New geochemical data integrated in a petrogenetic model indicate that the northern Ethiopia continental flood basalts (CFBs; 31-29 Ma) preserve record of magmas generated from the centre to the flanks of a plume head, currently corresponding to the "Afar hot spot". Basaltic lavas appear zonally arranged with Low-Ti tholeiites (LT) in the west, High-Ti tholeiites (HT1) eastwards and very High-Ti transitional basalts and picrites (HT2, TiO2 4-6 wt%) closer to the Afar triple point. Modelling elemental distribution provides estimates of the P-T-X conditions of magma sources showing that LT and HT melts could be generated in the pressure range of 3.0-1.3 GPa (ca. 40-100 km depth) from mantle sources increasingly metasomatized (ca. 3-4% to 7-12% amphibole + 1-2% Ti-phase for HT) and hotter (1200-1500 °C) from west to east. Metasomatizing agents can be envisaged as alkali-silicate melts which integrate various geochemical components (e.g.: Ti, Fe and related HFSE, LFSE, LREE, H2O, noble gases etc.) with different depths of provenance and mobility, scavenged and pooled along the plume axis. This has significant implications for the current debate on mantle plumes, since the modelled compositionally and thermically zoned plume head (temperature excess > 350 °C with respect to mantle xenoliths) is in accordance with seismic tomography, buoyancy flux characteristics, and the high 3He signal, thus supporting a deep provenance of the Afar plume, possibly originated in the lower mantle.
Continental flood basalts and mantle plumes: The case study of the Northern Ethiopian plateau
BECCALUVA, Luigi;BIANCHINI, Gianluca;NATALI, Claudio;SIENA, Franca
2008
Abstract
New geochemical data integrated in a petrogenetic model indicate that the northern Ethiopia continental flood basalts (CFBs; 31-29 Ma) preserve record of magmas generated from the centre to the flanks of a plume head, currently corresponding to the "Afar hot spot". Basaltic lavas appear zonally arranged with Low-Ti tholeiites (LT) in the west, High-Ti tholeiites (HT1) eastwards and very High-Ti transitional basalts and picrites (HT2, TiO2 4-6 wt%) closer to the Afar triple point. Modelling elemental distribution provides estimates of the P-T-X conditions of magma sources showing that LT and HT melts could be generated in the pressure range of 3.0-1.3 GPa (ca. 40-100 km depth) from mantle sources increasingly metasomatized (ca. 3-4% to 7-12% amphibole + 1-2% Ti-phase for HT) and hotter (1200-1500 °C) from west to east. Metasomatizing agents can be envisaged as alkali-silicate melts which integrate various geochemical components (e.g.: Ti, Fe and related HFSE, LFSE, LREE, H2O, noble gases etc.) with different depths of provenance and mobility, scavenged and pooled along the plume axis. This has significant implications for the current debate on mantle plumes, since the modelled compositionally and thermically zoned plume head (temperature excess > 350 °C with respect to mantle xenoliths) is in accordance with seismic tomography, buoyancy flux characteristics, and the high 3He signal, thus supporting a deep provenance of the Afar plume, possibly originated in the lower mantle.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
