RUI: Melt Viscosities in Silica-undersaturated Systems

Bates College, Lewiston ME

Investigators

Abstract

The movement of magma through the Earth, or magma transport, is an important agent of mass and heat transfer within the Earth. The efficiency of magma transport controls the separation of planets into different phases after their formation, the scale and frequency of volcanic eruptions, and the behavior of the planetary crust during mountain building. In turn, how far and how fast magma travels from its source before it hardens and the violence of volcanic eruptions are controlled by viscosity, the measure of magma's resistance to flow. Viscosity is one of the most variable physical properties in geological processes and its magnitude depends on temperature, chemistry, and on the magma's content of volatiles like water, carbon dioxide, and fluorine. Viscosity is subject to change rapidly in response to processes such as the formation of crystals or bubbles in the magma. These processes can cause chemical and thermal changes, leading to temperature and viscosity feedbacks in magmas. Modeling and understanding of geologic processes at all scales therefore requires accurate quantitative determination of magma transport and thermal properties. This can be done experimentally, by a combination of low- (<1000°C) and high-temperature (>1000°C) viscosity measurements and high-temperature (up to 1500°C) heat capacity measurements on the same suite of samples at atmospheric pressure. For a suite of silica-undersaturated aluminosilicate melts that are analogs for highly-alkalic mafic magmas, the PI and Bates College undergraduate students will quantify the effects of (1) Na-K mixing, of (2) Al/(Al+Si) ratio, and of (3) fluorine on viscosity and heat capacity. The effects of fluorine on viscosity and heat capacity of such melts are especially important to consider as fluorine's abundance correlates positively with the abundance of potassium and has a relatively high solubility at atmospheric conditions. This has implications for the transport behavior of terrestrial magmas that are nominally degassed with respect to other volatiles that are not significantly soluble at atmospheric pressure (e.g., water and carbon dioxide) and also for Martian magmas, which are thought to originate from the melting of a more fluorine-rich source relative to Earth. The results will be modeled using configurational entropy theory, which relates the timescale at which structural changes occur within a fluid (e.g., magma or melt) to the probability of these structural rearrangements. The results from this research will contribute to a better understanding of the chemical controls on viscosity and heat capacity, which in turn allows the construction of better predictive models of magma movement and emplacement in the Earth, and of lava eruption and flow at the Earth's surface.