The lithium isotopic composition of orogenic eclogites and deep subducted slabs Horst R. Marschall a, ⁎ , Philip A.E. Pogge von Strandmann a , Hans-Michael Seitz b , Tim Elliott a , Yaoling Niu c a Department of Earth Sciences, Wills Memorial Building, Queen's Road, University of Bristol, Bristol BS8 1RJ, UK b J.W. Goethe Universität, Institut für Geowissenschaften, F.E. Mineralogie, Altenhöferallee 1, 60438 Frankfurt, Germany c Department of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE, UK Received 17 May 2007; received in revised form 7 August 2007; accepted 14 August 2007 Editor: R.W. Carlson Available online 21 August 2007 Abstract We have modelled the Li budget of subducting oceanic crust during dehydration, using recently established input parameters. The results show that the entire prograde metamorphic process, up to anhydrous eclogite, can account for a decrease in δ 7 Li of only ≤ 3‰. Given that altered, oceanic crust entering the subduction zone should have Li isotopic compositions between - 3 and +14‰, dehydration cannot account for markedly light Li isotopic compositions (δ7Li b 0 to - 10‰) previously measured in some high- pressure metamorphic (HPM) rocks. We have analysed another 41 orogenic HPM rocks from 11 different localities. These samples show a wide range in Li abundances from 1 to 77 μg/g. Li isotopic compositions of the rocks display both very heavy (δ 7 Li N +6‰) and very light (δ 7 Li b 0‰) compositions, as low as - 21.9‰. Notably some of the samples with highest Li concentrations are also isotopically light, which would not be predicted by isotopic fractionation as a consequence of Li loss during dehydration. Li abundances in excess of 30 μg/g in orogenic HPM rocks of basaltic composition (eclogites) are higher than any value of altered MORB and presumably result from addition of Li after the onset of subduction, most probably during eclogitisation or exhumation. Hence we propose that light-δ 7 Li values are generated by kinetic fractionation of the Li isotopes during diffusive influx of Li from the country rocks into the exhuming eclogite bodies. Our conclusions are in stark contrast to the previously accepted model, as we predict the deeply subducted eclogites to have a Li isotopic signature heavier than the mantle. © 2007 Elsevier B.V. All rights reserved. Keywords: lithium isotopes; eclogite; high-pressure; modelling; subduction zone 1. Introduction The light alkali metal lithium with its two stable isotopes, 6 Li and 7 Li, has gained significant attention among geochemists during the last decade. Moderate incompatibility during mantle melting, high mobility in hydrous fluids and a strong isotopic fractionation at low temperatures make this trace element a potentially pow- erful tracer for recycled material in the Earth's mantle (Zack et al., 2003; Tomascak, 2004; Elliott et al., 2004, 2006). Lithium is enriched in low-temperature oceanic crust with an increase of 7 Li/ 6 Li ratios (noted as δ 7 Li, the deviation from the L-SVEC standard in permil) due to the interaction with the heavy-Li seawater (Donnelly et al., 1980; Ryan and Langmuir, 1987; Chan and Edmond, Available online at www.sciencedirect.com Earth and Planetary Science Letters 262 (2007) 563 – 580 www.elsevier.com/locate/epsl ⁎ Corresponding author. Tel.: +44 117 3315006; fax: +44 117 9253385. E-mail address: Horst.Marschall@bristol.ac.uk (H.R. Marschall). 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.08.005