Diamond–graphite relationship

The diamond matrix shows plastic deformation as evidenced by the high density of dislocations, stacking faults and a large number of {111} deformation twins (Supplementary Fig. 1). Despite no sign of graphitization for uninterrupted twins, the deformation twins that intersect an inclusion transform to graphite (Fig. 1, Supplementary Fig. 2), while keeping their original morphology. Thus, the diamond–graphite grain boundary forms parallel to the {111} planes of diamond (Supplementary Note 1).

Fig. 1 Graphitization of diamond along twinning directions. a The high-angle annular dark-field (HAADF) STEM image shows two twinning regions indicated as twin 1 and twin 2. Twin 1 is intersecting with two inclusions (indicated by orange arrows) and graphitized, while twin 2 is purely diamond. b The graphite-diamond EELS map (from the dashed blue rectangle in panel a) indicates that the graphitization is confined to the twinning region and around the inclusions (red = graphite, blue = diamond) Full size image

The sample shown in Fig. 2 consists of several diamond segments with close crystallographic orientations, and are separated by graphite bands. Inclusion trails can be seen extending from one diamond segment into the next, while disappearing in the in-between graphite band (Fig. 2b). This is undeniable morphological evidence that the inclusions existed in diamond before these were broken into smaller pieces by graphitization. Similar to the graphitized twins, the graphite bands in Fig. 2 have grain boundaries parallel to {111} planes of diamond (Supplementary Fig. 3 and Supplementary Note 1). Thus, the most likely cause of graphitization is the shock event where the diamond matrix has been severely deformed12,13. Elevated temperature during the shock, as well as stress concentration around the inclusion promotes the graphitization process13,14.

Fig. 2 Inclusion trails imaged inside diamond fragments. a HAADF-STEM image from diamond segments with similar crystallographic orientation. Dashed yellow lines show the diamond–graphite boundaries. b High-magnification image corresponding to the green square in a. Diamond and inclusion trails are cut by a graphite band. The dashed orange line shows the direction of the inclusion trails Full size image

Iron–sulfur type inclusions in diamond

The overwhelming majority of inclusions are iron-rich sulfides, found either as isolated grains with sizes up to a few 100 nanometers, or as trails of small particles ranging from 50 nm down to a few nanometers (Fig. 3 and Supplementary Fig. 4). All the inclusions are faceted indicating that they were trapped as solid crystalline phases rather than melts. However, they show evidence of transformation to low-pressure phases during decompression, similarly to those found in deep terrestrial diamond inclusions15. Both chemical and crystallographic analysis (Supplementary Table 1 and Supplementary Fig. 5) show that the sulfide inclusions have dissociated to three phases (Fig. 2c): FeS-troilite, (Fe,Ni)-kamacite, and minor amounts of (Fe,Ni) 3 P-schreibersite. The latter either dissociates to a separately detectable phosphide phase in larger inclusions (Fig. 3 and Supplementary Fig. 4), or concentrates at grain boundaries in smaller inclusions (Supplementary Fig. 4). It is noteworthy that troilite, kamacite, and schreibersite are never found as isolated mono-mineralic inclusions in the diamonds, but always together inside a very sharply defined polyhedral arrangement; two arguments promoting the idea that these inclusions crystalized as a single-Fe–Ni–S–P phase during diamond formation, that later decomposed into different phases. This is further confirmed by the constant and stoichiometric bulk chemical composition of these inclusions. In order to avoid any sampling bias in such multicomponent inclusions, the composition was measured only on those grains that were completely embedded inside the diamond host determined by electron tomography, leaving aside those that had been partially cut during focused ion beam (FIB) preparation. We found an average molar (Fe + Ni)/(S + P) ratio of 2.98±0.36 from 29 sulfide inclusions (Supplementary Table 2), which corresponds to an (Fe,Ni) 3 (S,P) initial mineralogy. (Fe,Ni) 3 P-schreibersite and (Fe,Ni) 3 S have the same space group (tetragonal I\(\bar 4\)) and their lattice parameters are very close16,17, allowing them to form a solid solution at high pressures as (Fe,Ni) 3 (S,P)16,17 across the entire compositional S–P join.

