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Subduction transfers surface carbon into the Earth's interior in a main form of carbonates that influences the global carbon cycles and surface climate through geologic time. Nevertheless, whether the fate of downgoing carbonates significantly varies in past subduction zones is rarely constrained by natural observations. Marine carbonates have remarkably higher zinc isotopic ratios (expressed as δ66Zn JMC-Lyon) relative to the mantle (0.99 ± 0.24‰ vs. 0.18 ± 0.05‰), making zinc isotopes a sensitive tracer for subducting carbonates. Here we examine this issue through a comparative zinc isotope study on basalts across the North-South Gravity Lineament (NSGL) in East Asia that were genetically related to two different oceanic slabs. Together with existing data, we show that all basalts in the east of the NSGL have high δ66Zn (∼0.3–0.6‰; n = 134) that do not vary with distances to the trench and are spatially coupled with the horizontally stagnated slab in the transition zone (410 – 660 km). This indicates that subducting carbonates survived shallow dissolution and were deeply buried during westward subduction of the Paleo-Pacific slab. By contrast, basalts in the west of the NSGL display a gradual decline of δ66Zn from 0.50 ± 0.04‰ to 0.28 ± 0.03‰ (n = 35) with increasing distances to the trench. No known magmatic processes (e.g., partial melting, crystal-melt differentiation, melt-peridotite interaction, and degassing) can account for the spatial Zn isotopic variation. The role of slab-derived sulfate rich fluids is also excluded because of the mantle-like Cu isotopic compositions of these basalts. Instead, the gradual decrease of δ66Zn, together with the coupled decline of CaO/Al 2 O 3 , are best explained as the diminished amounts of dissolved carbonates in their mantle sources. Thus, substantial carbonate dissolution must have occurred during southeastward subduction of the Paleo-Asian slab, which prevents deep burial of subducting carbon. The main differences between the two large slabs include: (i) the Paleo-Asian slab has an extended longevity (∼1.1 Ga) and slow spreading rate in comparison with the Paleo-Pacific slab, leading to the main incorporation of carbonate minerals into the altered oceanic crust, and (ii) the younger Paleo-Pacific slab contains abundant deep-sea Mg-rich carbonates that were not sufficiently dissolved at shallow depths. These differences demonstrate that subduction of different oceanic slabs can lead to contrasting fates of subducting carbon in ancient subduction zones, depending on the contents and species of carbonate sediments in the oceanic crust. [ABSTRACT FROM AUTHOR]

It has been suggested that the carbonated mantle reflected by Mg−Zn isotopic anomalies of Cenozoic intraplate basalts from East Asia coincides with the stagnant West Pacific slab in the mantle transition zone. However, the northern boundary of such carbonated domain beneath East Asia is uncertain. Late Mesozoic-Cenozoic intraplate basalts are widespread in far eastern Russia and thus provide an opportunity to examine this issue. Here we report major-trace element contents and Sr−Nd-−Mg−Zn isotopic compositions for 9 Late Mesozoic-Cenozoic basaltic samples from the Khanka Block and Sikhote-Alin accretionary complex. They are characterized by large variations in SiO2 contents (41 wt.% to 50 wt.%) and CaO/Al2O3 (0.50 to 0.97), enrichments of large-ion lithophile elements (LILE), positive Nb−Ta anomalies and strongly negative K, Pb, Zr, Hf, Ti, Y anomalies in primitive mantle-normalized trace element spider diagram. Furthermore, the rocks show good correlations of Ti/Ti* with Hf/Hf*, La/Yb, Fe/Mn and trace element contents (e. g., Nb). In addition, they have lighter Mg and heavier Zn isotope compositions than the BSE estimates, coupled with depleted Sr−Nd isotope compositions. These elemental and isotopic characteristics cannot be explained by alteration, magma differentiation or diffusion, but are consistent with the partial melting of carbonated peridotite. By and large, the Late Mesozoic-Cenozoic basalts from far eastern Russia bear very similar geochemical characteristics as those Na-series Cenozoic basalts from eastern China. The extended region of Mg−Zn isotopic anomalies is roughly coincident with the stagnant West Pacific slab beneath East Asia, and all of these alkali basalts can be generated from mantle sources hybridized by recycled Mg-carbonates from the Pacific slab stagnant in the mantle transition zone. We infer that (1) the carbonated big mantle wedge extends to the NE edge of the West Pacific slab and may have also appeared in the Late Mesozoic due to the effect of the Paleo-Pacific slab beneath this region, and (2) decarbonation of stagnant slabs in the mantle transition zone is a key mechanism for carbon outgassing from deep mantle to surface via intraplate alkali melts. [ABSTRACT FROM AUTHOR]