Large Lithium Isotopic Variations in Minerals from Peridotite Xenoliths from the Eastern North China Craton

Abstract : To investigate the effects of melt-rock interaction on Li isotope fractionation, we report in situ Li concentrations and δ7Li 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 from Cpx, 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 (−10.3) 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 diffusion-driven 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 (−10.3∼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 normal mantle. Olivines in Cpx-rich lherzolites have lower Fo contents (83.8–87.5), higher Li concentrations (1.4∼4.5 ppm), and heavier δ7Li (5.0∼22.0) than those in lherzolites and normal mantle. The δ7LiOl value correlates positively with Li concentration and negatively with Fo from lherzolites to Cpx-rich lherzolites, indicating a reaction between lherzolites and melts with 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.
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Submitted on : Wednesday, April 18, 2018 - 4:32:44 PM
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Yan Xiao, Hong-Fu Zhang, Etienne Deloule, Ben-Xun Su, Yan Tang, et al.. Large Lithium Isotopic Variations in Minerals from Peridotite Xenoliths from the Eastern North China Craton. Journal of Geology, University of Chicago Press, 2015, 123 (1), pp.79 - 94. ⟨10.1086/680222⟩. ⟨hal-01770181⟩

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