Are euhedral microdiamonds formed during ascent and decompression of kimberlite magma? Implications for use of microdiamonds in diamond grade estimation

Abstract

The relationship between euhedral microdiamonds (<1 mm in maximum dimension) and macrodiamonds is problematic. A major paradox is that euhedral microdiamonds, by virtue of their small size, should be especially susceptible to resorption compared to macrodiamonds, whereas the opposite is commonly observed. We discuss difficulties with published mechanisms for euhedral microdiamond formation and eruption, most of which involve two separate, possibly unrelated, events: first, euhedral microdiamond crystallization (by a variety of possible processes), and second, later entrainment in erupting kimberlitic or lamproitic magma. We propose an alternative hypothesis based on published experimental studies showing there is a significant decrease in the C-bearing volatile content of kimberlite magma as pressure decreases. During ascent of the kimberlite magma, this may result in a pressure-related transition from conditions at depth at which diamond is unstable in fluid-undersaturated kimberlite magma to shallower-level conditions at which diamond becomes stable (or metastable) as the magma reaches fluid saturation. In this scenario macrodiamond xenocrysts (of an early generation), entrained at asthenospheric or deep lithospheric depths, may partially or totally dissolve into the undersaturated magma upon disaggregation of their host rocks; upon ascent of the magma to shallower depths, and under appropriate oxidation conditions, the lowered capacity of the magma to dissolve C-bearing volatiles may result in euhedral microdiamond precipitation, probably immediately before final eruption. Although framed in qualitative terms, our hypothesis has testable consequences which we discuss. In view of these considerations, and the multiple possible relationships within and between microdiamonds and macrodiamonds in individual deposits, we suggest that the use of microdiamond abundances from small samples as a guide to economic macrodiamond grade is problematic as an exploration strategy.