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It is generally agreed that the Earth's mantle is chemically and isotopically heterogeneous. Many studies have shown that mantle heterogeneities may be a result of subduction, which introduces substantial quantities of oceanic crust along with compositionally heterogeneous sediment into the mantle. Therefore, understanding the origin of the geochemical heterogeneities in the mantle requires detailed knowledge of elemental partitioning and isotopic fractionation during material transfer from subduction zones to the mantle. Here we present lithium, oxygen, strontium, and neodymium isotope data showing a notable negative correlation between δ7Li and 87Sr/86Sr(T=240 Ma) ratios in fresh eclogites from the Chinese Continental Scientific Drilling (CCSD) Program, located in the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt in East China. Such co-variations of stable and radiogenic isotopes in rocks that have been subducted to depths of ∼ 200 km neither can result from diffusion, nor be inherited from protoliths. Rather, co-variations are likely to be due to progressive dehydration and mineral transformation during subduction, providing the first evidence for bulk 87Sr/86Sr(T) ratios that have been significantly modified due to subduction. Besides altered oceanic crust and sediments, selective mobilization of Sr with elevated 87Sr/86Sr ratios provides a third mechanism for causing Sr-isotope heterogeneity in the upper mantle. This finding can be applied for the interpretation of the Sr-isotope paradox that, although recycling oceanic crust has relatively low Rb/Sr ratios due to preferential removal of Rb to Sr during subduction dehydration, HIMU (high μ = 238U/204Pb; resulted from ancient subducted oceanic crust) has equivalent or even slightly higher 87Sr/86Sr ratios relative to present-day MORB. Moreover, the results show that lavas with moderately high 87Sr/86Sr ratios relative to MORB (similar to enriched mantle EM2) can be products of the recycling of subducted unaltered oceanic crust.
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