HYPERSPECTRAL MAPPING AND GEOCHEMICAL ANALYSES OF MULTIPLE ERUPTIVE PHASES OF THE BLUE DRAGON LAVA FLOW, CRATERS OF THE MOON NATIONAL MONUMENT

The polygenetic Craters of the Moon volcanic field in southern Idaho is the largest Holocene lava field in the contiguous U.S., where about 60 eruptive events have occurred since 15 k.a. The Blue Dragon lava flow is the largest (280 km²) and one of the youngest (2076 ± 45 years) flows produced at Craters of the Moon, and is characterized by fresh, iridescent dark- and light-blue surfaces. The Blue Dragon is a hawaiite produced by fissure eruptions from a dike-fed central rift zone, the NW-SE-trending Great Rift. Field observations show previously undocumented evidence for multiple eruptive phases of the Blue Dragon Flow, with each successive phase flowing a shorter distance than the previous one, resulting in 0.5-2 m stair-step flow front boundaries. The unique bluish color observed on much of the flow is caused by clusters of microscopic titanium magnetite crystals in the surface glass, and the intensity of this color varies across the flow boundaries. Landsat TM (6 Vis/NIR bands, 30 m spatial resolution) and AVIRIS (Airborne Visible Infrared Imaging Spectrometer, 224-bands, 0.4 to 2.5 micron spectral range, 15.3 m resolution) images reveal that these boundaries define distinct sub-areas of the flow that each have different visible and near-IR reflectance characteristics, due largely to differences in the amount of the blue color. Five eruptive phases have been mapped; the first 4 are dominated by slabby, shelly, and spiny pahoehoe morphologies, and the final, least voluminous episode was a tube-fed aa phase. Airborne Synthetic Aperture Radar (AIRSAR) C- and L-band data show roughness variability among the different spectral regions, also suggesting that each resulted from a distinct eruption event, possibly with different effusion rates and flow viscosities. Rock samples taken from multiple locations in each sub-region were analyzed for a suite of major and trace elements using XRF, and the results show distinct geochemical differences and a clear fractional crystallization trend from oldest to youngest phase. The first (lowest layer) and most voluminous phase exhibits the most primitive composition and the lavas become more evolved with time (e.g. wt. % MgO from 3.7% to 2.8%). These geochemical data and estimated volumes for each mapped flow phase have allowed us to constrain the thermal history of the source dike system.