Partial melting control of lithium concentrations and isotopes in the Cenozoic lithospheric mantle beneath Jiande area, the Cathaysia block of SE China

To investigate whether Li isotopic fractionation occurred during high temperature partial melting in the mantle, major and trace element, and Li isotopes were measured in minerals (olivine, Opx, and Cpx) from lherzolite xenoliths entrained in the Jiande Cenozoic basalts in the Cathaysia block. These lherzolites show correlations between Al2O3 and Mg# in Opx and between Fo and Cr# in spinel, indicating they experienced variable degrees of partial melting. Most of them show LREE-depleted patterns in Cpx, indicating little imprint from subsequent incipient metasomatism. The minerals in these lherzolites have narrow Li concentrations (olivine: 1.81-2.82 ppm; Cpx: 0.85-1.85 ppm; Opx: 0.45-1.44 ppm) but variable Li isotopic compositions (olivine: 2.06-10.00‰; Cpx: − 5.05-3.58‰; Opx: − 4.11-9.18‰). The relative concentrations among the different mineral phases in the Jiande lherzolites (LiOl > LiCpx ≥ LiOpx) are similar to those found in “equilibrated” mantle peridotites. The Li concentrations of olivine are a bit higher than the range of normal mantle (1.0-2.2 ppm). The negative correlations between indices of partial melting (Fo content and Cr# in spinel) and Li concentrations of olivine also implies that partial melting was the main process controlling the variations of olivine Li concentration and it generated depletion in Li concentration of < 1 ppm. In addition, the negative correlations between indices of partial melting and δ7Li values in olivine also occur. Thus, the observed variable δ7Li values in olivine could not have been produced by recent diffusive-driven isotopic fractionation, but potentially be explained through variable degree of partial melting of their source. Therefore, partial melting process could account for small but systematic Li isotopic fractionation in the mantle. In contrast to olivine, δ7Li values of Cpx and Opx in the Jiande lherzolites are a bit variable. They display isotopically heavy cores and light rims, suggesting a recent diffusive ingress of Li into a crystal from the entraining magma or during related intrusive magmatism. The restricted Li concentration in pyroxenes relative to isotope zonation may suggest faster isotopic self-diffusion relative to elemental diffusion.