Geothermobarometry

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Geothermobarometry
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• How do we know P and T?

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Geothermobarometry

• Need min. at equilibrium

• Good barometer rxn vs. good thermometer rxn
• Which has large DV and which has large DS?

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Good thermometer

• Measure distribution of elements in coexisting
  phases from experiments at known P and T
• Estimate P and T of equilibrium in natural
  samples
• Garnet-biotite exchange reaction
• K (Mg/Fe)Garnet

(Mg/Fe)Biotite
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Good thermometer

• K (Mg/Fe)Garnet
• Bigger K Mg-rich garnet or Fe rich biotite
• Smaller K Mg-rich biotite or Fe-rich garnet

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Garnet-biotite thermometer
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• The GASP geobarometer
• Garnet, aluminosilicate, silica, plag
• Large DV, shallow slope
• K Cagarnet

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• The GASP geobarometer
• Garnet, aluminosilicate, silica, plag
• Large DV, shallow slope
• K Cagarnet

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How to apply?

• Results of garnet-biotite thermometry and GASP
  barometry
• 1 sample, different calibrations

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• What if minerals zoned?
• Poikiloblastic garnet with plag inclusions

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• Zoned minerals can help with P-T-t paths
• GASP barometer garnet-biotite thermometer
• Look at intersection of P and T

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• Want multiple barometers for internal consistency

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• Worry about precision vs. accuracy

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Calculated uncertainties from various potential
sources

• Red instrumental
• Blue total instrumental/mathematical/calibration
• Yellow natural heterogeneity
• Dark green uncertainty in the K experiments
• Light green combination of all uncertainty

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Consider this reaction

• 3 Anorthite -gt Grossular 2 kyanite quartz
• Low P -gt High P

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