WEATHERING MECHANISM OF SILICATE PHENOCRYSTS IN TEPHRAS, INFERRED FROM SEM OBSERVATIONS OF SURFACE ETCHING

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In the present study we describe the surface etching features of plagioclase, pyroxene, olivine, amphibole, and biotite phenocrysts in tephras from Hachijo-Higashiyama, Daisen, and Niijima volcanoes with progress in weathering. The variations in the size and number of etch pits were quantified by image analysis as a function of depth in tephra column from Hachijo-Higashiyama. In Hachijo-Higashiyama tephras, pyroxene and plagioclase phenocrysts were completely covered with glassy materials and etching did not occur during the early weathering duration of about 10ky after deposition. After the glassy cover disappeared, etching began selectively at dislocations. The etch-pit shapes are crystallographically controlled on different faces, particularly in plagioclase. The etch-pit density decreased, the size increased with increase in depth of tephra column. This change is an overall result of etch-pit coalescence and neoformation of small etch pits. Eitching of phenocrysts is preceded by halloysitization of glass, which results in the increase of saturation levels in porewaters infiltrating through tephra column. Consequently, it is implied that mineral dissolution rates are strongly controlled by the saturation level of porewaters with respect to solute species supplied from rapid dissolution of glasses during chemical weathering of tephras.
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