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A Tale of Two Plumes – Magmatism and Mantle convection beneath the African Plate

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The African continent is characterised by a dramatic dynamic topography of broad domes and basins that relates to convection in the underlying mantle. Many of the domes are characterised by basaltic volcanism and in the case of east Africa, extensional tectonics heralding the earliest phases of continental break-up. Seismic tomography reveals intriguing structures in the shallow and deep mantle beneath Africa, but the relationship between these structures and topography and surface volcanism remains unclear. The east African Rift is associated with two of these domes, the Ethiopian and East African Plateaux. At issue is whether these two features are related to a single plume beneath the east African Rift or two plumes.

The geochemical and petrological evidence from the basaltic magmas along the length of the African Rift points to distinct features associated with each of the two domes. Basalts from Kenya are strongly influenced by the nature and age of the underlying lithosphere. Sr and Nd isotopes reflect the age of the lithosphere while aspects of their trace element contents are distinct from MORB -OIB. Identifying the composition of a plume end-member is not straightforward but the data suggest an εNd of +2.0. Basalts from Ethiopia, by contrast, are more akin to modern OIB -MORB and their trace element abundances can be broadly related to the amount of extension in the lithosphere. Isotopically, they are more systematic and focus on a composition with εNd of +5.0, which appears to be the present day composition of the Afar mantle plume. The existence of separate mantle plumes is also indicated by the geographical variation of 3He/4He ratios, which are high in both lavas and geothermal fluids from the area of the current Ethiopian plateau, but low (i.e.

The migration of magmatism through time also presents an intriguing pattern. The oldest basalts occur in southern Ethiopia during the Eocene (35-45Ma). Flood basalts erupt in N Ethiopia at 30 Ma, whereas basaltic magmatism in Kenya migrated from N to S between 35 Ma and the present day. This pattern is best explained in the context of two mantle plumes, the oldest initiating beneath S Ethiopia at 45 Ma, causing magmatism in S Ethiopia and Kenya as the African plate moves north. The second, the Afar plume initiated with the Ethiopian CFB at 30 Ma and continues to the present day feeding magmatism currently focussed on the Afar depression. The radiogenic isotopes in the erupted basalts are consistent with two distinct sources of magma in these two provinces.

The Ethiopian CFB include a sequence of high-Ti basalts and picrites that mark the first eruptives from the Afar plume. Their major and trace element compositions are distinctive with low Al2O3 and strongly fractionated REE , both consistent with an origin at 4-5GPa assuming derivation from a peridotite source region. Previous investigations reveal that the high-Ti lavas have elevated 3He/4He ratios (up to 19 R/R ) and εNd of +5.0, further supporting a deep origin. The lavas also have elevated Fe/Mn ratios, a characteristic that elsewhere has been used to signify a core component in the plume source. However, their Os isotopes ratios are consistently unradiogenic with the lowest values yet recorded from any flood basalt province, and are not consistent with a putative origin from the core. The combination of high 3He/4He and low 187Os/188Os ratios indicates an origin from a depleted but undegassed source region whereas the high Fe/Mn ratios and TiO2 contents suggest a significant pyroxenite contribution. However, the pyroxenites cannot be old recycled oceanic lithosphere as the addition of sufficient pyroxenite from such a source to satisfy the TiO2 and Fe/Mn characteristics would add too much radiogenic Os. Reconciling these two contrasting conclusions remains elusive!

This talk is part of the Sedgwick Club talks series.

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