GEOCHEMICAL CORRELATION OF IGNIMBRITES IN THE HEISE VOLCANIC FIELD, EASTERN SNAKE RIVER PLAIN, IDAHO

The Heise volcanic field in eastern Idaho represents the Miocene-Pliocene (6.6 – 4.5 Ma) period of volcanic activity along the Yellowstone hotspot track. It produced a series of large-volume, caldera-forming explosive eruptions of hot, dry rhyolite characteristically occurring in the Snake River Plain – Yellowstone volcanic province. The eruptive record at Heise consists of at least four regional ignimbrites, interbedded with local rhyolitic lava flows and small volume unwelded tuffs, all capped by more recent basaltic cover.

We are using new geological mapping coupled with detailed in-situ geochemistry of multiple mineral phases within the rhyolites to geochemically ‘fingerprint’ the products of individual eruptions. Here, we present data from multiple rhyolites from the Heise eruptive centre, with the Kilgore Tuff used as an example for how correlation of individual ignimbrites – while challenging in terms of field observations and major element chemistry – may be achieved via trace elemental and isotopic investigations. Our work uses major and trace elemental compositions of feldspars, pyroxenes and glass combined with in-situ analyses of ⁸⁷Sr/⁸⁶Sr in plagioclase and Pb isotopic compositions of both glass and sanidines to characterise individual eruptive units.

As observed in the older rhyolites of the Central Snake River Plain, the geochemical ‘fingerprinting’ of large-volume ignimbrites is aided by striking compositional homogeneity observed on all scales: within individual phenocryst phases as well as within an ignimbrite sheet both vertically and laterally. This homogeneity allows to establish precise geochemical correlations and to further constrain spatial extents and volumes of erupted material at Heise.