EARLY HISTORY OF THE YELLOWSTONE HOTSPOT: THE ASH FALL TUFF RECORD

Over the course of its history, the Yellowstone hotspot has produced silicic magmas that exhibit substantial systematic and often sympathetic variations in isotopic and chemical composition, temperature and frequency of eruption. The ash fall tuff record can be traced back to ~16 Ma. Earliest erupted silicic magmas were hot (up to 1080 °C), relatively less evolved, and have Nd isotopic ratios within the range of contemporaneous Columbia Plateau flood basalts, consistent with a source located in the accreted oceanic terrain west of the Precambrian craton. Early major units include the Dinner Creek Tuff (~15.9 Ma), the Lough ash (15.5 Ma) exposed in the Harper basin and at Succor Creek, OR, an unnamed ash (15.4 Ma) in sedimentary interbeds of the Columbia River basalts, and the Obliterator ash (14.9 Ma) at Succor Creek, OR, and Weiser, ID.

The transit of the hotspot across the lithospheric boundary between the western accreted oceanic terrain and the Precambrian craton at 15 Ma is marked by dramatic shifts in epsilon Nd from +4 to -11 and in epsilon Hf from +10 to –10 (measured on glass). At the same time, eruption temperatures declined by more than 100 °C on average, and magmas became systematically more evolved in composition as exemplified by decreased Fe and increased Rb. The 1.6 m.y. duration of the transect from fully off craton to fully on craton at 14.5 Ma, as defined by the isotopic record, constrains the lower crustal magma source to a diameter of ~60–80 km. The isotopic record provides good evidence for a focused and well-defined crustal melting anomaly relatively early in the history of the hotspot.

Nd and Hf isotopic ratios display a smooth and systematic variation from initial eruptions at ~16 Ma, contemporaneous with flood basalt volcanism on the Columbia Plateau to the present day Yellowstone Volcanic Plateau. Nd and Hf isotopic ratios co-vary and span the range of most terrestrial samples, reflecting mixing of crustal and mantle sources with the mantle component in silicic magmas varying between 25% and 60% over the history of the hotspot.