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 uplift and exhumation to the surface. 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. 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.
Metamorphic minerals are unstable upon changing P-T conditions. The original minerals are commonly destroyed during solid state metamorphism and react to grow into new minerals that are relatively stable. Water is generally involved in the reaction, either from the surroundings or generated by the reaction itself. Usually, a large amount of fluids (e.g. water vapor, gas etc.) escape under increasing P-T conditions e.g. burial. 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. Hence, the minerals are not fully in equilibrium when discovered on the surface. 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.
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; and anticlockwise P-T-t paths, which are usually of intrusion origin, and involve high temperatures before high pressures. (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.)
Stages in P-T-t paths P-T-t paths often reflect various stages of the metamorphic cycle. A metamorphic cycle implies the series of processes that a rock experienced from burial, heating to uplift and erosion. The P-T conditions experienced by a rock throughout these processes can be classified into three main stages according to temperature changes: Prograde (pre-peak) metamorphism: the process when the rock is buried and heated in environments such as basins or subduction zones. Devolatilization reactions (release of gases e.g. CO2, H2O) are common. Peak metamorphism: the maximum temperature reached throughout the metamorphic history. Retrograde (post-peak) metamorphism: the metamorphism occurred during uplift and cooling of the rock. However, retrograde metamorphism may not always be observed in metamorphic rocks. This is due to the loss of fluids (e.g. CO2, H2O) from prograde metamorphism, after which there is insufficient fluid to permit reverse reaction of the mineral assemblages. Another reason is that the rocks are of inappropriate composition to generate all the minerals that record their complete metamorphic events. On average, only one-in-twenty metamorphic rock samples display all the three stages of metamorphism.
P-T-t path trajectories P-T-t paths can generally be classified into two types: clockwise P-T-t paths and anticlockwise P-T-t paths.
Clockwise P-T-t paths Metamorphic rocks with clockwise P-T-t paths are commonly associated with a near-isothermal decompressional P-T trajectory.
Clockwise P-T-t path normally consists of three parts: Initial heating and compression until arriving a peak, a high pressure-low temperature peak is often observed. (Prograde metamorphism until peak) Near-isothermal decompression after the peak (Stage 1 retrograde metamorphism) Further decompression and cooling at a slow rate (Stage 2 retrograde metamorphism)
One might expect that the rock reaches its peak metamorphism at the peak temperature and pressure at similar time, and near-isothermal decompression P-T-t path is observed at its stage 1 metamorphism. However, in reality, the rocks commonly experience the peak pressure prior to the peak temperature. This is due to the relative insensitivity of rocks to thermal events, i.e. poor conductivity of rock upon external thermal changes, whereas the rocks instantaneously experience pressure changes.
Examples of metamorphic rocks that consist of clockwise P-T-t paths can be found at: Trans-North China orogen, North China Craton Bohemian Massif, Austria Southern Brittany, France Northern Gallatin Range, South-western Montana, USA Southern Prince Charles Mountains, East AntarcticaBoger, S. (n.d.). L. 2005. Early Cambrian crustal shortening and a clockwise PTt path from the southern Prince Charles Mountains, East Antarctica: Implications for the formation of Gondwana. Journal of Metamorphic Geology., 23, 603.
Anticlockwise P-T-t paths
Metamorphic rocks with anticlockwise P-T-t paths are commonly associated with a near-isobaric cooling P-T trajectory.
Anticlockwise P-T-t path normally consists of two parts: Initial heating and compression until reaching a peak, a low pressure-high temperature peak is often observed. (Prograde metamorphism until peak) Near-isobaric cooling after the peak (Retrograde metamorphism) It is commonly observed that the peak temperature is reached prior to the peak pressure in anticlockwise P-T-t paths, as the rocks usually experienced the heat from the heat source before being extensively pressurized.
Examples of metamorphic rocks that consist of anticlockwise P-T-t paths can be found at: Arunta Block, Central Australia Andes, South America Yinshan Block, North China Craton Coastal Cordillera, South-Central Chile
Reconstruction of P-T-t paths The reconstruction of P-T-t paths includes two types of approaches: Backward approach: the method of inversely inferring the metamorphic events from rock samples via traditional petrological investigation methods (e.g. optical microscopy, geothermobarometry etc.). Forward approach: using thermal modeling techniques to work on the geological evolutionary model of rocks, and is usually used to validate results obtained in the backward approach.
