Chemical zonation in the Upper Bandelier Tuff: Evidence from the chemistry of distal outcrops, Puye quadrangle
— David P. Dethier, Stephanie K. Kampe, David A. Sawyer, and James R. Budahn

Upper Bandelier Tuff (UBT) exposed beneath finger mesas on the northeastern Pajarito Plateau, New Mexico, includes some of the most distal outcrops of the compound cooling unit erupted from the Valles caldera at about 1.22 Ma. We report major and trace-element analyses for 35 samples of UBT from 8 sites in the Puye quadrangle as well as for 16 samples collected previously from 2 locations in the Los Alamos National Laboratory (LANL) area. Concentrations of silica, sodium, potassium and some other major oxides in UBT units 1 to 4 reflect the evolution from high-silica to low-silica rhyolite during the eruption, but changes are small and grouped data suggest that only TiO₂ varies systematically in the 10 stratigraphic sections. Multiple measurements from 10 sites indicate that concentrations of oxides and trace elements such as Eu, Sr and Zr in units 1 to 3 show little lateral or vertical variation across the area. In vertical sections through the tuff, trace elements such as Rb, Cs, U and Zn display a consistent pattern of zonation. Apparent concentration changes and patterns of relatively low versus high variance in the trace element content within UBT unit 1 may result from local mineralogic control or elemental redistribution during vapor-phase alteration. Trace element chemistry from tuff at Mesas 19 and 25 is anomalous and suggests that field identification of the units may be incorrect or that other processes may affect trace-element zonation in the thinnest distal tuffs.