-Al metagabbro units that underlie the mylonitic Fe-Ti eclogites. In this case, the Monviso eclogitic veins were potentially major channelways for fluid flow during subduction. Further whole rock oxygen isotope analysis is planned in order to better constrain fluid sources and fluxes. References: Page, Z.F., Kita, N.T., and Valley, J.W. (2010), Chemical Geology, 270, 9-19. Philippot, P., Agrinier, P., and Scambelluri, M. (1998), Earth and Planetary Science Letters, 161, 33-44. Spandler C. and Hermann J. (2006), Lithos, 89, 135-153.