Glasses in the Tshirege Member of the Bandelier Tuff: a record of how compositional zoning happens

Abstract:

The ~400 km³, 1.26 Ma Tshirege Member of the Bandelier Tuff, Valles caldera, New Mexico, USA is a well-known example of a compositionally zoned rhyolitic ignimbrite that exhibits systematic changes in crystallinity with whole-pumice, mineral, glass, and melt inclusion chemical variations throughout its thickness. The first-erupted pumice is highly differentiated, crystal-poor, high-silica rhyolite with strong enrichments in incompatible trace elements such as Rb, Th, Nb and U. Later-erupted material is successively more crystal-rich with lower concentrations of incompatible elements. Quartz in the first-erupted material is non-zoned, has low Ti contents, and contains melt inclusions with trace element compositions similar to the host glass. Quartz crystals from late-erupted pumice are strongly zoned, with Ti-rich rims overgrown on dissolution surfaces on Ti-poor cores; the latter have the same Ti abundances as the first-erupted quartz, and their melt inclusions have highly evolved compositions. Rare melt inclusions in the Ti-rich rims are depleted in incompatible elements and chemically match the host glass in late-erupted pumice. The range of zoning in the whole tuff unit is thus duplicated in single quartz crystals. These observations are consistent with melting and remobilization of a quartz-feldspar crystal mush followed by an increment of crystal regrowth. Melting was induced by invasion of hotter recharge magma, observed as dacite pumice scattered through the zoned rhyolite sequence. The model is supported by strong enrichments in fluorine (up to 1.6 wt. %) in later-erupted glasses that are attributed to observable biotite breakdown. Biotite was destabilized through a combination of heating and volatile dilution in the liquid as melting of quartz + feldspar proceeded. Recharge incrementally mobilized the crystal mush prior to the eruption and created a temperature gradient of ~100 °C at a pressure of ~0.18-0.30 GPa.