Abstract
To determine whether fractionation of Li isotopes could occur at high temperatures, we examined major element and Li isotopic compositions of olivine from the Xiadong intrusion, an Alaskan-type complex in the Central Asian Orogenic Belt. Olivine in thirteen dunites, displaying characteristic cumulus textures, yielded large variations in Li concentration (0.10 to 11.18ppm) and isotopic composition (δ7Li=−7.18 to +34.41‰). These variations are too large to be attributed entirely to diffusive processes. The correlations between Li elemental or isotopic composition and differentiation indices such as Fo and MnO contents of olivine, and NiO content of chromite, suggest probable Li isotope fractionation during early stage of differentiation. We speculate that while Li behaves mildly incompatible during differentiation, 7Li is preferentially incorporated into olivine relative to 6Li during early stage crystallization from arc magmas. Relatively high Li concentrations and low δ7Li in arc magmas might be the result of substantial olivine fractionation prior to eruption.
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