Li diffusion and isotopic fractionation in olivines crystals

Abstract

In order to constrain the behavior of lithium isotopes in crystals during magma cooling, the study of a fresh pillow•lava with large olivine crystals has been undertaken. An ion-microprobe was used to measure lithium abundances and isotopic compositions both in olivine phenocrysts and in the pillow rim glass. Profiles in δ7Li conducted through olivines show very different patterns correlated with their cooling history. Olivines embedded in the pillow rim glass display no variation in Li abundance ([Li] = 1.2 µg.g-1) and isotopic composition (δ7Li = +6‰). However, a large Li isotopic zoning is found from crystal cores (δ7Li = +6‰, [Li] = 1.1 µg.g-1) to rims (δ7Li = -11‰, [Li] = 1.4 µg.g-1) for olivines set in microcrystalline groundmass. The cores of large olivine phenocrysts are equilibrated with the pillow rim glass (δ7Li = +6‰, [Li] = 3.4 µg.g-1). Recently, detailed investigation of the Li isotopic composition of large phenocrysts in various magmatic environments demonstrated that microscale •7Li variations were due to fractionation of lithium isotopes during late stage chemical diffusion (Barrat et al., 2005; Beck et al., 2006; Jeffcoate et al., 2006). In our case, the diffusion-induced Li isotopes fractionation is the only process able to explain such diverse zoning in olivines from one single pillow lava. Our study brings new constraints on the Li fractionation during the diffusion process and suggests that the pristine δ7Li values can be completely erased in small phenocrysts, even in the case of rapidly cooled volcanics. The use of Li isotopes in crystals for tracking mantle heterogeneities should be taken with extreme caution.