Discovery of carbonatitic microinclusions in diamonds with highly aggregated nitrogen

Abstract:

It is accepted that fibrous diamonds grew from the high-density fluids (HDFs) they trapped as microinclusions. Such fluids are not found in monocrystalline (MC) diamonds, leaving their source of carbon a mystery. Fibrous diamonds carry nitrogen in A centers and are young (10⁶ years), while most MC diamonds carry aggregated nitrogen in both A and B centers and are older (10⁹ years). Weiss et al. (2014) found saline HDFs in the core of a coated diamond from Guinea and carbonatitic and saline fluids in a South African diamond, both with low concentrations of B centers (5-27%). Here we report finding microinclusions with high-Mg carbonatitic HDFs in MC diamonds. The infrared spectrum of these diamonds confirms their highly aggregated nature (>60%).

We studied a suite of twinned diamonds, macles, from the Venetia mine in South Africa. The diamonds were polished perpendicular to their twinning plane and cleaned in HF and HNO₃. We identified the twinning planes in cathodoluminescence images and methodically searched along and next to these planes, assuming that inclusions were preferentially trapped there. Shallow, sub-surface inclusions were analyzed using a JEOL JXA-8230 EPMA and an EDS detector.

In two out of 11 diamonds we found inclusions with high concentrations of MgO (26-31 wt%), CaO (21-34%), K₂O (9-16%), FeO (7-12%) and SiO₂ (8-13%). These compositions are similar to those of HDFs in fibrous diamonds from Guinea and Yakutia (Weiss et al., 2009; Klein-BenDavid et al., 2009; Zedgenizov et al., 2009). Eight carbonatitic inclusions were found in diamond ON-VNT-608 and three in ON-VNT-605.

Five microinclusions in 608 and one in 605 carry high concentrations of SiO₂, MgO and FeO, with little else. Their compositions fall close to that of orthopyroxene, suggesting that both diamonds belong to the peridotitic paragenesis. Other inclusions, rich in SiO₂ and Al₂O₃ in variable proportions were found in 8 of the 11 diamonds. The nature of these inclusions is not yet clear.

The presence of high-Mg carbonatitic inclusions in clear, gem-quality diamonds indicates that carbonate-bearing HDFs play a role in the formation of most diamonds. The high aggregation state of the nitrogen affirms a long mantle-residence time implying that the major element composition of diamond-forming fluids has not changed over time.