FROM CORE TO CHRONOLOGY: REFINING YELLOWSTONE'S VOLCANIC HISTORY

In the late 1960’s, 13 research wells were drilled in the major geyser basins of Yellowstone National Park. Stratigraphy of drilled cores was published decades before the geologic map of the Yellowstone Plateau Volcanic Field (YPVF) and prior to many geochronological constraints of the volcanic history. This new data has called into question the original core volcanic stratigraphy and raises the possibility that some subsurface rhyolite flows may have no surficial outcrops (for example, Y-3 has an “unidentified rhyolite” in the bottom of the core). This project aimed to further refine the known dates for rhyolite flows in the Y-2 and Y-3 drill cores. Cores were examined at the USGS Core Research Facility, three samples from each drillcore were selected, and minerals (zircon, sanidine) separated. U-series ages (²³⁰Th-²³⁸U) were analyzed on zircon rims, interpreted to be the eruption ages, at the USGS-Stanford SHRIMP-RG (Sensitive High Resolution Ion Microprobe-Reverse Geometry). Preliminary analysis indicate Y-2 rhyolite flows have similar eruption ages. However, more scatter in zircon ages in the deeper flows suggest sourcing from a longer-lived magmatic system than the shallower flow. Y-3 rhyolite flows have eruption ages ~130-150 ka, all derived from a shorter-lived magmatic system. The shallowest rhyolite in Y-2 has smaller (~25µm), mostly inclusion-free zircons whereas the deeper rhyolite flows have larger zircons with more inclusions. In Y-3, the relationship is flipped in that deeper derived zircons are much smaller and mostly inclusion-free.

New ages indicate the need for further investigation. Zircons were uranium-poor, posing a challenge for high precision ages. They were also small and variably broken, both of which could result in xenocrystic cores being dated rather than rims. Future work includes more zircon analyses and ⁴⁰Ar/³⁹Ar geochronology of sanidines to corroborate zircon rim eruption ages.

This undergraduate research experience (URE) was funded by the NSF-funded AGeS DiG program to promote access to geochronology among underrepresented undergraduate researchers while supporting and advancing inclusiveness. The URE allowed three undergraduates from Colorado State University to participate from start to finish in a project dating Yellowstone rhyolites.