UC Riverside

There is a large swath of slow seismic velocities in the upper mantle underneath the East African Rift, centered under the Main Ethiopian Rift segment. The presence of melts, H2O, or CO2 in the mantle can slow seismic velocities as well as lower mantle solidus, generating melts at higher pressures and lower temperatures than dry, carbon-free mantle. The volatile content of the mantle in the southern portion of the rift remains unconstrained, as the lithosphere is thick there, which leads to substantial differentiation during the formation of magmas and eruption of evolved, alkalic lavas that are difficult to link directly to the composition of the mantle. To understand the differentiation process, the water content of alkalic magmas erupted in this area has been constrained, and the H2O source whether from the asthenospheric mantle or lithospheric mantle and/or crustal assimilation has been tested. Major and volatile element concentrations of feldspar-hosted melt inclusions from tephras erupted at Suswa and Eburru have been measured to help understand the formation history of highly evolved magmas of the Central Peralkaline Province. The Suswa melt inclusions are in phonolites with 0.2-1.9 wt% H2O, while the Eburru melt inclusions are in trachytes with 3.45 to 5.80 wt% H2O. The Eburru trachytes are best described by the fractional crystallization of a dry (0.25 wt% H2O) basalt, assimilating the Pan-African crust after ~55% (by mass) of fractional crystallization. The assimilation resulted in the high-water content of Eburru magmas, as measured directly in melt inclusions. The history of Suswa phonolites is best explained by the fractional crystallization of a high-alkali basalt, though this model alone does not account for the whole range in major element composition. There is a shallow negative slope between SiO2 and Na2O+K2O contents in the Suswa melt inclusions and whole-rock concentrations that is best explained by magma mixing between an evolved Eburru-like magma and a Suswa-like evolved magma with higher alkali concentrations. The combination of fractional crystallization, magma mixing, and crustal assimilation is deemed essential for the development of the diverse and evolved magmas of the Central Peralkaline Province in East Africa.