Carbonate-silicate interaction in subducting slabs recorded by Zn isotopes in western Alps metasediments

Abstract

Carbonate-silicate interaction within subduction channels influences the chemical and physical properties of subducting carbon and is crucial to understanding the fate of carbonates in subduction zones. However, geochemical evidence for such interaction is still limited. Here we investigate zinc isotopes (expressed as δ66ZnJMC-Lyon) to decipher potential carbonate-silicate interaction in subducting slabs given the remarkable Zn isotopic difference between silicate and carbonate components. A suite of high- to ultrahigh-pressure metasediments containing variable amounts of carbonates from the Schistes Lustrés nappe (western Alps) record subduction processes to various depths (∼15–90 km) in a cool geothermal condition (∼8°C/km). Despite a broad range of metamorphic grades (0.8–2.9 GPa and 300–630°C) and carbonate contents (0–49 wt%), the bulk metasediments have nearly constant δ66Zn values (av. 0.21±0.05‰, 2sd, n=12), which are indistinguishable from their putative protoliths (0.25±0.07‰, 2sd, n=5). The lack of systematic variation of bulk sediment δ66Zn with metamorphic grades implies a limited transfer of Zn during prograde metamorphism. Therefore, sediments will preserve their original Zn isotopic compositions after subduction into the mantle. By contrast, the carbonate components in sediments obtained by leaching exhibit a progressive decline in δ66Zn from 0.82±0.01‰ to 0.20±0.04‰ and a modest increase in Zn concentration with pressure. These variations are best interpreted as a result of prolonged closed-system Zn isotopic exchange between isotopically heavy carbonates and light silicate components in the subducting slab. Our results demonstrate that a considerable amount of carbonates in subducting sediments can be retained at least down to the depths of the sub-arc mantle (>90 km). The strong pressure-dependent Zn isotopic fractionation (R2=0.87) during carbonate-silicate interaction makes Zn isotopes a novel proxy for the storage depth of subducting carbonates.