Probing the upper mantle transition zone under Africa with P520s conversions: Implications for temperature and composition

Abstract

We have investigated the nature of the 520km discontinuity under Africa by migrating P-wave receiver functions at 30 permanent broadband stations in the continent. Worldwide investigations with SS precursors have revealed that the detection of this discontinuity is intermittent and that it may split into two separate discontinuities, ~60km apart, in some regions. Several explanations have been proposed to account for both the intermittent detection and the splitting of the 520-km discontinuity, ranging from lateral temperature variations in the transition zone to compositional variations due to changes in minor phases containing Fe, Ca, and water. Our results reveal that detection of the 520km discontinuity is intermittent under Africa and that – when observed – it does not split. Detectability does not correlate with published maps of transition zone temperature, temperature estimates from transition zone thickness, or published potential temperature estimates from primitive magmas throughout East Africa. It does not correlate with published maps of transition zone water content, as well. Lateral variation in iron content due, for instance, to varying proportions of the transition zone׳s constituent minerals offers a plausible explanation for the observed intermittency in detection, but better maps of transition zone iron content need to be developed to assess a correlation. Focusing and defocusing effects due to small-scale variations around the stations are also a possibility. The lack of splitting, on the other hand, is found consistent with low-Ca contents and the general absence of present-day subduction around the continent. However, a regional increase in mantle fertility under West Africa – as previously reported with analysis of SS precursors – cannot be ruled out with our analysis.