LETTER https://doi.org/10.1038/s41586-018-0334-5 Blue boron-bearing diamonds from Earth’s lower mantle Evan M. Smith1*, Steven B. Shirey2, Stephen H. Richardson3, Fabrizio Nestola4, Emma S. Bullock5, Jianhua Wang2 & Wuyi Wang1 Geological pathways for the recycling of Earth’s surface materials in sublithospheric diamonds tend to destabilize during ascent in the into the mantle are both driven and obscured by plate tectonics1–3. mantle and break down to lower-pressure minerals, often unmixing Gauging the extent of this recycling is difficult because subducted into composite assemblages14,17. Many inclusions described here are crustal components are often released at relatively shallow depths, multiphase assemblages, as is the case with previously studied inclu- below arc volcanoes4–7. The conspicuous existence of blue boron- sions in super-deep diamonds and their high-pressure experimental bearing diamonds (type IIb)8,9 reveals that boron, an element analogues14,16,17. It is implausible that the same multiphase assemblages abundant in the continental and oceanic crust, is present in could be coincidentally replicated by random sampling of lower- certain diamond-forming fluids at mantle depths. However, both pressure mineral aggregates at shallower, lithospheric depths14. the provenance of the boron and the geological setting of diamond The most abundant inclusion identified, in 31 of 46 samples, was crystallization were unknown. Here we show that boron-bearing Ca-silicate dominated by CaSiO3 walstromite, sometimes with larnite diamonds carry previously unrecognized mineral assemblages (β-Ca2SiO4) and other phases of CaSiO3 composition (Extended Data whose high-pressure precursors were stable in metamorphosed Table 1). These inclusions are commonly interpreted as retrogressed 15,17,18 oceanic lithospheric slabs at depths reaching the lower mantle. We CaSiO3 perovskite (Ca-Pv) . As retrogression of pure Ca-Pv alone propose that some of the boron in seawater-serpentinized oceanic should maintain a bulk Ca:Si ratio of 1, the presence of (Ca-rich) larnite lithosphere is subducted into the deep mantle, where it is released in some inclusions may indicate that diamond growth occurred in a with hydrous fluids that enable diamond growth10. Type IIb chemically evolving system with variable calcium enrichment, as seen diamonds are thus among the deepest diamonds ever found and in other super-deep diamonds16,19,20. indicate a viable pathway for the deep-mantle recycling of crustal Other observed inclusions also correspond to retrogressed high- elements. pressure minerals (Extended Data Table 1). For example, inclusions Type IIb diamonds—including the Hope, a renowned blue of orthopyroxene, with sharp Raman spectra matching enstatite, and diamond—are mantle-derived minerals that contain boron at 0.01– minor amounts of coexisting olivine are interpreted as retrogressed 10 p.p.m. levels and show a lack of nitrogen absorption in infrared bridgmanite14,15, the lower-mantle Mg-silicate perovskite phase. spectroscopy8. Boron imparts their blue colour and p-type semi- Multiphase inclusions containing ortho- or clinopyroxene, coexisting conductivity, although they may not always appear blue when boron with jeffbenite (Mg3Al2Si3O12) or spinel ((Mg,Fe)Al2O4), are inter- concentrations are low or additional defects are present9. Because preted as aluminous bridgmanite14, although some bearing clinopy- boron is a quintessential crustal element with a low concentration in roxene may represent retrogressed majoritic garnet17. Earth’s mantle11, blue diamonds and their formation have long been a One inclusion provides a convincing example of retrogressed geochemical enigma. majorite. Being fortuitously exposed on a facetted diamond, the Type IIb diamonds have been recovered from worldwide local- two-phase assemblage of NaAl-clinopyroxene and jeffbenite (Fig. 1b) ities, including southern and central Africa, India, South America was confirmed with microanalysis by energy-dispersive X-ray spectros- and Borneo9, having been brought to the surface in kimberlite copy (Extended Data Fig. 3) and was interpreted as a former low-Ca, volcanoes ranging in age from the 1.15-billion-year-old Premier pipe12 high-Na majoritic garnet17. A separate inclusion of orthopyroxene in to the ~90-million-year-old Letseng deposit13. They can reach large this diamond, interpreted as former bridgmanite, would then make sizes, such as the 176.2-carat Brazilia diamond, the 122.5-carat rough a putative majorite–bridgmanite pair that would restrict its origin to diamond that yielded the (24.18-carat) Cullinan Dream, which was within17 ~660–750 km. Other observed inclusion phases are coesite examined as part of this study, and the 112.5-carat rough diamond (with accessory kyanite, interpreted as former stishovite) as well as from which the Hope was cut9. ferropericlase (found as relatively small, brown inclusions; see Extended The geological origin of blue diamonds has nevertheless remained Data Fig. 4 and Supplementary Table 1). unknown owing to their rarity (≤0.02% of mined diamonds; Another