Crystallization of Groundmass Spinel in Kimberlite

Abstract

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 magnesioulvo« 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 interpreted 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 was influenced by the high carbonate content of Group I kimberlites and the rapid crystallization of the minerals during the evolution of volatiles.The zoningTrend 2 involves increasing titanium and ferric iron as a function of increasing Fe 2þ /(Fe 2þ þ Mg). This trend is similar to the zoning of spinel in basalt and is thought to be due to co-crystallization of magnesium- and aluminum-rich silicate minerals such as olivine and phlogopite in kimberlites, or pyroxene and plagioclase in basalt. Zoning Trend 3 in kimberlite leads away from primary chromite and towards an aluminous pleonaste (Ple) spinel. This trend is characterized by a large decease of Cr/(Cr þAl) parallel to so-called olivine^spinel iso-potential lines. Similar trends of lesser magnitude and cyclic Al^Cr zoning have been identified in basaltic spinel. This trend is thought to be due to very rapid crystallization under conditions of supersaturation where the crystallization of spinel affects the local environment ahead of the growing spinel crystal (i.e. diffusion-controlled crystallization). The tendency for immiscibility between ferrite- or titanate-rich spinel, and aluminate-rich spinel (pleonaste) has a great influence on Trends 1 and 3 zoning and also on atoll-spinel formation. Very local conditions such as nucleation, or lack of nucleation, of other minerals can influence both the textural environment and composition of kimberlitic spinel.