Backward approach (Petrological P-T-t reconstruction)
Petrological reconstruction is a backward approach which utilizes mineral compositions of rocks samples to deduce the possible P-T conditions. Common techniques include optical microscopy, geothermobarometry, pseudosections, and geochronology.
Optical microscopy
In qualitative reconstruction of P-T conditions, geologists examine thin sections under polarized light microscope to determine the sequence of formation of the minerals. Due to incomplete replacement of the earlier formed minerals under changing P-T conditions, minerals formed at various P-T environments can be found in the same rock specimen. As different minerals have different optical characteristics and textures, determination of the mineral compositions in metamorphic rocks is made possible.
Common textures at different stages of metamorphism: Prograde (pre-peak) metamorphism Mineral inclusions (poikiloblastic texture): a mineral that is formed at a lower P-T condition is included in another mineral that is formed at a higher P-T condition. For example, in thin section examination, biotite crystal is included in a garnet grain, so biotite is considered to be formed at an earlier time. Peak metamorphism Porphyroblastic-matrix texture: large euhedral crystals inside a fine groundmass. Both the euhedral crystals and the matrix minerals of the porphyroblasts are formed at peak metamorphism. Retrograde (post-peak) metamorphism Corona (reaction rim): minerals formed at lower P-T conditions surrounding the higher grade mineral Symplectite (finger-like texture): intergrowth between retrograde minerals (formed at lower P-T conditions) and minerals formed at the peak stage (higher P-T conditions) Mineral cross-cutting: retrograde minerals cross-cut minerals that are formed at the peak stage Textures at different stages of metamorphism observed under a microscopeStages of metamorphismTypical textureExample of texturePrograde (Pre-peak)mineral inclusionsPeakporphyroblastsRetrograde (Post-peak)reaction rimssymplectitescross-cutting Not all rock samples exhibit all the P-T conditions they experienced throughout geological evolution. This is attributed to the complexity of the geological processes, which the samples may have undergone complicated thermodynamic histories, or of inappropriate mineral compositions to produce minerals that record their metamorphic events.
Geothermobarometry
Geothermobarometry is a quantitative measurement of the P-T conditions, which is widely used in analyzing the P-T conditions of metamorphic and intrusive igneous rocks.
The underlying principle of geothermobarometry is by utilizing the equilibrium constants of mineral assemblages in a rock to infer the metamorphic P-T conditions. An electron microprobe is usually used in geothermobarometry to measure the distribution of components in the minerals and give precise determination of the chemical equilibrium within the specimen.
Geothermobarometry is a combination of: Geothermometry: the measurement of temperature changes using equilibrium of minerals that are insensitive to pressure variations, and Geobarometry: the determination of the pressure changes using equilibrium of minerals that are of little dependence on temperature change. Geothermometers are usually represented by exchange reactions, which are sensitive to temperature but with little effect under changing pressure, such as exchange of Fe2+ and Mg2+ between garnet-biotite reaction:
Geobarometers are typically occurred as net-transfer reactions, which are sensitive to pressure but have little change with temperature, such as garnet-plagioclase-muscovite-biotite reaction that involves a significant volume reduction upon high pressure:
Since mineral assemblages at equilibrium are dependent on pressures and temperatures, by measuring the composition of the coexisting minerals, together with using suitable activity models, the P-T conditions experienced by the rock can be determined.
After one equilibrium constant is found, a line would be plotted on the P-T diagram. As different equilibrium constants of mineral assemblages would occur as lines with different slopes in the P-T diagram, therefore, by finding the intersection of at least two lines in the P-T diagram, the P-T condition of the specimen can be obtained.
Despite the usefulness of geothermobarometry, special attention should be paid to whether the mineral assemblages represent an equilibrium, any occurrence of retrograde equilibrium in the rock, and appropriateness of calibration of the results.
Garnet growth zoning Garnet growth zoning is a special type of geothermobarometry that focuses on the composition variations in garnet.
Zoning is a texture in solid-solution minerals which the minerals form concentric rings from core to rim upon changing P-T conditions. In a changing environment, minerals would be unstable and alter itself to reduce its Gibbs free energy to achieve stable states. However, sometimes the mineral core has not reached equilibrium upon the environmental change and zoning occurs. Zoning is also found in other minerals such as plagioclase and fluorite.