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An unusual occurrence of juxtaposed glassy and clay-altered ash was sampled to determine the extent of element mobility during glass diagenesis. The results are particularly interesting in that major mobilization of uranium is indicated. Closely spaced samples of glassy and clay-altered ash were collected from the same 20 to 50-cm-thick stratum in the Troublesome Formation (Miocene) of northwestern Colorado. Sharp contacts present between glassy ash and underlying pink montmorillonite may indicate water-saturated conditions restricted to basal ash layers or deposition in a body of water that dried up during the course of the eruption. Formation of montmorillonite instead of zeolites indicates that the water was not highly alkaline or saline. Multielement analysis of glass and clay-altered samples indicates three things. (1) Montmorillonite has 85 to 90% less uranium than the coexisting glass; similarly depleted elements include Cs, Rb, Na, and K; much smaller depletions of these elements in some glassy samples serve as particularly sensitive indicators of incipient alteration of this type. (2) The abundances of relatively insoluble elements such as Th or Ta are slightly higher (5 to 40%) in clay-altered ash and serve as indicators of the maximum levels of element enrichment in residual material; greater enrichment of elements such as Sc, Sr, and Co indicate adsorptive uptake by clay or by secondary oxides of iron and manganese. (3) The rare earth element (REE) patterns and abundances in glass are sufficiently mimicked by detritus-free montmorillonite to ocument the original compositional equivalency of the two.
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