Table 2. Modal abundances of phenocryst-rich and phenocryst-poor dacites.

[Phenocryst and matrix abundances are in volume percent on a vesicle-free basis. Modal abundances were measured with a JEOL 733 scanning electron microscope and a VOYAGER-NORAN image analysis system on polished pumice samples previously impregnated by a fluid epoxy resin under vacuum. Vesicles' abundances were easily measured from back-scattered electron images by using the high atomic number contrast between the low-atomic-number epoxy resin filling the pores and the silicate matrix. Phenocrysts and microphenocrysts (>5 m) were measured on the same areas by automated phase analysis with the energy dispersive spectrometer (counting time: 10 s/phase). Glassy matrix abundances were obtained as the difference between the total area of the image and the sum of the fractional areas occupied by vesicles+phenocrysts+microphenocrysts]

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Last updated 03.29.99