The olivine — clinopyroxene geobarometer: experimental results in the CaO-FeO-MgO-SiO2 system

1432-0967

Springer Online Journal Archives 1860-2000

Geosciences

Abstract The exchange of Ca and Mg between olivine and clinopyroxene has been studied in the CFMS system. Experiments were conducted in a piston-cylinder apparatus in the temperature range of 1,100–1,300° C and the pressure range 10–30 kbar. Results confirm the previous suggestion (Adams and Bishop 1982) that this reaction has a sufficiently largeΔ V° to be used as a geobarometer in several basic and ultrabasic systems. Experimental results were corrected for compositional effects using recent activity-composition models for quadrilateral pyroxenes and olivines. The corrected results indicate that the exchange reaction has aΔH 1 bar of 34,900 J, a ΔS° of -7.25J/deg, and a ΔV° of -0.489 J/bar. Corrected results agree well with calculations based on the thermodynamic properties of the endmembers. Application of the olivine-clinopyroxene geobarometer to many systems will require additional calibration of non CFMS components. Preliminary pressure estimates based on simple assumptions about the activity relations of these components have been made for spinel lherzolites from southwestern United States and coarse and porphyroclastic garnet lherzolites from southern Africa. A geotherm calculated from spinel lherzolites near the Rio Grande rift is consistent with a geophysical geotherm based on near-surface heat-flow measurements of 100 mW/m2 or greater. Results on garnet lherzolites yield a southern African geotherm with no inflection which falls at somewhat higher temperatures than pyroxene geotherms calculated for the same area.

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