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Three-dimensional models of Earth's seismic structure can be used to identify temperature-dependent phenomena, including mineralogical phase and spin transformations, that are obscured in 1-D spherical averages. Full-waveform tomography maps seismic wave-speeds inside the Earth in three dimensions, at a higher resolution than classical methods. By providing absolute wave speeds (rather than perturbations) and simultaneously constraining bulk and shear wave speeds over the same frequency range, it becomes feasible to distinguish variations in temperature from changes in composition or spin state. We present a quantitative joint interpretation of bulk and shear wave speeds in the lower mantle, using a recently published full-waveform tomography model. At all depths the diversity of wave speeds cannot be explained by an isochemical mantle. Between 1000 and 2500 km depth, hypothetical mantle models containing an electronic spin crossover in ferropericlase provide a significantly better fit to the wave-speed distributions, as well as more realistic temperatures and silica contents, than models without a spin crossover. Below 2500 km, wave speed distributions are explained by an enrichment in silica towards the core-mantle boundary. This silica enrichment may represent the fractionated remains of an ancient basal magma ocean. This study reveals that in the Earth's mid-mantle, ferropericlase (the second most abundant mineral) undergoes a major electronic reconfiguration. At the base of the mantle, an enrichment in silica may represent a crystallised ancient magma ocean. [ABSTRACT FROM AUTHOR]

An analysis of seismic data reveals the location and quantity of melted rock, known as melt, in Earth's upper mantle. The results show how these factors are correlated with the movement of the planet's tectonic plates. A possible explanation for the low-velocity zone in the mantle. [ABSTRACT FROM AUTHOR]