Sound velocities of majorite garnet and the composition of the mantle transition region

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Abstract

The composition of the mantle transition region, characterized by anomalous seismic-wave velocity and density changes at depths of 400 to 700 km, has remained controversial. Some have proposed that the mantle transition region has an olivine-rich ‘pyrolite’ composition1,2, whereas others have inferred that it is characterized by pyroxene- and garnet-rich compositions (‘piclogite’), because the sound velocities in pyrolite estimated from laboratory data are substantially higher than those seismologically observed3,4,5. Although the velocities of the olivine polymorphs at these pressures (wadsleyite and ringwoodite) have been well documented, those of majorite (another significant high-pressure phase in the mantle transition region) with realistic mantle compositions have never been measured. Here we use combined in situ X-ray and ultrasonic measurements under the pressure and temperature conditions of the mantle transition region to show that majorite in a pyrolite composition has sound velocities substantially lower than those of earlier estimates, owing to strong nonlinear decreases at high temperature, particularly for shear-wave velocity. We found that pyrolite yields seismic velocities more consistent with typical seismological models than those of piclogite in the upper to middle parts of the region, except for the potentially larger velocity jumps in pyrolite relative to those observed at a depth of 410 km. In contrast, both of these compositions lead to significantly low shear-wave velocities in the lower part of the region, suggesting possible subadiabatic temperatures or the existence of a layer of harzburgite-rich material supplied by the subducted slabs stagnant at these depths.

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Figure 1: Experimental conditions and results of the combined ultrasonic and in situ X-ray measurements on a polycrystalline majorite sample with pyrolite minus olivine composition.
Figure 2: Variations of sound velocities of majorite with temperature for the selected pressures in MTR.
Figure 3: A comparison of the sound velocities for pyrolite and piclogite compositions with representative seismological models in the MTR.

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Acknowledgements

We thank B. Li, R. C. Liebermann, J. Kung, I. Jackson for their help and advice in ultrasonic techniques, T. Shinmei, A. Yamada, T. Kunimoto, Y. Tange, N. Nishiyama for their assistance in synchrotron experiments, and C. A. McCammon and I. Jackson for comments on the manuscript. This paper was written while T. Irifune was on leave at the Bayreuth Geoinstitute, and supported by the Humboldt Foundation. This study is based on a research proposal to SPring-8 with a Grant-in-Aid for Scientific Research from the Japanese government (to T. Irifune).

Author Contributions T. Irifune directed the research project and wrote the manuscript. Y.H. did most of the experiments and analyses of the data with the help of T. Inoue, Y.K. and K.F. The TEM and XRD analyses of the recovered sample were conducted by H.O. and T. Inoue, respectively. All authors discussed the results and commented on the manuscript.

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Correspondence to T. Irifune.

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Irifune, T., Higo, Y., Inoue, T. et al. Sound velocities of majorite garnet and the composition of the mantle transition region. Nature 451, 814–817 (2008) doi:10.1038/nature06551

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