Lithium isotopic systematics of peridotite xenoliths from Hannuoba, North China Craton: Implications for melt–rock interaction in the considerably thinned lithospheric mantle

Abstract

Li concentrations and isotopic compositions of coexisting minerals (ol, opx, and cpx) from peridotite xenoliths entrained in the Hannuoba Tertiary basalts, North China Craton, provide insight into Li isotopic fractionation between mantle minerals during melt–rock interaction in the considerably thinned lithospheric mantle. Bulk analyses of mineral separates show significant enrichment of Li in cpx (2.4–3.6 ppm) relative to olivine (1.2–1.8 ppm), indicating that these peridotites have been affected by mantle metasomatism with mafic silicate melts. Bulk olivine separates (δ 7Li ∼ +3.3‰ to +6.4‰) are isotopically heavier than coexisting pyroxenes (δ 7Li ∼ −3.3‰ to −8.2‰ in cpx, and −4.0‰ to −6.7‰ in opx). Such large variation suggests Li elemental and isotopic disequilibrium. This conclusion is supported by results from in situ SIMS analyses of mineral grains where significant Li elemental and isotopic zonations exist. The olivine and opx have lower Li concentrations and heavier Li isotopes in the rims than in the cores. This reverse correlation of δ 7Li with Li concentrations indicates diffusive fractionation of Li isotopes. However, the zoning patterns in coexisting cpx show isotopically heavier rims with higher Li abundances. This positive correlation between δ 7Li and Li concentrations suggests a melt mixing trend. We attribute Li concentration and isotope zonation in minerals to the effects of two-stage diffusive fractionation coupled with melt–rock interaction. The earliest melts may have been derived from the subducted oceanic slab with low δ 7Li values produced by isotopic fractionation during the dehydration of the seawater-altered slab. Melts at later stages were derived from the asthenosphere and interacted with the peridotites, producing the Li elemental and isotopic zoning in mineral grains. These data thus provide evidence for multiple-stage peridotite–melt interaction in the lithospheric mantle beneath the northern North China Craton.