Mantle xenoliths from Bir Ali (Yemen)

Mantle xenoliths from the Bir Ali diatrema (southern Yemen) consist of spinel peridotites and clinopyroxenites. In this contribution we present bulk rock (XRF, ICP-MS) and mineral (EMPA, LA-ICP-MS) major and trace element analyses carried out on a collection of 62 samples in order to characterize the lithospheric mantle of the area and constrain its relative petrological evolution. Peridotites are mainly equilibrated at 950-1000 °C and include fertile lherzolites and more refractory harzburgites and dunites, suggesting that partial melting processes variably affected the pristine mantle composition. Subsequent metasomatic reactions are evidenced by: a) glassy fims and/or patches; b) interstitial plagioclase; c) rare pargasite amphibole (observed only in one sample). Further evidence of measomatic interactions is given by anomalously low Fo content (<0.87) of olivine and low Mg# of pyroxene (< 0.88). These metasomatic events are confirmed by the bulk rock trace element budget that reveal enrichments in the most incompatible elements, especially in the most refractory dunites (LaN/YbN up to 4.1). Coherently, enrichments in the most incompatible elements are also observed in diopsidic clinopyroxene (LaN/YbN up to 3.9) and glasses (LaN/YbN up to 9.9). Trace element discrimination ratios suggest that the causative metasomatic agents were mainly represented by alkali-silicate melts. Noteworthy, samples characterized by plagioclase impregnation show unfractionated (flat) bulk-rock and clinopyroxene REE patterns suggesting refertilization by subalkaline metasomatic melts. Therefore, the considered lithospheric section appears to be characterized by remarkable heterogeneity, in contrast with what observed in mantle xenoliths collected from the Ethiopian plateau area that display clear evidence of pervasive refertilization by CFB melts. This suggests that the lithospheric mantle of southern Yemen wasn’t affected by the thermochemical effects of the Afar plume.