Diffusion of Li in olivine. Part I: Experimental observations and a multi species diffusion model Ralf Dohmen a, * , Simone A. Kasemann b,1 , Laurence Coogan c , Sumit Chakraborty a a Institut fu ¨ r Geologie, Mineralogie und Geophysik, Ruhr Universita ¨ t Bochum, D-44780 Bochum, Germany b School of GeoSciences, Grant Institute of Earth Science, University of Edinburgh, King’s Buildings, West Mains Road, Edinburgh EH9 3JW, United Kingdom c School of Earth and Ocean Sciences, Petch Building, University of Victoria, P.O. Box 3055 STN CSC, Victoria, BC, Canada V8W 3P6 Received 17 February 2009; accepted in revised form 14 September 2009; available online 13 October 2009 Abstract There are an increasing number of studies that focus on the systematics of the distribution of Li and its isotopes among different geochemical reservoirs. These studies have found that Li is relatively mobile compared to many other elements (e.g., Fe, Mg), and diffusion has been considered as a mechanism to generate large isotopic fractionations even at high tempera- tures. In order to quantify some of these aspects, we have measured Li diffusion rates experimentally along [0 0 1] of single crystals of olivines from San Carlos, Arizona and Pakistan, at 800–1200 °C at a total pressure of 100 kPa and fO 2  WM buffer. A complex diffusion behavior of Li is observed, indicating that two mechanisms of diffusion (a fast and a slower one) operate simultaneously. The behavior is well described by a model that partitions Li between two different sites in olivine – an octahedral site (Li Me ) and an interstitial site (Li i ). Transport of Li is a combination of hopping within and between each of these kinds of sites involving also vacancies on the octahedral site (V Me ). It is assumed that the homogeneous reaction (Li Me =V Me + Li i ) that maintains equilibrium distribution of Li between the sites is instantaneous compared to the timescales of all other processes associated with diffusive transport. One consequence of this mode of transport of Li in olivine is that the shape and length of diffusion profiles depend on the boundary conditions imposed at the surface of a crystal; i.e., the chemical environment (e.g., fO 2 , aLi 4 SiO 4 ), in addition to temperature and pressure. Our model describes the variable experimentally determined Li-profile shapes produced at different temperatures and with different boundary conditions, as well as their time evolution, quantitatively. Modeling the observed isotopic fractionation shows that 6 Li diffuses about 5% faster than 7 Li on the interstitial site. Inspection of published data on Li distribution in natural olivines that are available until now indicates that the fast (interstitial) mechanism of Li diffusion is unlikely to be dominant in most natural systems; Li rich, oxidizing environ- ments (e.g., fluids?) may be exceptions. However, when it operates it can decouple the equilibration of Li isotopic gradients from the time scale of equilibration of overall Li concentrations. Diffusion dominated by the slower mechanism will occur on the average at a rate that is about an order of magnitude faster than diffusion of Fe, Mg and most other divalent cations in olivine; such diffusion of Li in olivine will be much slower than the rates of diffusion in clinopyroxene and plagioclase crystals at the same conditions. Fractionation of isotopes of Li by diffusion is likely to be a transient phenomenon and is more likely to be observed in crystals showing zoning of Li concentrations. Ó 2009 Elsevier Ltd. All rights reserved. 0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009.10.016 * Corresponding author. Present address: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, United Kingdom. E-mail addresses: r.dohmen@bristol.ac.uk (R. Dohmen), simone.kasemann@uni-bremen.de (S.A. Kasemann), lacoogan@uvic.ca (L. Coogan), Sumit.Chakraborty@rub.de (S. Chakraborty). 1 Present address: Department of Geosciences, University of Bremen, 28334. Bremen, Germany. www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 74 (2010) 274–292