Modelling mantle geodynamics in the Ethiopian Rift and Afar through
olivine thermometry and rare-earth element distributions
Abstract
The Ethiopian sector of the East African Rift system (the Main Ethiopian
Rift, MER) and Afar rift showcase advanced stages of continental
breakup. Here the interplay between active continental rifting and
rift-induced volcanism poses key questions regarding the mantle
geodynamics of late-stage rift development. A particular subject of
interest is the presence of hot mantle upwellings in the sub-rift
mantle, which are inferred from geophysical imaging. Magma generation in
the sub-rift asthenosphere depends on the temperature, lithology, and
composition of the upwelling mantle material. Geophysical observations
of the sub-rift mantle must therefore be supported by petrological
studies aimed at understanding the physico-chemical conditions of melt
production. In this study we investigate melt generation beneath the MER
and Afar using a mantle melting model constrained by olivine
crystallization temperatures and rare-earth element (REE)
concentrations, both observed in rift zone lavas. Olivine
crystallization, a proxy for magma liquidus temperature, is directly
related to the thermodynamic and geochemical conditions of the melting
mantle. Through application of an olivine-spinel aluminium exchange
thermometer, we provide the first petrological olivine crystallization
temperatures for MER and Afar basalts (1177±16°C and 1263±43°C
respectively). A multi-lithology mantle melting model subsequently
allows for inversion of our olivine crystallization temperatures and
observed REE concentrations of rift magmas to estimate the temperature,
lithology, and composition of the Ethiopian mantle. Our results suggest
that the crystallization temperatures and REE distributions measured at
the MER and Afar necessitate elevated mantle temperatures
(Tp ≥ 1450 °C) relative to ambient mid-ocean ridge
mantle. A thick mantle lithosphere (~60 km) is also
required to provide deep garnet-field mantle melting inferred from REE
distributions. We additionally conclude that an enriched and fusible
pyroxenitic mantle component is necessary to match crustal melt
thicknesses and observed REE concentrations. The composition of this
pyroxenitic lithology is further explored through our inversions, and
the contributions of enriched pyroxenitic melts to rift volcanism in the
MER and Afar are subsequently compared.