Trace element incorporation into growing augite phenocryst

Abstract

Since the introduction1 to geochemistry of crystallization theories developed in metallurgy, it has been thought that the effects of kinetic parmeters such as the rate of crystal growth and the rate of diffusive transport of elements in the melt could significantly affect the partition of trace elements between mineral and magma. This conclusion has not, however, been demonstrated unequivocally, mainly because the extent of variations of equilibrium partition coefficients with T, P and chemistry is not well known. Henderson and Williams2 found a correlation between morphology and apparent partition coefficient of uranium between olivine and basaltic melt. Data on diffusivity of elements (see ref. 3) and on crystal growth kinetics together with the development of secondary ion mass spectrometry (SIMS) techniques, enable crystal zoning to be studied with respect to trace elements and the kinetic effects to be evaluated quantitatively. Trace element zoning (or lack of it) of minerals is important, as many kinetic-based crystallization models predict trace element zoning in crystals. This letter presents trace element data on a sector-zoned augite phenocryst based on the ion-probe spot analyses. The fact that the slower-growing prism sector [100] is enriched in both compatible and incompatible elements relative to the faster-growing basal sector [111] strongly supports Dowty's4 model for crystal growth involving preferential adsorption of elements onto growing crystal faces in proportion to the charge/size ratio of the elements.