The control of lithium budgets in island arcs Paul B. Tomascak a;b;c ;  , Elisabeth Widom c;d , Laurie D. Benton c , Steven L. Goldstein b , Je¡rey G. Ryan e a Department of Geology, University of Maryland, College Park, MD 20742 USA b Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 USA c Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015 USA d Department of Geology, Miami University, Oxford, OH 45056 USA e Department of Geology, University of South Florida, Tampa, FL 33620 USA Received 16 July 2001; received in revised form 10 December 2001; accepted 18 December 2001 Abstract Measurements of the Li isotopic compositions of lavas from magmatic arcs worldwide suggest common processes at work that lead to the retention of isotopically heavy Li in the mantle. Samples from this study derive from the Kurile arc, eastern Russia, the Sunda arc, Indonesia, and a segment of the Aleutian arc, western Alaska. The overall range in N 7 Li is very restricted (+2.1 to+5.1 þ 1.1, 2c) for 34 of 36 samples. These values overlap the values of unaltered normal MORB glasses. The two samples with isotopic compositions that fall outside this range in N 7 Li have B/Be 6 13, and hence do not bear classical ‘slab’ trace element signatures. Considering the high N 7 Li in altered ocean crust, marine and terrigenous sediments, and forearc fluids, aqueous components lost by subducting slabs are expected to have similarly heavy enriched Li isotope signatures. If Li behaves similarly to a fluid-mobile element such as B, N 7 Li should correlate strongly with, for example, B/Be. As such, samples with high B/Be should show elevated N 7 Li. The sample set we have examined does not show such correlations and is interpreted to reflect a globally significant process. Although Li is a fluid-mobile element, its partitioning into Mg-silicates may cause it to be effectively removed during equilibration with subarc mantle peridotite. Elements with stronger fluid/mantle partitioning behavior, such as B, are not so affected. The convergence of Li isotope ratios on MORB-like values is interpreted to result from the sequestration of slab-derived Li in the subarc mantle before it reaches the zone of melting. The results indicate conditions appropriate for mantle ‘buffering’ of slab-derived Li are widespread in magmatic arcs. Alternately, some proportion of Li could be retained on the slab in high Li/B minerals. Either way, this indicates that regions of the upper mantle with N 7 Li s MORB may be common, as a direct consequence of the subduction process. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: lithium; stable isotopes; island arcs; mantle; mobilization 1. Introduction The fundamental importance of light stable isotope systems such as Li and B in studies of magmatic arcs lies in the strong isotopic con- trast between materials of broadly mantle prov- 0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII:S0012-821X(01)00614-8 * Corresponding author. Tel.: +1-301-405-4054; Fax: +1-301-314-9661. E-mail address: tomascak@geol.umd.edu (P.B. Tomascak). Earth and Planetary Science Letters 196 (2002) 227^238 www.elsevier.com/locate/epsl