The Earth's mantle

Seismological images of the Earth's mantle reveal three distinct changes in velocity structure, at depths of 410, 660 and 2,700 km. The first two are best explained by mineral phase transformations, whereas the third-the D" layer-probably reflects a change in chemical composition and thermal structure. Tomographic images of cold slabs in the lower mantle, the displacements of the 410-km and 660-km discontinuities around subduction zones, and the occurrence of small-scale heterogeneities in the lower mantle all indicate that subducted material penetrates the deep mantle, implying whole-mantle convection. In contrast, geochemical analyses of the basaltic products of mantle melting are frequently used to infer that mantle convection is layered, with the deeper mantle largely isolated from the upper mantle. We show that geochemical, seismological and heat-flow data are all consistent with whole-mantle convection provided that the observed heterogeneities are remnants of recycled oceanic and continental crust that make up about 16 and 0.3 per cent, respectively, of mantle volume.     

Inner-core shear-wave anisotropy and texture from an observation of PKJKP waves

Since the discovery of the Earth's core a century ago, and the subsequent discovery of a solid inner core (postulated to have formed by the freezing of iron) seismologists have striven to understand this most remote part of the deep Earth. The most direct evidence for a solid inner core would be the observation of shear-mode body waves that traverse it, but these phases are extremely difficult to observe. Two reported observations in short-period data have proved controversial. Arguably more successful have been studies of longer-period data, but such averaging limits the usefulness of the observations to reported sightings. We present two observations of an inner-core shear-wave phase at higher frequencies in stacked data from the Japanese High-Sensitivity Array, Hi-Net. From an analysis of timing, amplitude and waveform of the 'PKJKP' phase we derive constraints on inner-core compressional-wave velocity and shear attenuation at about 0.3 Hz which differ from standard isotropic core models. We can explain waveform features and can partially reconcile the otherwise large differences between core wavespeed and attenuation models that our observations apparently suggest if we invoke shear-wave anisotropy in the inner core. A simple model of an inner core composed of hexagonal close-packed iron with its c axis aligned perpendicular to the rotation axis yields anisotropy that is compatible with both the shear-wave anisotropy that we observe and the well-established 3 per cent compressional-wave anisotropy.     

Outer-core compositional stratification from observed core wave speed profiles

Light elements must be present in the nearly pure iron core of the Earth to match the remotely observed properties of the outer and inner cores. Crystallization of the inner core excludes light elements from the solid, concentrating them in liquid near the inner-core boundary that potentially rises and collects at the top of the core, and this may have a seismically observable signal. Here we present array-based observations of seismic waves sensitive to this part of the core whose wave speeds require there to be radial compositional variation in the topmost 300?km of the outer core. The velocity profile significantly departs from that of compression of a homogeneous liquid. Total light-element enrichment is up to five weight per cent at the top of the core if modelled in the Fe-O-S system. The stratification suggests the existence of a subadiabatic temperature gradient at the top of the outer core.