Deep volatile cycling and diamond formation in the cratonic lithosphere

The Earth’s cratonic mantle is a major reservoir for carbon stored as carbonates as well as graphite/diamond. Over the billions of years of the cratons’ existence, migrating melts/fluids from the convecting mantle or released from subducted slabs have deposited carbon and other volatiles in the Thermal Boundary Layer (TBL) in the cratonic roots. This geodynamic history has created a geochemical environment that is variable on a small scale, where oxygen fugacity and lithology are heterogeneous and carbon storage and release are spatially controlled.

This complex history of carbon storage in, and release from the cratonic mantle is recorded in some suites of diamonds, such as small-grained polycrystalline diamond aggregates (PCAs). Due to their rapid crystallization, PCAs and their silicate paragenesis preserve chemical and isotopic heterogeneity that are testament to their episodic formation by small-scale melts in the TBL and the cratonic roots. While economically unimportant, PCAs represent sizable proportions of up to 20% of the diamond production in some kimberlites, attesting to the prevalence and importance of their formation mechanism in the deep carbon cycle. We present here a large dataset on PCAs and their silicate paragenesis from South Africa (Jacob et al., 2025) and discuss insights into the deep cycles of carbon and nitrogen that these samples offer.

 

Jacob, D.E., Stern, R.A., Czas, J., Reutter, M., Piazolo, S., Stachel, T. (2025) Polycrystalline diamond aggregates and their role in Earth’s deep carbon cycle. Geochimica et Cosmochimica Acta, 389, 136-156.