09-28-2021-1846 - Pressure-Temperature-time path (P-T-t path)

 The Pressure-Temperature-time path (P-T-t path) is a record of the pressure and temperature(P-T) conditions that a rock experienced in a metamorphic cycle from burial and heating to upliftand exhumation to the surface.^[1] Metamorphism is a dynamic process which involves the changes in minerals and textures of the pre-existing rocks (protoliths) under different P-T conditions in solid state.^[2] The changes in pressures and temperatures with time experienced by the metamorphic rocks are often investigated by petrological methods, radiometric dating techniques and thermodynamic modeling.^[1][2]

Metamorphic minerals are unstable upon changing P-T conditions.^[1][3] The original minerals are commonly destroyed during solid state metamorphism and react to grow into new minerals that are relatively stable.^[1][3] Water is generally involved in the reaction, either from the surroundings or generated by the reaction itself.^[3] Usually, a large amount of fluids (e.g. water vapor, gas etc.) escape under increasing P-T conditions e.g. burial.^[1] When the rock is later uplifted, due to the escape of fluids at an earlier stage, there is not enough fluids to permit all the new minerals to react back into the original minerals.^[1] Hence, the minerals are not fully in equilibrium when discovered on the surface.^[1] Therefore, the mineral assemblages in metamorphic rocks implicitly record the past P-T conditions that the rock has experienced, and investigating these minerals can supply information about the past metamorphic and tectonic history.^[1]

The P-T-t paths are generally classified into two types: clockwise P-T-t paths, which are related to collision origin, and involve high pressures followed by high temperatures;^[4] and anticlockwise P-T-t paths, which are usually of intrusion origin, and involve high temperatures before high pressures.^[4] (The "clockwise" and "anticlockwise" names refer to the apparent direction of the paths in the Cartesian space, where the x-axis is temperature, and the y-axis is pressure.^[3])

A schematic clockwise P-T-t path. Metamorphicminerals alter with the changing P-T condition with time without reaching complete phase equilibrium, making P-T-t path tracking possible. From 1910 Ma (i.e. 1910 million years ago) to 1840 Ma, the rock experienced an increase in P-T conditions and formed mineral garnet, which is attributed to burial and heating. After that, the rock was continuously heated to the peak temperature and formed mineral cordierite. Meanwhile, it went through a great decrease in pressure around 1840 Ma due to an uplift event. Finally, the continuous drop in pressure and temperature in 1800 Ma resulted from further erosion and exhumation. The peak pressure is found to be reached before the peak temperature, owing to the relatively poor thermal conductivity of the rock upon increasing P-T condition, while the rock instantaneously experienced the pressure changes. Garnet and cordierite do not reach complete equilibrium when discovered on the surface, leaving a print of the past P-T environments.

https://en.wikipedia.org/wiki/Pressure-temperature-time_path