Intense Shallow Magma Sediment Mingling Within Dikes at Guffey Butte Maar, Idaho

Abstract

The interactions between magma and 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 an eroded Pleistocene basalt maar complex, Guffey Butte, Idaho to investigate extreme sediment magma mingling in a non-explosive environment. Exposed dikes cut through both lake Idaho sands and silts and Guffey Butte pyroclastic deposits. 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), the scales of interactions and diversity of mixing styles between the basaltic dikes and host sediments were constrained. 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. GB basalts contain less lithics than mixed samples. The intensity of mingling is diverse and the style of mingling over a spatial area is not systematic. Magma penetrated through wet unconsolidated siliciclastic sediment and due iv to their differing densities mixing occurs through Kelvin-Helmholtz Instability. Mingling is separated into three categories: 1) Least Homogenized, 2) Moderate Homogenization, and 3) Homogenized. The different levels of mixing preserve the different stages of Kelvin-Helmholtz instability over a distance of a few cm to meters. 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. Basaltic and siliciclastic mineral preservation suggest mixed dike temperatures to be slight below 900°C -1,000°C. This study expands the observed range of interactions between wet sediments and shallow basaltic intrusions that can occur in a non-explosive environment. v APPROVAL PAGEThe faculty listed below, appointed by the Dean of the College of Arts and Sciences, have examined a thesis titled "Intense Shallow Magma Sediment Mingling within Dikes at Guffey Butte Maar, Idaho" presented by Hannah Grachen, candidate for the Master of Science degree, and certify that in their opinion it is worthy of acceptance.