Tracing fluid transfers in subduction zones an integrated thermodynamic and δ18O fractionation modelling approach

摘要

Oxygen isotope geochemistry is a powerful tool forinvestigating rocks that interacted with fluids, to assess fluid sources andquantify the conditions of fluid–rock interaction. We present an integratedmodelling approach and the computer program PTLoop that combinethermodynamic and oxygen isotope fractionation modelling for multi-rock opensystems. The strategy involves a robust petrological model performingon-the-fly Gibbs energy minimizations coupled to an oxygen fractionationmodel for a given chemical and isotopic bulk rock composition; both modelsare based on internally consistent databases. This approach is applied tosubduction zone metamorphism to predict the possible range of δ ~(18)O values for stable phases and aqueous fluids at various pressure(P ) and temperature (T ) conditions in the subducting slab. The modelled systemis composed of a mafic oceanic crust with a sedimentary cover of knowninitial chemical composition and bulk δ ~(18)O. The evolution ofmineral assemblages and δ ~(18)O values of each phase is calculatedalong a defined P –T path for two typical compositions of basalts and sediments.In a closed system, the dehydration reactions, fluid loss and mineralfractionation produce minor to negligible variations (i.e. within 1?‰) in the bulk δ ~(18)O values of the rocks, whichare likely to remain representative of the protolith composition. In an opensystem, fluid–rock interaction may occur (1)?in the metasediment, asa consequence of infiltration of the fluid liberated by dehydration reactionsoccurring in the metamorphosed mafic oceanic crust, and (2)?in themetabasalt, as a consequence of infiltration of an external fluid originatedby dehydration of underlying serpentinites. In each rock type, theinteraction with external fluids may lead to shifts in δ ~(18)O up to 1 order of magnitude larger than those calculated for closed systems. Suchvariations can be detected by analysing in situ oxygen isotopes in keymetamorphic minerals such as garnet, white mica and quartz. The simulationsshow that when the water released by the slab infiltrates the forearc mantlewedge, it can cause extensive serpentinization within fractions of 1?Myr andsignificant oxygen isotope variation at the interface. The approachpresented here opens new perspectives for tracking fluid pathways insubduction zones, to distinguish porous from channelled fluid flows, and todetermine the P –T conditions and the extent of fluid–rock interaction.