page_title: Pressure-temperature-time path
section_title: Optical microscopy
hierarchical_section_title: Pressure-temperature-time path / Reconstruction of P-T-t paths / Backward approach (Petrological P-T-t reconstruction) / Optical microscopy
caption_attribution_description: English: Backscattered electron image of a fayalite-pyroxene symplectite against an apatite crystal, from the Los Angeles Martian Shergottite meteorite
context_page_description: The Pressure-Temperature-time path is a record of the pressure and temperature 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 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 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.
context_section_description: 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
  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.
caption: Intergrowth of fayalite-pyroxene symplectite (grey) against apatite (white) exhibits a symplectitic texture on the right.
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