Evaluation of differences in minor-element composition between coexisting obsidian, perlite, and felsite from four Cenozoic rhyolite lava flows in the Rocky Mountain region indicates that significant errors can be made in estimating the original composition of rhyolitic obsidian simply by relying on abundances of elements in associated perlite and/or felsite.

Perlites have low Li, high Sr and Ba, and variable F relative to obsidian. Variations of Li, Sr, and Ba in perlites are controlled by low-temperature ion exchange with groundwater. Felsites have low Li, Cs, U, Mo, and F, and high Sr, Ba, and Eu relative to obsidian. Variations of elemental abundances in felsites result from a combination of high-temperature (crystal-melt fractionation and volatile transport) and low-temperature processes (ion exchange, differential solution, and absorption by secondary phases). For rhyolitic rocks that evolve by major feldspar fractionation, the rare-earth elements (REE), and particularly Eu, are useful for evaluating abundance variations produced by magmatic heterogeneity of feldspar. The component of Sr and Ba variations caused by magmatic heterogeneity of feldspar may be estimated using the Eu data. Young felsites have significantly different abundances of F, REE, Cs, and Li than coexisting obsidian, and this indicates that variations are produced by either crystal-melt fractionation or processes active during or shortly after eruption. Compared to obsidian, felsites exhibit relative depletions of U and Mo that increase with age, which indicates that losses are caused by weathering.

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