2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 3
Presentation Time: 2:05 PM


MARSH, Bruce, Earth & Planetary Sciences, Johns Hopkins Univ, Baltimore, MD 21218, bmarsh@jhu.edu

Magma chambers are the conceptual cornerstones of every magmatic process. They are high-level staging platforms for volcanoes; cavernous pools wherein complex patterns and cycles of crystallization take place to transform initially homogeneous melt into spatially differentiated sequences; enormously versatile in size, shape, and function and can produce layered intrusions, layered oceanic crust, homogeneous granitic plutons, and featureless diabase sills. The central distinctive property of this dynamic versatility is the pervasive nucleation and growth of crystals throughout the chamber that can be separated, sorted, deposited, or entrained in a vast array of processes. This valuable concept of magma has been adhered to for over one hundred years. It is based on exceedingly fundamental and misleading premises regarding the form, transport, common state of magma, and, above all, the spatial and temporal solidification regime. Good ideas in science are uncommon, and new ideas often grow legs and travel far and wide in spite of their true value. When George Becker (a chemist) saw Shonkin Sag Laccolith in the 1890's he set forth the basic concept of magma chamber differentiation. He saw magma crystallizing as an over-chilled bottle of wine. Crystals grow inward from the margins while the melt circulates, yielding nutrients, becoming strongly enriched; fractional crystallization was known for many centuries. He missed the fundamental petrologic observations that the magma carried about 35% phenocrysts upon emplacement, magmatic crystals are not dendritic, and the laccolith simply crystallized inward from the margins as the initial crystals settled to the floor. Yet the idea took off, and when found wanting in the face of observation it mutated to needing giant reservoirs of magma emplaced free of crystals with heavy crystallization at the roof and crystals settling to the floor, and then for crystallization throughout the system at all times. Bowen's appearance on the scene with phase equilibria and the dire need for a model to explain Skaergaard and Stillwater fanned the flames. But the power of combining critical field observations with theory and experiment on an equal footing has now produced though subtle changes in perspective a revolution in understanding magmatic processes. Magmatic systems function by a common, basic, and simple set of processes.