Cluster analysis reveals increasing plume-like magmatism during progressive rifting in Afar (Ethiopia)

Abstract

Reconstructing chemical variations of magma during rifting is challenging due to the heterogeneous mantle sources of the melt and different evolution pathways that magma potentially takes. Therefore, how and when the magma generation process evolves to that typical of an oceanic ridge (MORB-like composition) is still unclear. The Afar depression is an ideal place for studying magma changes during rift evolution, with North Afar close to breakup and older rift products preserved across the region. To investigate magma sources, we applied clustering analyses to a vast geochemical dataset of more than 1000 samples from the Afar rift. Combinations of different clustering methods (K-means and hierarchical) and assessment of correlation between features (Pearson coefficient) show that both trace element and isotope clustering group the North Afar samples, identifying a mantle source containing residual MREE-bearing minerals and an enriched mantle component. This suggests that North Afar, where the rift is closest to breakup, has a stronger influence of the Afar plume and more extensive partial melting of metasomatized lithosphere than the rest of Afar. We show that geochemical variations during rifting do not always follow a progressive transition toward a MORB-like composition but, instead, plume-like magmatism can increase until the most advanced stages of rifting (i.e., North Afar), potentially because the mantle plume is focused towards regions of thinnest lithosphere.

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