BUILDING AND RE-BUILDING A SUPER-ERUPTION: ID-TIMS U-PB ZIRCON GEOCHRONOLOGY OF THE BANDELIER TUFF AND CERRO TOLEDO RHYOLITES

The Jemez Mountains in North-Central New Mexico USA are the site of two Catastrophic Caldera Forming (CCF) eruptions during the Pleistocene with the Toledo Caldera eruption producing the Lower Bandelier Tuff (LBT), and the Valles Caldera producing the Upper Bandelier Tuff (UBT). Multiple smaller volume tephra of the Cerro Toledo Rhyolite interval lies stratigraphically between the LBT and UBT and help to capture the geochemical evolution of the melts present over this super-eruption building and re-building cycle. A high-precision Isotope Dilution – Thermal Ionization Mass Spectrometry (ID-TIMS) U–Pb study of zircon was undertaken to provide a more robust chronological framework in order to assess if the magma chamber supplying eruptive material is characterized as being geometrically interconnected (large melt pockets in communication with each other) as seen by a broad range of individual zircon ages in each sampled tephra (‘memory’ of previous eruptions), or if the magma chamber is quiescent (isolated melt pockets) in between eruptions with each sampled tephra represented by a narrow range of individual zircon ages with little ‘memory’ of previous eruptions. Fifteen samples of airfall tephra were collected from two stratigraphic sections of the Cerro Toledo Rhyolite (CTR) as well as samples of LBT and UBT. Nine of the CTR samples directly correlate to previous tephra sampled for K/Ar and 40Ar/39Ar dating and allow for direct comparison of U/Pb and K-Ar/Ar geochronology. We report, for the first time, high-precision U-Pb geochronology for all 15 CTR samples as well as the UBT and LBT and found that the tephra units older than ~1.37Ma have evidence of older antecrystic zircons (some ‘memory’ of previous eruptions), and the tephra younger than ~1.37Ma show little evidence for older population zircons (no ‘memory’ of previous eruptions). Our geochronology reveals local unconformities in the sections, which prevent us knowing if the entire magmatic history of the LBT, CTR and UBT sequence has been captured. Regardless, the changes in abundance and age of antecrystic zircons in the CTR units appear to mark the transition between the remnant LBT magmatic system and the renewed accumulation of magma leading up to the catastrophic eruption of the UBT.