VARIABLE SOURCE ROCKS FOR LOW OXYGEN ISOTOPE RHYOLITES OF THE EASTERN UPPER BASIN MEMBER OF THE PLATEAU RHYOLITES, YELLOWSTONE CALDERA, WY

Isotopic variations within the eastern Upper Basin Member of the Plateau Rhyolites can be used to determine if the source of the flows was from within the Yellowstone Caldera, or just outside of the margins. The eastern Upper Basin Member is comprised of three high silica rhyolites that are thought to have erupted from within in present Caldera, near the northern ring fracture. Eruption ages have been estimated at 480 ka, which is approximately 160,000 years after the last caldera forming eruption. These flows differ from most other intra-caldera rhyolites due to the low δ ¹⁸O isotopic signature, radiogenic isotope compositions, and increased plagioclase abundance. This study has identified that the youngest eastern Upper Basin Member Rhyolite, the Dunraven Road Flow, contains isotopic compositions that are similar to extra-caldera rhyolites. For example, the ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb isotopic composition of the Dunraven Road Flow is similar to the Cougar Creek and Riverside Flows of the Roaring Mountain Member, which erupted north of the Caldera. The ⁸⁷Sr/⁸⁶Sr and ¹⁴⁴Nd/¹⁴⁷Nd isotopic composition is similar to the previously mentioned Roaring Mountain Members and multiple domes from the Obsidian Cliff Member, both members erupted north of the Caldera. These findings suggest that the Dunraven Road Flow erupted from along the northern ring fracture, or just outside of the caldera. The source for the Dunraven Road Flow was hydrothermally altered and possibly older than the source for the rest of the eastern Upper Basin Member.