Iron and zinc stable isotope evidence for open-system high-pressure dehydration of antigorite serpentinite in subduction zones. (1st March 2021) - Debret, Baptiste
Abstract: 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 (δ 56 Fe and δ 66 Zn) 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 δ 56 Fe 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 δ 56 Fe and indices of peridotite protolith fertility (e.g., Al2 O3 /SiO2 ) suggests that the δ 56 Fe 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 (δ 66 Zn = +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; δ 66 Zn = +0.16 ± 0.06‰). The Atg-serpentinites ([Zn] = 34–46 ppm; δ 66 Zn = +0.23 ± 0.06‰) display similar [Zn] and δ 66 Zn values to those of slab serpentinites from other high-pressure meta-ophiolites. Both [Zn] and δ 66 Zn increase in transitional lithologies ([Zn] = 45–67 ppm; δ 66 Zn = +0.30 ± 0.06‰) and Chl-harzburgites with granofels ([Zn] = 38–59 ppm; δ 66 Zn = +0.33 ± 0.04‰) or spinifex ([Zn] = 48–66 ppm; δ 66 Zn = +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 δ 66 Zn. 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 δ 66 Zn 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 δ 66 Zn 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 (H2 O) 5+ ] or [ZnCO3 (H2 O)3 ]). Our models further demonstrate that Zn complexation with reduced carbon species cannot produce fluids with heavy δ 66 Zn signature and hence explain the δ 66 Zn variations observed in the Chl-harzburgites. The most straightforward explanation for the heavy δ 66 Zn 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.