Dre Céline Martin - Tracking the origin of fluids encountered in subduction zones with boron isotopes

Subduction zones are the main interface on Earth for chemical exchanges between the surface and the mantle. The processes occurring in subduction zones involve huge amount of fluids, including aqueous fluids, released from the dehydrating metabasites and metasediments. The sources of these aqueous fluids, as well as their role in recycling element into the mantle and towards arc magma generation areas, are not yet well known. Serpentine minerals, which contain up to 14% of water in mass, are the main water carrier in subduction zones, and also a significant reservoir for Fluid Mobile Elements (FME) like Li, B, As, Sb, Cl, Sr, U, Th. Recently it has been shown that B isotopic signature can be used to determine the source of fluid(s) responsible for serpentinization, and consequently in which tectonic settings a given serpentinite formed. We now know the B isotopic signature of serpentinites forming from subduction-derived metamorphic fluid, which is very different from that of serpentinites forming from seawater. The coexistence of both serpentinites at great depth, where arc magmas are generated, permits explaining the wide range of B isotopic signatures observed in arc volcanoes. The most recent studies have also evidenced significantly different fluids, recording different events, in serpentinites from ophiolites.