MICROLITES AND NANOLITES IN RHYOLITIC GLASS - MICROSTRUCTURAL AND CHEMICAL CHARACTERIZATION

We used transmission electron microscopy (TEM) and scanning electron m icroscopy (SEM) to study magmatic crystals in the Ben Lomond rhyolite lava dome, Taupo Volcanic Center, New Zealand. Using TEM and SEM to in vestigate the size distributions of these crystals, we identified thre e size populations: microphenocrysts (>1.2 mu m wide), microlites (>0. 6 mu m wide), and smaller crystals (<0.6 mu m wide) which we term ''na nolites''. The predominant mineral phases of the microlites and nanoli tes are augites, pigeonites, and hypersthenes. The compositions and mi crostructures within these pyroxenes indicate disequilibrium crystalli zation at approximately 850-900 degrees C and under-coolings as high a s 300 degrees C from equilibrium crystallization temperatures. Complex microstructures resulting from subsolidus reactions in augite and pig eonite are consistent with moderate cooling rates within the upper obs idian layer of the Ben Lomond rhyolite dome. This study demonstrates t he existence of sub-micron magmatic crystals in a rhyolite and illustr ates the potential of TEM to provide unique information about the crys tallization and cooling histories of glassy volcanic rocks.