Geological pathways for the recycling of Earth’s surface materials into the mantle are both driven and obscured by plate tectonics1,2,3. Gauging the extent of this recycling is difficult because subducted crustal components are often released at relatively shallow depths, below arc volcanoes4,5,6,7. The conspicuous existence of blue boron-bearing diamonds (type IIb)8,9 reveals that boron, an element abundant in the continental and oceanic crust, is present in certain diamond-forming fluids at mantle depths. However, both the provenance of the boron and the geological setting of diamond crystallization were unknown. Here we show that boron-bearing diamonds carry previously unrecognized mineral assemblages whose high-pressure precursors were stable in metamorphosed oceanic lithospheric slabs at depths reaching the lower mantle. We propose that some of the boron in seawater-serpentinized oceanic lithosphere is subducted into the deep mantle, where it is released with hydrous fluids that enable diamond growth10. Type IIb diamonds are thus among the deepest diamonds ever found and indicate a viable pathway for the deep-mantle recycling of crustal elements.
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This research was supported by a GIA Liddicoat Postdoctoral Research Fellowship to E.M.S. Support to S.B.S. and F.N. was provided by the Deep Carbon Observatory (DCO). The European Research Council supported F.N. (INDIMEDEA, number 307322). Sincere thanks to K. S. Moe, T. Moses, M. Breeding, U. D’Haenens-Johansson, P. Johnson, K. Smit, J. Liao, S. Persaud, E. Myagkaya, A. Balter and B. Torres for analytical/logistical assistance, to N. Renfro for the micrograph of Fig. 1a, to Ascot Diamonds for lending rough samples and to M. Alvaro for discussions about geobarometry.
Nature thanks E. Gaillou and T. Stachel for their contribution to the peer review of this work.
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Images are not to scale. Refer to noted dimensions (max diam., maximum diameter).
Extended Data Fig. 2 Mineralogy of mantle rocks with peridotitic and basaltic bulk composition as a function of depth.
Numbers in boxes denote the number of diamonds observed with inclusions of the given phase, and blue shading in the boxes indicates that a thin fluid CH4 ± H2 jacket was found with the phase. We note that the division of samples between the left and right panel is for illustrative purposes, and in reality some samples (for example, with Ca-Pv alone) are not firmly categorized. See Extended Data Table 1 for a breakdown of inclusions by sample. Adapted from ref. 17.
a, Optical microscope image of the inclusion, exposed on a polished facet of sample 880000037816. b, Secondary-electron image of the same inclusion, grooved with nearly horizontal polishing lines. c, d, EDS spectra of the two phases, consistent with their Raman identification as jeffbenite and NaAl-pyroxene (monoclinic, with composition between enstatite and jadeite). High Na content suggests a metabasaltic paragenesis, while low Ca content may reflect Ca partitioning into coexisting Ca-Pv at the base of the mantle transition zone or the uppermost lower mantle.
a, Optical microscope image of the whole diamond, showing multiple dark inclusions of ferropericlase. b, Polishing down the table facet slightly exposed this group of four ferropericlase inclusions, shown here in an electron backscatter image. The smeared texture on the largest inclusion is a small amount of iron inadvertently deposited on the surface during polishing on a conventional cast iron scaife. c, Cathodoluminescence image of the whole diamond, revealing a complex dislocation network pattern, with interspersed healed fractures, that records a combination of both plastic and brittle deformation.
a, Optical microscope image of inclusion B, while still contained within the diamond host. b, Electron backscatter image of the inclusion, after polishing to expose a cross-section through it. The three main phases are colour-coded in the right panel. c, X-ray spectra obtained with EDS, showing the qualitative elemental composition of each of the three phases. Carbon is present in all spectra owing to the diamond host, diamond particles embedded in the polished surface (black specks, especially in the sulphide phase), as well as the carbon inherent to the Fe-carbide phase. d, X-ray elemental maps obtained with EDS, showing the spatial variation in signal in the region of Fe, S and O peaks (Kα1). Sulphur delineates the Fe-sulphide phase. Oxygen marks the Fe-oxide phase, while also showing the variable oxidation/tarnish layer on the sulphide portion of the inclusion.
a, CaSiO3 walstromite (thought to be former Ca-Pv) in sample 110203744064, inclusion A, with the three main peaks shifted to higher wavenumbers compared to a zero-pressure reference spectrum. This inclusion also contains CH4. b, Coesite (SiO2, thought to be former stishovite) in sample 890000180198. The inclusion analysed (circled) is about 2 µm wide and lies in a planar lobate healed crack, along with other related ‘satellite’ inclusions that presumably surrounded a nucleus coesite inclusion that was polished away when this diamond was facetted. Neighbouring coesite inclusions in b also have high, but variable, remnant pressures, as reflected by the Raman spectra. Reference spectra are from ref. 19 and RRUFF-X050094, and zero-pressure reference peak positions are taken from refs22,41.
Extended Data Fig. 7 Dislocation network pattern in sample 110208245246, as seen in panchromatic cathodoluminescence.
Each of the bright web-like lines are made up of many dislocations, and these cathodoluminescent boundaries surround darker, low-strain domains. The dark curved feature on the right of the centre is a crack (not healed).
Electron microprobe results for exposed ferropericlase inclusions in sample 110208425476 (also see Extended Data Fig. 5)
Electron microprobe analyses for a multiphase Fe-S-C-O inclusion in sample 110208245246.
Carbon isotopic compositions for 3 Type IIb diamond samples (110208425476, 110208425246, DVBT).
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Smith, E.M., Shirey, S.B., Richardson, S.H. et al. Blue boron-bearing diamonds from Earth’s lower mantle. Nature 560, 84–87 (2018). https://doi.org/10.1038/s41586-018-0334-5
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