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Empirical calibration of the silica–Ca‐tschermak’s–anorthite (SCAn) geobarometer
Author(s) -
MCCARTHY* T. C.,
PATIÑO DOUCE A. E.
Publication year - 1998
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1998.00164.x
Subject(s) - anorthite , granulite , enthalpy , mineralogy , quartz , mineral , geology , plagioclase , closure temperature , thermodynamics , analytical chemistry (journal) , chemistry , materials science , geochemistry , metamorphic rock , metallurgy , physics , paleontology , chromatography , facies , structural basin
The pressure‐sensitive equilibrium among anorthite, quartz and the Ca‐tschermak component in clinopyroxene (CaAl 2 SiO 6 ; CaTs), CaAl 2 SiOCpx6+SiOQtz2=CaAl 2 Si 2 OPl8 (SCAn) ,can be used as a geobarometer in granulites with the proper assemblage, and has been calibrated using mineral composition data from partial melting experiments of natural assemblages and from phase equilibrium experiments on the end‐member CMAS system. The experimental data cover the P – T range 4–32 kbar and 900–1400 °C. Linear least‐squares regression analysis of the experimental data resulted in the following empirical expressions for pressure in terms of composition and temperature: P = 5.066 [±0.760]+ 1300 [±800] T −ln K 276 [±16] · T [±2.5 kbar]or P = 6.330 [±0.116]−ln K 301 [±9]· T [±1.0 kbar] ,where K = a PlAn a CpxCaTs .The first equation incorporates an enthalpy term, but is less accurate than the second equation, in which the enthalpy of reaction is ignored. Application of these expressions to natural and experimental equilibrium mineral assemblages demonstrates that the empirical barometers are applicable over a wide range of pressures (≥4 kbar), temperatures (≥700 °C) and bulk compositions (Mg#≥32.5).