Tracing carbonate dissolution in subducting sediments by zinc and magnesium isotopes

Abstract

Carbonate dissolution in subduction zones transfers surface carbon to the mantle and plays an important role in regulating global carbon cycles through geologic time. Petrological observations and thermodynamic models predict substantial dissolution of subducting carbonates induced by slab-derived fluids at arc mantle depths, but this dissolution process has rarely been verified by existing geochemical tracers. In this paper we show that zinc and magnesium isotopes can serve as such tracers through a comparative study on carbonate-bearing and carbonate-free sediments from the South China Sea and the Philippine Sea that are going to be subducted. Leaching experiments are further performed to characterize the isotopic difference between dissolved carbonates and silicate components in sediments. Bulk carbonate-bearing sediments and carbonate-free sediments surprisingly have indistinguishable δ66Zn (0.26 ± 0.04‰ versus 0.25 ± 0.04‰), but the former display systematically lower δ26Mg (−0.85‰ to −0.17‰, n = 10) than the latter (−0.26‰ to +0.13‰, n = 13). Leaching experiments show that dissolved carbonates have much higher δ66Zn (0.82 ± 0.26‰) and lower δ26Mg (−2.70 ± 0.35‰) compared with the silicate residues (δ66Zn = 0.21 ± 0.06‰; δ26Mg = −0.06 ± 0.14‰). These results indicate that the presence of carbonate components has insignificant influence on Zn isotopic compositions of bulk sediments but prominent influence on their Mg isotopic compositions. In this respect, the final fate of carbonates in subducting sediments can be tracked by a combined Mg and Zn isotope analysis on mantle-derived magmas. For instance, bulk carbonate-bearing sediment addition (i.e., no carbonate dissolution) during subduction would produce low δ26Mg but normal δ66Zn values (i.e., Mg-Zn isotopic “decoupling”) in mantle-derived magmas. By contrast, if carbonate dissolution occurs during subduction, the high δ66Zn and low δ26Mg (i.e., Mg-Zn isotopic “coupling”) signals of dissolved carbonates will be transferred to the sources of mantle-derived magmas. We test this application for a Cenozoic subduction-related, arc-like volcanic exposure from Tengchong, Southwestern China and find significant Mg-Zn isotopic coupling, which demonstrates carbonate dissolution during Indian Oceanic slab subduction. Our results also shed constraints on the amount of dissolved carbonates in the sub-arc mantle. Available data reveal the absence of high δ66Zn and low δ26Mg in global arc lavas, for which we suggest to be the result of a relatively low proportion (<20%) of dissolved calcium carbonates in the sub-arc mantle.