Iron and zinc stable isotope evidence for open-system high-pressure dehydration of antigorite serpentinite in subduction zones

Subducted serpentinites have the potential to control the exchange of volatile and redox sensitive elements (e.g., Fe, S, C, N) between the slab, the mantle wedge and the deep mantle. Here we examine the mobility of iron and zinc in serpentinite-derived fluids by using their stable isotopes (δ56Fe and δ66Zn) in high-pressure subducted meta-serpentinites from the Cerro del Almirez massif (Spain). This massif preserves a metamorphic front between antigorite (Atg-serpentinite) and antigorite-olivine-orthopyroxene (transitional lithologies) -bearing serpentinites, and chlorite-bearing harzburgite (Chl-harzburgite), displaying granofels, spinifex and fine-grained recrystallized textures. Those rocks were formed at eclogite facies conditions (1.6–1.9 GPa and 680–710 °C). The mean δ56Fe of all the Cerro del Almirez meta- serpentinites (+0.05 ± 0.01 ‰) is identical within an error to that of primitive mantle (+0.03 ± 0.03 ‰). A positive correlation between δ56Fe and indices of peridotite protolith fertility (e.g., Al2O3/SiO2) suggests that the δ56Fe values of Cerro del Almirez samples predominantly reflect protolith compositional variations, likely produced by prior episodes of melt extraction. In contrast, the Zn concentrations ([Zn] = 34–67 ppm) and isotope signatures (δ66Zn = +0.18 – +0.55 ‰) of the Cerro del Almirez samples show a broad range of values, distinct to those of the primitive mantle ([Zn] = 54 ppm; δ66Zn = +0.16 ± 0.06 ‰). The Atg- serpentinites ([Zn] = 34–46 ppm; δ66Zn = +0.23 ± 0.06 ‰) display similar [Zn] and δ66Zn values to those of slab serpentinites from other high-pressure meta-ophiolites. Both [Zn] and δ66Zn increase in transitional lithologies ([Zn] = 45–67 ppm; δ66Zn = +0.30 ± 0.06 ‰) and Chl-harzburgites with granofels ([Zn] = 38– 59 ppm; δ66Zn = +0.33 ± 0.04 ‰) or spinifex ([Zn] = 48–66 ppm; δ66Zn = +0.43 ± 0.09 ‰) textures. Importantly, Cerro del Almirez transitional lithologies and Chl-harzburgites display abnormally high [Zn] relative to abyssal peridotites and serpentinites (29–45 ppm) and a positive correlation exists between [Zn] and δ66Zn. This correlation is interpreted to reflect the mobilization of Zn by subduction zone fluids at high pressures and temperatures coupled with significant Zn stable isotope fractionation. An increase in [Zn] and δ66Zn from Atg-serpentinite to Chl-harzburgite is associated with an increase in U/Yb, Sr/Y, Ba/Ce and Rb/Ce, suggesting that both [Zn] and δ66Zn record the interaction of the transitional lithologies and the Chl- harzburgites with fluids that had equilibrated with metasedimentary rocks. Quantitative models show that metasediment derived fluids can have isotopically heavy Zn as a consequence of sediment carbonate dissolution and subsequent Zn complexation with carbonate species in the released fluids (e.g., [ZnHCO3(H2O)5+] or [ZnCO3(H2O)3]). Our models further demonstrate that Zn complexation with reduced carbon species cannot produce fluids with heavy δ66Zn signature and hence explain the δ66Zn variations observed in the Chl-harzburgites. The most straightforward explanation for the heavy δ66Zn of the Cerro del Almirez samples is thus serpentinite dehydration accompanied by the open system infiltration of the massif by oxidized, carbonate-rich sediment-derived fluids released during prograde subduction-related metamorphism.This work was supported by a Natural Environment Research Council (NERC) Deep Volatiles Consortium Grant (NE/M000303/1), a European Research Council (ERC) Starting Grant (HabitablePlanet; 306655) to H. Williams