Squeezing the sponge: the role of serpentinites in subduction zones
Clarke, Eleri Rachel
Subduction zones are the main drivers of global volatile recycling through the supply of water to the mantle wedge which, in turn, triggers arc volcanism. Serpentinites may contribute greatly to this process due to their high water content and widespread presence in oceanic lithosphere. B isotopes are a powerful tracer of serpentinites due to their distinct signature (delta11B = +5 to +40 permil) and the signature of their expelled fluids (delta11B up to +15 permil), compared to the mantle which has a low delta11B (~-7.1 permil). However, despite its fluid mobile nature, B-rich metamorphic olivine has been found in dehydrated serpentinites in the field as well as in experiments. This retention of B in olivine potentially fractionates B isotopes during serpentinite dehydration, but details of this process remain largely unconstrained. The aim of this thesis is to analyse the B and delta11B distribution between phases in dehydrated serpentinites from two different geological settings: a subduction zone and a contact aureole, in order to constrain B isotope behaviour during deserpentinization. In addition we ran olivine-fluid B partitioning experiments to quantitatively constrain the partitioning behaviour of isotopes between olivine and fluid. This project provides an insight into serpentinite-associated fluid movements during subduction and contact metamorphism and the partitioning of B and its isotopes between olivine and fluid. We find that open system dehydration significantly effects the B isotope signature of dehydrated serpentinites and 11B partitioning preferences are fluid>olivine>serpentine.