The internal structure and dynamics of Earth have been shaped by the 660 km boundary between the mantle transition zone and lower mantle. However, due to the paucity of natural samples from this depth, the nature of this boundary—its composition and volatile fluxes across it—remain debated. Here we analyse the mineral inclusions in a rare type IaB gem diamond from the Karowe mine (Botswana). We discovered recovered lower-mantle minerals ringwoodite + ferropericlase + low-Ni enstatite (MgSiO3) in a polyphase inclusion, together with other principal lower-mantle minerals and hydrous phases, place its origin at ~23.5 GPa and ~1,650 °C, corresponding to the depth at the 660 km discontinuity. The petrological character of the inclusions indicates that ringwoodite (∼Mg1.84Fe0.15SiO4) breaks down into bridgmanite (∼Mg0.93Fe0.07SiO3) and ferropericlase (∼Mg0.84Fe0.16O) in a water-saturated environment at the 660 km discontinuity and reveals that the peridotitic composition and hydrous conditions extend at least across the transition zone and into the lower mantle.
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This research was supported by a GIA Liddicoat Postdoctoral Research Fellowship to T.G. Sincere thanks to T. Moses from GIA for access to this sample; J. W. Valley and B. L. Dutrow for their support on GIA research program; E. Yazawa and C. Zhou from GIA for technical assistance; K. Moe from GIA for assistance with Raman data collection; J. I. Koivula, N. D. Renfro and J. Liao from GIA for photomicrography; U. D’Haenens-Johansson from GIA for sample selection; E. Smith, K. Smit and M. Y. Krebs from GIA for valuable discussion; R. Passeri, Jr from Hitachi and A. Chan from GIA for assisting in sample preparation; and S. D. Jacobsen from NU for discussions and providing supplementary Raman data. M.G.P. has received funding from the European Union’s Horizon 2020 Marie Skłodowska-Curie grant agreement no. 796755. D.N. acknowledges the ‘Rita Levi Montalcini’ programme of the Italian Ministry of University and Research. M.A. is supported by a European Research Council (ERC) grant agreement no. 714936 TRUE DEPTHS. F.N. thanks the ERC Starting Grant no. 307322 and the Alexander von Humboldt Foundation. Sincere thanks to O. Navon for constructive comments.
The authors declare no competing interests.
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The spectrum was baselined in GRAMS software and normalized by two-phonon diamond lattice bands at 2460 cm−163, showing that it is a pure type IaB diamond with only B center nitrogen. Inset figure (a) shows hydrogen related peaks at 3107 and 3085 cm−1, which are typical in milky type IaB diamonds64,65 and consistent with the milky feature observed around inclusion 3. Inset figure (b) shows B center nitrogen at 1172 cm−1 with no detectable A center nitrogen and very weak residual IR absorption at 1367 cm−1 caused by hydrogen related defects incorporated into platelets that quench the platelet IR absorption66.
Inclusion 3 is a cluster of submicron sized inclusions (inc3a), surrounded by pin points that generate a milky appearance (inc3b), which could be octahedral or elongated nano sized inclusions filled with nitrogen rich fluids65. Inclusion 7 and 12 did not show any detectable Raman bands. Inclusion 9 is opaque in the center and close to the mineral assemblage of inclusion 2. Its appearance and Raman spectra (Extended databases) imply that it is likely a ferropericlase phase as the by-product from the decomposed ringwoodite. Inclusion 13 is a polyphasic assemblage without diagnostic Raman spectra, while the individual minerals are too small to be precisely probed by XRD (the shortest dimension is less than 10 μm). Field views of each image from inc3a to inc13 are 2.87, 1.40, 1.60, 0.91, 0.91, 0.96 mm.
Scratches on the surface are marks in the Ir coating, after EMPA. Field view is 0.91 mm.
Coesite (cs) peaks (RRUFF ID: X05009467) have been observed in almost all spectra; the peaks in area (a) and (c) at ~800 and 858 cm−1 are assigned to ringwoodite (rw) or partially retrogressed olivine (ol), possibly accompanied by enstatite (En) in (a). Diagnostic Raman peaks have been observed in (b) at ~787, 735, 626, 375 cm−1, which could be assigned to phase D (D). The accompanied vibration at ~2849 cm−1 in (b) is consistent with the OH stretching of phase D23,68,69. A few peaks at ~781 and 735 cm−1 seem recurring in (d). Spectra were collected at GIA. Red circle, triangle, orange stars: undefined peaks. The orange stars with the peak around 831 cm−1 could be overlapped with other hydrous phase such as super hydrous phase B70,71.
Fe partition between bridgmanite and ferropericlase at different amounts of Al content in bridgmanite in atoms per formula unit adopted from previous study at 24 GPa, 1650 °C29. Red cross marks the data observed in this study based on the molar Fe/(Fe+Mg) ratio of bridgmanite and ferropericlase, which falls into the range of Al ~0.024 per formula unit.
Solidus curve is from72. a Solid blue diamonds and lines are from pyrolitic mantle composition KLB-1 with slightly depleted bulk Al2O3 ~3.6 wt.%24. Coexisting mineral assemblages have been marked by dark blue text. Note that the transition of ringwoodite (Rw) to bridgmanite (Bm), ferropericlase (Fp) and Ca-perovskite (Cpv) has a negative Clapeyron slope (Rw out line), while the transition of majorite garnet (Mj) to bridgmanite and ferropericlase has a positive Clapeyron slope (Mj out line). b Solid blue pentagons are from pyrolitic mantle composition and the solid purple line is the estimated line for ringwoodite phase transition which is between 24 to 25 GPa at 1500 °C31. c Solid brown circles and lines33 are from pyrolitic mantle composition73 with ~4.5 wt.% bulk Al2O3 content. The phase transition pressure of majorite garnet to bridgmanite and ferropericlase is slightly higher than that with lower Al content in the bulk sample24. d Dashed green line represents the transition of ringwoodite (Mg2SiO4) to bridgmanite and periclase constrained by a precise internal MgO pressure scale30. e Dashed dark green line is from the most recent data of the phase boundary of ringwoodite (Mg2SiO4) bridgmanite and periclase constrained by in situ X-ray diffraction in a multi-anvil press74.f Phase relations of ringwoodite, wadsleyite (Wd), bridgmanite and periclase in a Mg2SiO4 system (dashed light blue lines) as well as akimotoite (Ak), majorite garnet, bridgmanite in a MgSiO3 system (dashed orange lines) calculated by density functional theory are plotted for comparison24,75. Results from previous studies on the pyrolitic mantle composition with green76 and purple triangles77 are also plotted. *Note that the dashed lines represent data constrained in a Mg-Si-O system (without Fe or Al). The pink star is the reference spot indicating the phases expected in a pyrolitic mantle composition that falls in the range of our calculated temperature and the corresponding pressure at the phase equilibrium.
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Gu, T., Pamato, M.G., Novella, D. et al. Hydrous peridotitic fragments of Earth’s mantle 660 km discontinuity sampled by a diamond. Nat. Geosci. 15, 950–954 (2022). https://doi.org/10.1038/s41561-022-01024-y
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