Seismic evidence for a chemically distinct thermochemical reservoir in Earth's deep mantle beneath Hawaii

Abstract

Nearly antipodal continent-sized zones of reduced seismic shear wave velocities exist at the base of Earth's mantle, one beneath the Pacific Ocean, the other beneath the South Atlantic Ocean and Africa. Geophysicists have attributed the low velocity zones to elevated temperatures associated with large-scale mantle convection processes, specifically, hot mantle upwelling in response to cooler subduction-related downwelling currents. Hypotheses have included superplumes, isochemical heterogeneity, and stable as well as metastable basal thermochemical piles. Here we analyze waveform broadening and travel times of S waves from 11 deep focus earthquakes in the southwest Pacific recorded in North America, resulting in 8500 seismograms studied that sample the deep mantle beneath the Pacific. Waveform broadening is referenced to a mean S-wave shape constructed for each event, to define a relative-misfit‖. Large misfits are consistent with multipathing that can broaden wave pulses. Misfits of deep mantle sampling S-waves infer that the structure in the northeast

Morphology of seismically slow lower-mantle structures

“…Our results are consistent with the inference of steeper sides of the West Pacific anomaly. Zhao et al (2015) analyse the same anomaly beneath the northern Pacific using observations of waveform broadening and suggest it extends to 600-900 km above the CMB and also argue that it has shallowly sloped sides. The specific geometries used by He & Wen (2012) and Zhao et al (2015) constrain the shallow slopes only on the northern and eastern edge of this anomaly.…”

“…Zhao et al (2015) analyse the same anomaly beneath the northern Pacific using observations of waveform broadening and suggest it extends to 600-900 km above the CMB and also argue that it has shallowly sloped sides. The specific geometries used by He & Wen (2012) and Zhao et al (2015) constrain the shallow slopes only on the northern and eastern edge of this anomaly. Our vote map suggests that this anomaly, which we dubbed the Superswell anomaly, does indeed have shallowly sloped sides, but also continues further to the south where it extends throughout the lower mantle (Fig.…”

“…4dD). Therefore, the vertical extents inferred by He & Wen (2012) and Zhao et al (2015) would represent apparent heights along the corridor where their data are sensitive to the anomaly, and would not be representative of the maximum height of the anomaly. The height of the Superswell anomaly up to the mantle transition zone beneath the southern Pacific Ocean is independently confirmed by the P-wave tomography models of Tanaka et al (2009) and Suetsugu et al (2009).…”

Continent-sized anomalous zones with low seismic velocity at the base of Earth's mantle

“…A number of high-resolution, forward modelling seismic studies have shown that shear waves travelling in the deep mantle near LLSVP edges become broadened and/or shifted in time 2,5,[9][10][11] . This is consistent with a phenomenon called multi-pathing, where a seismic wave incident on strong heterogeneity can parse into energy that travels through the heterogeneity, as well as energy that refracts around it.…”

Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

“…Long-lived thermochemical mantle upwellings (e.g. Zhao et al, 2015) could continuously advect peridotite that is oxidising enough to enable redox melting at 250 km depth. Alternatively, low-density methane-rich fluids continually rising from the deep upper mantle and transition zone could be oxidised at the depth of metal saturation, providing a continuous supply of reduced carbon from which carbonated melt could be generated near the base of thick lithospheres (Rohrbach et al, 2007;Frost and McCammon, 2008;Rohrbach and Schmidt, 2011) (Fig.…”