Abstract
Regional seismic studies of the Earth's deep mantle indicate the presence of P- and S-wave reflectors at the top of the D′′ layer. Such reflectors appear to be laterally intermittent, occur at a range of depths, and produce waveforms whose amplitude and polarity differ at different locations. A number of
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possible explanations for D′′ reflections have been proposed, including: a phase change from perovskite to post-perovskite; thermal or thermochemical discontinuities generated by subducted slabs; alignment of anisotropic minerals and global or small-scale chemical heterogeneity arising perhaps from core–mantle interactions or primitive material. However, some of the observed heterogeneity in D′′ structure may be related to lack of consistency between the style, quality and number of seismic observations in different regions, as well as complexity in wave propagation due to 3-D structure. In this study, we present a compilation of observations of D′′ reflections for regions in which (1) a reflector is seen in both P- and S-wave arrivals and (2) consistent array seismology techniques have been applied to infer the properties of the reflectors. Using a recent Monte Carlo thermodynamic modelling method to convert changes in chemistry and mineralogy into changes in seismic velocity, we compare the fit of different isotropic thermochemical structures to the observed seismic properties of D′′ reflections, namely the polarity and amplitude of the waveforms. Our results indicate that it may be possible to discriminate between different possible origins for D′′ reflections on the basis of polarity observations alone. Amplitude measurements may provide further constraints but must be treated with caution due to a large number of factors, which can influence waveform amplitudes. In regions such as the Caribbean, which are characterized by high-amplitude, negative polarity P waveforms and positive polarity S waveforms, a phase change from perovskite to post-perovskite provides the most likely explanation for the discontinuity. However, in other regions, notably Siberia and the central Pacific, P- and S waveforms both display a positive polarity, and multiple explanations for the discontinuity are possible, including subduction-related chemical heterogeneity and anisotropy.
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