Measuring t and p in rocks

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 Geothermometry • 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) Geobarometers • Most involve dismantling plagioclase and reassembling into denser phases • GASP (Garnet-Aluminosilicate-SilicaPlagioclase) – An  Grossularite + Ky + Qz – P(Mpa) = 2.28 T(C) – 109.3 ... 009 6P) /T • Note weak sensitivity to P • T = (2089 + 009 6P) /(0.782 - ln KD) The Biotite-Garnet Thermometer • Many Fe-Mg minerals but Biotite and Garnet differ enough in Mg/Fe ratio Other Geothermometers... Later Diffusion? Using Mineral Reactions Geothermometry • Most depend on cation exchange • Most common: Mg/Fe in garnet and biotite • Similarity in radius means P has little influence • Many other... 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