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A thin mantle transition zone beneath the equatorial Mid-Atlantic Ridge

Abstract

The location and degree of material transfer between the upper and lower mantle are key to the Earth’s thermal and chemical evolution. Sinking slabs and rising plumes are generally accepted as locations of transfer1,2, whereas mid-ocean ridges are not typically assumed to have a role3. However, tight constraints from in situ measurements at ridges have proved to be challenging. Here we use receiver functions that reveal the conversion of primary to secondary seismic waves to image the discontinuities that bound the mantle transition zone, using ocean bottom seismic data from the equatorial Mid-Atlantic Ridge. Our images show that the seismic discontinuity at depths of about 660 kilometres is broadly uplifted by 10 ± 4 kilometres over a swath about 600 kilometres wide and that the 410-kilometre discontinuity is depressed by 5 ± 4 kilometres. This thinning of the mantle transition zone is coincident with slow shear-wave velocities in the mantle, from global seismic tomography4,5,6,7. In addition, seismic velocities in the mantle transition zone beneath the Mid-Atlantic Ridge are on average slower than those beneath older Atlantic Ocean seafloor. The observations imply material transfer from the lower to the upper mantle—either continuous or punctuated—that is linked to the Mid-Atlantic Ridge. Given the length and longevity of the mid-ocean ridge system, this implies that whole-mantle convection may be more prevalent than previously thought, with ridge upwellings having a role in counterbalancing slab downwellings.

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Fig. 1: The PI-LAB network, data coverage and mantle transition zone delay time.
Fig. 2: Vertical cross-section from the 3D depth-migrated receiver functions.
Fig. 3: Horizontal cross-sections from the 3D depth-migrated receiver functions.
Fig. 4: Mantle flow beneath the Mid-Atlantic Ridge.

Data availability

Data are available from the Incorporated Research Institutions for Seismology (IRIS) Data Management Center (DMC) website https://ds.iris.edu/ds/nodes/dmc/ under the XS network for 2016‐2017 (https://doi.org/10.7914/SN/XS_2016). Source data are provided with this paper.

Code availability

The methods and codes used are standard and widely used42 and are detailed in the Methods section. Figures were made using Generic Mapping Tools61 and MATLAB. Correspondence and requests for materials should be addressed to M.R.A.

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Acknowledgements

C.A.R. and N.H. acknowledge funding from the Natural Environment Research Council (NE/M003507/1 and NE/K010654/1) and the European Research Council (GA 638665). J.-M.K. was funded by the Natural Environment Research Council (NE/M004643/1). We thank the captain and crew of the R/V Marcus G. Langseth and the RRS Discovery, and also the scientific technicians.

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Contributions

M.R.A. processed the data and wrote the manuscript. C.A.R. conceived the experiment, acquired funding, managed the project and wrote the manuscript. N.H. contributed to conceptualization, funding acquisition, project management and writing of the original manuscript. S.T. organized the raw data and assisted in initial data quality control. J.-M.K. contributed to funding acquisition and the writing of the manuscript.

Corresponding author

Correspondence to Matthew R. Agius.

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The authors declare no competing interests.

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Peer review information Nature thanks Brandon Schmandt and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Relationship of MTZ shear velocity with distance to ridge.

Distance-binned average velocity of the mantle transition zone for global4,5,7,56,57,58,59 and regional9,60 models. Bin averages are shown as cyan circles, with error bars showing the standard error of the mean. Red lines show the averages for distances <300 km and >300 km. Background shading shows a 2D histogram of transition zone velocities. Model name and t-statistic are given above each panel. The top two rows are presented in percentage anomaly, whereas the bottom row is in absolute velocity, as reported in the original publications. Probability at 95% occurs when the absolute value of the t-statistic is >1.67; 99% occurs at a value >2.37 for the given degrees of freedom. A negative t-statistic indicates that the mean of the sub-ridge bin is less than the mean of more distant bins. Map shows the distance to ridge binning as coloured circles, with the MAR shown in black. White circles centred on black dots show the hotspot locations, with the size of the black circle proportional to their deep origin ranking29,55. Black box shows our study area. Source data

Extended Data Fig. 2 Global shear-velocity models of the mantle transition zone.

The three models PRI-S057, S40RTS5 and SGLOBE4 show average MTZ shear velocities beneath the Atlantic Ocean. The MAR is shown in black. White circles centred on black dots show the hotspot locations, with the size of the black circle proportional to their deep origin ranking29,55. Black box shows our study area.

Extended Data Fig. 3 Temperature estimates from relationships with respect to the 410 and 660 discontinuity topography, and mantle TZT.

Grey lines are the Clapeyron slopes +2.9 MPa K−1 (ref. 48), −2.5 MPa K−1 (ref. 49) and −0.13 km K−1 (ref. 51) for the 410, 660 and TZT, respectively. Black diamonds, estimates from this study. Other symbols, average estimates from the Azores, the Canary islands, Cape Verde, northwest Africa28, Iceland (orange62 and brown31 pentagons), the Southwest Indian Ridge27 and Hawaii32.

Extended Data Fig. 4 Example of waveform corrections and receiver functions.

Left panels, the unfiltered and uncorrected (black) and the tilt- and compliance-corrected (cyan) vertical waveforms (Vert.) of a recorded earthquake on stations I04D and I28D. Middle, zoomed in, filtered waveforms also showing the radial (red) component. Right, receiver functions (RF) from the deconvolution of the vertical uncorrected (black) and corrected (blue) components with the radial component, here essentially identical for corrected and uncorrected data. Top and bottom are examples of waveforms from earthquakes of magnitude 7.4 and 6.6, respectively.

Extended Data Fig. 5 Maps showing the number of waveforms stacked at 410-km and 660-km depths (grey shade).

Dark red line marks the plate boundary37.

Extended Data Fig. 6 Depth errors of the 410 and 660 discontinuities.

Standard errors are determined from the depth migration of each waveform. Dark red line marks the plate boundary37.

Extended Data Fig. 7 Migration tests.

Receiver function migration tests using 1D crust-corrected PREM8, 3D VS model SEMUM26 (using PREM VP/VS ratio for VP) and 3D VP and VS model from PRI7. Horizontal cross-sections from the 3D depth migration: 410 and 660 discontinuities (top and middle, respectively), and mantle TZT (bottom). The TZT is given as the difference from a thickness of 250 km. Semi-transparent shades are poorly constrained areas. Dark red line marks the plate boundary37.

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Agius, M.R., Rychert, C.A., Harmon, N. et al. A thin mantle transition zone beneath the equatorial Mid-Atlantic Ridge. Nature 589, 562–566 (2021). https://doi.org/10.1038/s41586-020-03139-x

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