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Geothermobarometry

Geothermobarometry. How do we know P and T?. Need min. at equilibrium. Good barometer rxn vs. good thermometer rxn Which has large D V and which has large D S?. D. dP. S. =. D. dT. V. Geothermobarometry.

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Geothermobarometry

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  1. Geothermobarometry

  2. How do we know P and T?

  3. Need min. at equilibrium Good barometer rxn vs. good thermometer rxn Which has large DV and which has large DS? D dP S = D dT V Geothermobarometry

  4. 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 Good thermometer (Mg/Fe)Biotite

  5. K: (Mg/Fe)Garnet Bigger K: Mg-rich garnet or Fe rich biotite Smaller K: Mg-rich biotite or Fe-rich garnet (Mg/Fe)Biotite Good thermometer

  6. Garnet-biotite thermometer

  7. Caplag • The GASP geobarometer • Garnet, aluminosilicate, silica, plag • Large DV, shallow slope • K: Cagarnet

  8. Caplag • The GASP geobarometer • Garnet, aluminosilicate, silica, plag • Large DV, shallow slope • K: Cagarnet

  9. How to apply? • Results of garnet-biotite thermometry and GASP barometry • 1 sample, different calibrations

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

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

  12. Want multiple barometers for internal consistency

  13. Worry about precision vs. accuracy

  14. 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

  15. Consider this reaction: • 3 Anorthite -> Grossular + 2 kyanite + quartz • Low P -> High P

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