Rapid Recycling of Subducted Sediments in the Subcontinental Lithospheric Mantle

Subduction recycling of sediments plays a key role in the geochemical evolution of Earth. The presence of recycled terrigenous sediments in upwelling plumes has been cited to explain the EM2 signature in ocean island volcanics, characterized by particularly high Sr-87/Sr-86 (>0.706). However, the origin of such isotopic anomalies in continental regions and the role of subducted sediments in the subcontinental lithospheric mantle (SCLM) remain unclear. The Himalaya-Tibet orogen is one of the world's best places for deciphering continental subduction and the fate of subducted crustal materials in the mantle. Here we present a systematic study of the geochronology, mineral chemistry (especially clinopyroxene), whole-rock chemistry and Sr-Nd-Pb-Hf-O isotopic compositions of Cenozoic potassic-ultrapotassic lavas from the western Kunlun area of northwestern Tibet. New secondary ion mass spectrometry (SIMS) zircon U-Pb dating, coupled with published age results, constrain the timing of volcanism from similar to 8.3 Ma to the present. These lavas show geochemical characteristics that closely resemble the EM2 mantle end-member represented by the Samoan hotspot. Both whole rocks and individual magmatic clinopyroxenes display arc-like trace-element patterns and remarkably enriched Sr-Nd-Pb-Hf isotope compositions (Sr-87/Sr-86 <= 0.7080; epsilon(Nd) <= -4.8; Pb-206/Pb-204 >= 18.704; epsilon(Hf) <=-2.6). Together with high zircon Delta O-18 values (6.3-10.4 & PTSTHOUSND;), the data point to a mantle source enriched by recycled sedimentary materials. Geochemical modeling and geophysical evidence further indicate that the sediments were directly derived from the subducted Indian continental lithosphere during India-Eurasia collision. Partial melting models assuming a hybridized mantle source that contains similar to 5% Indian continental crust suggest that the primary melts of the potassic-ultrapotassic lavas could be formed by melting of a phlogopite-bearing garnet lherzolite at low melting degrees (1-5%). The magma geochemistry is consistent with the model of melange melting, implying that the subducted sediments may detach from the downgoing Indian slab and rise up diapirically into the overlying mantle lithosphere. Unlike traditional models of subducted sediments entering the deep mantle, the western Kunlun EM2-like lavas reveal that subducted sediments can be rapidly recycled into the SCLM during continental subduction (probably <50 Myr). We suggest that the SCLM could be an important reservoir for subducted sediments. The findings are important to our understanding of mantle circulation rates and chemical heterogeneities.