The nature of the Lehmann discontinuity from its seismological Clapeyron slopes

While the existence of the Lehmann discontinuity at 220 km in Earthʹs mantle has been known for over 40 years from seismology, it is still debated what causes this discontinuity. Here we report measurements of seismological Clapeyron slopes for the Lehmann discontinuity, which are key to explaining the seismic discontinuity as either a manifestation of phase transitions or of other mineral physical processes. The Clapeyron slopes are measured by correlating discontinuity depths with local velocity perturbations from a tomographic model, assuming that the velocity perturbations are solely due to temperature variations. We find that in most regions the Lehmann discontinuity is characterised by a regionally varying negative seismological Clapeyron slope. Reflections from greater depths (250–350 km), which we associate with the X-discontinuity, appear in clusters without a clear Clapeyron slope. In seeking mineralogical explanations of the Lehmann and X-discontinuities, we can reject hypotheses in disagreement with these observations. Known phase transitions in the upper mantle above 400 km depth all have positive Clapeyron slopes. In the case of the Lehmann discontinuity, the only remaining hypothesis for a negative Clapeyron slope is that it represents the transition in deformation mechanism from dislocation to diffusion creep. The only exception is the Middle East region, where a positive Claperyon slope is observed which could be explained by a phase transition from coesite to stishovite. In the case of the X-discontinuity, we are not aware of mineral physical mechanisms possessing the required behaviour.