LLSVP can be explained by recycled oceanic crust

A.R. Thomson, W.A. Crichton, J.P. Brodholt, I.G. Wood, N.C. Siersch, J.M.R. Muir, D.P. Dobson, S.A. Hunt

Seismic velocities of CaSiO3 perovskite can explain LLSVPs in Earth’s lower mantle

Nature, 572, 643-647 (2019), https://www.nature.com/articles/s41586-019-1483-x

The origin of heterogeneities in the Earth’s lower mantle recorded by seismological investigations ‒ also known as large low-shear-velocity provinces (LLSVP) ‒ has remained enigmatic for several years. In order to interpret the seismic velocities observed in this region, knowledge about the elastic properties of all constituents of the lower mantle is required. Investigations of the third most abundant lower-mantle mineral CaSiO3 perovskite have been found to be especially challenging, due to its unrecoverable nature that makes quenching of samples to ambient conditions impossible.

High pressure and temperature ultrasonic experiments combined with synchrotron X-ray diffraction and radiography were performed on CaSiO3 perovskite in the large volume press at beamline ID06 of the ESRF. Since CaSiO3 perovskite cannot be recovered, and therefore cannot be synthesized prior to the ultrasonic run, an alternative approach was used. Walstromite-structured CaSiO3 was directly transformed to CaSiO3 perovskite inside the experiment at pressure and temperature. Moreover, the influence of Ti on the stability of the CaSiO3 structure and elastic properties was investigated.

Experiments revealed that a modest enrichment of recycled oceanic crust can explain the seismic properties of LLSVP’s in the Earth’s lower mantle. Additionally, it was demonstrated that the tetragonal to cubic phase transition in Ti-bearing CaSiO3 perovskite may be responsible for seismologically observed mid mantle discontinuities.

Schematic representation

Compressional-wave velocity (a) and shear-wave velocity (b) of cubic CaSiO3 determined in this and previous studies. Predictions of the wave velocity of cubic CaSiO3 at lower mantle conditions are shown here with the thick colored curve, which is based on finite-strain modelling.

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