The evolution of magma during continental rifting: New constraints from the isotopic and trace element signatures of silicic magmas from Ethiopian volcanoes

Abstract

Magma plays a vital role in the break-up of continental lithosphere. However, significant uncertainty remains about how magma-crust interactions and melt evolution vary during the development of a rift system. Ethiopia captures the transition from continental rifting to incipient sea-floor spreading and has witnessed the eruption of large volumes of silicic volcanic rocks across the region over ∼45 Ma. The petrogenesis of these silicic rocks sheds light on the role of magmatism in rift development, by providing information on crustal interactions, melt fluxes and magmatic differentiation. We report new trace element and Sr–Nd–O isotopic data for volcanic rocks, glasses and minerals along and across active segments of the Main Ethiopian (MER) and Afar Rifts. Most δ18Odata for mineral and glass separates from these active rift zones fall within the bounds of modelled fractional crystallization trajectories from basaltic parent magmas (i.e., 5.5–6.5h) with scant evidence for assimilation of Pan-African Precambrian crustal material (δ18Oof 7–18h). Radiogenic isotopes (εNd= 0.92–6.52; 87Sr/86Sr = 0.7037–0.7072) and incompatible trace element ratios (Rb/Nb 100 km3), and estimate that crystal cumulates fill at least 16–30% of the volume generated by crustal extension under the axial volcanoes of the MER and Manda Hararo Rift Segment (MHRS) of Afar. At Erta Ale only ∼1% of the volume generated due to rift extension is filled by cumulates, supporting previous seismic evidence for a greater role of plate stretching in mature rifts at the onset of sea-floor spreading. We infer that ∼45 Ma of magmatism has left little fusible Pan-African material to be assimilated beneath the magmatic segments and the active segments are predominantly composed of magmatic cumulates with δ18O indistinguishable from mantle-derived melts. We predict that the δ18Oof silicic magmas should converge to mantle values as the rift continues to evolve. Although current data are limited, a comparison with ∼30 Ma ignimbrites (with δ18Oup to 8.9h) supports this inference, evidencing greater crustal assimilation during initial stages of rifting and at times of heightened magmatic flux