Mineralogy and Microtextures of Basaltic Glass Alteration in Hyaloclastite, Jeju Island, Korea

Basaltic glass in the subsurface hyaloclastite from Jeju Island, Korea, reacted with meteoric groundwater. The alteration products of the basaltic glass were examined using electron microscopy, in situ sample preparation using focused ion beam, and electron microprobe analysis. An alteration rim of hemispherical oscillatory zoning was formed around the hyaloclastite particles by the selective alteration of the glass. Leached glass of the hemispherical shape was developed at the glass alteration front by partial to complete leaching of cations relative to Si. The order of cation leaching from glass was Na>Mg>K>Ca>Al>Fe>Ti, roughly consistent with the increasing ionic potentials. Leached cations were partly incorporated into secondary phases of the alteration rim, and partly removed from hyaloclastite–water system. The porous leached glass changed gradually into either nanogranular amorphous aggregates with a partial crystallization of primitive smectite, retaining its hemispherical shape or abruptly to pure well-crystallized smectite via solution-precipitation to form the oscillatory zoning. Systematic microtextural, microchemical, and mineralogical analyses of natural glass alteration are critical to understanding the long-term behavior of glass alteration, which cannot be reproduced in short-term laboratory experiments.