Date/heure
Date(s) - 5 avril 2024
13 h 00 min - 14 h 00 min
Catégories
Fluid paths in subduction zones: insights from boron isotopes in serpentinites
Subduction zones are the main interface on Earth for chemical exchanges between the surface and the mantle. The processes occurring in subduction zones involve important amounts of fluids, including aqueous fluids, which locally change the chemistry of the mantle, and likely has an impact on the genesis of arc volcano magmas. The sources of these aqueous fluids, as well as their role in recycling elements into the mantle, are still not yet well constrained. Serpentine, which forms by alteration of the mantle by aqueous fluids, contains up to 14 wt. % of water, and is commonly encountered in subduction zones, and therefore represents a key mineral to study the source and role of aqueous fluids within convergent plate boundaries. Boron is a highly fluid-mobile element whose isotopes fractionate easily in function of pH, temperature, and mineralogy, and that is concentrated in serpentine minerals (~ 2-800 ppm), which makes it a key isotopic system to study the fluid path(s) in the mantle. Recently, it has been demonstrated that serpentine minerals hydrated by seawater and seawater-derived fluid have δ11B above +10 ‰, while serpentine minerals hydrated by subducted crust-derived metamorphic fluid (i.e., the fluids resulting from the progressive dehydration of sediments and oceanic crust during subduction) have δ11B above +10 ‰ and that both fluids can be present simultaneously at great depths in subduction zones.