Earth sciences: Novel mineral measurements explain mantle mystery

The first lab-based measurements of calcium silicate perovskite, a major mantle mineral that is not stable on the Earth’s surface, are reported in this week’s Nature. The findings support the proposal that regions of subducted oceanic crust exist at the base of the upper mantle - through which seismic waves are thought to travel at anomalous velocities.

Discrepancies between models of how seismic waves pass through the Earth and equivalent lab tests suggest that at the boundary between the upper and lower mantle (560-800 km deep), chemically distinct material is present, which might be subducted oceanic crust. The mineral calcium silicate (CaSiO3) perovskite would be a major component of this material. However, lab-based velocity measurements of CaSiO3 perovskite have never been made, as it takes a different, cubic form at high temperatures than on the Earth’s surface.

Steeve Greaux and colleagues created cubic CaSiO3 perovskite in the lab and took ultrasonic sound velocity measurements of the CaSiO3 perovskite at various pressures and temperatures - up to conditions equivalent to the base of the Earth's upper mantle. They find that CaSiO3 perovskite is about 25% less rigid than theoretically predicted and that waves would therefore travel more slowly through it than previously thought. The authors suggest that these results provide evidence for the accumulation of oceanic crust in the uppermost lower mantle as previously hypothesized.

doi: 10.1038/s41586-018-0816-5

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