Fig. 3 Electron micrograph and compositional maps of diamond inclusions in ureilite. HAADF-STEM images (a, b, c, and d) and associated Fe and S elemental maps (e, f, g, and h) of inclusions in diamond. All chemical (EDX) maps show Fe (light blue) and S (red) distribution. Kamacite and troilite phases appear as light blue and reddish-pink respectively Full size image

The pressure stability of the Fe 3 (S,P) phase depends18 on its composition (Supplementary Note 2 and Supplementary Fig. 6), and ranges from 21 GPa for the Fe 3 S to room pressure for Fe 3 P, allowing to use the P/(S + P) ratio as an internal thermo-barometer. Phosphorus has no effect on the stability for P/(S + P) between 0 and 0.2, Fe 3 (S,P) is only stable above 21 GPa18 (Supplementary Fig. 6) just like Fe 3 S. The average P/(S + P) of the inclusions observed here is 0.12±0.02 (Supplementary Table 2), and therefore these can only have formed above 21 GPa. Similarly, the inclusions contain nickel, with Ni/(Fe + Ni) = 0.068 ± 0.011, which could also have an effect on the stability pressure of (Fe,Ni) 3 (S,P), with Ni 3 S (isostructural with Fe 3 S19) stable only above 5.1 GPa. We lack the experimental work to evaluate the pressure effect of Ni substitution for Fe, but assuming a linear dependence of pressure stability on Ni content, the (Fe,Ni) 3 (S,P) inclusions would only form above ~20 GPa (Supplementary Note 2 and Supplementary Fig. 7). It is noteworthy that pressure-composition phase diagrams are often concaved downward, and there could be, just as with S–P substitution, no effect on pressure at those low Ni concentrations, so that 20 GPa is actually a lower bound for the inclusions’ formation pressure (Supplementary Fig. 7).

Chromite and phosphate inclusions in diamond

A second type of inclusions, Cr 2 FeO 4 chromite, are rare (with only a few identified in the samples) but rather large with grains a few hundred nanometers across (Supplementary Fig. 8). The mineralogy of chromite grains is well preserved and chemical analysis confirms a stoichiometric Cr 2 FeO 4 chromite (Supplementary Note 3), with no Mg or Al substitution for Fe and Cr, respectively. While chromite is often observed in meteorites, Mg- and Al-free end-members are only found in iron meteorites20,21,22. It has been proposed that such end-members must form in a metallic melt with low Cr and O concentration close to the Fe–FeS join22,23. Therefore, these chromites must have formed in an iron-rich environment.

Finally, rare Ca–Fe–Na phosphate inclusions were found, roughly ~20 nanometer or smaller (Supplementary Fig. 8), which were only characterized chemically due to their small size (not structurally due to overlap with the surrounding diamond). These inclusions are chemically similar to the ones observed in iron meteorites where they are the most common companions of pure Cr 2 FeO 4 chromites24 (Supplementary Note 3).

Iron–sulfur type inclusions in graphite

Whereas the polyhedral shapes and consistent bulk composition of inclusions in diamond shows that these phases were a single-homogeneous solid phase at the time of diamond formation, the morphology of inclusions in neighboring graphitized bands shows evidence of melting (Fig. 2a and 4, Supplementary Fig. 9). Indeed, Fe- and S-bearing phases of varying composition and arbitrary shapes are dispersed in the graphitized areas and between graphite layers (Fig. 2a and 4, Supplementary Fig. 9), which provides an evidence for melting of inclusions at the time of graphitization, and yet another indication that graphitization is subsequent to diamond formation. This also provides an explanation for the transformation of original (Fe,Ni) 3 (S,P) solid solution to kamacite, troilite and schreibersite phases while keeping the polyhedral shape and bulk composition of the initial parental phase. Graphitization is likely caused by a shock event, which is followed by separation from the parent body and, therefore a pressure drop. That same shock event should melt the inclusions, which then recrystallize after the pressure drop as kamacite, troilite and schreibersite, which are the equilibrium phases at low pressures. The volume change during melting would also add to the strain concentration around them, which in turn facilitates the graphitization process.