CATHODOLUMINESCENCE AND TRACE ELEMENT CHEMISTRY OF SILLIMANITE: EVIDENCE FOR MULTIPLE METAMORPHIC REACTIONS

Sillimanite, typically assumed to be essentially a chemically ‘pure phase’ in high-grade metapelitic rocks, displays textures and contains trace elements that reflect its reaction history. Cathodoluminescent (CL) imaging and trace element analyses (electron microprobe) of sillimanite from aluminous migmatites (bt + grt + sil + qtz + ilm + pl ± ap ± zrn ± mnz) of the Sawtooth Metamorphic Complex (SMC), Idaho, USA, reveal chemical zonation in nearly all of the sillimanite analyzed. Optical CL examination of sillimanite typically shows a somewhat rounded, darker core with irregular, brighter overgrowths surrounded by another darker zone. CL intensity varies from bright to dull orange-red along and across blades. These color variations correlate with variable trace amounts of Fe³⁺(~1100-5320 ppm), Cr³⁺(~ 40-460 ppm), V³⁺ (~ 80-340 ppm) and Mn²⁺ (~ 40-260 ppm). For example, areas of brighter luminescence typically correspond to lower Fe³⁺ contents (ca. 1400-1445 ppm), with higher Cr³⁺ (~ 285-592 ppm). Alternatively, areas of darker luminescence correspond with the higher abundance of Fe³⁺(~ up to 2230 ppm) with slightly lower Cr³⁺(~ 283-482 ppm) contents.

Results indicate that changes in the CL color intensity are visual indicators of chemical variability and textures that can relate to multiple episodes of growth and dissolution of sillimanite during metamorphism. For example, the original grains are likely to be related to prograde growth of sillimanite followed by dissolution related to melt-forming reactions. Additional sillimanite growth may be related to fluid infiltration or growth on the retrograde path. The dissolution and stabilization of sillimanite produces overgrowths within the grain that are preserved as red regions with darker and lighter CL signatures. Consequently, by combining these methods, sillimanite can be used as an effective indicator of mineral reactions at high temperatures and pressures (~800 °C, ~7 kbars) where other indicator minerals (i.e. garnet) would typically homogenize.