THE SECRET LIFE OF KYANITE

Trace element zoning in metamorphic minerals offers a unique window into metamorphic processes, as it reflects reactions progressing during mineral growth. Kyanite from amphibolite to eclogite facies conditions has been analyzed by cathodoluminescence (CL) to characterize trace element zoning. All studied kyanite reveal zoning patterns, which group into four main types: pseudomorphs, sector, growth, and anomalous zoning.

Kyanite trace element zoning has been characterized by both a Gatan MonoCL system attached to a Cameca SX100 electron microprobe and an ELM-3R luminoscope with an attached Canon digital camera. Images of zoning captured with the Gatan system through red and blue filters display submicron zoning behavior whereas green filtered images were contaminated by UV, violet, red and IR light. Luminoscope images display the true CL color of the kyanite on a micron scale and can be split into red, green and blue channels to show zoning represented by each luminescent color.

Trace element zoning was quantified by X-ray mapping and spot analysis with the electron microprobe and LA-ICPMS. Trace elements in kyanite include Fe, Cr, Ti, and V. Fe is most abundant, with concentrations ranging 600-1200 ppm. Cr concentrations range from below detection limit to 300 ppm, and V concentrations are 150 ppm to below detection limit. Ti concentrations typically range from below detection limit to 50 ppm although discreet zones contain Ti concentrations ranging 100-200 ppm.

Red luminescence correlates with Cr concentration determined from X-ray maps and spot analyses whereas blue luminescence correlates with Ti concentrations. Zones with high concentrations of Fe generally correlate with low luminescence, indicating Fe may suppress luminescence. However, in some samples, Fe appears to correlate with green luminescence. Additional work is being done to conclusively correlate Fe and V to specific luminescence.

Trace element zoning in kyanite, as well as preliminary work on andalusite and sillimanite, suggest that trace element zoning patterns are a powerful tool for the analysis of TE behavior during metamorphic reactions or to infer trace element mobility and partitioning during metamorphism.