Tracing fluid transfers in subduction zones: an integrated thermodynamic and δ<sup>18</sup>O fractionation modelling approach

Abstract

Abstract. Oxygen isotope geochemistry is a powerful tool for investigating rocks that interacted with fluids, to assess fluid sources and quantify the conditions of fluid-rock interaction. We present an integrated modelling approach and the computer program PTLOOP that combine thermodynamic and oxygen isotope fractionation modelling for multi-rock open systems. The strategy involves a robust petrological model performing on-the-fly Gibbs energy minimizations coupled to an oxygen fractionation model both based on internally consistent databases. This approach is applied to subduction zone metamorphism to predict the possible range of δ18O values for stable phases and aqueous fluids at various pressure-temperature (P-T) conditions in the subducting slab. The modelled system is composed by a sequence of oceanic crust (mafic) with sedimentary cover of known initial chemical composition and bulk δ18O. The evolution of mineral assemblage and δ18O values of each phase is calculated along a defined P-T path. Fluid-rock interactions may occur as consequence of (1) infiltration of an external fluid into the mafic rocks or (2) transfer of the fluid liberated by dehydration reactions occurring in the mafic rocks into the sedimentary rocks. The effects of interaction with externally-derived fluids on the mineral and bulk δ18O of each rock are quantified for two typical compositions of metabasalts and metasediments with external fluid influx from serpentinite. The dehydration reactions, fluid loss and mineral fractionation produce minor to negligible variations in bulk δ18O values, i.e. within 1 ‰. By contrast, the interaction with external fluids may lead to shifts in δ18O up to one order of magnitude larger. Such variations can be detected by analysing in-situ oxygen isotope in key metamorphic minerals such as garnet, white mica and quartz. The simulations show that, when the water released by the slab infiltrates the forearc mantle wedge, it can cause extensive serpentinization within fractions of a Myr and significant oxygen isotope variation at the interface. This technique opens new perspectives to track fluid pathways in subduction zones, to distinguish porous from channelized fluid flows, and to determine the P-T conditions and the extent of fluid/rock interaction.

Greenstone burial–exhumation cycles at the late Archean transition to plate tectonics

“…Notably, garnet typically attains lower δ 18 O isotope signatures compared to the whole-rock values, due to isotope fractionation between other minerals, which is controlled by oxygen bond types 54 . Modelling has shown that the maximum oxygen isotope fractionation between whole rock and garnet at high temperature (>400 °C) is <2‰, with particularly low fractionation in mafic rocks that are poor in quartz and calcite 44,55 , such as the amphibolites studied here. The whole-rock δ 18 O value of EA sample 155899, at the time of garnet formation, therefore likely overlapped with a mantlelike δ 18 O signature (Fig.…”