A widely accepted explanation for deep-focus earthquakes is that they are caused by the delayed transformation kinetics of dry olivine, which would seem to require dry subducting slabs. However, many geochemical and geophysical observations and mineral physics data indicate that water is present within both hydrous and nominally anhydrous minerals, implying hydrated subducting slabs. The presence of metastable olivine in wet slabs is therefore paradoxical, and the hydration state of the slabs remains an open question. Here, we report results of water-partitioning experiments between olivine, wadsleyite and a major dense hydrous magnesium silicate in slabs, hydrous phase A, under water-undersaturated conditions. We show that olivine and wadsleyite coexisting with hydrous phase A are kinetically dry and contain less than 1 ppm and approximately 300 ppm water, respectively. Our results suggest that olivine and wadsleyite show dry transformation kinetics even in wet slabs. It is therefore possible that olivine transformation as a cause of deep-focus earthquakes and large slab deformation creating stagnant slabs could occur in the water-undersaturated wet slabs. These processes could be caused jointly by dehydration of hydrous minerals and the subsequent rapid phase transformation when the dehydration starts at lower temperatures than the phase transformation.
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All data used in this paper are available on Zenodo (https://doi.org/10.5281/zenodo.3936232). Any other data can be requested by emailing the corresponding author.
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We thank H. Fischer for preparation of cell assemblies, R. Njul for preparation of thin-section samples, D. Wiesner and F. Ferreira for measurements of EBSD patterns of the samples and H. Keppler for measurements of water in the samples by using FTIR, at the Bayerisches Geoinstitut. We also appreciate D. J. Frost and T. Katsura for valuable discussion on this topic and T. Withers for useful comments on the manuscript. This research was supported by the Grants-in-Aid of the German Research Foundation (no. IS350/1-1) to T.I. and the Kakenhi Grants JP15H05748 and JP20H00187 from Japan Society for the Promotion of Science to E.O. E.O. was supported also by the research award from the Alexander von Humboldt foundation.
The authors declare no competing interests.
Peer review information Nature Geoscience thanks Jean-Philippe Perrillat and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Rebecca Neely.
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Water-partitioning experiments between a, forsterite and hydrous phase A and b, wadsleyite and hydrous phase A. In the experiments for forsterite, oriented crystals with 200 μm a axis × 400 μm b axis × 400 μm c axis were put in the capsules as shown in the right capsule in Extended Data Fig. 1a.
Extended Data Fig. 2 Representative X-ray diffraction patterns of the recovered samples at the surrounding part of the single crystals.
a, 10 GPa and 800 °C (S7423). b, 12 GPa and 900 °C (H5073). Fo, forsterite; Wd, wadsleyite; Cen, clinoenstatite; PhA, hydrous phase A.
Experimental conditions and results of phase identification and water contents in single crystals.
Details of FTIR measurements of forsterite and wadsleyite single crystals.
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Ishii, T., Ohtani, E. Dry metastable olivine and slab deformation in a wet subducting slab. Nat. Geosci. 14, 526–530 (2021). https://doi.org/10.1038/s41561-021-00756-7