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\begin{document}

\title{The X-discontinuity as a Tracer for Chemical Heterogeneity: Observations
from East Africa}



\author[1]{Stephen Pugh}%
\author[1]{Alistair Boyce}%
\author[2]{Ian Bastow}%
\author[3]{Cynthia Ebinger}%
\author[1]{Sanne Cottaar}%
\affil[1]{University of Cambridge}%
\affil[2]{Imperial College London}%
\affil[3]{Tulane University of Louisiana}%


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  \date{May 25, 2024}


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\begin{abstract}
Previous studies of the East African upper mantle have invoked one or
more mantle upwellings with varying thermochemical nature to underly the
distribution of surface volcanism. For example, Boyce and Cottaar (2021)
suggest that a hot, chemically distinct upwelling beneath the southern
East African Rift (EAR) is sourced from the African Large Low Velocity
Province (LLVP), while magmatism in Ethiopia may lie above an additional
purely thermal upwelling. Constraints on chemical heterogeneities in the
upper mantle may be derived from studying the seismically observable
impedance contrasts that they produce. Away from subduction zones, two
causal mechanisms are possible to explain the X-discontinuity (X;
230-350km): the coesite-stishovite phase transition and/or carbonate
silicate melting, both of which require entrainment of basalt from the
lower mantle. Intriguingly, carbonate silicate melt was invoked by
Rooney et al., (2012) to explain the discrepancy in upper mantle
temperature anomalies predicted by seismic wavespeed and petrological
estimates beneath East Africa. Further, active carbonatite magmatism
occurs along the edge of the Tanzanian craton (Muirhead et al., 2020).
Several recent regional to continental receiver function (RF) studies
have identified potential observations of the X in East Africa. These
studies are not focused on the presence of these upper mantle phases or
lack the spatial sampling needed to robustly identify the X and its
causal mechanism. Targeted high-resolution observations of the X are
required to confirm the presence of exotic converted phases in the East
African upper mantle and their relationship to mantle upwellings. We
capitalise on the new TRAILS dataset from the Turkana depression
(Bastow, 2019; Ebinger, 2018) and an adjacent network in neighbouring
Uganda (Nyblade, 2017), to supplement our existing RF database and
characterise the X across active continental rift setting in
unprecedented detail. The prevalence of the X is mapped beneath East
Africa, and subsequently compared to other areas of the African
continent.%
\end{abstract}%



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