B2 · Publication Volume 7

Controls on Metamorphism

temperature, pressure, fluid, time and deformation

Temperature, pressure, fluid, time and deformation acting on a protolith
Temperature, pressure, fluid, time and deformation acting on a protolith

Learning objectives

After this lesson, you should be able to define metamorphism as dominantly solid-state change, explain the roles of temperature, pressure, fluid, time, deformation and bulk composition, estimate lithostatic pressure, and recognise why one rock may preserve several incomplete equilibrium domains.

Start with a field problem

A pelitic rock contains aligned mica, garnet with inclusion-rich cores, late chlorite along fractures and quartz ribbons. A nearby mafic layer contains amphibole and plagioclase but no garnet. Did both layers experience the same pressure and temperature? Which minerals grew together, which were inherited, and which formed during cooling or fluid ingress?

Spatial proximity does not guarantee identical assemblages because protolith composition and fluid access differ. Nor does coexistence in a thin section guarantee equilibrium. The task is to identify textural domains, reaction relations and mineral compositions before converting assemblages into metamorphic conditions.

Core process model

Metamorphism comprises mineralogical, chemical and microstructural change in a rock, dominantly in the solid state, under conditions between diagenesis and melting as defined by the adopted nomenclature. If melt becomes important, the system enters partial melting and migmatite terminology. The boundary is geological and operational, not a universal temperature.

Temperature changes reaction equilibrium, diffusion rates, recrystallisation and mineral stability. It can rise through burial, crustal thickening, intrusion, shear heating or advective fluid flow. Pressure includes mean stress and may be approximated by lithostatic load at depth, while differential stress drives deformation. Pressure inferred from an equilibrium assemblage is not simply a depth gauge if tectonic overpressure, density structure or later re-equilibration matters.

Bulk composition defines which reactions are possible. A mafic, pelitic, carbonate-rich and ultramafic protolith can experience the same pressure–temperature path and produce different mineral assemblages. Fluid activity and composition affect hydration, dehydration, dissolution, transport and reaction rates. A rock can be fluid limited even when a hydrous reaction is thermodynamically favoured.

Time and deformation determine how closely a system approaches equilibrium and how products are distributed. Deformation creates grain boundaries, fractures and fabrics, shortens diffusion distances and can localise fluid. Conversely, strong minerals or dry domains may retain metastable phases. Metamorphism is therefore a history of reaction progress, not only a peak point.

Evidence and measurement

Describe mineral relations before measuring compositions. Record inclusions, overgrowths, reaction rims, pseudomorphs, coronas, symplectites, pressure shadows, cleavage and lineation. A garnet core isolated from matrix minerals may record an earlier equilibrium domain; a rim adjacent to biotite may record a later one. Use element maps and profiles to connect analysis spots with textures.

Pressure and temperature methods require a stated equilibrium basis. Conventional thermometers and barometers use calibrated reactions or exchange equilibria. Phase-equilibrium models predict stable assemblages and compositions for a chosen bulk composition, thermodynamic dataset and activity models. Both approaches require uncertainty in analyses, calibration and geological interpretation.

The effective bulk composition of a reacting domain may differ from the whole-rock analysis. Fractionated garnet, segregated melt, veins or local fluid pathways can remove components. Model the domain that plausibly equilibrated, and document any adjustment rather than tuning composition until a desired result appears.

Worked example

As a first-order approximation, lithostatic pressure is


P=\int_0^z \rho(z)g\,dz.

For constant density \rho=2700\ \mathrm{kg\,m^{-3}}, gravity g=9.81\ \mathrm{m\,s^{-2}} and depth z=20\ \mathrm{km},


P\approx2700\times9.81\times20{,}000
=5.30\times10^8\ \mathrm{Pa}
=0.53\ \mathrm{GPa}.

This value is a scale estimate. Density varies with crustal level and composition, surface elevation may matter, and tectonic stress can perturb mean pressure. If a mineral assemblage yields 0.8 GPa, do not simply convert it to 30 km and declare burial depth. First evaluate equilibrium, calibration uncertainty, pressure convention and possible tectonic context.

Now compare the pelitic and mafic layers. If the pelite has garnet + biotite + muscovite + quartz while the mafic rock has hornblende + plagioclase, both may be consistent with one regional regime, but the assemblages do not by themselves prove identical peak conditions. Mineral compositions, reaction textures and phase relations must overlap within uncertainty.

Misinterpretations and uncertainty

Metamorphic grade is not a thermometer reading independent of composition. An index mineral can be absent because the bulk rock lacks a required component, because reaction was incomplete or because retrogression removed it. A peak mineral may persist metastably during cooling. A retrograde mineral along fractures may record local fluid access rather than whole-rock re-equilibration.

Pressure shadows and foliation can identify deformation but not automatically quantify differential stress. Grain size reflects nucleation, growth, strain, fluids and later annealing. A mineral pair separated by a reaction rim may no longer be in equilibrium. Analytical precision should not be confused with geological accuracy.

Practical investigation

Annotate a thin-section image with at least four textural domains. For each mineral analysis, record domain, spot position, analytical uncertainty and proposed equilibrium partners. Build two competing interpretations: one in which the matrix and garnet rim equilibrated at peak conditions, and another in which the rim grew during decompression.

Calculate lithostatic pressure at 10, 20 and 35 km for densities of 2600, 2800 and 3100 kg m⁻³. Plot the range and explain why a petrological pressure estimate should not be converted to one exact depth.

Mastery check

  1. Why is metamorphism described as dominantly solid-state change?
  2. How do bulk composition and fluid access alter the assemblage produced at one P–T condition?
  3. What makes two mineral analyses plausible equilibrium partners?
  4. Why is a whole-rock analysis not always the effective reacting composition?
  5. Which assumptions limit conversion of pressure to depth?

Sources and further reading