Volatiles and Redox Along the East African Rift

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2024-08, Vol.25 (8), p.n/a
• Main Authors: Brounce, Maryjo, Scoggins, Sara, Fischer, Tobias P., Ford, Heather, Byrnes, Joseph
• Format: Article
• Language: English
• Subjects: Carbon dioxide; continental rifting; Depth; High temperature; Isotope composition; Major elements; Mantle plumes; melt inclusions; Melting; Ocean floor; oceanic spreading; Olivine; Oxidation; oxygen fugacity; P-waves; Plagioclase; redox; Seafloor spreading; Seismic waves; Spreading centres; Upper mantle; volatiles; Volcanoes
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2024GC011657

The upper mantle under the Afar Depression in the East African Rift displays some of the slowest seismic wave speeds observed globally. Despite the extreme nature of the geophysical anomaly, lavas that erupted along the East African Rift record modest thermal anomalies. We present measurements of major elements, H2O, S, and CO2, and Fe3+/ΣFe and S6+/ΣS in submarine glasses from the Gulf of Aden seafloor spreading center and olivine‐, plagioclase‐, and pyroxene‐hosted melt inclusions from Erta Ale volcano in the Afar Depression. We combine these measurements with literature data to place constraints on the temperature, H2O, and fO2 of the mantle sources of these lavas as well as the initial and final pressures of melting. The Afar mantle plume is C/FOZO/PHEM in isotopic composition, and we suggest that this mantle component is damp, with 852 ± 167 ppm H2O, not elevated in fO2 compared to the depleted MORB mantle, and has temperatures of ∼1401–1458°C. This is similar in fO2 and H2O to the estimates of C/FOZO/PHEM in other locations. Using the moderate H2O contents of the mantle together with the moderate thermal anomaly, we find that melting begins at around 93 km depth and ceases at around 63 km depth under the Afar Depression and at around 37 km depth under the Gulf of Aden, and that ∼1%–29% partial melts of the mantle can be generated under these conditions. We speculate that the presence of melt, and not elevated temperatures or high H2O contents, are the cause for the prominent geophysical anomaly observed in this region. Plain Language Summary The mantle under the Afar Depression and Gulf of Aden in Northeastern Africa is geophysically distinct from the mantle elsewhere on Earth. Typically, these geophysical distinctions are thought to arise from elevated temperatures, but the composition of lavas that erupted in this region demonstrate that the mantle is only moderately warm and cannot fully explain the geophysical nature of the mantle in this region. We produce new measurements of submarine pillow glasses erupted from Erta Ale volcano and find that in addition to being somewhat warm, the mantle in this region is also somewhat hydrated compared to the mantle that feeds mid‐ocean ridge volcanoes but is not substantially different in bulk oxidation state. These conditions together produce a region of partial melt that exists between 93 km and 63–36 km depth under the surface. We speculate that this lens of melt can explain the geophysical observations o