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Geological Society, London, Special Publications; 1989; v. 43; p. 63-81;
DOI: 10.1144/GSL.SP.1989.043.01.04
© 1989 Geological Society of London

Relative thermobarometry and metamorphic P-T, paths

Frank S. Spear

Department of Geology, Rensselaer Polytechnic Institute, Troy, NY 12180, USA

Relative thermobarometry involves the determination of changes in P and T experienced by a rock through analysis of mineral zoning, inclusion suites and reaction textures. For this purpose, the differential forms of the thermodynamic equations and mass balance constraints are particularly useful because the equations are linear, and direct computation of changes in dependent variables may be made by matrix inversion of the system of equations defining the thermodynamic constraints and multiplication by changes in independent variables. The number of independent variables equals the phase rule variance if only intensive variables are considered and equals two if mass balance constraints are imposed. Isopleth diagrams may be readily constructed for mineral composition, mineral moles or equilibrium constants provided that entropies, volumes and solution models for all phases as well as a reference assemblage equilibrated at a known P and T are available. An example is presented demonstrating the use of such diagrams in the interpretation of diffusion zoning and retrograde reaction textures in a high-grade gneiss from western New Hampshire, USA.

The P-T paths computed from relative thermometry may be sensitive monitors of the tectonic history because errors of ± 5–10% propagate to small errors in the P-T path for small changes in P and T. The P-T paths from the lower plates of thrust sheets may reveal isothermal compression culminating at pressures representative of the maximum depth of burial. Subsequent heating or cooling of such rocks reveals how the thermal structure evolves after thrusting and may permit speculation about the pre-thrusting thermal structure.





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