Measuring T and P in Rocks

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Measuring T and P in Rocks
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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

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What Are We Measuring?

• Peak Conditions?
• Closure Conditions?
• Later Diffusion?

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Using Mineral Reactions
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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 wont work
  because Mg/Fe ratios too similar
• Exsolution reactions (feldspar, pyroxene,
  magnetite-ilmenite) also useful

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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)

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The Biotite-Garnet Thermometer

• Many Fe-Mg minerals but Biotite and Garnet differ
  enough in Mg/Fe ratio

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Other Geothermometers

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

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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