Cordierite-spinel-quartz assemblages: A potential geobarometer

• Main Authors: F. Seifert, J.C. Schumacher
• Format: Article
• Language: English
• Published: Geological Society of Finland 1986-06-01
• Series: Bulletin of the Geological Society of Finland
• Subjects: geobarometer; cordierite; spinel; quartz; phase relations; thermodynamic data
• Online Access: http://www.geologinenseura.fi/bulletin/Volume58/sgs_bt_058_1_pages_095_108.pdf

The composition of (Mg-Zn-Fe+2)-aluminate-spinel coexisting with cordierite and quartz is potentially a sensitive geobarometer in rock types such as cordierite ‒ anthophyllite rocks which are not well suited to most other geobarometers. Calculations based on thermodynamic data and the end member reactions demonstrated that the compositions of (Mg-Fe) ‒ cordierites and (Mg-Fe-Zn) ‒ aluminate spinels are only slightly affected by temperature but the effect of pressure is dramatic. With increasing pressure the XMg of the spinel changes from less than 0.1 at 1 bar to about 0.8 at 9 kbars. Comparison of pressures derived from this model for natural assemblages with known PT conditions shows good agreement but also points to the necessity of treating the effects of water on the stability of cordierite more quantitatively.