Tracing fluid transfers in subduction zones: an integrated thermodynamic and <i>δ</i><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 for a given chemical and isotopic bulk rock composition; both models are 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 (P) and temperature (T) conditions in the subducting slab. The modelled system is composed of a mafic oceanic crust with a sedimentary cover of known initial chemical composition and bulk δ18O. The evolution of mineral assemblages and δ18O values of each phase is calculated along a defined P–T path for two typical compositions of basalts and sediments. In a closed system, the dehydration reactions, fluid loss and mineral fractionation produce minor to negligible variations (i.e. within 1 ‰) in the bulk δ18O values of the rocks, which are likely to remain representative of the protolith composition. In an open system, fluid–rock interaction may occur (1) in the metasediment, as a consequence of infiltration of the fluid liberated by dehydration reactions occurring in the metamorphosed mafic oceanic crust, and (2) in the metabasalt, as a consequence of infiltration of an external fluid originated by dehydration of underlying serpentinites. In each rock type, the interaction with external fluids may lead to shifts in δ18O up to 1 order of magnitude larger than those calculated for closed systems. Such variations can be detected by analysing in situ oxygen isotopes 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 1 Myr and significant oxygen isotope variation at the interface. The approach presented here opens new perspectives for tracking fluid pathways in subduction zones, to distinguish porous from channelled fluid flows, and to determine the P–T conditions and the extent of fluid–rock interaction.

Pervasive fluid-rock interaction in subducted oceanic crust revealed by oxygen isotope zoning in garnet

“…This sharp zoning is compatible with slow rates of oxygen intergranular diffusion (Vielzeuf et al 2005;Page et al 2019) and suggests that the original δ 18 O composition of garnet core is preserved during exhumation and cooling. It has been demonstrated that, in a closed-system evolution over a temperature range of 150-200 °C, dehydration reactions, fluid loss and mineral fractionation have only minor influence on the mineral δ 18 O value (< 1‰) and will not influence the δ 18 O protolith signature (Kohn 1993;Vho et al 2020a). Consequently, the magnitude of δ 18 O variation observed in the small garnet grains can only be explained by ingress of external fluids with low δ 18 O values and in isotopic disequilibrium with the rock, during the TGU first decompression stage.…”

“…Fluid-rock interaction during rock metamorphic history was investigated using the computer program PTLOOP (Vho et al 2020a), which combines equilibrium thermodynamic calculations with oxygen isotopic fractionation modelling. The model performs successive Gibbs energy minimizations computed using Theriak-Domino (De Capitani and Brown 1987;De Capitani and Petrakakis 2010) along a given P-T trajectory and calculates at each simulation step the oxygen isotopic fractionation between the mineral predicted to be stable.…”

“…The model geometry was set to a two-rock stack with the possibility for any free fluid released by the underneath lithology to migrate and interact with the lithology situated above. As discussed in Vho et al (2020a), an additional external free fluid phase can be introduced in the lower lithology at any simulation step.…”

“…The TGU unit underwent multiple fluid influxes at the oceanic and subduction stage and thus the original bulk rock δ 18 O cannot be retrieved from measuring the bulk rock δ 18 O composition. The original bulk rock δ 18 O composition was instead calculated for each lithology at peak pressure conditions, before the ingress of external fluids (Online Resource 4), using the software PTLOOP (Vho et al 2020a) and the bulk rock compositions for major and minor elements (Online Resource 5). The use of bulk rock chemistry for phase equilibrium modelling requires the assumption of minor or insignificant bulk chemical variations, outside water, during fluid-rock interaction (see discussion in Bovay et al 2021).…”

“…Advances in thermodynamic modelling and underpinning experiments provide a detailed framework on fluid production and mineral δ 18 O change in single rock types (Kohn 1993;Zheng 1993;Manning 2004;Kessel et al 2005). Recent studies have enabled major improvement in numerical modelling of petrological and isotopic systems by coupling thermodynamic and δ 18 O simulations (Vho et al 2019(Vho et al , 2020a. With this approach, the effect of fluid interaction between multiple rock types of contrasting composition can be quantitatively evaluated along a given P-T metamorphic path, allowing monitoring of variables of interest such as fluid-rock ratios and δ 18 O isotopic variations at the rock and mineral scale (Vho et al 2020b).…”