How can geologists determine when a deformed metamorphic rock was changed by heat and pressure?

To determine when a deformed metamorphic rock was altered by heat and pressure, geologists use geochronology to date the metamorphic events. When a rock is subjected to significant heat or deformation, daughter isotopes can diffuse out of host minerals and redistribute, effectively "resetting" the isotopic clock.1

Geologists employ several techniques to reconstruct these events:

• Isotopic Dating: Methods such as rubidium–strontium dating or uranium–lead dating are used to measure the time elapsed since the heating or deformation event. By analyzing the isotopic ratios in different minerals within a single rock, researchers can determine the time since the rock cooled below specific "blocking temperatures".12
• Pressure-Temperature-Time (P-T-t) Paths: By combining isotopic ages of successive metamorphic events with mineral chemistry, geologists construct P-T-t curves. These curves chart the rock's movement through the crust over time.34
• Petrologic Analysis: Geologists use geobarometry and geothermometry to calculate the specific pressures and temperatures of recrystallization. They also study mineral assemblages and fluid inclusions to constrain the conditions the rock experienced during its burial and uplift history.34