U-Pb Zircon Geochronology of the Columbia River Basalt

The Columbia River Basalt (CRB) is the youngest and best-preserved continental flood basalt province, but its eruptive chronology is disputed. Published geochronology by the K-Ar and 40Ar/39Ar methods show that the majority of the basalt erupted in <2 Ma, but large analytical uncertainties cannot distinguish different formations nor correlate paleomagnetic reversals to the Geomagnetic Polarity Timescale (GPTS). To improve on this, we use CA-ID-TIMS U-Pb zircon geochronology, with 2σ uncertainties on single analyses of ca. 10 kyr, to accurately and precisely resolve the timing and duration of CRB volcanism. While basalt does not typically saturate zircon, interflow sediments throughout the CRB stratigraphy contain felsic zircon-bearing ash, presumably unrelated to the CRB, whose dates can be used to bracket the age of basalt flows. Preliminary results show that 88% of the total volume of the CRB (the Imnaha, Grande Ronde, and Wanapum Basalts) erupted in 700 kyr, beginning 16.6 Ma. We use these results to drastically improve correlation of the CRB with GPTS. Additionally, improved knowledge of the timeframe for the CRB will allow for calculation of volumetric eruption rates and improved understanding of volatile release, as degassing from the CRB is often invoked as a cause of the Mid-Miocene Climate Optimum, a concurrent period of elevated global temperatures and atmospheric CO2.