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 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 KelvinHelmholtz 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.
Table of Contents
Introduction -- Guffey Butte -- Results: Field descriptions, petrography, and geochemistry -- Discussion -- Conclusions
Degree
M.S. (Master of Science)