HR: 09:30h
AN: V41I-07    [Abstracts]
TI: Chemical Controls on Magma Viscosity: Changing Residual Liquid Composition and Volatile Content During Crystallization of Silicic Magmas
AU: * Whittington, A G
EM: whittingtona@missouri.edu
AF: University of Missouri-Columbia, 101 Geological Sciences University of Missouri, Columbia, MO 65211-1380 United States
AU: Getson, J M
EM: jmgcn2@mizzou.edu
AF: University of Missouri-Columbia, 101 Geological Sciences University of Missouri, Columbia, MO 65211-1380 United States
AU: Hellwig, B M
EM: bmh862@mizzou.edu
AF: University of Missouri-Columbia, 101 Geological Sciences University of Missouri, Columbia, MO 65211-1380 United States
AB: The viscosity of magma exerts a fundamental control over the rates and styles of many geological processes, including melt segregation from source regions, magma ascent rates, magma chamber processes, and style of emplacement or eruption. The combination of magma viscosity and strain rate also determines whether magma will flow or fracture. The viscosity of silicate liquids depends primarily on temperature, liquid composition, and dissolved volatile content, while the viscosity of natural magmas depends also on crystal and bubble content. Here we calculate viscosity changes accompanying crystallization for two case studies: (i) leucogranite magmas of near-minimum melt composition undergo only minor changes in residual liquid composition during crystallization. The viscosity of the magma is then controlled by the physical effect of phenocrysts (increasing viscosity), and increasing volatile content in the liquid phase (decreasing viscosity), over a narrow temperature interval. Magma viscosity may decrease during the early stages of crystallization, especially for low initial volatile contents. (ii) silicic calc-alkaline magmas, crystallizing and fractionating from an initial liquid of andesitic-dacitic composition, and evolving into a crystal-bearing rhyolitic liquid. Under initially water-undersaturated conditions, liquid viscosity can decrease quite dramatically during crystallization, due to increased water content, so that bulk magma viscosity remains nearly constant for crystal contents up to about 40%. Magma viscosity always increases if the liquid is initially water saturated, or if cooling occurs during crystallization, but this increase will be small (≤ 1 log unit). These case studies demonstrate that it is essential to consider the residual liquid composition, and to apply the mechanical effect of crystals separately, when calculating magma viscosity for modeling many petrologic processes. Failure to do this results in calculated magma viscosities that may be in error by more than an order of magnitude. Furthermore, the role of viscosity in controlling rates of crystal settling and bubble growth, leads to complex feedback relations between magma physics and chemistry.
DE: 3618 Magma chamber processes (1036)
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8429 Lava rheology and morphology
DE: 8434 Magma migration and fragmentation
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005