Molybdenum and boron isotopic evidence for carbon-recycling via carbonate dissolution in subduction zones

Abstract

Subduction zones are critical sites for carbon cycling between Earth’s surface and interior. However, how subducted carbon is released and transferred to the surface is not well understood, especially regarding the role of slab-derived fluids in the deep carbon cycle. Here we report Mo and B isotopic data for the Silurian normal arc andesites and adakitic andesites from the Chinese North Tianshan, which represent partial melts of fluid-modified mantle wedge and dehydrated oceanic crust, respectively. The normal arc andesites yielded δ98Mo values (0.33–1.08‰) significantly higher than that (about –0.20‰) of the depleted mantle. Because their limited range of SiO2 (53.8–55.3 wt.%) precludes differentiation as a cause for their variable δ98Mo values and Mo isotopic fractionation solely by fluid mobilization is limited (≤0.3‰), the elevated δ98Mo values could be ascribed to the incorporation of crustal material with heavy Mo isotopes in the mantle source. Since marine carbonate is featured by both heavy Mo and B isotopes and our normal arc andesites also give heavy δ11B (−1.63 to +4.00‰) values, we consider that marine carbonate was possibly involved as a component of the subducted slab, which modified Mo–B isotopic compositions of the mantle source. The positive correlations between δ98Mo and δ11B and between δ98Mo and Ba/Rb suggest transport of subducted carbonates to the overlying mantle wedge via slab fluids, thus providing robust evidence for transfer of subducted carbon to the overriding plate by carbonate dissolution. In contrast, the younger adakitic andesites have light δ98Mo (−0.48 to −0.27‰) and δ11B (−9.43 to −2.05‰) values, implying an isotopically Mo- and B-light source. Given the preferential transport of heavy 98Mo and 11B to the fluid phase during slab dehydration, their remarkably light δ98Mo and δ11B values support a dehydrated oceanic crust as their magma source. The contrasting Mo–B isotopes for such two kinds of andesites highlight that most carbonates can be removed from the subducted slab to the overriding plate during oceanic subduction.