Origins of non-equilibrium lithium isotopic fractionation in xenolithic peridotite minerals: Examples from Tanzania

Abstract

Olivine, clinopyroxene and orthopyroxene in variably metasomatised peridotite xenoliths from three lithospheric mantle sections beneath the East African Rift in Tanzania (Lashaine, Olmani, Labait) show systematic differences in their average Li concentrations (2.4 ppm, 2.0 ppm and 1.5 ppm, respectively) and intermineral isotopic fractionations, with olivine being heaviest (δ 7Li = + 2.3 to + 13.9‰, average + 5.0‰), followed by orthopyroxene (− 4.1 to + 6.5‰, average + 0.8‰) and clinopyroxene (− 6.7 to + 4.1‰, average − 1.6‰). These features are ascribed to the effects of kinetic Li isotope fractionation combined with different Li diffusivities in mantle minerals. Two main mechanisms likely generate diffusion-driven kinetic Li isotope fractionation in mantle xenoliths (1) Li diffusion from grain boundary melt into minerals during recent metasomatism or entrainment in the host magma and (2) subsolidus intermineral Li-redistribution. The latter can produce both isotopically light (Li-addition) and heavy (Li-loss) minerals and may occur in response to changes in pressure and/or temperature. Modelling shows that non-mantle-like δ 7Li in clinopyroxene (< + 2‰), combined with apparent equilibrium olivine-clinopyroxene elemental partitioning in most peridotite xenoliths from all three Tanzanian localities probably reflects incipient Li addition during interaction with the host magma. Low δ 7Li (< − 3‰), combined with high Li concentrations (> 3 ppm) in some clinopyroxene may require very recent (minutes) Li ingress from a Li-rich melt (100s of ppm) having mantle-like δ 7Li. This might happen during late fragmentation of some mantle xenoliths caused by a volatile- (and Li-) rich component exsolved from the host basalt. In contrast, high Li concentrations (> 2 ppm) and δ 7Li (> 4‰) in olivine from many Labait and Olmani samples are attributed to an older, pre-entrainment enrichment event during which isotopic equilibrium was attained and whose signature was not corrupted during xenolith entrainment. Low Li concentrations and mantle-like isotopic composition of olivine from most Lashaine xenoliths indicate limited metasomatic Li addition. Thus, Li concentrations and isotope compositions of mantle peridotites worldwide may reflect two processes, with olivine mainly preserving a signature of depletion in refractory samples (low Li contents and δ 7Li) or of older (precursory) melt addition in metasomatised samples (high Li contents and δ 7Li), while non mantle-like, low δ 7Li in almost all clinopyroxene can be due to Li ingress during transport in the host magma and/or slow cooling, if the samples were erupted in lavas. In Tanzania, the peridotites experienced rift-related heating prior to entrainment and were quenched upon eruption, so Li ingress is the most likely process responsible for the isotopically light clinopyroxene here.