Calcic amphibole thermobarometry in metamorphic and igneous rocks: New calibrations based on plagioclase/amphibole Al-Si partitioning and amphibole/liquid Mg partitioning

Abstract

Dependencies of plagioclase/amphibole Al-Si partitioning, DAl/Siplg/amp, and amphibole/liquid Mg partitioning, DMgamp/liq, on temperature, pressure and phase compositions are investigated employing robust regression methods based on MM-estimators. A database with 92 amphibole-plagioclase pairs – temperature range: 650–1050°C; amphibole compositional limits: >0.02 apfu (23O) Ti and >0.05 apfu Al – and 148 amphibole-glass pairs – temperature range: 800–1100°C; amphibole compositional limit: CaM4/(CaM4+NaM4) >0.75 – compiled from experiments in the literature was used for the calculations (amphibole normalization scheme: 13-CNK method).Statistical analysis reveals a significant dependence of DAl/Siplg/amp on pressure, temperature, Al fraction in amphibole T1-site, XAlT1, and albite fraction in plagioclase, XAb, leading to the barometric expression: Pkbar=8.3144TKlnDAl/Siplg/amp−8.7TK+23377XAlT1+7579XAb−11302/−274 (precision: ±1.5 to ±2.3kbar; expressed at 1s).The DMgamp/liq was found to have a significant dependence on temperature and the logarithm of XCaliq/(XCaliq+XAlliq) ratio (glass composition expressed as anhydrous mole fraction of cation components) that led to the thermometer: T℃=71975−11896lnXCaliq/XCaliq+XAlliq8.3144lnDMgamp/liq+58−273 (precision: ±35 to ±45°C).Amphibole-liquid compositional relationships revealed that the temperature dependence of DMgamp/liq was mostly controlled by a positive correlation of lnXMgliq with temperature that led to the thermometer: T(℃)=107 lnXMgliq−108ln[XCaliq/(XCaliq+XAlliq)]+1184 (precision: ±37 to ±42°C).The plagioclase-amphibole barometer is suitable for a large diversity of amphibole-plagioclase-bearing assemblages from metamorphic (amphibolites and mafic granulites) and igneous (metaluminous granitoids to gabbros) rocks, whereas the liquid-only and the amphibole-liquid thermometers are applicable to alkaline and subalkaline igneous rocks.The calibrated expressions yield P-T estimates consistent with those obtained with widely used barometers and thermometers. Plagioclase-amphibole barometry extends pressure determinations to quartz-free and/or garnet-free assemblages.