Distinct slab interfaces imaged within the mantle transition zone
Oceanic lithosphere descends into Earth's mantle at subduction zones and drives material exchange between Earth's surface and its deep interior. The subduction process creates chemical and thermal heterogeneities in the mantle, with the strongest gradients located at the interfaces between subducted slabs and the surrounding mantle. Seismic imaging of slab interfaces is key to understanding slab compositional layering, deep-water cycling and melting, yet the existence of slab interfaces below 200 km remains unconfirmed. Here, we observe two sharp and slightly dipping seismic discontinuities within the mantle transition zone beneath the western Pacific subduction zone that coincide spatially with the upper and lower bounds of the high-velocity slab. Based on a multi-frequency receiver function waveform modelling, we found the upper discontinuity to be consistent with the Mohorovičić discontinuity of the subducted oceanic lithosphere in the mantle transition zone. The lower discontinuity could be caused by partial melting of sub-slab asthenosphere under hydrous conditions in the seaward portion of the slab. Our observations show distinct slab–mantle boundaries at depths between 410 and 660 km, deeper than previously observed, suggesting a compositionally layered slab and high water contents beneath the slab.
© 2020 Nature Publishing Group. Received 22 October 2018; Accepted 30 September 2020; Published 09 November 2020. We thank L. Chen, H. Kawakatsu, T.-R. A. Song, S. P. Grand, J. Yang and J. Hu for numerous discussions during this work. We thank all the people who installed and service the NECsaids array. We also thank V. Lambert and P. Adamek for linguistic suggestions. This study was supported by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (grant number XDB18000000) and the National Natural Science Foundation of China (grant numbers 91958209, 41974057, 41130316 and 41630209). J.B. was supported by the National Science Foundation's Collaborative Study of Earth's Deep Interior (EAR-1161046 and EAR-2009935). Data availability: Seismic data from CSN were provided by the Data Management Center of China National Seismic Network at Institute of Geophysics, China Earthquake Administration (https://doi.org/10.11998/SeisDmc/SN, http://www.seisdmc.ac.cn, registration is required to download the data, in Chinese). The NECESSArray data were downloaded through the Incorporated Research Institutions for Seismology (https://doi.org/10.7914/SN/YP_2009). Waveforms of the NECsaids data are deposited in the Seismic Array Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences (https://doi.org/10.12129/IGGSL.Data.Observation, http://www.seislab.cn) and can be downloaded via ftp://184.108.40.206/data/NECsaids/RF2020. The Seismic Array Laboratory will make the NECsaids Array data publicly available from October 2021 (three years after the completion of the NECsaids project). In addition, the raw multi-frequency (with Gaussian low-pass filters of 0.5, 0.75, 1.0 or 1.5) receiver function waveform data (2.7Gb) obtained in this study can be downloaded either from https://doi.org/10.12197/2020GA012 (World Data System for Geophysics; http://www.geophys.ac.cn) or from ftp://220.127.116.11/data/NECsaids/RF2020. Code availability: The RF CCP stacking code was downloaded from http://www.eas.slu.edu/People/LZhu/home.html. The other codes used in this paper are available upon request from the corresponding authors. Author Contributions: X.W. and Q-F.C. designed the research. X.W. conducted the seismic analysis. X.W., Q-F.C., F.N., J.B. and L.L. contributed to the interpretation of seismic observations. X.W. wrote the manuscript, and all co-authors discussed the results and commented on the manuscript. The authors declare no competing interests. Peer review information: Primary Handling Editors: Stefan Lachowycz; Melissa Plail.
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