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

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Abstract

We report the measurements of aggregate shear velocity (VS) of CaSiO3 perovskite (CaPv) at high pressure (P) between 32 and 133 GPa and room temperature (T) on the basis of Brillouin spectroscopy. The sample had a tetragonal perovskite structure throughout the experiments. The measured PVS data show the shear modulus and its pressure derivative at ambient condition to be G0=115.8 GPa and G'=1.20, respectively. The zero-pressure shear velocity is determined to be VS0=5.23 km/s, in good agreement with the previous estimate inferred from the ultrasonic measurements on Ca(Si,Ti)O3 perovskite at 1 bar. 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 PT 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.

Highlights

► We first measured sound velocities of CaSiO3 perovskite (CaPv) to 133 GPa. ► They are much slower than those of the other lower mantle phases. ► The results are generally in good agreement with the previous calculations. ► Subducted harzburgite may explain fast shear and slow bulk sound velocities anomalies in D”.

Introduction

CaSiO3 perovskite (CaPv) is an important mineral in both transition zone and lower mantle. It’s mineral proportions are supposed to be about 5 and 25 vol% in pyrolitic mantle and subducted mid-oceanic ridge basalt (MORB) materials, respectively, under the lower mantle conditions (e.g., Kesson et al., 1998, Murakami et al., 2005, Hirose et al., 2005). Therefore, elastic properties of CaPv are of great importance to interpret the seismic wave velocity structure in the lower mantle. First principles calculations by Karki and Crain (1998) demonstrated that sound velocities of cubic CaPv, in particular shear velocity, are much faster than MgSiO3 perovskite. Karato and Karki. (2001) argued that the high shear to longitudinal wave velocity heterogeneity ratio observed in the deep lower mantle can be reconciled with the variation in the abundance of CaPv in addition to the Fe/(Mg+Fe) ratio.

CaPv adopts the cubic perovskite structure at high temperature, while it distorts to tetragonal symmetry with decreasing temperature (e.g., Stixrude et al., 1996, Shim et al., 2002, Akbar-knutson et al., 2002, Kurashina et al., 2004, Caracas et al., 2005). The P–T condition of such tetragonal-cubic phase transition is still under debate (e.g., Li et al., 2006, Stixrude et al., 2007, Komabayashi et al., 2007). While the most recent calculations by Tsuchiya (2011) demonstrated that the sound velocity of tetragonal CaPv is comparable to that of the cubic phase, and the earlier theoretical works predicted that cubic CaPv exhibits much higher velocity of ∼30% than tetragonal CaPv (Li et al., 2006, Stixrude et al., 2007).

Despite its importance in the lower mantle, sound velocities of CaPv were measured at high pressure only up to 12 GPa by ultrasonic method (Li et al., 2004). In addition, sound velocity measurements were performed only on CaTiO3 and Ca(Si0.5,Ti0.5)O3 perovskites at 1 bar (Sinelnikov et al., 1998), because CaPv is unstable at ambient condition. In this study, we synthesized pure CaSiO3 perovskite at high pressure in a laser-heated diamond-anvil cell (DAC) and subsequently measured its shear velocity by Brillouin spectroscopy. The measurements were made at high pressure ranging from 32 to 133 GPa at room temperature. With previously reported bulk modulus of CaPv, longitudinal velocity (VP) is also estimated to deep lower mantle pressures.

Section snippets

Experimental method

High-pressure Brillouin scattering measurements were conducted at room temperature in a symmetric DAC with 60° angular aperture (see Murakami et al., 2009a for details). We used CaSiO3 gel as a starting material, which was dehydrated completely beforehand by heating at 1273 K in a furnace. The powder of the starting material and NaCl pressure medium were loaded into a hole in a rhenium gasket (NaCl:CaSiO3:NaCl=1:4:1 in thickness). They were compressed to high pressure with flat 300 μm culet (<60 

Results

We obtained the sharp Brillouin peaks for shear acoustic mode of CaPv over the entire pressure range explored, although longitudinal mode overlapped with the diamond peak. Representative spectra collected at 33 and 80 GPa are shown in Fig. 2. The peaks from NaCl pressure medium were not found in all the measurements, probably due to its small thickness. The aggregate shear velocities of tetragonal CaPv measured in this study are summarized in Table 1 and plotted as a function of pressure in Fig.

Comparison with theoretical predictions

Present measurements on tetragonal CaPv at room temperature show lower velocity of ∼4% than earlier theoretical calculations at 0 K and equivalent pressure (Stixrude et al., 2007, Tsuchiya, 2011) (Fig. 3). Such difference is not reconciled with the temperature difference by 300 K. The discrepancy may be caused by the fact that our sample included some defects, similar to natural samples, which diminish the velocity. In addition, crystal structure of tetragonal CaPv in the present experiments

Acknowledgments

We thank T. Komabayashi, E. Sugimura and N. Sata for their support in the Brillouin spectroscopy measurements and helpful discussions. The reviewers’ comments were very helpful to improve the manuscript. Experiments were conducted at SPring-8 (proposal nos. 2010A0087, 2010B0087, and 2011A0087). Y.K. was supported by the Global COE program “From the Earth to Earths”, MEXT, Japan.

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