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3.9 e. Microlites and "nanolites" in a rhyolitic glass: Microstructural and chemical characterisation (R.J. Stevenson, T.G. Sharp and D.B. Dingwell)

Microlites can provide important information on the crystallisation kinetics of rhyolitic lavas. We have used transmission electron microscopy (TEM) and SEM to study magmatic crystals in two obsidians from the Ben Lomond dome, New Zealand. With TEM, crystal size distribution (CSD) information can be obtained for crystals < 20 nm size, far below the resolution of petrographic and backscatter SEM image analysis. Furthermore, microstructural and compositional information can be obtained from microlites to provide unique information about the crystallisation and cooling histories of glassy volcanic rocks.

Based on CSDs, we have identified three size populations: microphenocrysts (> 1.2 µm width); microlites (> 0.6 µm width); and smaller crystals (< 0.6 µm width) that we term "nanolites" (Fig. 3.9-1). These populations imply three crystallisation events which may correspond to near-magma chamber, conduit, and lava flow environments for the
 

Fig. 3.9-1: CSD plots for TEM data (a) and SEM data (b). The natural log of population density is plotted as a function of crystal width (nm). The pronounced break in slope at 600 nm indicates the change from microlites to nanolites (a) and at 1200 nm (b) represents the change from microlite to microphenocryst populations.

 
microphenocrysts, microlites and nanolites, respectively. The predominant mineral phases of both microlites and nanolites are augite, pigeonite, and hypersthene, whereas plagioclase dominates in the microphenocryst assemblage. The compositions and structures within these pyroxenes indicate disequilibrium crystallisation at approximately 850 - 900 °C and undercoolings as great as 300 °C based on equilibrium crystallisation textures. The observation of nanolites is consistent with nucleation theory, and implies that nanolites may be common in rhyolitic obsidians.