Measuring t and p in rocks

No Pure Geothermometers or Geobarometers • All mineral equilibria depend on T, P and other variables... Using Mineral Reactions... • Most depend on cation exchange • Most common: Mg/Fe i

Measuring T and P in Rocks

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

What Are We Measuring?

• Peak Conditions?

• Closure Conditions?

• Later Diffusion?

Using Mineral Reactions

• 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 won’t work because Mg/Fe ratios too similar

• Exsolution reactions (feldspar, pyroxene, magnetite-ilmenite) also useful

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)

The Biotite-Garnet Thermometer

• Many Fe-Mg

minerals but

Biotite and Garnet

differ enough in

Mg/Fe ratio

Other Geothermometers

• Most involve garnet and Fe-Mg ratios

• Garnet-Clinopyroxene

• Garnet-Orthopyroxene

• Garnet-Hornblende

• Garnet-Chlorite

• Garnet-Olivine

• Garnet-Ilmenite (Fe-Mn)

• 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