Lithium elemental and isotopic disequilibrium in minerals from peridotite xenoliths from far-east Russia: Product of recent melt/fluid–rock reaction

Abstract

Lithium concentrations and isotopic compositions of coexisting olivine and clinopyroxene (cpx) in well-characterized peridotite xenoliths from Tok (SE Siberian craton) and samples from two other far-east Russian localities reveal strong elemental and isotopic disequilibria, which correlates with bulk rock composition. Lithium concentrations in cpx from Tok (1–12 ppm) are equal to or significantly greater than those in coexisting olivines (1–5 ppm). The Li-rich cpx show core to rim zoning, indicative of Li infiltration from the grain boundaries. Olivines are generally unzoned, although Li concentrations can vary significantly from grain to grain. ol/cpx D varies from 0.2 to 1.0, which is lower than that expected for equilibrium partitioning ( ol/cpx D eq = 1.1 to 2.0), and reflects preferential Li enrichment in cpx. The Li isotopic compositions of both minerals range far beyond normal mantle δ 7Li of ∼ + 4 ± 2. δ 7Li cpx (− 0.8 to − 14.6) is systematically lighter than δ 7Li of coexisting olivine (− 1.7 to + 11.9), and Δ 7Li ol-cpx varies from 2.8 to 22.9‰. The greatest elemental and isotopic disequilibria occur in the most fertile samples (lherzolites) and may reflect longer equilibration times and/or enhanced melt permeability in the more refractory samples. Collectively, these observations suggest that the peridotite minerals experienced Li addition via diffusion from a grain boundary melt or fluid shortly before or coincident with their entrainment into the host basalt (i.e., within tens of thousands of years, based on published diffusion coefficients for Li in cpx at the temperatures of equilibration). This diffusional ingress of Li generated large kinetic isotopic fractionation, leading to unusually light cpx and heavier olivines. Thus, low δ 7Li cpx do not reflect the influence of an exotic mantle component related to crustal recycling.