STRONTIUM ISOTOPES AND MAGMA DYNAMICS: WHAT WE CAN LEARN FROM HIGH-TEMPERATURE RHYOLITES

Volcanic rocks often exhibit internal heterogeneity in radiogenic isotopes. Isotopic disequilibrium between co-existing phenocrysts and isotopic zoning within single crystals has been demonstrated in basalts, andesites, dacites, rhyolites and alkaline magmas. High-temperature “Snake River-type” rhyolites appear to be an exception. Despite the occurrence of Snake River Plain rhyolites in a region of isotopically highly variable crust and mantle, and significant differences from rhyolite unit to rhyolite unit, little to no isotopic zoning is found within their feldspar phenocrysts, and feldspars within a single unit define tightly grouped unimodal populations.

High-temperature rhyolitic magmas possess a unique combination of temperature and melt viscosity. Although typically 200°C hotter than common rhyolites, the temperature effect on viscosity is offset by lower water contents, hence melt viscosities are in the same range as common, water-rich, cool rhyolites. However, magmatic temperatures are in the same range as basaltic andesites and andesites, consequently cation diffusion rates are orders of magnitude greater than common rhyolites. We hypothesize that this combination of characteristics promotes crystal isotopic homogeneity: viscosities are too high to permit crystal transfer and magma mixing on timescales shorter than those required for diffusive homogenization of Sr between phenocrysts and matrix (100 – 1000 years). This is not true for most magma types, in which either crystal transfer is rapid (<<100 years) due to low melt viscosities (basalts and intermediate magmas), or Sr diffusion rates are so slow that equilibration times are longer than the lifetime of the system (e.g. cool, wet rhyolites: 100 k.y. – 1 m.y.).