Multistage melt/fluid-peridotite interactions in the refertilized lithospheric mantle beneath the North China Craton: constraints from the Li-Sr-Nd isotopic disequilibrium between minerals of peridotite xenoliths

Elemental and Li-Sr-Nd isotopic data of minerals in spinel peridotites hosted by Cenozoic basalts allow us to refine the existing models for Li isotopic fractionation in mantle peridotites and constrain the melt/fluid-peridotite interaction in the lithospheric mantle beneath the North China Craton. Highly elevated Li concentrations in cpx (up to 24 ppm) relative to coexisting opx and olivine (< 4 ppm) indicate that the peridotites experienced metasomatism by mafic silicate melts and/or fluids. The mineral delta(7)Li vary greatly, with olivine (+0.7 to +5.4aEuro degrees) being isotopically heavier than coexisting opx (-4.4 to -25.9aEuro degrees) and cpx (-3.3 to -21.4aEuro degrees) in most samples. The delta(7)Li in pyroxenes are considerably lower than the normal mantle values and show negative correlation with their Li abundances, likely due to recent Li ingress attended by diffusive fractionation of Li isotopes. Two exceptional samples have olivine delta(7)Li of -3.0 and -7.9aEuro degrees, indicating the existence of low delta(7)Li domains in the mantle, which could be transient and generated by meter-scale diffusion of Li during melt/fluid-peridotite interaction. The (143)Nd/(144)Nd (0.5123-0.5139) and (87)Sr/(86)Sr (0.7018-0.7062) in the pyroxenes also show a large variation, in which the cpx are apparently lower in (87)Sr/(86)Sr and slightly higher in (143)Nd/(144)Nd than coexisting opx, implying an intermineral Sr-Nd isotopic disequilibrium. This is observed more apparently in peridotites having low (87)Sr/(86)Sr and high (143)Nd/(144)Nd ratios than in those with high (87)Sr/(86)Sr and low (143)Nd/(144)Nd, suggesting that a relatively recent interaction existed between an ancient metasomatized lithospheric mantle and asthenospheric melt, which transformed the refractory peridotites with highly radiogenic Sr and unradiogenic Nd isotopic compositions to the fertile lherzolites with unradiogenic Sr and radiogenic Nd isotopic compositions. Therefore, we argue that the lithospheric mantle represented by the peridotites has been heterogeneously refertilized by multistage melt/fluid-peridotite interactions.