Evaluation of kimberlite diamond potential using FTIR spectroscopy of xenocrystic olivine

Xenocrysts or phenocrysts of olivine in kimberlite show a diversity of hydroxyl-related infrared absorption bands that reflect compositional variation of their mantle source and conditions of last equilibration. Interpretation of infrared spectra of olivine in terms of equilibration/crystallization conditions may help to isolate spectral features associated with good diamond carrying and diamond preservation potential. As an exploratory study we examine by FTIR the spectroscopic features of olivine inclusions in diamond, olivine coexisting with diamond in peridotite xenoliths and olivine from diamond-free peridotite xenoliths. The three environments are respectively characterized by spectroscopically anhydrous olivine, olivine with low water concentrations (H2O < 60 wt. ppm) and olivine that may be fairly rich in water (H2O up to 260 ppm). Previous studies have shown that the majority of olivine macrocrysts in kimberlite contain > 100 ppm H2O, with a significant proportion containing > 250 ppm H2O. Our new results suggest that hydrogenation of olivine occurs in the mantle during equilibration with kimberlite-related hydrous fluids and/or during equilibration with, or crystallization from, kimberlite magmas during ascent. The association of gem diamonds with water-poor mantle olivine suggests that such samples experienced limited interaction with hydrous fluids/melts over a short time span, thus preventing diffusional equilibration. Future FTIR studies of olivine from kimberlites with widely variable diamond grades and reconciliation with diamond resorption features may permit development of new FTIR based procedures to assist in the evaluation of new kimberlite discoveries.