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Measuring T and P in Rocks

Measuring T and P in Rocks. No Pure Geothermometers or Geobarometers. All mineral equilibria depend on T, P and other variables. Some minerals (staurolite) appear over a short T range Numerous reactions expand stability field Good field indicators Few good simple field barometers

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Measuring T and P in Rocks

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  1. Measuring T and P in Rocks

  2. No Pure Geothermometers or Geobarometers • All mineral equilibria depend on T, P and other variables. • Some minerals (staurolite) appear over a short T range • Numerous reactions expand stability field • Good field indicators • Few good simple field barometers • Most are high pressure transitions • Andalusite and cordierite are low P indicators

  3. What Are We Measuring? • Peak Conditions? • Closure Conditions? • Later Diffusion?

  4. Using Mineral Reactions

  5. Geothermometry • Most depend on cation exchange • Most common: Mg/Fe in garnet and biotite • Similarity in radius means P has little influence • Many other common mineral pairs won’t work because Mg/Fe ratios too similar • Exsolution reactions (feldspar, pyroxene, magnetite-ilmenite) also useful

  6. Garnet-Biotite Geothermometer • KD = (Mg/Fe)gar/(Mg/Fe)bio • ln KD = 0.782 – (2089 + .0096P)/T • Note weak sensitivity to P • T = (2089 + .0096P)/(0.782 - ln KD)

  7. The Biotite-Garnet Thermometer • Many Fe-Mg minerals but Biotite and Garnet differ enough in Mg/Fe ratio

  8. Other Geothermometers • Most involve garnet and Fe-Mg ratios • Garnet-Clinopyroxene • Garnet-Orthopyroxene • Garnet-Hornblende • Garnet-Chlorite • Garnet-Olivine • Garnet-Ilmenite (Fe-Mn)

  9. Geobarometers • Most involve dismantling plagioclase and reassembling into denser phases • GASP (Garnet-Aluminosilicate-Silica-Plagioclase) • 3 An  Grossularite + 2 Ky + Qz • P(Mpa) = 2.28 T(C) – 109.3

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