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Polycrystalline Diamonds from Kimberlites: Snapshots of Rapid and Episodic Diamond Formation in the Lithospheric Mantle

Monocrystalline diamonds are the most valuable diamond type, economically. However, there are other varieties of diamond forged in Earth’s lithospheric mantle, which, while not economically profitable, are of considerable value to the geosciences. Most prominent amongst these are fibrous diamonds (Weiss et al. 2022, this volume) and polycrystalline diamond aggregates (PDAs). Polycrystalline diamond aggregates are rocks in which the dominant mineral phase is diamond (Fig. 1), whereas fibrous diamonds are cuboid samples, sometimes with monocrystalline diamond cores and cloudy overgrowths (‘coats’), or octahedral diamonds with fibrous cores (Weiss et al. 2022, this volume). The fibrous growth sectors are highly imperfect single crystals hosting millions of fluid and solid microinclusions (Navon et al. 1988). Polycrystalline diamond aggregates (PDAs) from kimberlites are the least well-studied of the diamond family. This chapter aims to showcase what we know of PDA-formation in the context of monocrystalline diamond formation, the origin of carbon enrichment in the cratonic lithosphere, and the identify the relationship(s) between polycrystalline diamond formation and plate tectonics.