Large lithium isotopic variations in minerals from peridotite xenoliths from the eastern north China craton

To investigate the effects of melt-rock interaction on Li isotope fractionation, we report in situ Li concentrations and d7Li of olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx) for six peridotite xenoliths from the eastern North China Craton. These xenoliths contain two lherzolites, two Cpx-rich lherzolites, and two wehrlites and are variably metasomatized. Lithium isotope zonation is observed in most peridotite minerals. The majority of Cpx grains display isotopically light cores with lower Li concentrations than the heavy rims. However, the Opx grains show a different style of zonation fromCpx, where higher Li concentrations in the cores are associated with much lighter δ⁷Li. Olivines in most peridotites have a restricted range of Li concentration and δ⁷Li within individual grains, whereas the olivines in a lherzolite show isotopically light cores (210.3) with high Li concentrations (2.3 ppm) and heavy rims (5.5) with low Li concentrations (1.7 ppm). These Li isotopic variations in mineral phases may reflect the combined effects of diffusiondriven kinetic fractionation of Li isotopes during melt/fluid-peridotite interactions and slow cooling. Intersample heterogeneity of Li isotopes is also apparent. Olivine with forsterite (Fo) content of 91.3 in one lherzolite sample has “normal” mantle-like Li concentrations (1.1∼2.4 ppm) and light δ⁷Li (210.3∼5.5), while Ol with Fo content of 89.7 in another lherzolite has slightly high Li concentrations (2.0∼3.0 ppm) but similar δ⁷Li (1.6∼6.4) relative to normalmantle. Olivines in Cpx-rich lherzolites have lower Fo contents (83.8–87.5), higher Li concentrations (1.4∼4.5 ppm), and heavier d7Li (5.0∼22.0) than those in lherzolites and normalmantle. The δ⁷Li_Ol value correlates positively with Li concentration and negativelywith Fo fromlherzolites toCpx-rich lherzolites, indicating a reaction between lherzolites andmeltswith isotopically heavy Li- and Fe-rich signatures. By contrast, olivines in wehrlites have extremely lower Fo contents (82.2∼83.2) and higher Li concentrations (2.4∼4.2 ppm) than those in normal mantle, while their δ⁷Li values are within the range of normal mantle, reflecting metasomatism of the peridotites by asthenospheric melt. Overall, the large intragrain and intersample variations in Li concentrations and isotopic compositions reflect kinetic isotope fractionation during multiple metasomatisms.