Crystallization of Groundmass Spinel in Kimberlite

Groundmass spinel grains in 46 kimberlite and related rocks have been analyzed and compared. The majority of the spinel analyses are classified as high-chromium chromite (Chr) and magnesio-ulvo « spinel^magnetite (Mum) and represent two significant stages of spinel growth. There are also a significant number of spinel grains that are classified as xenocryst spinel (Xen), pleonaste spinel (Ple) and magnetite (Mag). Eight different spinel zoning trends are identified.The majority of the Chr spinel grains are inter-preted as a primary phase that crystallized as small octahedra from kimberlite magma on the journey from the upper mantle to the final resting place in the upper crust. Three zoning trends lead directly away from primary chromite. The major zoning trend, Trend 1, is from chromite to magnesio-ulvo « spinel^magnetite.This zoning trend is unique to spinel in kimberlite, carbonatites and lamprophyres.We suggest that this somewhat oxidizing, and more magnesian, trend