ORIGIN OF THE CRYSTAL LOAD IN SILICIC MAGMA SYSTEMS: TANDEM ZIRCON GEOCHEMISTRY AND GEOCHRONOLOGY IN RHYOLITES OF THE WESTERN MOUNT BENNETT HILLS, SNAKE RIVER PLAIN, ID

One deficiency in geochronology applied to silicic magma systems is the apparent discrepancy between U-Pb and ⁴⁰Ar/³⁹Ar ages, with the U-Pb zircon method consistently yielding ages >1 Ma older than ⁴⁰Ar/³⁹Ar sanidine ages. Is this discrepancy a product of long residence time in the magma chamber, or do these older ages result from crystal inheritance? A long residence time questions the accuracy of U-Pb applied to silicic magma systems, whereas crystal inheritance supports the accuracy of the method and indicates the need for reassessment of the K decay constant. In order to address this problem, samples were collected from a stack of 10 rhyolite units in the western Mount Bennett Hills (WBH), which is an area of voluminous, high-temperature silicic volcanic stratigraphy along the northern margin of the central Snake River Plain, Idaho. We posit that combining cathodoluminescence (CL) imaging and LA-ICPMS trace element results with CA-TIMS ages can provide the ideal data set for assessing the magmatic evolution of these rhyolites in terms of their genesis, differentiation, and residence times. Zircon grains were mounted, imaged via CL, and analyzed for trace elements and U-Pb isotopes via LA-ICPMS. Three zoning patterns were identified from the CL images: weak oscillatory zoning, planar zoning, and dark cores. Each zoning pattern displays unique trace element signatures and/or ages. Miocene-aged oscillatory zoned grains display continuous differentiation trends supported by Ti-in-zircon thermometry. Miocene planar zoned crystals are geochemically discordant to the oscillatory zoned grains and must be older due to the presence of oscillatory overgrowths. Eocene to Cretaceous dark cores have the highest trace element concentrations relative to other zoning patterns and support xenocrystic inheritance in the rhyolites. However, the resolution of these LA-ICPMS ages is inadequate for resolving the <1 Ma age differences necessary for testing crystal recycling, thus requiring the tandem approach of applying a higher precision dating method. We will report results for select zircon grains plucked from grain mounts and analyzed via U-Pb CA-TIMS in order to obtain high precision ages (+/- 0.01 to 0.03 Ma), which will aid in distinguishing recycling from slow growth in the Miocene crystal populations.