Phase Equilibria and Melting

Phase diagrams describe how minerals and melt coexist as functions of temperature, pressure, and composition, providing the quantitative framework for melting and crystallization.

Definition

The study of the equilibrium relationships among minerals and silicate melt as functions of temperature, pressure, and composition, used to predict melting and crystallization paths.

Scope

This topic covers the Gibbs phase rule, binary and ternary phase diagrams with eutectic, peritectic, and solid-solution behavior, the contrast between equilibrium and fractional crystallization and melting, and the effects of pressure and volatiles on melting. It is the experimental foundation linking thermodynamics to the petrology of igneous rocks.

Core questions

• How does the Gibbs phase rule constrain the number of coexisting phases?
• How are eutectic and peritectic relations read from binary phase diagrams?
• How do equilibrium and fractional crystallization paths differ?
• How do pressure and water content shift melting temperatures?

Key theories

Gibbs phase rule

The relation F = C - P + 2 ties the number of degrees of freedom to the components and coexisting phases, governing how many phases can coexist and how invariant, univariant, and divariant equilibria appear on phase diagrams.

Experimental phase equilibria of silicate systems

Laboratory determination of melting relations in systems such as forsterite-diopside-silica and the plagioclase binary, pioneered by Bowen, established eutectic and peritectic behavior and quantified the crystallization sequences of natural magmas.

Clinical relevance

Phase equilibria allow petrologists to predict crystallization orders, estimate magma temperatures and depths, model partial melting in the mantle, and interpret the mineral assemblages and zoning preserved in igneous rocks.

History

Gibbs's phase rule provided the theory in the 1870s; the Geophysical Laboratory under Bowen and colleagues determined the melting relations of silicate systems experimentally in the early twentieth century, and Morse's textbook later codified their quantitative use in igneous petrology.

Key figures

• Norman L. Bowen
• J. Willard Gibbs
• S. A. Morse

Seminal works

• bowen1928
• morse1980
• winter2013

Frequently asked questions

What is a eutectic?

The composition and temperature at which a liquid crystallizes two or more minerals simultaneously, the lowest melting point in a system; many magmas approach eutectic compositions as they crystallize.

Why does adding water lower the melting temperature of rock?

Dissolved water disrupts the silicate melt structure and stabilizes the liquid, depressing the solidus so that hydrous rocks begin to melt at significantly lower temperatures than dry ones.