ESR DATING OF QUARTZ PHENOCRYSTS IN THE EL-CAJETE AND BATTLESHIP-ROCKMEMBERS OF VALLES RHYOLITE, VALLES CALDERA, NEW-MEXICO

The electron spin resonance (ESR) dating method was employed on quartz phenocrysts separated from pumice of the El Cajete and Battleship Roc k Members of the Valles Rhyolite in the Valles caldera, New Mexico. Th e results of heating experiments indicate that Ti impurity centers hav e two components; a thermally stable one and a less stable, temperatur e sensitive one. ESR dates using the stable Ti center yield eruption a ges of 59 +/- 6 ka for the Battleship Rock Member and 53 +/- 6 ka for the El Cajete Member while recent C-14 dates (S. Reneau and J. Gardner , unpub. data) from carbonized logs in the El Cajete pumice indicate t hat its age is older than 50 ka. Our results indicate that volcanism i n the Valles caldera is much younger than previously thought (greater than or equal to 130 ka) and that recent revisions to the post-0.5 Ma stratigraphy of Valles caldera are probably in error. The results sugg est that ESR dating of quartz may be a useful method for obtaining age s of units in other Quaternary volcanic areas.