Geochemical and Sr–Nd–Pb isotopic compositions of lithospheric mantle: Spinel lherzolites in alkaline basalts from the northwestern Ethiopian plateau

Abstract

The geochemical characteristics of mantle xenoliths (spinel lherzolites) in Quaternary alkaline basalts from the northwestern Ethiopian plateau provide new insight into the conditions of melting associated with the Afar plume. The major element compositions of the spinel lherzolites are in the range of those of the primitive mantle. The high modal content of the clinopyroxene (14–23%) indicate that the mantle xenoliths are fertile spinel lherzolites that have experienced insignificant partial melting. However, enrichment in highly incompatible elements (LREE, Ba, Pb, Th, and U) and the absence of any typical metasomatic minerals indicate that the spinel lherzolites underwent an event of later cryptic metasomatic enrichment induced by plume–related, hydrous fluid–rich silicate melts. The 87Sr/86Sr (0.70184–0.70324), 143Nd/144Nd (0.51313–0.51338), and 206Pb/ 204Pb (17.17–19.06) values of the clinopyroxene separate from the spinel lherzolites display systematic variations between those of depleted MORB mantle (DMM) and enriched compositions interpreted as related to the Afar mantle plume. The mineral compositions yield an estimated temperature and pressure in the range of 921– 973 °C and 13–17 kbar, respectively, indicating that the mantle xenoliths originated at shallow depths of 40 to 55 km above the Afar mantle plume.