The Lower Bandelier Tuff, erupted from the Valles Caldera at 1.51 Ma, is composed of a Plinian tephra and associated ignimbrite. Based on ion and electron microprobe analyses of melt inclusions (MI) in quartz, sanidine, and pyroxene phenocrysts, a strong volatile gradient was present in the upper portion of the magma chamber. In the 20 km3 of magma that produced the Plinian tephra, most H2O and F contents of MI are between 4 to 5 wt % and 0.18 to 0.20 wt %, respectively, and between 3 to 4 wt % and 0.12 and 0.14 wt % in the first-deposited basal ignimbrite. Measurements of the H2O and F contents of the 380 km3 of magma which produced the bulk of the ignimbrite are concentrated between 2 to 3 wt % and 0.05 to 0.14 wt %, respectively. The invariance of H2O (and possibly Cl and B) relative to F in the magma which formed the Plinian tephra suggests that this portion of the magma chamber was saturated with respect to an H2O-rich vapor phase. The trace element composition of MI are varied, and overlap with composition of pumice lumps. A group of MI are rich in Ti, Sr, Ba and low in B, Cl, Rb, Y, Nb, and Th, and are compositionally similar to analyses of matrix glass from some ignimbrite pumice lumps. These represent the mixing of a second rhyolite into the base of the Lower Bandelier magma chamber. MI influenced by the second magma have not been found in Plinian samples, but occur in ignimbrite samples from base to top of the deposit. When these MI are removed from the data set, the remainder show strong linear correlations between Nb and Rb, Y, Zr, and Th.

These correlations can be most easily explained by fractionation of approximately ~40% quartz and alkali feldspar (with trace amounts of chevkinite). MI from the Plinian tephra are similar to bulk Plinian pumice composition, suggesting that magmatic evolution was well-progressed at the time that the MI were trapped. However, fractional crystallization is difficult to reconcile with the observed distribution of some other elements, including H, B, Li, F, and Cl. The enrichment of H2O and F in the upper portion of the magma chamber, along with their lack of correlation with Nb, Rb, Zr, Y, and Th, suggest that these sets of elements were decoupled during magmatic evolution. The upward enrichment of H2O and F must have occurred at a faster rate than other trace elements. The observed volatile gradient may have influenced the eruption dynamics of the magma, initially causing discrete layers of magma to be removed when the gradient was large, followed by chaotic eruption of the bulk of the ignimbrite when the gradient was small. Comparison with the Bishop Tuff eruption, however, suggests that the chaotic eruption style of the LBT may be related to vent geometry and conduit evolution, promoting high discharge rates, not differences in the absolute concentration of volatiles. ¿ American Geophysical Union 1992