doi:10.1016/j.gca.2004.08.004 Observations of Li isotopic variations in the Trinity Ophiolite: Evidence for isotopic fractionation by diffusion during mantle melting CRAIG C. LUNDSTROM, 1, *MARC CHAUSSIDON, 2 ALBERT T. HSUI, 1 PETER KELEMEN, 3 and MARK ZIMMERMAN 4 1 Department of Geology, University of Illinois–Urbana Champaign, Urbana, IL 61801, USA 2 CPRG-CNRS, 54501 Vandoeuvre-lès-Nancy, France 3 Department of Marine Geology, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA 4 Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA (Received February 19, 2004; accepted in revised form August 11, 2004) Abstract—The Trinity peridotite (northern CA) contains numerous lithologic sequences consisting of dunite to harzburgite to spinel lherzolite to plagioclase lherzolite. Previous workers have documented geochemical gradients in these sequences consistent with melt-rock reaction processes occurring around dunites, interpreted to reflect conduits for melt ascent. We have undertaken a study of Li isotope compositions of clinopyroxene and some olivine within these sequences using ion probe techniques to test the hypothesis that the geochemical gradients are related to diffusive fluxing of alkali elements into or away from the melt conduit. Results show large variations in 7 Li/ 6 Li occurring in a consistent pattern across three transects from dunite to plagioclase lherzolite within the Trinity peridotite. Specifically, measurements of average  7 Li for single thin sections along the traverse reveal a low in  7 Li in the harzburgite adjacent to the dunite returning to higher values farther from the dunite with a typical offset of 10 per mil in the low  7 Li trough. This pattern is consistent with a process whereby Li isotopes are fractionated during diffusion through a melt either from the dunite conduit to the surrounding peridotite, or from the surrounding peridotite into the dunite conduit. The patterns in 7 Li/ 6 Li occur over a length scale similar to the decrease in REE concentration in these same samples. Explaining both the trace element and Li isotopic gradients requires a combined process of alkali diffusion and melt extraction. We develop a numerical model and examine several scenarios of the combined diffusion-extraction process. Using experimentally constrained values for the change in Li diffusion coefficient with isotope mass, large changes in  7 Li as a function of distance can be created in year to decade timescales. The addition of the melt extraction term allows larger Li concentration gradients to be developed and thus produces larger isotopic fractionations than diffusion only models. The extraction aspect of the model can also account for the observed decrease in rare earth element concentrations across the transects. Copyright © 2005 Elsevier Ltd 1. INTRODUCTION The importance of reactions between ascending melts and mantle in affecting both the physics of magma ascent in the mantle (Aharonov et al., 1997; Kelemen et al., 1997; Spiegelman et al., 2001) and the geochemical signatures of erupted lavas (Navon and Stolper, 1987; Asimow et al., 1999; Lundstrom, 2000a; Jull et al., 2002) is increasingly realized. Geochemical evidence suggests that ascending melts beneath midocean ridges are channelized in dunites created by melt- rock reactions (Kelemen et al., 1995, 1997, 2000). Because of channelization, ascending melts do not directly react with the peridotite surrounding the melt conduit, resulting in orthopy- roxene undersaturation in MORB (O’Hara, 1968; Stolper, 1980) and highly depleted trace element concentrations in abyssal peridotites (Johnson et al., 1990; Dick and Natland, 1996). However, ascending, channelized melts could affect melting in the surrounding peridotite if alkali elements diffuse away from the conduit into the surrounding peridotite (Lund- strom, 2000b). Laboratory experiments demonstrate that alkali elements, particularly sodium, will rapidly diffuse from an alkali basalt into the interstitial melt within peridotite at low pressure, causing orthopyroxene to preferentially dissolve (Lundstrom, 2000b, 2003). Because the effect of increasing bulk sodium content on melt-peridotite equilibria dramatically increases at lower pressures (Kushiro, 1975; Hirschmann et al., 1998), this process could have fundamental importance to the overall production of the oceanic crust. However, the hypoth- esis that sodium diffusively fluxes the peridotite surrounding a melt conduit, based solely on laboratory experiments, has little observational evidence supporting its occurrence. Lithium, an alkali element of smaller ionic radius than so- dium, is known to diffuse extremely rapidly. Recent experi- ments have demonstrated that Li isotopes are fractionated dur- ing diffusion through silicate melts (Richter et al., 2003); thus Li isotopes should serve as an indicator of the alkali diffusive fluxing process. Here, we test the hypothesis that melt-rock reactions related to alkali diffusion occur adjacent to mantle melt conduits through measurement of Li isotopes in lithologic transects in the Trinity Ophiolite. Our results show that the Li isotopic compositions of samples vary in a systematic pattern across three transects, consistent with alkalis diffusing through interconnected melt within the different lithologies. 2. USING Li ISOTOPES TO IDENTIFY ALKALI DIFFUSION PROCESSES Lithium is an element under increasing geochemical scrutiny due to rapid advances in analytical techniques and interest in * Author to whom correspondence should be addressed (lundstro@ uiuc.edu). Geochimica et Cosmochimica Acta, Vol. 69, No. 3, pp. 735–751, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 + .00 735