40AR/39AR SANIDINE DATING OF THE BLUE CREEK FLOW (YELLOWSTONE VOLCANIC FIELD) AND THE POTENTIAL RECORDING OF THE PRE-OLDUVAI GEOMAGNETIC POLARITY EXCURSION

While the major volcanic eruptions of the Yellowstone Volcanic Field have been extensively studied and dated, minor explosive and effusive eruptions have been examined with less scrutiny. The effusive Blue Creek rhyolite flow at Big Bend Ridge erupted between the ~ 2.1 Ma Huckleberry Ridge and ~1.3 Ma Mesa Falls Tuffs. However, the temporal and genetic relationship between the Blue Creek Flow, other minor rhyolitic flows, and caldera-forming eruptions remains unclear. Previous K/Ar and paleomagnetic data were obtained on the Blue Creek Flow, but placement of the flow within the stratigraphic, chronologic, and paleomagnetic record of the Yellowstone Volcanic Field was not resolved. We present new single crystal ⁴⁰Ar/³⁹Ar incremental heating analyses of potassium-rich feldspars from the Blue Creek Flow to provide substantive constraints on the timing of the eruption. Published paleomagnetic analyses assigned a normal polarity to the Blue Creek Flow during the Matuyama reversed polarity chron. Our dating results, combined with the paleomagnetic data, suggest that the Blue Creek Flow may have erupted during the Pre-Olduvai geomagnetic polarity excursion. This short period of normal polarity within the reversed Matuyama chron was previously identified exclusively in the sedimentary record, particularly at Ocean Drilling Program sites 983 and 984 in the North Atlantic as well as several African locations. Thus, the Blue Creek Flow may be the first terrestrial lava identified that preserves this excursion.