Age of the Lava Creek supereruption and magma chamber assembly at Yellowstone based on ⁴⁰Ar/³⁹Ar and U-Pb dating of sanidine and zircon crystals

The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the >1000 km³ of rhyolite that composes the Lava Creek Tuff. Tephra from the Lava Creek eruption is a key Quaternary chronostratigraphic marker, in particular for dating the deposition of mid Pleistocene glacial and pluvial deposits in western North America. To resolve the timing of eruption and crystallization history for the Lava Creek magma, we performed (1) ⁴⁰Ar/³⁹Ar dating of single sanidine crystals to delimit eruption age and (2) ion microprobe U-Pb and trace-element analyses of the crystal faces and interiors of single zircons to date the interval of zircon crystallization and characterize magmatic evolution. Sanidines from the two informal members composing Lava Creek Tuff yield a preferred ⁴⁰Ar/³⁹Ar isochron date of 631.3 ± 4.3 ka. Crystal faces on zircons from both members yield a weighted mean ²⁰⁶Pb/²³⁸U date of 626.5 ± 5.8 ka, and have trace element concentrations that vary with the eruptive stratigraphy. Zircon interiors yield a mean ²⁰⁶Pb/²³⁸U date of 659.8 ± 5.5 ka, and reveal reverse and/or oscillatory zoning of trace element concentrations, with many crystals containing high U concentration cores that likely grew from highly evolved melt. The occurrence of distal Lava Creek tephra in stratigraphic sequences marking the Marine Isotope Stage 16–15 transition supports the apparent eruption age of ∼631 ka. The combined results reveal that Lava Creek zircons record episodic heating, renewed crystallization, and an overall up-temperature evolution for Yellowstone's subvolcanic reservoir in the 10³−10⁴ year interval before eruption.