The influence of δ-(Al,Fe)OOH on seismic heterogeneities in Earth's lower mantle

Creators: Ohira, Itaru; Jackson, Jennifer M.; Sturhahn, Wolfgang; Finkelstein, Gregory J.; Kawazoe, Takaaki; Toellner, Thomas S.; Suzuki, Akio; Ohtani, Eiji

Abstract

The high-pressure phases of oxyhydroxides (δ-AlOOH, ε-FeOOH, and their solid solution), candidate components of subducted slabs, have wide stability fields, thus potentially influencing volatile circulation and dynamics in the Earth's lower mantle. Here, we report the elastic wave velocities of δ-(Al,Fe)OOH (Fe/(Al + Fe) = 0.13, δ-Fe13) to 79 GPa, determined by nuclear resonant inelastic X-ray scattering. At pressures below 20 GPa, a softening of the phonon spectra is observed. With increasing pressure up to the Fe³⁺ spin crossover (~ 45 GPa), the Debye sound velocity (v_D) increases. At higher pressures, the low spin δ-Fe13 is characterized by a pressure-invariant v_D. Using the equation of state for the same sample, the shear-, compressional-, and bulk-velocities (v_S, v_P, and v_Φ) are calculated and extrapolated to deep mantle conditions. The obtained velocity data show that δ-(Al,Fe)OOH may cause low-v_Φ and low-v_P anomalies in the shallow lower mantle. At deeper depths, we find that this hydrous phase reproduces the anti-correlation between v_S and v_Φ reported for the large low seismic velocity provinces, thus serving as a potential seismic signature of hydrous circulation in the lower mantle.

Additional Information

© The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Received 19 November 2020; Accepted 17 May 2021; Published 08 June 2021. We warmly thank Y. Ito for his help with polishing and performing the EPMA-analysis of the crystals used in this work, and Natalia V. Solomatova, Rachel A. Morrison, and Vasilije Dobrosavljevic for help during the experiments. This work was supported by JSPS KAKENHI Grant Numbers JP16J04690, JP20K22369 to I.O., JP15H05828, JP19H01985, JP19K21890 to A.S., JP15H05748, JP20H00187 to E.O., the International Research and Training Group "Deep Earth Volatile Cycles" funded by the German Science Foundation (Grant Number: GRK 2156/1), the JSPS Japanese-German Graduate Externship, the International Joint Graduate Program in Earth and Environmental Science, Tohoku University (GP-EES), and by Grants awarded to J.M.J by the National Science Foundation (NSF-CSEDI-EAR-1600956 and 2009935) and the W.M. Keck Institute for Space Studies. The NRIXS experiments were conducted at 3-ID-B, Advanced Photon Source, the United States, which is partially supported by COMPRES. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Data availability: The data that support the findings of this study are available from the corresponding author(s) upon reasonable request. Author Contributions: I.O., E.O. and J.M.J. designed and planned the project of this research. T.K. and I.O. synthesized the experimental samples. I.O., J.M.J, W.S., G.J.F., and T.S.T. conducted the NRIXS experiments at 3-ID-B, APS. I.O. and J.M.J analyzed the NRIXS spectra using the software developed and maintained by W.S. I.O., J.M.J., A.S., and E.O. discussed the interpretation of the experimental results. All authors contributed to the preparation the manuscript and have approved the submitted version of the manuscript. The authors declare no competing interests.

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