CONTAMINATION VS. LONG-TERM STORAGE OF HIGH-SILICA RHYOLITE MAGMA: HIGH-PRECISION PB ISOTOPE RATIO EVIDENCE FROM THE BANDELIER TUFF

The Otowi Member of the Bandelier Tuff represents ~400 km³ of compositionally zoned high-silica, high Rb/Sr rhyolite magma erupted from the Valles caldera at 1.61 Ma. Prior work has established that large ⁸⁶Sr/⁸⁷Sr_i variations (0.7052 – 0.7095) exist among Otowi glasses and feldspar phenocrysts. Some of this variability is due to contamination of the magma. However, one population of feldspars, representing >90% of the erupted volume, defines a positive trend in ⁸⁷Sr/⁸⁶Sr_i – ⁸⁷Rb/⁸⁶Sr, which may be interpreted either as a "magmatic isochron" or as a mixing line. This "isochron" age is 300 ky older than the eruption age and coincides with the age of the previous eruption from the system.

To resolve this ambiguity, we have measured high-precision Pb isotopes of Otowi feldspars using MC-ICPMS. Precisions are typically ± 0.002 on ²⁰⁶Pb/²⁰⁴Pb ratios. At this resolution, the tuff is clearly zoned in Pb isotopes (²⁰⁶Pb/²⁰⁴Pb: early, most evolved=17.790, late, least evolved=17.831; ²⁰⁸Pb/²⁰⁴Pb=37.535 and 37.592 respectively). These variations cannot be produced by radiogenic ingrowth at the U/Pb and Th/Pb ratios of the host magma on any reasonable timescale, and hence are unequivocal evidence that the more evolved Otowi magma is contaminated by Proterozoic country rock. Country rock fragments from the margin of the Otowi magma chamber have the Pb isotope ratios required of the contaminant. Furthermore, we have been able to determine that selected feldspars are isotopically zoned in Pb isotopes by in-situ measurement of ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb using laser ablation. Feldspar zoning is consistent with increasing contamination during feldspar growth as reflected in phenocryst rims having higher ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb isotope ratios. These observations completely undermine any reasonable age significance of ⁸⁷Sr/⁸⁶Sr - ⁸⁷Rb/⁸⁶Sr variations.

Finally, initial work on Pb isotopes of feldspar from the later Tshirege Member (1.24 Ma) indicates it to be isotopically identical to that of late-erupted glomerocrysts from the Otowi, raising the possibility that the later-erupted tuff contains remobilized Otowi crystal-rich residual magma.