Anomalous compressibility in (Fe,Al)-bearing bridgmanite: implications for the spin state of iron
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ORIGINAL PAPER
Anomalous compressibility in (Fe,Al)‑bearing bridgmanite: implications for the spin state of iron Yoshiyuki Okuda1 · Kenji Ohta1 · Ryosuke Sinmyo2 · Kei Hirose3,4 · Yasuo Ohishi5 Received: 13 May 2020 / Accepted: 26 August 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The valence and spin states of Fe in (Fe,Al)-bearing bridgmanite (bdg) affect its physical properties, which is important for the interpretation of geophysical observations. Currently, tens of studies on the compressibility and spin states of Fe-bearing bdg have been reported. A consensus is that Fe-bearing bdg shows spin transition, which affects its elastic parameters. However, there is a conflict between reports on the compressibility and spin states of (Fe,Al)-bearing bdg in experiments using samples pre-synthesized in a multi-anvil apparatus (MA), and samples directly synthesized in a diamond anvil cell (DAC). There are no reports showing evidence of spin transition of Fe in compression experiments using (Fe,Al)-bearing bdg samples pre-synthesized in a MA, while those synthesized at relatively high pressure (at least above 45 GPa) in a DAC all exhibited the spin transition. Here, we performed synchrotron X-ray diffraction measurements on M g0.85Fe0.09Al0.21Si0.86O3 and Mg0.85Fe0.14Al0.05Si0.96O3 bdg synthesized at relatively high pressure in a laser-heated DAC from amorphous starting material up to 47 and 56 GPa, respectively, at room temperature. The obtained pressure (P)–lattice volume (V) relations show noticeable softening at 22–30 GPa and 35–45 GPa, respectively, which is probably due to the spin transition of Fe. Combining our results and previous reports, we suggest that the lower mantle bdg is capable of containing low-spin Fe3+, which questions the general view. Such a transition changes density and may affect the physical properties of bridgmanite such as thermal conductivity and iron partitioning coefficient, thus having profound implications for mantle dynamics, and the chemical composition of the Earth. Keywords Bridgmanite · Compressibility · Spin transition of iron · Lower mantle
Introduction
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00269-020-01109-3) contains supplementary material, which is available to authorized users. * Yoshiyuki Okuda [email protected] 1
Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152‑8551, Japan
2
Department of Physics, Meiji University, Kawasaki, Kanagawa 214‑85713, Japan
3
Department of Earth and Planetary Science, The University of Tokyo, 7‑3‑1 Hongo, Bunkyo‑ku, Tokyo 113‑0033, Japan
4
Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo 152‑8551, Japan
5
Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679‑5198, Japan
Bridgmanite (bdg), (Fe,Al)-bearing M gSiO3 perovskite is accepted to be the most abundant mineral in the Earth’s lower mantle. Bdg in the pyrolitic lower mantle is likely to
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