Behaviour of Li and its isotopes during metasomatism of French Massif central lherzolites

Li behaviour and distribution in the mantle were investigated by ion microprobe in situ measurements on co-existing olivine (ol), orthopyroxene (opx), clinopyroxene (cpx) and amphibole (amp) in xenoliths from the French Massif Central. The fertile spinel lherzolites of this study record increasing degrees of mantle metasomatism, from unmetasomatised anhydrous samples through cryptically metasomatised samples to highly metasomatised amphibole-rich samples. In anhydrous lherzolites, Li is preferentially incorporated into olivine (1.1–1.4 ppm, average values) compared to pyroxenes (0.2–0.9 ppm). The hydrous samples clearly show enrichment of Li in ol (1.5–5.0 ppm), opx (1.1–2.4 ppm) and cpx (2.4–5.4 ppm), while amphibole incorporates less Li than the co-existing phases (0.8–1.3 ppm). Average dLi values range from +7.6 to +14.5‰ in ol, from 5.1 to +13.7‰ in opx and from 8.8 to +10.3‰ in cpx from the anhydrous lherzolites. A layered peridotite sample (Sdi) shows higher Li content in all phases, with lighter isotopic composition in opx and cpx ( 0.6 and 2‰ average dLi values, respectively). In the hydrous lherzolites average dLi values both overlap and extend beyond these ranges in ol (up to 17.5 ‰) and in opx (up to 22.9‰), and vary widely in cpx ( 2.7 to +9.7‰). Low dLi values are observed in some opx ( 10.4‰) and cpx ( 13‰) from sample Sdi, and in cpx from three hydrous samples (from 9.7 to 5.3‰). The different anhydrous phases from the hydrous samples show large intra-grain variations in Li isotopic ratios (e.g., up to 18‰) compared to the same phases from the anhydrous samples (mostly less than 6‰), excepting sample Sdi which has up to 20.4‰ variation in cpx. Similar to the anhydrous silicates, amphiboles show a wide variation of dLi values on the intra-grain scale (2–27‰). These variations are interpreted to result from fractionation processes during metasomatism by a silicate melt undergoing compositional changes as it percolates through and reacts with the peridotite phases. Thus Li abundances and isotopic in situ measurements are useful for tracing metasomatic processes but the heterogeneities observed in the samples preclude any identification of a specific mantle source by its Li signature. 2007 Elsevier Ltd. All rights reserved.