Orthopyroxene-clinopyroxene equilibria in the system CaO-MgO-Al2O3-SiO2 (CMAS): new experimental results and implications for two-pyroxene thermometry |
| |
| Authors: | K G Nickel G P Brey L Kogarko |
| |
| Institution: | (1) Abteilung Kosmochemie, Max-Planck-Institut für Chemie, Saarstraße 23, D-6500 Mainz, Federal Republic of Germany;(2) Vernadski Institut, Akademia Nauk USSR, Moskau, USSR |
| |
| Abstract: | A new set of reversal experiments for coexisting ortho- and clinopyroxenes in the system CMAS at conditions between 1,000–1,570° C and 30 to 50 kb is presented and combined with literature data. Pyroxene behaviour, particularly that of clinopyroxene, is very complicated and different styles of Al incorporation into the pyroxene structure for low and high concentrations of Al are indicated, strongly influencing the exchange of the enstatite component between ortho- and clinopyroxene. Thermodynamic modelling of this exchange is problematic because of the large number of unknown coefficients compared to the number of experiments. Thermometry based on such models becomes very dependent on accuracy of experimental data and analyses of small quantities of elements. Despite this complexity very simple empirical thermometric equations are capable of reproducing experimental conditions in the systems CMS and CMAS over a wide range of P, T conditions. We derived the equation
which gives a mean error of estimate of 25° C when applied to CMS and CMAS data.Abbreviations Used in the Text cpx
clinopyroxene
- di
diopside, CaMgSi2O6
- en
enstatite, Mg2Si2O6
- opx
orthopyroxene
- px
Pyroxene
- py
pyrope
-
a
i
j
activity of component i in phase j
-
activity coefficient
-
 G
P,T
(A)
molar Gibbs free energy difference of reaction (A) at P, T
-
X
i
j
mole fraction of component i in phase j |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
