In-situ trace elements and Li and Sr isotopes in peridotite xenoliths from Kuandian, North China Craton: Insights into Pacific slab subduction-related mantle modification

Abstract

The trace element, Li and Sr isotopic compositions of major minerals in peridotite xenoliths from Kuandian, North China Craton (NCC) were analyzed in situ to investigate Pacific slab subduction-related mantle modification beneath the eastern NCC. The 87Sr/86Sr ratios (0.70282–0.70553) are positively correlated with the La/Nb and U/Nb ratios for clinopyroxenes (Cpx). Based on the trace element distribution patterns, three types of Cpx were identified. Type 1 Cpx are characterized by significant Nb–Ta–Ti depletions and the highest Sr isotopic ratios (up to 0.70553); Type 2 Cpx display Nb (or Ta) depletions and highly variable Nb and Ta fractionation; and Type 3 Cpx show no significant Nb or Ta depletion or even weak Ta enrichment and have the lowest Sr isotopic ratios (0.70282–0.70323). Some Cpx display a pattern of increasing Ca+Mg±Si from the cores to the rims, indicating addition of Ca+Mg±Si-rich fluids derived from a serpentinized peridotite layer above the subducting slab. These features indicate an ancient mantle metasomatic event associated with the subduction of the altered Pacific oceanic crust, which may contribute substantially to the destruction of the eastern NCC.The variations in the Li contents and isotopic compositions of major minerals in the Kuandian peridotite xenoliths encode both the melt/fluid metasomatism and the cooling process. The Li contents in the Cpx (up to 34.8ppm) and orthopyroxenes (Opx) (up to 28.0ppm) are typically higher than those in the coexisting olivines (Ol) (<9.19ppm), suggesting silicate melt metasomatism. Considering the rapid diffusion rate of Li in minerals at mantle temperature, the Li enrichment and low δ7Li values in the rims of most Ol, Cpx and Opx may have arisen from diffusive fractionation due to faster diffusion of 6Li compared to 7Li during a latest Li-rich melt/fluid metasomatic process shortly before the host magma eruption. On the other hand, a few Ol grains show decreased Li contents and higher δ7Li values from the cores to the rims. This could imply Li isotope fractionation caused by cooling-induced Li redistribution during and/or after the host magma eruption, because DLiOl/Cpx decreases significantly with decreasing temperature and 6Li diffuses faster than 7Li.