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A poorly mixed mantle transition zone and its thermal state inferred from seismic waves

Abstract

The abrupt changes in mineralogical properties across the Earth’s mantle transition zone substantially impact convection and thermochemical fluxes between the upper and lower mantle. While the 410-km discontinuity at the top of the mantle transition zone is detected with all types of seismic waves, the 660-km boundary is mostly invisible to underside P-wave reflections (P660P). The cause for this observation is debated. The dissociation of ringwoodite and garnet into lower-mantle minerals both contribute to the ‘660’ visibility; only the garnet reaction favours material exchanges across the discontinuity. Here, we combine large datasets of SS and PP precursors, mineralogical modelling and data-mining techniques to obtain a global thermal map of the mantle transition zone, and explain the lack of P660P visibility. We find that its prevalent absence requires a chemically unequilibrated mantle, and its visibility in few locations is associated with potential temperatures greater than 1,800 K. Such temperatures occur in approximately 0.6% of Earth, indicating that the 660 is dominated by ringwoodite decomposition, which tends to impede mantle flow. We find broad regions with elevated temperatures beneath the Pacific surrounded by major volcanic hotspots, indicating plume retention and ponding of hot materials in the mantle transition zone.

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Fig. 1: SS and PP precursory reflections underneath the 410 and 660, and global data coverage.
Fig. 2: Global observations of the 410 and 660 in SS and PP data.
Fig. 3: Effects of temperature on MTZ mineralogical phase transitions, and SS and PP precursors.
Fig. 4: Thermal model of the MTZ and implications for global mantle circulation.

Data availability

Waveform data are publicly available from the IRIS Data Management Center (NSF grant EAR-1063471). Measurements of the 410 and 660 km discontinuities, the thermal map and velocity models are available from the ISC Repository (https://doi.org/10.31905/7M3LMG8X).

Code availability

A downsampled database of synthetic seismograms for mineral-physics models is available from https://zenodo.org/record/5512035. A simplified version of the adaptive stacking code is available from https://zenodo.org/record/5512805. The full database and software are available from the corresponding author upon request.

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Acknowledgements

L.W. acknowledges the Australian National University as host institution for the majority of this research, as well as inspiration through discussion at the Cooperative Institute for Dynamic Earth Research summer programme. CIDER-II is funded as a ‘Synthesis Center’ by the Frontiers of Earth Systems Dynamics (FESD) programme of the National Science Foundation (NSF) under grant no. EAR-1135452. The authors thank I. Campbell, L. Moresi and E. Debayle for insightful discussions and comments, and acknowledge support from a Discovery Early Career Research Award (project no. DE170100329), funded by the Australian Government (L.W.), the NSF under grant nos. EAR-1661985 and EAR-1853662 (L.W.), the European Union’s Horizon 2020 Research and Innovation programme under the Marie Skłodowska-Curie grant agreement 793824 (B.T.) and Australian Research Council grants LP170100233 and DP190102940 (J.C.A.). Calculations were performed on the ANU Terrawulf cluster, a computational facility developed with support from the AuScope initiative. AuScope Ltd is funded under the National Collaborative Research Infrastructure Strategy (NCRIS), an Australian Commonwealth Government Programme.

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The study was conceived by L.W. and B.T. The datasets were compiled by L.W., partially supervised by N.C.S. B.T. wrote the adaptive stacking and synthetic modelling codes, with some contribution by L.W. B.T. and L.W. ran the synthetic experiments. L.W. implemented the analysis on observed data. All authors discussed the results and interpretations, and contributed to the manuscript.

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Correspondence to Lauren Waszek.

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Peer review information Nature Geoscience thanks Sidao Ni, Saskia Goes and Johannes Buchen for their contribution to the peer review of this work. Primary Handling editors: Stefan Lachowycz and Simon Harold.

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Supplementary methods, discussion, references, Figs. SI1–SI31 and Table SI1.

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Waszek, L., Tauzin, B., Schmerr, N.C. et al. A poorly mixed mantle transition zone and its thermal state inferred from seismic waves. Nat. Geosci. 14, 949–955 (2021). https://doi.org/10.1038/s41561-021-00850-w

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