OrientXplot calculationsThe information we report below is extracted from the "Users manual" of OrientXplot (Angel et al., 2015) and explains how it is possible to plot the orientation data from single-crystal X-ray diffraction or from other sources (i.e. Electron Backscatter Diffraction, hereafter EBSD).One of the main problems when plotting the relative crystallographic orientations of inclusions in different hosts is the possible ambiguity in indexing the diffraction patterns (or data from other sources, e.g. EBSD) coming from the symmetry of both the inclusion and the host. For example, in an orthorhombic system, the symmetry makes the [100] direction equivalent to [-100], and [010] equivalent to [0-10], and others. There is no physical approach to distinguish between these two directions. Therefore, in this case, an 'a-axis' pointing vertically upwards could equally be described as '-a' or [-100]. The possible equivalent orientations that maintain a right-hand description of the unit-cell axes of a crystal with a certain symmetry point group are described by the symmetry elements of the point sub-group that does not include any inversion symmetry and mirror planes. Accordingly, both garnet and diamond will be considered as having a point group 432 instead of m3m. In detail, to eliminate the symmetry ambiguity, we rotate the orientation matrix (determined by single-crystal X-ray diffraction) of each garnet inclusion by the symmetry elements of point group 432, oriented to coincide with the unit-cell axes of the diamond host. This generates 24 possible crystallographic orientations of garnet with respect to its diamond host (symmetrically equivalent and physically indistinguishable by any method). For each of them, there are 24 orientations, which are symmetrically-equivalent orientations of the garnet because of its symmetry.

Diamonds and their inclusions are unique fragments of deep Earth, which provide rare samples from inaccessible portions of our planet. Inclusion-free diamonds cannot provide information on depth of formation, which could be crucial to understand how the carbon cycle operated in the past. Inclusions in diamonds, which remain uncorrupted over geological times, may instead provide direct records of deep Earth’s evolution. Here, we applied elastic geothermobarometry to a diamond-magnesiochromite (mchr) host-inclusion pair from the Udachnaya kimberlite (Siberia, Russia), one of the most important sources of natural diamonds. By combining X-ray diffraction and Fourier-transform infrared spectroscopy data with a new elastic model, we obtained entrapment conditions, P trap = 6.5(2) GPa and T trap = 1125(32)–1140(33) °C, for the mchr inclusion. These conditions fall on a ca. 35 mW/m 2 geotherm and are colder than the great majority of mantle xenoliths from similar depth in the same kimberlite. Our results indicate that cold cratonic conditions persisted for billions of years to at least 200 km in the local lithosphere. The composition of the mchr also indicates that at this depth the lithosphere was, at least locally, ultra-depleted at the time of diamond formation, as opposed to the melt-metasomatized, enriched composition of most xenoliths.

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Diamonds are witnesses of processes that have operated in Earth’s mantle over more than 3 b.y. Essential to our understanding of these processes is the determination of diamond crystallization ages. These cannot be directly determined on diamond, but they can be calculated using radiogenic isotopic systematics of suitable minerals included in a diamond. This method relies on the assumption that the mineral inclusions were in isotopic equilibrium with the diamond-forming medium. We evaluated the validity of Sm-Nd ages yielded by clinopyroxene inclusions by combining crystallographic orientation analyses and Nd diffusion modeling at the relevant conditions for Earth’s cratonic mantle. We investigated the crystallographic orientation relationships (CORs) for 54 clinopyroxene inclusions within 18 diamonds from South Africa and Siberia. Clinopyroxene inclusions in some diamonds showed specific CORs with their hosts, indicating possible syngenesis. Other samples had clusters of clinopyroxene inclusions sharing the same orientation but no specific orientation relative to their hosts, indicating that the inclusions are older than the diamond (i.e., they are protogenetic). Diffusion modeling in the temperature range typical for lithospheric diamonds (900–1400 °C) showed that resetting of the Sm-Nd isotopic system in clinopyroxene grains larger than 0.05 mm requires geologically long interaction with the diamond-forming fluid/melt (>3.5 m.y. at average temperature of ~1150 °C). Depending on inclusion size and temperature regime, protogenetic clinopyroxene inclusions may not fully reequilibrate during diamond-formation events. We suggest that small clinopyroxene inclusions (<0.2 mm) that equilibrated at temperatures higher than 1050–1080 °C may be the most suitable for age determinations.

Crowningshieldite is the natural analog of the synthetic compound α-NiS. It has a NiAs-type structure and is the high-temperature polymorph relative to millerite (β-NiS), with an inversion temperature of 379 °C. Crowningshieldite is hexagonal, space group P63/mmc, with a = 3.44(1) Å, c = 5.36(1) Å, V = 55.0(2) Å3, and Z = 2. It has an empirical formula (Ni0.90Fe0.10)S and dcalc = 5.47(1) g/cm3. The five strongest lines in the powder X-ray diffraction data are [dmeas in angstroms (I) (hkl)]: 1.992 (100) (102), 1.718 (55) (110), 2.978 (53) (100), 2.608 (35) (101), and 1.304 (17) (202). Crowningshieldite was found as part of a multiphase inclusion in a gem-quality, colorless, type IIa (containing less than ~5 ppm N) diamond from the Letseng mine, Lesotho. The inclusion contains crowningshieldite along with magnetite-magnesioferrite, hematite, and graphite. A fracture was observed that extended from the inclusion to the diamond exterior, meaning that fluids, possibly kimberlite-related, could have penetrated into this fracture and altered the inclusion. Originally, the inclusion might have been a more reduced, metallic Fe-Ni-C-S mixture made up of cohenite, Fe-Ni alloy, and pyrrhotite, akin to the other fracture-free, pristine inclusions within the same diamond. Such metallic Fe-Ni-C-S primary inclusions are a notable recurring feature of similar type IIa diamonds from Letseng and elsewhere that have been shown to originate from the sublithospheric mantle. The discovery of crowningshieldite confirms that the α-NiS polymorph occurs in nature. In this case, the reason for its preservation is unclear, but the relatively iron-rich composition [Fe/(Fe+Ni) = 0.1] or the confining pressure of the diamond host are potential factors impeding its transformation to millerite. The new mineral name honors G. Robert Crowningshield (1919–2006) (IMA2018-072).