Sound velocity measurements of CaSiO3 perovskite to 133 GPa and implications for lowermost mantle seismic anomalies

Abstract

We report the measurements of aggregate shear velocity (VS) of CaSiO3 perovskite (CaPv) at high pressure (P) between 32 and 133GPa and room temperature (T) on the basis of Brillouin spectroscopy. The sample had a tetragonal perovskite structure throughout the experiments. The measured P–VS data show the shear modulus and its pressure derivative at ambient condition to be G0=115.8GPa and G'=1.20, respectively. The zero-pressure shear velocity is determined to be VS0=5.23km/s, in good agreement with the previous estimate inferred from the ultrasonic measurements on Ca(Si,Ti)O3 perovskite at 1bar. Our experimental results are broadly consistent with the earlier calculations on tetragonal CaPv but exhibit lower velocity at equivalent pressure. Such tetragonal CaPv is present in cold subducting slabs and possibly in wide areas of the lowermost mantle. While primitive mantle includes certain amount of CaPv, a depleted peridotite (former harzburgite) layer in subducted oceanic lithosphere is deficient in CaPv and enriched in ferropericlase in the lower mantle. Such harzburgite exhibits 0.9% faster VS and 0.7% slower bulk sound velocity (VΦ) at the lowermost mantle P–T conditions if CaPv is present in the tetragonal form in the surrounding mantle. The observed fast VS and slow VΦ anomalies in the D” layer underneath the circum-Pacific region might be attributed in large part in the presence of subducted harzburgitic materials.