Geochemical tracking of fluid sources in metamorphic veins: an example from eclogites of the Western Alps

Utilising modern in situ micro-analytical techniques, key information on fluid flow, fluid source and element mobility in veins systems can be extracted. Applied to mineralized systems, these techniques may provide crucial insights into how fluids mobilize metals and how these metals can become concentrated. Here, we examine high-pressure metamorphic veins in the Monviso Ophiolite, Italian Western Alps. Previous studies demonstrate that these veins formed from fluids derived from the local host rock (Fe-Ti metagabbro), with fluid flow limited to the cm scale. Detailed electron microprobe mapping and analysis reveal distinct Cr-rich zones within veins minerals that cannot have been derived from the surrounding host rocks. Targeted in situ trace element analysis of these minerals reveal correlations between Cr, Ni, As, Sb and B: an element combination that uniquely fingerprints the fluid source to nearly serpentinite. Hf isotope compositions of zircons in the veins and host rocks also support involvement of a primitive external fluid source for the veins. These data show that the veins formed from internally and externally sourced fluids, and fluid flow length-scales varied several orders of magnitude during vein growth. Without detailed micro-geochemical analysis, such a complex history to these veins would not be recognized