LA-ICP-MS AS A HIGH PRECISION IDENTIFICATION METHOD FOR THE TEPHRA LAYERS OF THE WILSON CREEK FORMATION, MONO BASIN, CALIFORNIA

The Wilson Creek (WC) Formation of the Mono Basin in eastern California consists of lacustrine silt and sand beds interbedded with 19 tephra layers. The WC family of tephra are composed of intra-volcanic eruptions from the Mono Craters, which span 100 - 13 ka, and have highly similar major-element compositions, as determined by electron microprobe analysis. The eastern-most extent of WC tephra is currently Utah, demonstrating their potential as late Pleistocene chrono-markers for the western United States. Further, Wilson Creek ash #15 or WC-15 is closely associated with the Mono Lake paleomagnetic excursion, extending the usefulness of this tephra as a regional time-line. Previous work has shown the Wilson Creek tephra layers are distinguishable by minor and trace-element compositions as measured by instrumental neutron activation analysis (INAA). In this study, minor and trace element concentrations from volcanic glass shards of 18 WC tephra layers were analyzed by laser ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS). Results show that higher concentration trace elements are more accurate using LA-ICP-MS. Our results also show that Ba and Sr concentrations for WC-15 clearly distinguish this tephra layer from the other WC tephra layers.