Internal and external fluid sources for eclogite-facies veins in the Monviso meta-ophiolite, Western Alps: Implications for fluid flow in subduction zones

To contribute to our understanding of the mechanisms and path-ways of fluid movement through deeply subducted crust, we investigate high-pressure veins cutting eclogite-facies (2·0 GPa and 6008C) metagabbros of the Monviso Ophiolite, Italian Western Alps. The veins consist mainly of omphacite with minor garnet, rutile, talc and accessory zircon. Most of the vein minerals have major and trace element compositions that are comparable with the host-rock minerals, and vein and host-rock zircons have similar Hf isotopic compositions.These observations support the conclusions of previous studies that these veins largely formed from a locally sourced hydrous fluid during prograde or peak metamorphism. However, the bulk-rock Cr and Ni contents of the veins are signifi-cantly higher than those of the surrounding host eclogites. We also document distinct Cr-rich (up to weight per cent levels) zones in omphacite, garnet and rutile in some vein samples.Vein garnet an