[S-IT14] Deep Earth Sciences

[SIT14-P07] Substitutional effect of iron on water incorporation in hydrous ringwoodite constrained by multiple methods

*Youyue Zhang^1,2, Takashi Yoshino², Xuejing He³, Hiroyuki Kagi³, Asami Sano-Furukawa⁴, Jun Abe⁵ (1.Geodynamics Research Center, Ehime University , 2.Institute for Planetary Materials, Okayama University, 3.Geochemical Research Center, Graduate School of Science, The University of Tokyo, 4.J-PARC Center, Japan Atomic Energy Agency, 5.Neutron Science and Technology Center, Comprehensive Research Organization for Science and Society (CROSS))

Keywords:Ringwoodite, Neutron diffraction, High pressure, Planetary mantle, Water

Water is an important volatile that affects the physical and chemical properties of planetary interiors, such as those of Earth and Mars [1]. Experimental studies have demonstrated that hydrogen in minerals has considerable effects on electrical conductivity, thermal conductivity, viscosity, etc. The involvement of water therefore critically alters the planetary evolution over geological time, such as triggering plate tectonics. Ringwoodite, the high-pressure polymorph of olivine, constitutes about 60 vol% of the Earth's mantle transition zone (MTZ) [2], and about 80 vol% of the harzburgitic slab [3]. Recent seismological observation also indicates the existence of Fe-rich ringwoodite in the deep part of the Martian mantle [4]. Its abundance, combined with its water solubility of up to 2.7 wt% [5], makes ringwoodite a potential water reservoir in the Earth's and Mars's interior. Though the comparison of the aforementioned studies [6][7] has suggested that the substitution of Fe in the ringwoodite structure can affect the sites of proton, the effects of Fe on the behaviors of water in ringwoodite under high pressure have not been well constrained.

In this study, deuterated Fe-bearing ringwoodite samples with various Fe contents ((Mg_0.9Fe_0.1)₂SiO₄ and (Mg_0.7Fe_0.3)₂SiO₄) were synthesized under a water-saturated condition at 20 GPa and 1475 K using a Kawai-type multi-anvil apparatus. The chemical compositions of the synthesized samples were characterized by electron microprobe analyses (EPMA). The amounts of water were quantified by Fourier Transform Infrared Spectroscopy (FTIR). Mössbauer spectroscopy measurements were carried out to determine the valence states of the samples. The crystal structures and the proton sites of (Mg_0.9Fe_0.1)₂SiO₄ and (Mg_0.7Fe_0.3)₂SiO₄ were worked out using neutron powder diffraction and single crystal X-ray diffraction up to 7 GPa. Preliminary results suggest that the increase of iron content in ringwoodite can lead to the increase of water solubility and ferric iron content in the samples. Such effects of iron on water incorporation may indicate the different physical chemical performances of ringwoodite in Earth's mantle and Mars's core-mantle boundary.


Reference: [1] Karato & Jung, Earth Planet. Sci. Lett., 157,193-207 (1998). [2] Frost, Elements 4 (3): 171-176 (2008). [3] Irifune & Ringwood, Geophys. Res. Lett. 14, 1546-1549 (1987). [4] Huang et al., Proceedings of the National Academy of Sciences 119.42 (2022). [5] Kohlstedt et al., Contributions to Mineralogy and Petrology, 123, 345-357. (1996). [6] Purevjav et al., Geophysical Research Letters 41.19: 6718-6724 (2014). [7] Purevjav et al., Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials 74.1: 115-120 (2018).