Questions: Metamorphic Equilibrium and Phase Diagrams

Question 1

A geologist finds a metamorphic rock containing both kyanite and sillimanite, which have a reaction boundary separating their stability fields on a P-T phase diagram. What is the most geologically reasonable interpretation?

• A. The rock formed simultaneously from two different magmas with incompatible mineral compositions
• B. The rock crossed the kyanite-sillimanite reaction boundary during metamorphism, and one mineral is a relic preserved from earlier conditions
• C. Both minerals are stable across all metamorphic facies, so their coexistence is unremarkable
• D. The sample was contaminated during collection and the minerals did not form in the same rock

Answer: (B)

Coexistence of minerals across a reaction boundary records a changing P-T history. As the rock passed through the kyanite-to-sillimanite transition, the reaction may not have gone to completion, leaving kyanite relics enclosed in sillimanite overgrowths (or vice versa). This textural evidence — which mineral is the inclusion and which is the host — tells geologists the direction of P-T change. This is precisely why metamorphic rocks are powerful recorders: they preserve snapshots of past conditions, not just current ones.

Question 2

A rock is found at the surface containing diamond, which is only stable at pressures exceeding ~40,000 atmospheres (mantle depths). A student concludes the rock currently equilibrates at mantle conditions. What is the fundamental flaw in this reasoning?

• A. Diamonds form through ordinary organic processes and don't require high pressure
• B. Phase diagrams only apply to sedimentary rocks, not metamorphic ones
• C. Diamond is metastably preserved at surface conditions because conversion to graphite requires activation energy that is kinetically unavailable at surface temperatures
• D. The student has misread the phase diagram — diamond is actually stable at low pressure

Answer: (C)

This is the critical caveat of metamorphic petrology: equilibrium is an idealization. Diamond is thermodynamically unstable at the surface (graphite has lower free energy), but the conversion requires breaking and reforming strong C-C bonds — a kinetically hindered process at low temperatures. Diamond persists as a metastable relic because the activation energy barrier is enormous. This is why identifying which minerals achieved equilibrium (and which are metastable relics) is the central interpretive skill, not just reading stability fields off a phase diagram.

Question 3

A metamorphic rock that equilibrated in the amphibolite facies will generally display mainly amphibolite-facies mineral assemblages when examined at Earth's surface.

• T. True
• F. False

Answer: False

Metamorphic minerals can be preserved metastably outside their stability fields during exhumation. As a rock is uplifted from depth, it passes through lower P-T conditions where its high-grade minerals are no longer thermodynamically stable. However, if temperature drops quickly enough that reaction kinetics are too slow, the original minerals survive as relics. This is why high-pressure minerals like eclogite assemblages and even ultra-high-pressure phases (coesite, diamond) are found in surface outcrops.

Question 4

The metamorphic facies of a rock identifies the pressure-temperature region in which the rock's mineral assemblage reached thermodynamic equilibrium.

• T. True
• F. False

Answer: True

Metamorphic facies (greenschist, amphibolite, granulite, blueschist, eclogite, etc.) are defined as regions of P-T space where specific mineral assemblages are stable. Assigning a rock to a facies means identifying which stability fields its coexisting minerals share — i.e., finding the P-T conditions where the whole assemblage could have been at equilibrium simultaneously. The facies concept is a shorthand for this P-T location.

Question 5

Why does the presence of a mineral in a metamorphic rock not necessarily indicate that the rock currently occupies that mineral's thermodynamic stability field?

Model answer: Mineral reactions require activation energy and sufficient atomic mobility, which depend on temperature, fluid presence, and time. During exhumation, as a rock cools and decompresses, temperatures may drop too quickly for minerals to re-equilibrate. Minerals that were stable at peak P-T conditions persist metastably because the kinetics of converting them to the lower-temperature stable phase are too slow at ambient surface conditions. The presence of a mineral records where the rock was, not where it is now.

This kinetic barrier is geologically fortunate — it is the reason we can read P-T histories from surface outcrops. If metamorphic minerals always equilibrated to surface conditions, we would find only zeolites and clay minerals in all metamorphic rocks, and the entire record of deep-crustal and mantle conditions would be erased.