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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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Geothermobarometry l.jpg

Geothermobarometry


Slide2 l.jpg

  • How do we know P and T?


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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


Good thermometer l.jpg

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


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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


Garnet biotite thermometer l.jpg

Garnet-biotite thermometer


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Caplag

  • The GASP geobarometer

  • Garnet, aluminosilicate, silica, plag

  • Large DV, shallow slope

  • K: Cagarnet


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Caplag

  • 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 -> Grossular + 2 kyanite + quartz

  • Low P -> High P


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