diamond contains a multiphase inclusion dominated by see Methods), high value and general lack of mineral inclusions. To over- nepheline and spinel, interpreted as former calcium-ferrite-type (CF) come this problem, prospective samples were screened from the extensive phase or possibly new aluminous (NAL) phase, which is compelling grading operations of the Gemological Institute of America. Over two evidence of derivation from host rocks of basaltic composition at years, this approach allowed examination of 46 type IIb diamonds with lower-mantle depths (Fig. 1)14,16,17. The same diamond also contains a inclusions, an invaluable suite for analysis (Extended Data Fig. 1). multiphase inclusion of Fe carbide, Fe sulphide and wüstite (Extended Inclusions were characterized using Raman spectroscopy (Fig. 1) and Data Fig. 5, Supplementary Table 2) that does not correspond to a were found to differ substantially from the common minerals found known mineral but may represent a former metallic melt similar to in diamonds from the cratonic lithosphere (<200 km), such as olivine those recently discovered in (boron-lacking) CLIPPIR (Cullinan-like, and Cr-rich pyrope from peridotite or grossular-almandine-pyrope and Large, Inclusion-Poor, relatively Pure, Irregularly shaped and Resorbed) omphacitic clinopyroxene from eclogite. Instead, the inclusion mineral- diamonds21. Three other type IIb samples also contain metallic-looking, ogy is typical of super-deep diamonds originating from the mantle tran- magnetic inclusions that may be similar to those of CLIPPIR diamonds, sition zone to the lower mantle (Extended Data Fig. 2)14–17. Inclusions although it should be stressed that these are a minor part of the type IIb 1Gemological Institute of America, New York, NY, USA. 2Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC, USA. 3Department of Geological Sciences, University 4 5 of Cape Town, Rondebosch, South Africa. Department of Geosciences, University of Padova, Padua, Italy. Geophysical Laboratory, Carnegie Institution for Science, Washington, DC, USA. *e-mail: [email protected] 84 | NATURE | VOL 560 | 2 AUGUST 2018 © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. LETTER RESEARCH 663 a a CH4 990 2,917 cm-1 CaSiO3 walstromite 1,049 y Reference Intensit 684 100 μm NaAl-pyroxene 1,023 357 b 200 μm 2,850 2,9002,950 Raman shift (cm-1) b 866 H 927 CH4 2 Jeffbenite 642 990 2,918 cm–1 588 cm–1 506 320 H2 4,157 cm–1 y 100 μm y 531 c 550 600 Intensit Intensit 195 Coesite 139 273 430 Kyanite 2,9002,950 4,100 4,1504,200 100 μm Raman shift (cm–1) Fig. 2 | Inclusions jacketed by thin films of fluid CH4 and H2, revealed 100 μm 430 Nepheline by Raman spectroscopy. a, CaSiO3 walstromite (former Ca-Pv), 400 with the main part of the inclusion circled, in sample 110208780369. 471 987 d b, Orthopyroxene (former bridgmanite) in sample 110208773706. Olivine Lobate sprays of small inclusions are thought to reflect expansion and proliferation of inclusion material into its own decompression crack. 560 645 753 404 Spinel of at least ~9 GPa in the mantle (we note that these entrapment pres- sures are severe underestimates due to diamond deformation; Extended 100 μm Data Fig. 6)22. Similarly, a coesite inclusion with its main Raman peak shifted from 520.6 cm−1 up to 537.9 ± 0.5 cm−1 (Extended Data Fig. 6) 200 400 600 800 1,000 1,200 indicates extreme remnant pressure, far exceeding the 4 GPa highest- -1 Raman shift (cm ) pressure benchmark for coesites observed in lithospheric diamonds Fig. 1 | Selected Raman spectra of inclusions in type IIb diamonds. (see Methods). X-ray diffraction also reveals remnant pressures of a, Former Ca-Pv, now CaSiO3 walstromite, in sample 110205945970. ~1.8 GPa in ferropericlase inclusions, requiring a minimum entrap- b, Former majoritic garnet, now a composite of NaAl-pyroxene and ment pressure of 10.3 to 14.1 GPa, calculated at 1,200 K and 2,000 K, jeffbenite (verified by scanning electron microscopy/energy-dispersive respectively, corresponding to a depth beyond 300 km, well below the X-ray spectroscopy; see Extended Data Fig. 3) in sample 880000037816. base of the deepest continental lithospheric mantle keels23. c, Former stishovite, now coesite, in sample 101024478345. Also shown on Additional pressure observations come from the silicate inclusions the left is a composite coesite plus kyanite spectrum (green) from sample 890000180201. d, Former CF, now composite of nepheline and spinel in blue diamonds, which typically have large decompression cracks (with CH4 fluid, not shown), in sample 110208245246. Dashed lines are and often lobate sprays of tiny droplet-like satellite inclusions in 19 30 reference spectra of CaSiO3 walstromite , jeffbenite , nepheline and healed fractures emanating from the main inclusion (Figs. 1, 2). These spinel14, plus omphacite R061129 and coesite X050094 from the RRUFF satellite inclusions formed during exhumation, as high internal inclu- database. Spectra are stacked vertically for clarity. Source Data. sion stresses20 relative to decreasing external confining pressure led to rupturing of the host diamond.