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Oxygen isotope analyses are reported from co-existing quartz and feldspar from the Bandelier Tuff and Cerro Toledo high-silica rhyolitic pyroclastic deposits erupted from the Valles caldera, New Mexico. Quartz shows little variation outside analytical error, but δ 18 O in feldspar varies over >1‰. In most samples, 18 O/ 16 O fractionation between quartz and feldspar is significantly less than is predicted for equilibrium at temperatures appropriate for rhyolitic magma. In the Tshirege (upper) Member of the Bandelier Tuff, isotopic fractionation between mineral pairs is close to equilibrium in the later erupted ignimbrite, but non-equilibrium in the initial Plinian deposit. These relationships are interpreted in terms of a model where most phenocrysts are derived from a highly porphyritic carapace around the magma chamber that was disrupted by eruption, thus scattering crystals throughout the magma. Carapace quartz and feldspar are initially isotopically lighter than the bulk aphyric magma, due to chemical communication with low-δ 18 O country rock in the meteoric/hydrothermal system surrounding the chamber. We assume that quartz and feldspar were initially in isotopic equilibrium. Diffusive re-equilibration of crystals begins when the carapace disintegrates and the minerals are immersed in the bulk magma just prior to and during eruption. Feldspar is isotopically lighter than quartz at equilibrium, but responds more rapidly than quartz to an external change, due to a higher diffusion coefficient for oxygen. Hence, immersion in the isotopically heavier bulk magma causes feldspar and quartz δ 18 O values to initially converge over ∼10 2 years, and then diverge over 10 3 –10 4 years as first feldspar, and then quartz, re-equilibrate with the new magma. Higher δ 18 O variability of feldspar than quartz indicates that the shorter timescale applies to the Bandelier and Cerro Toledo rhyolites. Two important implications of this interpretation are (1) that the Bandelier magmas developed in an aphyric condition, and their porphyritic character is an artifact of eruption; and (2) that a protective, mechanically rigid cognate carapace around a silicic magma chamber may limit interaction with low-δ 18 O hydrothermally altered crust, thus hindering the development of significant volumes of low-δ 18 O silicic magma. |
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