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

Baptiste Debret (1) , Carlos J Garrido , Marie-Laure Pons (2) , Pierre Bouilhol , Edward Inglis , Vicente López Sánchez-Vizcaíno , Helen Williams

1 IPGP (UMR_7154) - Institut de Physique du Globe de Paris
2 CEREGE - Centre européen de recherche et d'enseignement des géosciences de l'environnement

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 serpentinitederived fluids by using their stable isotopes (delta(56) Fe and delta Zn-66) in high-pressure subducted meta-serpentinites from the Cerro del Almirez massif (Spain). This massif preserves a metamorphic front between antigorite (Atg-serpentinite) and antigoriteolivine-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 (1.61.9 GPa and 680-710 degrees C). The mean delta Fe-56 of all the Cerro del Almirez meta-serpentinites (+0.05 +/- 0.01 parts per thousand) is identical within an error to that of primitive mantle (+0.03 +/- 0.03 parts per thousand). A positive correlation between delta Fe-56 and indices of peridotite protolith fertility (e.g., Al2O3/SiO2) suggests that the delta Fe-56 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] = 3467 ppm) and isotope signatures (delta Zn-66 = +0.18 - +0.55 parts per thousand) of the Cerro del Almirez samples show a broad range of values, distinct to those of the primitive mantle ([Zn] = 54 ppm; delta Zn-66 = +0.16 +/- 0.06 parts per thousand). The Atg-serpentinites ([Zn] = 34-46 ppm; delta Zn-66 = +0.23 +/- 0.06 parts per thousand) display similar [Zn] and delta(66) Zn values to those of slab serpentinites from other high-pressure metaophiolites. Both [Zn] and delta Zn-66 increase in transitional lithologies ([Zn] = 45-67 ppm; delta Zn-66 = +0.30 +/- 0.06 parts per thousand) and Chlharzburgites with granofels ([Zn] = 38-59 ppm; delta Zn-66 = +0.33 +/- 0.04 parts per thousand) or spinifex ([Zn] = 48-66 ppm; delta Zn-66 = +0.43 +/- 0.09 parts per thousand) 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 delta Zn-66. 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 delta(66) Zn from Atg-serpentinite to Chlharzburgite is associated with an increase in U/Yb, Sr/Y, Ba/Ce and Rb/Ce, suggesting that both [Zn] and delta(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(H2O)(5+)] or [ZnCO3(H2O)(3)]). Our models further demonstrate that Zn complexation with reduced carbon species cannot produce fluids with heavy delta Zn-66 signature and hence explain the delta Zn-66 variations observed in the Chl-harzburgites. The most straightforward explanation for the heavy delta Zn-66 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.

Mots clés: Subduction; Serpentinite; Fe and Zn stable isotopes; Carbon cycle

Domaines: Sciences de la Terre

Baptiste Debret, Carlos J Garrido, Marie-Laure Pons, Pierre Bouilhol, Edward Inglis, et al.. Iron and zinc stable isotope evidence for open-system high-pressure dehydration of antigorite serpentinite in subduction zones. Geochimica et Cosmochimica Acta, 2021, 296, pp.210-225. ⟨10.1016/j.gca.2020.12.001⟩. ⟨hal-03065997⟩