Selenium isotope evidence for pulsed flow of oxidative slab fluids

Abstract

Isotope systematics of the redox sensitive and chalcophile element selenium (Se) were investigated on exhumed parts of subducted oceanic lithosphere to provide new constraints on slab dehydration conditions during subduction. The samples show increasing δ82/76SeNIST3149 with higher abundances of fluid mobile elements, comprising a larger range (−1.89 to þ0.48 %) than that of mantle (−0.13 ± 0.12 %) and altered ocean crust (−0.35 to −0.07 %). Our data point to pronounced, local scale redox variations within the subducting crust, wherein oxidative fluids dissolve sulfides and mobilise oxidised Se species. Subsequently recrystallising sulfides preferentially incorporate isotopically lighter, reduced Se, which shifts evolving fluids and late stage sulfides to higher δ82/76SeNIST3149. Redistribution of Se by repeated cycles of sulfide reworking within the subducted crust can be reconciled with episodes of oxidised fluid pulses from underlying slab mantle in modern subduction zones.