Liquid and magma viscosity in the anorthite‐forsterite‐diopside‐quartz system and implications for the viscosity‐temperature paths of cooling magmas

Abstract

We measured the viscosity of 17 liquids in the systems anorthite‐forsterite‐quartz and anorthite‐diopside‐forsterite, representing analogs of dacitic, basaltic andesitic, and basaltic magmas. The three series lie in the anorthite liquidus fields and represent liquid lines of descent. The viscosity of evolving basaltic and basaltic andesite liquids changes little during cooling because the viscosity decrease associated with removal of polymerized anorthite component from the melt offsets the viscosity increase associated with cooling. A minimum liquidus viscosity in the basaltic system is encountered in the anorthite stability field. In contrast, dacitic liquids appear insensitive to the compositional changes during anorthite crystallization, and liquidus viscosity increases monotonically between anorthite and the anorthite‐forsterite‐quartz eutectic. Using the Einstein‐Roscoe equation to calculate the rheological effect of crystals, we show that magma viscosity always increases during crystallization for closed systems and that crystallinity is the dominant control on the viscosity of all three magma series. We introduce the concept of viscosity paths, which may approach one of four end‐members: (1) equilibrium crystallization (closed system) and (2) perfect fractional crystallization (open system) are both associated with changing residual liquid compositions; (3) supercooling without crystal growth maintains constant liquid composition, while (4) increasing crystallinity at constant liquid composition is only permissible for eutectic starting compositions. Because of the dependence of crystal‐liquid segregation rates on liquid viscosity and crystal buoyancy, feedback relations exist between evolving liquid composition, crystallinity, and magma viscosity. Crystal‐liquid segregation rates may increase or decrease during progressive cooling and crystallization of basaltic liquids.