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Contributions to Mineralogy and Petrology

, Volume 114, Issue 4, pp 510–518 | Cite as

Two-feldspar geothermometry: a review and revision for slowly cooled rocks

  • Herbert Kroll
  • Christos Evangelakakis
  • Gerhard Voll
Article

Abstract

Recent improvements in the experimental and thermodynamic basis of two-feldspar geothermometry allow one to recover temperatures of coexistence more reliably. Some problems, however, persist: (1) the experimental solvi by Seck (1971a) and Elkins and Grove (1990) differ from each other; (2) it is not known to what extent Na−K−Ca exchange equilibrium is approached; (3) both solvi are probably metastable with regard to Al, Si order; (4) it is difficult to judge how closely high-temperature natural feldspars compare to this situation; (5) the thermodynamic treatment neglects phase transformations; (6) the temperature dependence of the Margules parameters used to model non-ideal mixing behaviour may not be linear; (7) it is not clear which expressions should be used to describe ideal activities. With these caveats in mind we treat the problem of retrograde resetting in high-grade metamorphic rocks that were slowly cooled under essentially dry conditions. Coexisting feldspars from such rocks commonly do not plot on a common isotherm. Thus temperatures derived from such pairs using any of the proposed two-feldspar geothermometers will necessarily be in error. We suggest that the non-equilibrium compositions result from retrograde intercrystalline K−Na exchange. This exchange continues after the plagioclase and alkali feldspar have already become essentially closed systems with respect to Al−Si exchange, which is a prerequisite for (Na,K)−Ca exchange. We use a modified version of the Fuhrman and Lindsley (1988) programme to reverse the K−Na exchange and derive concordant temperatures.

Keywords

Mineral Resource Close System Metamorphic Rock Recent Improvement Thermodynamic Basis 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer-Verlag 1993

Authors and Affiliations

  • Herbert Kroll
    • 1
  • Christos Evangelakakis
    • 1
  • Gerhard Voll
    • 2
  1. 1.Institut für MineralogieWestfalische Wilhelms-UniversitätMünsterGermany
  2. 2.Institut für MineralogieUniversität zu KölnKoln 1Germany

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