The melting and crystallisation behaviour of a natural clinopyroxene-ilmenite intergrowth

The anhydrous solidus of a natural clinopyroxene-ilmenite intergrowth from the kimberlite pipe at Monastery was found to be 1300° C at 20 kb increasing to 1470 ° C at 47 kb. The slope of the solidus is 6 ° C/kb, with a constant melting interval of approximately 140 ° C. Clinopyroxene was the liquidus phase in all but one case, and the ilmenite-out curve coincided with the beginning of melting. Controlled cooling experiments resulted in liquidus clinopyroxene crystals surrounded by intergrowths of clinopyroxene and ilmenite. The experimental and natural intergrowths are texturally, crystallographically and chemically similar. These results support the view that the natural clinopyroxene-ilmenite intergrowths found in kimberlites are a product of eutectic crystallisation in the upper mantle. Compositional similarities between the liquidus clinopyroxene and discrete clinopyroxene nodules from kimberlite suggest that the latter are possibly related to the clinopyroxene-ilmenite intergrowths by fractional crystallisation. Differences between the temperatures of equilibration obtained for clinopyroxenes from the natural intergrowths, and the beginning of melting on the clinopyroxene-ilmenite join, may be the result of hydrous melting in the former case, or as a consequence of the natural intergrowths existing as mantle veins or pegmatites at the ambient temperature prior to their incorporation in the kimberlite.