Lithium abundance and isotope composition of Logudoro basalts, Sardinia: Origin of light Li signature

Abstract

Basalts from Logudoro, Sardinia (Italy), showing geochemical features similar to EMI-type mantle source, have been analyzed for their Li isotopic composition to provide data related to a heretofore unknown reservoir. The Li concentrations of the Logudoro basalts vary between 7.7 and 10.8 ppm, which falls within the common range of Li abundances in OIB, but their Li/Dy ratios are markedly higher than those of most OIB. Based on our calculations, such a feature is ascribed to partial melting processes occurred at higher pressure than those of ordinary OIB, and it is likely to result from the increasing amount of residual garnet present during partial melting. The Li isotopic compositions of Logudoro basalts range from δ7Li = + 1.5 to + 3.6% (δ7Li(%) = ([7Li/6Li]sample/[ 7Li/6Li]L-SVEC standard - 1) x 1000). These ratios are similar to or slightly lower than those of ordinary mantle materials. The diffusion calculations for recycled subducted oceanic crust and delaminated lower continental crust indicate that the fast diffusion of Li prevents the recycled materials from preserving their original Li isotopic ratio, thus it is difficult to constrain the specific recycled material in OIB source in terms of Li isotopes. Nevertheless, it is likely that the observed Li isotopic ratios have derived from the mantle or lower oceanic crust whose Li isotopic compositions were modified by Li diffusion from a Li-enriched part to Li-poor parts. Copyright © 2011 by The Geochemical Society of Japan.