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Mineral composition (especially Fe-Mg ratios, Al content) varies systematically with temperature and pressure. Calibrated geothermometers and geobarometers use mineral chemistry to estimate metamorphic P-T conditions. Multiple independent estimates constrain both P and T and reveal whether rocks cooled or reheated during uplift.
Analyze electron microprobe data from metamorphic minerals. Compare multiple thermobarometric methods to assess uncertainty.
From your work with metamorphic phase diagrams, you know that different mineral assemblages are stable at different pressures and temperatures. Thermobarometry takes this idea one step further: it uses the chemical composition of coexisting minerals — not just which minerals are present — to pinpoint where on a P-T diagram a rock equilibrated. The underlying principle is that certain element exchanges between mineral pairs are sensitive to temperature or pressure in well-characterized, experimentally calibrated ways.
A geothermometer exploits a temperature-sensitive exchange reaction. The classic example is the Fe-Mg exchange between garnet and biotite. At higher temperatures, more magnesium partitions into garnet relative to biotite; at lower temperatures, iron dominates garnet. By measuring the Fe/Mg ratio in each mineral with an electron microprobe and plugging those values into a calibrated equation, you recover the temperature at which the two minerals last exchanged atoms. A geobarometer works similarly but targets a pressure-sensitive reaction — for instance, the amount of aluminum that dissolves into orthopyroxene when it coexists with garnet increases with pressure. Together, one thermometer and one barometer give you a point in P-T space.
In practice, petrologists never rely on a single mineral pair. Different thermobarometers have different closure temperatures — the temperature below which diffusion effectively stops and the mineral composition is "frozen in." A garnet-biotite thermometer might record peak conditions, while a feldspar thermometer records a later cooling stage. By applying multiple independent methods to the same rock, you build a P-T path that traces the rock's journey through the crust during burial, heating, and exhumation. Discrepancies between methods are not failures; they are information about the rock's thermal history.
The critical pitfall is assuming that mineral compositions faithfully preserve peak conditions. Retrograde diffusion during slow cooling can reset compositions, especially in minerals with fast diffusion rates like biotite. Garnet cores may preserve peak temperatures while rims re-equilibrate during cooling, so microprobe traverses across a single grain can reveal zoning that maps directly onto the P-T path. Recognizing which compositions to trust — and which have been overprinted — is where the real skill in thermobarometry lies.