A HIMU-Like Endmember Hiding in the Turkana Depression Continental Lithospheric Mantle

Abstract

The fate of continental lithosphere during rifting is central to the process of continental extension. The continental lithospheric mantle comprises both depleted and enriched domains that may contribute to magma generation during extension. The East African Rift System is the archetypal example of a magma-rich continental rift, with the Turkana Depression containing the most extensive temporal record of mafic magmatism. There is debate as to the contribution of continental lithosphere to this mafic magmatism, with suggestions that HIMU-like isotopic signatures, often attributed to the continental lithosphere, are derived instead from a heterogeneous mantle plume. We focus on Miocene lavas that are characterized by radiogenic ²⁰⁶Pb/²⁰⁴Pb > 19.3, requiring a contribution from an HIMU-like endmember in their origin. We present a novel two-stage chromatographic metasomatism model that demonstrates that a HIMU-like endmember can be generated through time-integrated evolution within the continental lithospheric mantle. The first model stage uses an initial composition for the metasomatizing agent equivalent to a subduction magma to generate metasomes within the continental lithosphere during the Pan-African stabilization of the regional lithosphere (∼700 Ma). During Mesozoic rifting, the second model stage simulates destabilization and melting of these initial metasomes, re-enriching the surrounding lithosphere to generate new Mesozoic metasomes. Melts of these metasomes, when combined with melts of the regional asthenosphere, are consistent with the observed trace element and isotopic signatures of Turkana Miocene lavas. These findings suggest an important role for the continental lithospheric mantle during rifting and obviate the need for a complex, heterogenous plume.