COMBINING SEM-EBSD AND CL ANALYSES FURTHERS IGNEOUS INTERPRETATIONS AND UNDERGRADUATE RESEARCH

Combining the scanning electron microscopy (SEM) techniques of electron backscatter diffraction (EBSD) and cathodoluminescence (CL) has led to new insights into igneous processes. Both techniques are useful to interpret mineral textures: EBSD primarily has been used to analyze mineral orientations and CL to reveal slight variations in mineral chemistry. For example, recent studies have shown the following. Quartz orientations (EBSD) combined with Ti zoning histories (CL) showed that oriented synneusis of quartz in granite porphyry formed as a result of crystal accumulation in a magma chamber followed by partial melting during rejuvenation (Vinalhaven intrusive complex, Maine). Similarly oriented quartz clusters with Ti zoning histories showing multiple episodes of resorption in granitoid lithic clasts from the Kaharoa eruption revealed pluton formation from a crystal mush synchronous with volcanic eruptions (Okataina, Taupo Volcanic Zone, New Zealand). This process likely was similar for quartz from a plutonic lithic in a nearby ignimbrite deposit that showed resorption and abrupt temperature fluctuations along with disparate histories for adjacent crystals (Rotoiti, Taupo Volcanic Zone, New Zealand). Switching to more mafic igneous systems, an inverse correlation between plagioclase foliation (EBSD) and the amount of late-stage interstitial material (CL) in plutonic lithics was used to interpret melt extraction through compaction (Akaroa Volcanic Complex, New Zealand). Undergraduate students who have acquired and interpreted EBSD and CL data have gained valuable research experience as they contributed to these and other collaborative projects. The author is immensely grateful to Bud Wobus for first introducing her as an undergraduate to igneous studies, the SEM, and the benefits of collaborative research through his skillful guidance in Williams College courses and the Keck Geology Consortium’s Pike Peak Project.