Pressure-temperature-time path

[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.

[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 metamorphic cycle implies the series of processes that a rock experienced from burial, heating to uplift and erosion.

[11] Anticlockwise P-T-t path normally consists of two parts:[2] 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.

[12] Examples of metamorphic rocks that consist of anticlockwise P-T-t paths can be found at: The reconstruction of P-T-t paths includes two types of approaches:[1] Petrological reconstruction is a backward approach which utilizes mineral compositions of rocks samples to deduce the possible P-T conditions.

[1] In qualitative reconstruction of P-T conditions, geologists examine thin sections under polarized light microscope to determine the sequence of formation of the minerals.

[16] Common textures at different stages of metamorphism: Not all rock samples exhibit all the P-T conditions they experienced throughout geological evolution.

[20] The underlying principle of geothermobarometry is by utilizing the equilibrium constants of mineral assemblages in a rock to infer the metamorphic P-T conditions.

[1][20] 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.

[21] In a changing environment, minerals would be unstable and alter itself to reduce its Gibbs free energy to achieve stable states.

[23] It attempts to simulate the garnet growth zoning numerically by solving a set of differential equations involving variables pressure (P), temperature (T), chemical potential (μ), mineral composition (X).

[1][23] The aim of this analysis is to search for the absolute P-T condition during different zonal growth and matches the observed composition of zones in the sample.

[28] It utilizes the idea of radioactive decay of long-lived unstable isotopes in minerals to search for the age of events.

[29][30] Monazite is a phosphate mineral containing light rare-earth-elements (LREE) that occurs in a wide range of rock types.

[34][35] Since garnets are quite stable upon changing temperature, the included monazite grains are well preserved and prevented from the re-setting of decay system and age.

[34] Apart from occurring as inclusion in garnets, monazite also display zonal growth pattern itself upon changing P-T conditions.

[1][2] An advantage of thermal modeling is that it provides a holistic estimation of the duration of different stages of metamorphism, which is somehow difficult to completely extract from geochronological methods.

[1] The model simulation involves solving the continuous time-dependent differential heat transfer equation by its approximate discrete finite difference form using computer programs such as FORTRAN.

[1] By combining petrological methods and thermal modeling techniques, the understanding of metamorphic processes due to tectonic events is facilitated.

[1] Areas with collision-related tectonic events or under subduction zones commonly produce metamorphic rocks with clockwise P-T-t paths with near-isothermal decompressional P-T trajectories,[5][6] and the reason is as follows:

[42] The P-T-t paths provide in-depth investigations and implications of the mechanisms in the lithosphere, and further support the plate tectonic theory[42][46] and the formation of supercontinents.

[11][49] Anticlockwise P-T paths with near-isobaric cooling after the peak are normally found in the Archean rocks, suggesting an intrusion origin.

[11][50] Evidenced together by a large doming structure, widespread of komatiites and bimodal volcanism, it is proposed that plume tectonics is the major crust-forming process in the Archean.

[1][3] For example, during the formation of fault-bend-fold, the rocks in the lower segment (footwall) are heated due to contact with the hotter upper thrust sheet (hanging wall), while the upper thrust sheet is cooling because of losing heat in a downward direction.

[52] Depending on the location of the rock, a variety of complex P-T trajectories can be found, which may make interpretation of a terrain challenging.

[2] Therefore, both the evidence of the maximum pressures and temperatures experienced by the buried layers can be imprinted in the underlying metamorphic rocks.

A schematic clockwise P-T-t path. Metamorphic minerals 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.
A common anticlockwise P-T-t path.
Microcline (cross-hatched twinning ) included in magnetite (black, opaque ) in plagioclase ( polysynthetic twinning ). Therefore, the sequence of formation is: microcline → magnetite → plagioclase .
A garnet - mica schist with porphyroblastic garnet (black) in fine-grained mica matrix
A reaction rim (light grey area) is formed around the host mineral (dark grey) when the temperature and pressure decrease.
Intergrowth of fayalite - pyroxene symplectite (grey) against apatite (white) exhibits a symplectitic texture on the right.
Light-colored serpentine veins cross-cut dark-colored mafic minerals, thus serpentine veins should be formed later than the dark minerals.
An illustration of geothermobarometry . A line of temperature equilibrium (orange) and a line of pressure equilibrium (blue) of selected mineral assemblages found in the specimen are plotted on the P-T diagram. The intersection represents the likely P-T condition experienced by rock in its metamorphic history.
An example of a pseudosection. The above shows the areas of stable mineral assemblages at different P-T ranges for a single bulk-rock composition (red dot) of the CaO-SiO 2 -Al 2 O 3 rock composition ternary diagram (white triangle).
Monazite crystals (white dots) are often included in a concentrically zoned garnet (each colored ring represents a zone). Dating of monazite inclusions can therefore allow estimation of the age of each garnet zone.
An example of using thermal modeling in P-T-t path reconstruction. The above diagram shows the calculated geothermal gradients upon crustal thickening at 0 million year (m.y.) followed by an immediate uplift event at a rate of 1 mm per year. The P-T-t evolution of a rock originally at 40 km below ground is marked as red dots on the diagram. The corresponding P-T-t path trajectory is also inferred (blue dotted line). Edited from Peacock(1989). [ 1 ]
Convergent plate boundaries with subduction zones and volcanic arcs , where paired metamorphic belts with contrasting metamorphic mineral assemblages are found. Clockwise P-T-t paths are commonly found in the forearc , while anticlockwise P-T-t paths are found in the volcanic arc or back-arc basin .
A diagram of plume tectonics. A mantle plume rises from the core to the surface.
The different metamorphic facies under various P-T conditions.