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 δ 82/76 Se NIST3149 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 δ 82/76 Se NIST3149. 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.

Supplemental Material: Extreme fractionation of selenium isotopes and possible deep biospheric origin of platinum nuggets from Minas Gerais, Brazil

“…Measurements of the inter-laboratory standard solution MH-495 in four different analytical sessions over a period of more than 6 months yielded an average value of δ 82/76 Se of -3.23 ± 0.15 ‰ (2 SD, N = 23, 30 ng mL −1 solutions, Table S3), in agreement with previous studies (Kurzawa et al, 2017(Kurzawa et al, , 2019Labidi et al, 2018;Yierpan et al, 2018Yierpan et al, , 2019, and our long-term reproducibility in our lab over 5 years. International rock reference material USGS BCR-2 was repeatedly digested and analyzed during the course of this study, and gave δ 82/76 Se of 0.14 ± 0.15 ‰ (2 SD, N = 4) and Se concentration of 78 ng g ˗1 , respectively (Table S4, see also König et al, 2021). This is in agreement with previously published Se concentration and isotope composition (Kurzawa et al, 2017;Yierpan et al, 2018Yierpan et al, , 2019.…”

Slab fluid redox potential of subducted metasomatized heterogeneous oceanic crust: Insights from Sulfur–Selenium systematics in metarondingites

“…Subsequently, recrystallizing sulfides within the metamorphic assemblage scavenge reduced, isotopically lighter Se. As such, redistributed Se with a large isotopic range traces repeated cycles of sulfide reworking within the subducted crust by episodic, highly oxidized fluid pulses from underlying slab serpentinite [2]. Our preliminary results in Almirez show that metarodingites and associated metamorphic rocks are strongly depleted in Se-S relative to eclogites and MORBs, while reaction rims are commonly enriched in these elements.…”

Extreme fractionation of selenium isotopes and possible deep biospheric origin of platinum nuggets from Minas Gerais, Brazil

“…2A; Stüeken, 2017;and crustal 187 Os/ 188 Os ratios between 0.90 and 1.30; Table S1), the highly negative δ 82/76 Se values in the nuggets require a large isotopic fractionation process and a small extent of Se-oxyanion reduction-e.g., <30%, for which δ 82/76 Se values are still low. Larger extents of Se-oxyanion reduction would have tended to drive the aqueous fluid to higher δ 82/76 Se values, which in turn would have led the Se 0 precipitated in the nuggets to higher and likely more variable δ 82/76 Se values similar to those found in metamorphic sulfides (König et al, 2021). As for hydrothermal sulfides with δ 82/76 Se values of ∼−3‰ (König et al, 2019), even the lowest δ 82/76 Se values may also be explained by abiogenic reduction of high Se-oxyanion concentrations.…”