Constraining late stage melt-peridotite interaction in the lithospheric mantle of southern Ethiopia: evidence from lithium elemental and isotopic compositions

Abstract

Lithium (Li) elemental and isotopic compositions for mineral separates of coexisting olivine, orthopyroxene and clinopyroxene of mantle xenoliths from the Quaternary volcanic rocks of southern Ethiopian rift (Dillo and Megado) reveal the influence of late stage melt-peridotite interaction on the early depleted and variably metasomatized lithospheric mantle. Two types of lherzolites are reported (LREE-depleted La/Sm(N) = 0.11–0.37 × Cl and LREE-enriched, La/Sm(N) = 1.88–15.72 × Cl). The depleted lherzolites have variable range in Li concentration (olivine: 2.1–5.4 ppm; opx: 1.1–2.3 ppm; cpx: 1.0–1.8 ppm) and in Li isotopic composition (δ7Li in olivine: −9.4 to 1.5‰; in opx: −4.5 to 3.6‰; in cpx: −17.0 to 4.8‰), indicating strong disequilibrium in Li partitioning and Li isotope fractionation between samples. The enriched lherzolites have limited range in both Li abundances (olivine: 2.7–3.0 ppm; opx: 1.1–3.1 ppm; cpx: 1.1–2.3 ppm) and Li isotopic compositions (δ7Li in olivine: −1.3 to +1.3‰; in opx: −2.0 to +5.0‰; in cpx: −7.5 to +4.8‰), suggest that the earlier metasomatic event which lead to LREE enrichment could also homogenize the Li contents and its isotopes. The enriched harzburgite and clinopyroxenite minerals show limited variation in Li abundances and variable Li isotopic compositions. The Li enrichments of olivine and clinopyroxene correlate neither with the incompatible trace element enrichment nor with the Sr-Nd isotopic compositions of clinopyroxene. These observations indicate that the metasomatic events which are responsible for the LREE enrichment and for the Li addition are distinct, whereby the LREE-enrichment pre-dates the influx of Li. The presence of large Li isotopic disequilibria within and between minerals of depleted and enriched peridotites suggest that the lithospheric mantle beneath the southern Ethiopian rift has experienced recent melt-peridotite interaction. Thus, the Li data set reported in this study offer new additional evidence for the existence of late stage metasomatism, which probably occurred at shallow depth briefly before and/or during entrainment and ascent of mantle xenoliths to the surface.