COMPLEX MAGMATIC EVOLUTION OF THE 2.1 KYR BLUE DRAGON FLOW, CRATERS OF THE MOON NATIONAL MONUMENT, IDAHO

The Blue Dragon lava flow erupted as part of the late Pleistocene-Holocene sequence of flows in the Craters of the Moon volcanic field on the Eastern Snake River Plain in southern Idaho. With an area of 280 km² and volume of 3.4 km³, it is the largest flow at Craters of the Moon and one of the largest Holocene basaltic flows in the contiguous U.S., flowing over 20 km from its source vents via a complex network of lava tubes. Parts of the Blue Dragon flow have a unique blue color, attributed to absorption caused by Fe and Ti charge transfers in clusters of microscopic titanium magnetite crystals in the surface glass. The coloration is variable across the surface of the flow, ranging from dark grays typical of young basaltic lavas to vivid blues. Multispectral (visible and near-infrared) remote sensing in this study shows the surface of the flow is subdivided into six large regions each with distinct surface coloration, and field observations show they were emplaced during sequential discharge episodes of the Blue Dragon eruption. These major flow lobes are separated by stair-step offsets, allowing for their stratigraphic and temporal order in the eruptive sequence to be established. These color-coded flow lobes offer a unique opportunity to reconstruct the chemical and mineralogical evolution of the magmas as the eruption progressed. In this first comprehensive study of the compositional diversity of the Blue Dragon flow, major and trace element data from 31 rock samples from across the flow show the lavas progressively changed in composition over the course of the eruption (e.g. MgO 2.7-3.7 wt. %). The changes are best explained by eruption from a zoned magma source and binary mixing, with the initial eruption of more evolved and enriched magmas that may have been residual from the previous eruption, followed by progressively higher proportions of a more primitive and depleted recharge component from deeper in the system. New Pb isotope data suggest subtle differences in assimilation of Archean crust between the two mixing end-members. The ability to characterize compositional changes in single eruptions can provide detailed insight into the relative roles and rates of processes that affect individual batches of magma, which may be homogenized and undetectable in studies of longer-term, multi-eruption time scales and processes.