Seismic and Mineral Physics Constraints on the D" Layer

Abstract

We present a synthesis of results from seismic and mineral physics studies of the D" layer to improve constraints on Earth's lowermost mantle. We focus on structures of two length scales, the seismic reflector at the top of this region and the structures of smaller scale known as ultralow velocity zones (ULVZs). A review of seismic observations for the D" layer and their interpretation is followed by a case study, where we combine new data with published results for the D" region beneath the Bering Sea and Alaska. Our evaluation of plausible interpretations includes features related to subducted slab debris that trade-off with petrology (mineral phase assemblage) and post-perovskite crystal chemistry. For the smaller-scale structures that exhibit lower than average seismic wavespeeds, the ULVZs, we consider hypotheses involving either iron-rich (Mg,Fe)O magnesiowüstite or iron-rich (Mg,Fe)SiO₃ post-perovskite. We use mineral physics data and previously published seismic reduction ratios of S- and P-wavespeeds with their respective uncertainties for a best-fit optimization approach with parameter correlations. Seismic wave reduction ratios ranging from 2 to 3 are well explained by phase assemblages containing elastically anisotropic iron-rich (Mg,Fe)O. Observations outside this range require different explanations than those considered here, thus suggesting diverse origins for ULVZs.

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