Features of apatite in kimberlite from Ekati Diamond Mine and Snap Lake: modelling kimberlite composition

Abstract

Magmatic volatiles (H2O, CO2, F, Cl) and the timing of fluid exsolution have implications for kimberlite melt composition, eruption and preservation of diamonds. Apatite, Ca5(PO4)3(F,Cl,OH), is a common groundmass mineral in kimberlite, often used as an indicator of volatile behaviour in igneous rocks because it is a major host for halogens, rare earth elements (REEs) and Sr. This study reports on the diversity of apatite in kimberlite, and explores its for indicating kimberlite volatile histories, using pipes from Ekati Diamond Mine and Snap Lake Mine, located in the Northwest Territories, Canada. Experimental partition coefficients from the literature for apatite and various crystallization media (silicate melt, carbonate melt, aqueous fluid), are employed to estimate concentrations of trace elements in hypothetical melts coexisting with kimberlitic apatite. The constructed models are consistent with kimberlitic-apatite crystallization from a melt nearer to carbonate composition, while some apatite grains preserve evidence of magmatic fluid separation from the kimberlite melt. Evidence from groundmass apatite supports the current models for kimberlite eruption and can be a robust way to assess volatile histories in kimberlite pipes.