Sr isotope disequilibrium during differentiation of the Bandelier Tuff: Constraints on the crystallization of a large rhyolitic magma chamber

The 1.61 Ma Otowi Member of the Bandelier Tuff (Jemez Mountains volcanic field, New Mexico) is a caldera-forming high-silica rhyolitic ignimbrite with a precursor fallout deposit. The Otowi Member has high Rb/Sr ratios; the ⁸⁷Rb/⁸⁶Sr ratios from sanidines and glasses range from 14 to 570. Most sanidines from glomerocrysts in the ignimbrite have ⁸⁷Sr/⁸⁶Sr_I = 0.7052–0.7056, whereas the ratios from glasses from glomerocrysts range from 0.7052 to 0.7079. Quartz phenocrysts containing glass inclusions from both the initial fallout and the ignimbrite are markedly more radiogenic, at ⁸⁷Sr/⁸⁶Sr_I = 0.7105–0.7113, despite having much lower Rb/Sr ratios than glomerocryst glasses. These relations require that the inclusion glasses are more contaminated with Proterozoic country rock than are glomerocryst glasses. Textural, isotopic, and trace element data support a model in which crystals grow in a boundary layer with the most inclusion-rich quartz grains closest to the magma–country rock contact. Phenocrysts and glomerocrysts represent fragments from different zones of the chamber's crystalline carapace, disseminated throughout the magma prior to eruption. An important implication of these results is that glass inclusions do not necessarily represent precursor magma compositions; hence extrapolation of measured volatile contents of inclusion glasses to the entire volume of an erupted magma should be approached with caution.