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Pre-Erupted Water Concentrations in Erta Ale Lavas and Gulf of Aden Glasses from the East African Rift


The compositions of lavas in East African Rift suggest that the upper mantle temperatures in this region are high (~1360-1490°C), however, seismic wave speeds are very low, which if due to elevated mantle temperatures alone, would require mantle potential temperatures >1700°C. This suggests a significant role for H2O, CO2, or melt in the mantle under the East African Rift. The volatile contents of submarine glasses from the Gulf of Aden are unknown but are key to constraining H2O contents in the mantle in this area. The radiogenic isotope (Sr-Nd-Hf-Pb) compositions of these glasses point to three distinct mantle sources for the lavas erupted along the East African Rift: the depleted upper mantle, the Afar mantle plume, and the Pan-African lithosphere. We present major and volatile element compositions and S6+/ΣS of glassy melt inclusions and their phenocryst hosts from Erta Ale, and S, H2O, S6+/ΣS, and Fe3+/ΣFe of selected Gulf of Aden glasses to constrain primary melt and mantle source H2O and melt fractions along the rift. Erta Ale inclusions are basaltic, have 0.05-0.4 wt% H2O, and ~0.16 Fe3+/ΣFe. Gulf of Aden glasses are basaltic, have ~0.16 Fe3+/ΣFe, and are divided into groups based on K2O. “Low K2O” glasses have 0.15-0.36 wt% H2O, “Medium K2O” have 0.05-0.22 wt% H2O, and “High K2O” have 0.84 wt% H2O. We calculate primary melt compositions for samples with MgO > 8 wt% and find that Gulf of Aden samples represent 7-10% melts of the mantle. Assuming a H2O content for the solid depleted upper mantle of 0.012 wt%, our calculated melt fractions, primary melt H2O contents, and known partition coefficients, we calculate the H2O of the Pan-African lithosphere (0-0.4 wt%) and the Afar mantle plume (0.06 wt%). These values enable a more nuanced characterization of the geophysical anomaly under the East African Rift.

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