Geochemical Evidence of Varying Melt Production at the Erta Ale Volcanic Segment

The Erta Ale Volcanic Segment (Ethiopia) includes seven volcanic centres and lies in the Danakil Depression, a region undergoing a transition from continental rifting (Southern Afar) to seafloor spreading (Red Sea). This area is characterised by extensive magma-assisted rifting, and has experienced numerous stages of volcanism beginning at 30 Ma and continuing to the present day. This study investigates how melt generation varies during the evolution of this magmatic segment, and how this relates to rifting processes more broadly. We present trace element and high-precision Pb isotope compositions from three volcanoes within the Erta Ale Volcanic Segment: Alu, Dalafilla and Borale. The basaltic products from all three volcanoes have Nb/Y and TiO2 in the range 0.78 1.31 and 2.11-2.97 wt% respectively, which are comparable to the High-Ti (HT1) basalts previously observed within the Ethiopian plateau (Pik et al., 1998). The strong influence of a HiMu mantle plume source beneath these volcanoes is indicated by the high Ce/Pb (33-43) and positive Nb (0.25-0.38). Pb isotopes reflect interaction between HiMu mantle magmas and the lower crust, with only a minor influence from depleted MORB mantle (DMM) component. To verify this result, we also used [Sm/Yb]PM, [Dy/Yb]PM and [Ce/Sm]PM providing further insights into melting conditions. Melting is estimated to be typically ~0.8%; consisting of a mixture of spinel lherzolite (~90%) and garnet lherzolite (~10%). However, one period (Stage 2, Watts et al., 2020) during the evolution of the segment was characterised by a slightly higher degree of melting (~1%) predominantly of the shallower spinel lherzolite facies. Overall, this indicates that whilst the Afar plume is the dominant mantle component fuelling volcanism, the chemical characteristics and depth of melt production and supply beneath the rift varied during its evolution. References: Pik, R. et al., (1998) JVGR. 81, pp.91-111. Watts E.J. et al., (2020) JVGR. 408, p. 107094