How can you use geothermobarometry to reconstruct metamorphic rocks?

1 What is geothermobarometry?

Geothermobarometry is based on the principle that the distribution of elements and isotopes between coexisting minerals or phases depends on the temperature and pressure of the system. By measuring the chemical composition of these minerals or phases, and applying thermodynamic models or empirical calibrations, you can calculate the temperature and pressure at which they formed or equilibrated. Geothermobarometry can provide information about the depth, geothermal gradient, tectonic setting, and duration of metamorphism.

2 How to select geothermometers and geobarometers?

When selecting geothermometers and geobarometers, you need to consider the compatibility of the minerals or phases involved, the sensitivity and accuracy of the equations or diagrams, and the availability and representativeness of the data. The minerals or phases should be in chemical equilibrium, stable, and not affected by retrograde reactions or diffusion. The geothermometer or geobarometer should have a large and linear response to temperature or pressure changes, with a low uncertainty or error range. Additionally, you should have enough reliable data to perform the calculations and reflect the bulk composition and metamorphic grade of the rock.

3 How to use mineral assemblages as geothermobarometers?

Mineral assemblages are combinations of minerals that coexist in a metamorphic rock under a certain range of temperature and pressure. To use mineral assemblages as geothermobarometers, you must first identify the minerals or phases present in the rock, and their modal proportions and compositions. Then, you must consult a phase diagram or pseudosection for the rock system, which is a graphical representation of the stability fields of different minerals or phases as a function of temperature, pressure, and composition. Finally, you need to locate the region or point on the phase diagram or pseudosection that matches the observed mineral assemblage, and read off the corresponding temperature and pressure values. By comparing the observed mineral assemblage with this phase diagram or pseudosection, you can constrain the possible conditions of metamorphism.

4 How to use mineral compositions as geothermometers and geobarometers?

Mineral compositions are the concentrations of elements or isotopes in a single mineral or a pair of minerals that coexist in a metamorphic rock. By applying the geothermometers and geobarometers that relate the temperature or pressure to the composition of a mineral or a mineral pair, you can estimate the conditions of metamorphism. To use mineral compositions as geothermometers and geobarometers, you need to analyze the mineral or mineral pair using analytical techniques such as electron microprobe, X-ray fluorescence, or mass spectrometry. You should also select the appropriate geothermometer or geobarometer for the mineral or mineral pair based on criteria discussed, and then plug in the measured composition into the equation or plot it on the diagram to solve for the temperature or pressure value. Geothermometers and geobarometers are usually expressed as algebraic equations or graphical plots, which can help you determine metamorphic conditions.

5 How to interpret geothermobarometric results?

Geothermobarometric results, which are temperature and pressure values obtained from the application of geothermometers and geobarometers to metamorphic rocks, can provide insights into the history and evolution of metamorphic rocks. However, in order to interpret geothermobarometric results correctly, you need to compare and cross-check the results from different geothermometers and geobarometers to evaluate their consistency and reliability. Additionally, you must consider possible effects of non-equilibrium, heterogeneity, re-equilibration, and closure temperature on the results and adjust them accordingly. Finally, you should integrate the results with other geological and geochronological evidence such as structural, petrographic, isotopic, and radiometric data to construct a coherent metamorphic scenario.

6 Here’s what else to consider

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