Effect of Temperature Cycling on Crystal Size, Crystal Alignment, and Approach to Equilibrium in Magmas

The Earth's crust is composed largely of volcanic rocks that were erupted onto the Earth's surface and plutonic rocks that formed when unerupted magma crystallized within the earth. One method by which these rocks are interpreted is to examine the size distribution of minerals that make up the rocks. For example, many volcanic rocks have contain small amounts of tiny crystals, indicating rapid cooling at the Earth's surface, whereas most granites contain large crystals that indicate slow cooling at depth. These basic relationships have been known for over a century and guide much of what we know about the Earth's crust, but recent work has pointed out many puzzling inconsistencies with these interpretations. In particular, many granites contain huge crystals (10 cm or more in length) that must have grown very late in the cooling history of the rock. The source of these anomalies in crystal size distribution may lie in temperature fluctuations. Preliminary experiments show that oscillating temperature can play a profound role in cannibalizing small crystals and promoting growth of large ones. This process plays an important role in many related fields of materials science, including food technology, semiconductors, metallurgy, and studies of snow and ice on the earth and other planets.

This study will examine the effects of temperature cycling on crystal size and alignment in magmas. Crystal size relationships suggest that these large crystals grow by cannibalism of smaller crystals. It is planned to use a four-fold approach to studying the effects of temperature cycling on crystal size relationships: (1) experiments in the ammonium thiocyanate-cobalt chloride magma analog system; (2) temperature cycling experiments in natural basaltic and andesitic magmas in a one-atmosphere gas-mixing furnace; (3) temperature cycling experiments in the granite-water system using cold-seal pressure vessels; and (4) pilot studies of temperature cycling in a piston-cylinder device. The goal of these experiments is to determine what role temperature cycling plays in the textural evolution of igneous rocks. Experiments in the magma analog system will be continued in order to develop a quantitative dataset on crystal size development, and experiments at 1-atm and in high-pressure furnaces will examine the role of varying temperature of crystal growth. An important facet of these experiments is the possibility that oscillating temperature will dramatically increase crystal growth rates, as is seen in other materials; if so, then the kinetic problems that plague experiments in high-silica systems may be partially avoided.