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

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 delta Mo-98 values (0.33-1.08 parts per thousand) significantly higher than that (about -0.20 parts per thousand of the depleted mantle. Because their limited range of SiO2 (53.8-55.3 wt.%) precludes differentiation as a cause for their variable delta Mo-98 values and Mo isotopic fractionation solely by fluid mobilization is limited (<= 0.3 parts per thousand), the elevated delta Mo-98 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 delta B-11 (-1.63 to +4.00 parts per thousand) 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 delta Mo-98 and delta B-11 and between delta Mo-98 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 delta Mo-98 (-0.48 to -0.27 parts per thousand) and delta B-11 (-9.43 to -2.05 parts per thousand) values, implying an isotopically Mo- and B-light source. Given the preferential transport of heavy Mo-98 and B-11 to the fluid phase during slab dehydration, their remarkably light delta Mo-98 and delta B-11 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. (C) 2019 Elsevier Ltd. All rights reserved.