DECOUPLED ASSIMILATION-FRACTIONAL CRYSTALLIZATION IN THE HENRY’S FORK CALDERA OF THE YELLOWSTONE-SNAKE RIVER PLAIN AS REVEALED BY SR-ND-HF-PB ISOTOPES AND 40AR/39AR ERUPTION AGES

The Yellowstone Hotspot’s 17 Ma history has resulted in numerous caldera-forming eruptions of ash-fall tuffs (>250 km³) and concomitant rhyolitic flows. Basaltic flows are often interspersed with large-scale eruptive cycles and concentrated along caldera rims, suggesting that mafic magmas play a critical role in driving the long-term evolution of the Yellowstone Hotspot. Still, the vast majority of Yellowstone research has focused on the felsic extrusive units, specifically those located within the confines of Yellowstone National Park, leaving the basaltic lavas, albeit erupted less frequently, comparatively under-constrained. This dearth in research represents a critical aspect of Yellowstone’s plumbing system in need of further investigation, as mantle-derived, basaltic magmas are often argued to provide both the heat and mass necessary to maintain upper crustal, rhyolitic magma chambers which fuel continental, bimodal volcanism.

Here we report major and trace element abundances, coupled with Sr-Nd-Hf-Pb isotopic compositions and ⁴⁰Ar/³⁹Ar eruption ages for a suite of geochemically and isotopically diverse lavas (MgO = 0.561-11.2 wt%, ⁸⁷Sr/⁸⁶Sr = 0.705600-0.709316) collected from in and around the Henry’s Fork caldera of eastern Idaho (~1.3 Ma), an area of recent Quaternary volcanism situated at the intersection of the upwelling mantle plume and the structural manifestation of Basin & Range extension. Until now, this region has remained largely understudied regarding its geochemical, geochronological, and isotopic implications, leaving the exact number, timing, source(s), and contaminant(s) of mafic lavas fundamentally unknown. By coupling binary mixing and crystal fractionation models with ⁴⁰Ar/³⁹Ar eruption ages, we reveal that the Henry’s Fork region preserves a previously unrecognized diversity in basaltic compositions which may stem from a similar mantle source but have subsequently followed divergent paths of magmatic differentiation over similar geologic periods. Specifically, basaltic magmas undergo fractional crystallization prior to eruption along the western reaches of the caldera rim, while crustal contamination appears to play the dominant role in shaping the isotopic composition of basalts that erupt proximal to the Yellowstone Plateau Volcanic Field.