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Extreme redox variations in a superdeep diamond from a subducted slab

Abstract

The introduction of volatile-rich subducting slabs to the mantle may locally generate large redox gradients, affecting phase stability, element partitioning and volatile speciation1. Here we investigate the redox conditions of the deep mantle recorded in inclusions in a diamond from Kankan, Guinea. Enstatite (former bridgmanite), ferropericlase and a uniquely Mg-rich olivine (Mg# 99.9) inclusion indicate formation in highly variable redox conditions near the 660 km seismic discontinuity. We propose a model involving dehydration, rehydration and dehydration in the underside of a warming slab at the transition zone–lower mantle boundary. Fluid liberated by dehydration in a crumpled slab, driven by heating from the lower mantle, ascends into the cooler interior of the slab, where the H2O is sequestered in new hydrous minerals. Consequent fractionation of the remaining fluid produces extremely reducing conditions, forming Mg-end-member ringwoodite. This fractionating fluid also precipitates the host diamond. With continued heating, ringwoodite in the slab surrounding the diamond forms bridgmanite and ferropericlase, which is trapped as the diamond grows in hydrous fluids produced by dehydration of the warming slab.

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Fig. 1: Room temperature Mössbauer spectrum of the ferropericlase inclusion in diamond KK203 showing that its Fe3+/ΣFe is negligible and indicating fO2 conditions near the IW buffer.
Fig. 2: Schematic pressure–temperature plot illustrating processes in a warming, hydrated, subducted slab over time and the relationships between ringwoodite, ferropericlase and bridgmanite.
Fig. 3: Diamond growth events through a schematic cross-section of Earth.

Data availability

The original reflection data for olivine in KK203 diamond and Supplementary Tables 13 have been deposited at EarthChem  (https://doi.org/10.26022/IEDA/112541).

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Acknowledgements

This research used resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We acknowledge the support of GeoSoilEnviroCARS (Sector 13), which is supported by the US National Science Foundation (NSF) – Earth Sciences (EAR-1128799), and the Department of Energy, Geosciences (DE-FG02-94ER14466), and staff scientists M. Newville, T. Lanzirotti and M. Rivers. S.D.J. acknowledges support from NSF grant no. EAR-1853521. NSERC Discovery grants to R.W.L., D.G.P. and T.S. funded aspects of this research. The authors acknowledge A. Rohrbach and K. Kiseeva for very valuable comments that prompted a re-think of our fO2 estimate and formation model.

Author information

Authors and Affiliations

Authors

Contributions

M.E.R., C.A. and F.N. collected and interpreted the XRD data. M.E.R. characterized the inclusions by EPMA and Raman spectroscopy. F.N. and M.E.R. wrote the original manuscript. R.W.L. and T.S. developed the diamond growth model. F.N. and L.B performed the geobarometric calculations on olivine. C.M. characterized the inclusions by Mössbauer spectroscopy. M.D.W., M.E.R. and S.D.J. conducted tomographic image collection and analyses. R.W.L. and M.E.R. did the fO2 calculations. J.W.H. provided the diamond and helped describe and break the sample. F.N., M.R. and D.G.P. wrote the original manuscript. All coauthors improved interpretations and provided editing.

Corresponding author

Correspondence to Fabrizio Nestola.

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The authors declare no competing interests.

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Nature thanks Kate Kiseeva and Arno Rohrbach for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Raman spectrum of enstatite measured in diamond KK203 (in blue solid line).

Reference spectrum of enstatite is in solid red line (RRUFF ID: R070641). The data reduction software is OMNIC 9 (Thermo Fisher Scientific Inc.).

Extended Data Fig. 2 X-ray tomographic image of KK203, collected at GSECARS, showing no cracks leading to inclusions.

The entire diamond could not fit into the field of view.

Extended Data Fig. 3 Calculated values of log fO2 relative to the IW buffer at 10, 15, and 20 GPa and 1,200, 1,400, and 1,800 °C necessary to stabilize a (Mg,Fe)2SiO4 polymorph with Mg# = 99.9.

The different values at each condition reflect the range of assumed activities of Fe and SiO2 (see text for details). The stars denote the case with both activities equal to one. See tabulated values in the accompanying spreadsheet for details of the calculations.

Extended Data Table 1 Hyperfine parameters of ferropericlase from KK203 determined from room temperature Mössbauer spectrum

Supplementary information

Supplementary Table 1

Chemical analyses are provided in wt% oxides.

Supplementary Table 2

Standards, analyzing crystals and detection limits for EPMA analyses.

Supplementary Table 3

Calculations of fO2 for diamond KK203 using methodology modified as previously described34.

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Nestola, F., Regier, M.E., Luth, R.W. et al. Extreme redox variations in a superdeep diamond from a subducted slab. Nature 613, 85–89 (2023). https://doi.org/10.1038/s41586-022-05392-8

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