EXPERIMENTAL STUDY OF CLINOPYROXENE + QUARTZ + ALBITE EQUILIBRIUM IN THE SYSTEM NA2O-CAO-FEO-AL2O3-SIO2 AT 900-1100°C AND 14-25 KBAR

Abstract

The content of the jadeite end member (Jd) in clinopyroxene (Cpx) in equilibrium with plagioclase (P1) and quartz (Qz) is defined by a reaction given in the paper, and is an efficient indicator of pressure for metamorphic rocks of basic composition. The equilibrium of reaction is very well understood in the marginal system Na2-O-Al2O3-SiO2. However, the pyroxenes from these rocks typically are rich in the hedenbergite end member, and this must be taken into account in estimating pressure on the basis of the above geobarometer. To obtain quantitative information on the effect of Fe in Cpx in equilibrium (1), the authors conducted experiments on the system Na2O-CaO-FeO-Al2O3-SiO2. Data on the maximum solubility of Jd in a jadeite-hedenbergite (Hd) solid solution in the presence of albite and quartz were used to calculate its mixing properties, in order to model a multicomponent clinopyroxene solid solution on that basis. Study materials, methods and results are discussed.