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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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geothermobarometry3
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
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

good thermometer5
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
slide8

Caplag

  • The GASP geobarometer
  • Garnet, aluminosilicate, silica, plag
  • Large DV, shallow slope
  • K: Cagarnet
slide9

Caplag

  • The GASP geobarometer
  • Garnet, aluminosilicate, silica, plag
  • Large DV, shallow slope
  • K: Cagarnet
how to apply
How to apply?
  • Results of garnet-biotite thermometry and GASP barometry
  • 1 sample, different calibrations
slide11
What if minerals zoned?
  • Poikiloblastic garnet with plag inclusions
slide12
Zoned minerals can help with P-T-t paths
  • GASP barometer & garnet-biotite thermometer
  • Look at intersection of P and T
calculated uncertainties from various potential sources
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
consider this reaction
Consider this reaction:
  • 3 Anorthite -> Grossular + 2 kyanite + quartz
  • Low P -> High P
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