Impact of residual zircon on Nd-Hf isotope decoupling during sediment recycling in subduction zone

The subduction factories in convergent plate margins exert crucial control on recycling terrestrial components and returning to the overlying crust. The Nd and Hf isotopic systems provide potential tracers to evaluate these processes. Here we present a case where these isotopic systems are decoupled in a suite of granites from the Chinese Altai, showing a wide range of ε_Hf(t) values (from -4.7 to +10.8) in contrast to a limited range of εNd(t) values (from -5.8 to -1.9). The zircon xenocrysts occurring frequently in these rocks show markedly negative ε_Hf(t) values (from -34.3 to -6.5) and positive δ⁷Li values (from +12.5 to +18.2). We propose a model to explain the observed relationship between residual zircon and Nd-Hf isotope decoupling. We suggest that the Altai granites originated from partial melting of subducted slab components under relatively low temperature conditions which aided the residual zircon from oceanic sediments to inherit and retain a significant amount of ¹⁷⁷Hf in the source, thereby elevating the ¹⁷⁶Hf/¹⁷⁷Hf ratio of the melt, and decoupling from the ¹⁴³Nd/¹⁴⁴Nd ratio during the subsequent magmatic processes. Our study illustrates a case where sediment recycling in subduction zone contributes to decoupling of Nd and Hf isotopic systems, with former providing a more reliable estimate of the source characteristics of granitic magmas.