Textural coarsening in igneous rocks

Abstract

The initial growth of crystals in magma is driven by kinetic forces, and the resulting textures can be preserved in rapidly cooled igneous rocks. However, crystals in such rocks have a high surface area with respect to their volume, and hence an excess surface energy. This energy can be dissipated by textural equilibration. At advanced stages, this is represented by textural coarsening, in which smaller crystals dissolve simultaneously with the growth of larger crystals. These textural changes occur commonly in slowly cooled plutonic rocks and may be important for the development of some volcanic rocks as well. Textural coarsening is clearly an important petrologic process, but may not have received the attention it deserves inasmuch as it does not change the chemical composition of the rock, and hence cannot be quantified by the geochemical methods that currently dominate petrology.

Keywords:

• coarsening
• Ostwald ripening
• textural equilibration
• crystal size distribution
• textural quantification

Acknowledgements

I thank NSERC (Canada) for funding my textural research, Universidad de Chile for hosting me when I wrote this article, Université du Québec à Chicoutimi for giving me sabbatical leave, and Judit Ozoray for editorial comments. Dougal Jerram and Thomas Muller gave thoughtful reviews.

Notes

1. Most petrologists’ definition of the solidus of a magma is somewhat arbitrary. For example, if a granitic magma is cooled in a sealed chamber, then initially the silicate and oxide phases will crystallize from a silicate fluid. As the temperature goes down, the fluid becomes richer in water. This fluid will still be present when the system is at 20°C; hence the system is still above the liquidus, using a strict definition. Most petrologists consider the solidus to be the temperature at which the fluid component of the system changes from silicate-dominated to water-dominated.