CATHODOLUMINESCENCE DOMAINS CORRELATE WITH COMPOSITIONAL ZONING IN KYANITE

Cathodoluminescence (CL) imaging has been used to document zoning patterns within kyanite (e.g. Kendrick and Indares, 2017), and, in some cases, variations in CL intensity can be linked to trace element concentrations (Müller et al., 2011; Horkley et al., 2013). In this study, we analyzed kyanite from three localities to evaluate correlations among CL intensity, trace element concentrations, and absolute orientation. Samples were selected from metapelites within the ultrahigh-pressure Rhodope Metamorphic Complex (Greece), ultrahigh-pressure whiteschists from Kokchetav (Kazakhstan), and high-pressure metapelites from the Goshen Dome (western MA, USA).

Kyanite ranges in size from a few mm to a few cm, with the largest grains being commonly poikiloblastic and smaller grains being skeletal fragments or subhedral single grains. All analyzed grains exhibit multiple CL zones as measured on a Tescan RainbowCL. The interiors of most grains display a range of CL intensities and colors, sometimes revealing apparent sector zoning, whereas others exhibit “feathered” CL oscillations. A few grains preserve CL-dark patches. In many cases, CL-bright domains with curved to lobate boundaries truncate the zoning observed within the kyanite cores. Many grains also feature CL-bright rims that cross-cut interior zoning. Electron backscatter diffraction analysis demonstrates that absolute orientation does not correlate with CL intensity, suggesting that trace element variations provide the primary control on CL zoning.

Electron microprobe analysis (mapping and point analysis) shows that CL intensity correlates most strongly with variation in P. CL-dark domains yield the highest enrichment in P₂O₅ (up to 0.07 wt%) whereas CL-bright domains yield the lowest concentrations of P₂O₅ (<0.01 wt%). CL-dark domains also yield MgO and FeO enrichment (>0.02 wt % and >0.20 wt%, respectively), but boundaries of these compositional domains are more diffuse and the relative enrichment/depletion is not as large. For comparison, CL-bright domains are typically <0.01 wt% MgO and <0.15 wt% FeO. Ti, Mn, Y, Cr, Zr, and V do not correlate with observed zoning patterns. These data and the cross-cutting relationships observed in CL suggest that kyanite can preserve compositional zones that may correspond to discrete metamorphic events.