SEVERE LEACHING OF TRACHYTIC GLASS WITHOUT DEVITRIFICATION, TERCEIRA,AZORES

Pumice produced during the subplinian phase of eruption of a peralkali ne trachyte lava on the island of Terceira, Azores, has undergone exte nsive leaching at ambient conditions by groundwater, while remaining i n a glassy state. Over 25% of F, Na, K, Si, Fe, Ti, and Mn originally present has been removed, whereas U, Al, Nb, Ca, Y, and Rb show smalle r but significant losses. Thorium, Zr, Hf, Ta, and Lu have remained im mobile. Water content (measured as LOI) has increased up to tenfold, w hereas the Sr content has risen by up to 80%. The absence of devitrifi cation requires that leached elements escaped by diffusing through the glass; a coefficient of chemical diffusion D(Na) between 1.8 X 10(-19 ) cm2 s-1 and 6.1 X 10(-19) cm2 s-1 has been calculated, in excellent agreement with previously published experimentally derived values for comparably low temperatures. Water has not replaced the leached cation s on a charge-equivalent basis, a finding that requires an explanation other than simple binary interdiffusion. We suggest that a simultaneo us influx of molecular water and cation exchange between Na+ in the gl ass and H+ in the groundwater opens the glass structure and frees cati ons, which can diffuse outward by hydrolysis of bridging oxygen atoms. Reported mobilities have important implications for such diverse fiel ds as igneous petrology, nuclear waste management, and agriculture.