Lithium elemental and isotopic disequilibrium in minerals from peridotite xenoliths from far-east Russia: Product of recent melt/fluid–rock reaction Roberta L. Rudnick a, ⁎ , Dmitri A. Ionov b,c,1 a Geochemistry Laboratory, Department of Geology, University of Maryland, College Park, MD 20742, USA b Institut für Mineralogie, J.W. Goethe-Universität, Senckenberganlage 28, 60054 Frankfurt/Main, Germany c Max-Planck-Institut für Chemie, Postfach 3060, D-55020 Mainz, Germany Received 10 September 2006; received in revised form 26 January 2007; accepted 26 January 2007 Editor: R.W. Carlson Available online 3 February 2007 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 7 Li of ∼ +4±2. δ 7 Li cpx (- 0.8 to - 14.6) is systematically lighter than δ 7 Li of coexisting olivine (- 1.7 to +11.9), and Δ 7 Li 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 7 Li cpx do not reflect the influence of an exotic mantle component related to crustal recycling. © 2007 Elsevier B.V. All rights reserved. Keywords: upper mantle; metasomatism; Li diffusion; Li isotopes; isotopic disequlibrium; peridotite xenolith 1. Introduction Studies of Li and its isotopes ( 6 Li: 7.5% and 7 Li: 92.5% atomic abundance) in the solid Earth are rapidly increasing in number, as there is great interest in determining whether the Li isotope system will be Earth and Planetary Science Letters 256 (2007) 278 – 293 www.elsevier.com/locate/epsl ⁎ Corresponding author. Tel.: +1 301 405 1311; fax: +1 301 405 3597. E-mail address: rudnick@geol.umd.edu (R.L. Rudnick). 1 Current address: Departement Géologie-Pétrologie-Géochimie, Université Jean Monnet, 23 rue Paul Michelon, 42023 Saint-Etienne, France. 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.01.035