Metamorphic Equilibrium and Phase Diagrams

Core Idea

Mineral assemblages in metamorphic rocks reflect equilibrium at specific pressure-temperature (P-T) conditions. Phase diagrams show which minerals are stable at different P-T; mineral boundaries define metamorphic facies. Comparing observed minerals to phase diagrams reveals the P-T path rocks followed during metamorphism.

Explainer

From your study of metamorphic rocks, you know that pre-existing rocks transform when subjected to elevated temperature and pressure, producing new mineral assemblages and textures. From thermodynamics, you know that Gibbs free energy determines which phase is stable at given conditions, and from phase diagrams, you know how to read stability fields separated by reaction boundaries. Metamorphic phase diagrams bring these concepts together: they map out which mineral assemblages are thermodynamically stable at each combination of pressure and temperature, turning a metamorphic rock into a recorder of the conditions it experienced.

The fundamental principle is chemical equilibrium. At any given P-T condition, the mineral assemblage with the lowest total Gibbs free energy is the one that should form, given enough time and sufficient atomic mobility. A boundary line on a P-T diagram represents a reaction — say, the transformation of kyanite to sillimanite — where both phases have equal free energy. Cross that boundary, and one phase becomes unstable while the other becomes favored. In practice, metamorphic rocks contain multiple minerals whose mutual stability fields overlap, and identifying which combination of minerals coexists allows you to locate the rock's conditions within a specific region of P-T space. These regions are called metamorphic facies: greenschist facies (low-moderate T, low-moderate P), amphibolite facies (moderate-high T, moderate P), granulite facies (high T), blueschist facies (low T, high P), and so on. Each facies name tells an experienced geologist approximately where in P-T space the rock equilibrated.

The real power of this approach emerges when you consider that metamorphic rocks often preserve evidence of *changing* conditions — not just a single P-T point. As a rock is buried during mountain building, heated, and eventually exhumed, it passes through different stability fields. Early-formed minerals may be preserved as inclusions inside later-grown crystals, or reaction rims may develop around minerals that became unstable. By identifying these textural relationships and matching each mineral assemblage to its stability field on a phase diagram, petrologists reconstruct the rock's P-T path — the trajectory it followed through pressure-temperature space over millions of years. A path that shows increasing pressure followed by increasing temperature (a clockwise loop in P-T space) tells a story of burial followed by heating, characteristic of continental collision. A path showing high pressure at low temperature (upper-left region of the diagram) indicates subduction.

One important caveat: equilibrium is an idealization. Real metamorphic reactions require activation energy, fluid catalysts, and time. Some minerals persist metastably outside their stability fields because the reaction kinetics are too slow — this is why diamonds, which are only stable at mantle pressures, survive at Earth's surface. The art of metamorphic petrology lies in recognizing which minerals achieved equilibrium and which are metastable relics, and using that judgment to extract reliable P-T estimates from the phase diagram framework.