On anharmonic and pressure corrections to the equilibrium isotopic constants for minerals
Abstract
Specifies of the calculations of the reduced isotopic partition function ratios (βfactor) of minerals are discussed. Comparative calculations in the framework of the fully harmonic, quasiharmonic, and intrinsic anharmonic approximations show minor anharmonic corrections to the harmonic values of the βfactor. In the case of calcite, the difference between the fully harmonic and intrinsic anharmonic values of 10 ^{3}lnβ varies from 0.60 at 300 K to 0.37 at 1200 K and is close to typical values of the anharmonic correction in gas molecules. A new treatment for calculating isotopic effects in molar volumes of minerals and pressure effects on their βfactors is developed on the basis of the MieGrüneisen equation of state. There is no significant difference between the quasiharmonic and intrinsic harmonic values of (∂lnβ/∂ P) _{T}. For calcite, the pressure derivative of the βfactor is positive, decreases monotonically with temperature, and becomes small at T ∼ 1000 K (10 ^{3}(∂lnβ/ ∂P) _{T} ≈ 0.10.15 GPa ^{1}). These results contradict the large anharmonic and pressure effects to the βfactor of calcite calculated by Gillet et al. (1996) as well as their conclusion that the pressure correction to the βfactor of calcite is negative at higher temperatures and increases in its absolute value with increasing temperature.
 Publication:

Geochimica et Cosmochimica Acta
 Pub Date:
 September 1998
 DOI:
 10.1016/S00167037(98)002208
 Bibcode:
 1998GeCoA..62.3077P