Research press release





今回、Steeve Greauxたちの研究グループは、実験室内で立方晶CaSiO3ペロブスカイトを作製し、地球の上部マントル底部に相当する条件に至るさまざまな圧力と温度の条件下で、超音波速度測定を行った。その結果、CaSiO3ペロブスカイトの剛性が理論予測値より約25%低く、そのため、超音波はこれまで考えられていたよりもゆっくり移動することが明らかになった。Greauxたちは、この結果が、下部マントル最上部に海洋地殻が蓄積しているというこれまでの仮説の証拠になると考えている。

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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