INTENSE SHALLOW MAGMA SEDIMENT MINGLING WITHIN DIKES AT GUFFEY BUTTE MAAR, IDAHO

The way that magma interacts with shallow unconsolidated sediments on the way to the surface influences the eruption behavior and products. Phreatomagmatic eruptions are a result of the interaction of magma and water or wet sediment and their deposits are evidence of this interaction. The relative influence of internal magmatic and external environmental factors that control magma sediment interaction are not well constrained. This study focuses on exposed dike structures in a Pleistocene eroded basalt maar complex, Guffey Butte, Idaho to investigate extreme sediment magma mingling. Exposed dikes cut through both lake Idaho sands and silts and Guffey Butte pyroclastic deposits. The dikes range in thickness (20 -170 cm) and have variable trends and convoluted geometries. The dike margins are both distinct and gradational into the host material. Through a combination of field mapping, microtexture measurements from 36 thin sections, and geochemistry (whole rock major and trace elements using XRF and ICP-MS), we constrain the scales of interactions and diversity of mixing styles between the basaltic dikes and host sediments. The two host materials were characterized for componentry and grain size. Field observations reveal the scales of these interactions range from blocks (< 10 cm) to individual crystals. The intensity of mingling is diverse and the style of mingling over a spatial area is not systematic. Local sections (up to 25 m long) appear highly homogenous, where basaltic and sedimentary material cannot be visually distinguished. In thin section, changes in crystal, sedimentary grain, and basalt fragment sizes occur as well as injections of sediment into basalt and basalt into sediment at the mm scale. This study expands the observed range of interactions between wet sediments and shallow basaltic intrusions that can occur during phreatomagmatic eruptions.