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 fi eld, New Mexico) is a caldera-forming high-silica rhyolitic ignimbrite with a precursor fallout deposit. The Otowi Member has high Rb/Sr ratios; the R b-87/Sr-86 ratios from sanidines and glasses range from 14 to 570. Most san idines from glomerocrysts in the ignimbrite have Sr-87/Sr-86(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 ini tial fallout and the ignimbrite are markedly more radiogenic, at Sr-87/Sr-8 6(i) = 0.7105-0.7113, despite having much lower Rb/Sr ratios than glomerocr yst glasses. These relations require that the inclusion glasses are more co ntaminated with Proterozoic country rock than are glomerocryst glasses. Tex tural, isotopic, and trace element data support a model in which crystals g row 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, disse minated throughout the magma prior to eruption. An important implication of these results is that glass inclusions do not necessarily represent precur sor magma compositions; hence extrapolation of measured volatile contents o f inclusion glasses to the entire volume of an erupted magma should be appr oached with caution.