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Rare-earth-element abundances have been determined for 26 rhyolite obsidians and glass separates of both peralkaline and subalkaline chemical character for which a history of marked differentiation had been inferred from very low Sr, Ba, Mg, and minor transition-element contents. All but five of the specimens possess very strong negative Eu anomalies, with values of Eu/Eu* of 0.06 or less. Eight have values of Eu/Eu* of 0.02 or less, and subalkaline rhyolites from the Fish Creek Mountains Tuff, Nevada, have Eu/Eu* of <0.003. Calculations using coefficients appropriate for the distribution of Eu between Ca-poor feldspar and silicic melt suggest that, if the Eu anomalies are the result of crystal fractionation, many of the glasses represent 10% to 20% or less of hypothetical unfractionated parent liquids. Distribution coefficients for Eu between Ca-poor feldspar and peralkaline silicic melt are significantly lower than for subalkaline systems; this implies high degrees of differentiation for peralkaline rocks having even modest negative Eu anomalies.

A number of the specimens studied represent relatively large rock units, and some are from the lower parts of large, compositionally zoned ash-flow sheets. The mechanism—whether crystal fractionation or thermogravitational diffusion—called upon to produce large-scale vertical compositional variations within bodies of highly silicic magmas must be able to explain the profound depletion of large volumes of magma in such elements as Sr, Eu, Ba, Mg, Co, and Ni.

The moderate negative Eu anomaly of glass from the Summit Lake Tuff, Nevada, can be explained by crystallization of the anorthoclase phenocrysts present in the rock. This suggests that this compositionally unzoned ash-flow sheet was produced from rhyolitic magma that possessed little or no Eu anomaly before it rose to a high-level chamber where it subsequently underwent appreciable intratelluric crystallization, but little or no crystal segregation, prior to eruption. In contrast, the strong Eu depletion (Eu/Eu* = 0.05 to 0.15) of crystal-rich specimens of the Fish Creek Mountains Tuff indicates that considerable differentiation had taken place before growth of the present generation of phenocrysts.

Peralkaline specimens from the northern Great Basin and Snake River Plain have lower La/Lu ratios (2.4 to 6.2) than do specimens from the southern Great Basin (La/Lu = 6.4 to 15.7), which suggests that they may have been derived from source regions that have significantly different rare-earth-element abundance patterns.

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