An olivine-clinopyroxene geothermometer

The iron-magnesium exchange reaction between olivine and calcium-rich clinopyroxene is formulated as a geothermometer. It is shown that the clinopyroxene M1 site must be nonideal and it is expressed as a regular solution. The appropriate mixing parameters are calculated from a set of groundmass olivine-clinopyroxene pairs from lavas for which there are groundmass iron-titanium oxide temperatures. The pressure dependence of the geothermometer is calculated from abailable experimental work, and is approximately 5°C per kilobar. Layered gabbros from the Kap Edvard Holm Complex, East Greenland, show no significant variation of temperature with structural height in the intrusion, while those of Skaergaard give temperatures which do not have a consistent variation with height. Continued equilibration during post-crystallisation cooling is a possibility in slowly cooled intrusions. Inclusions in diamond give a pressure-temperature line consistent with formation at 1300°C at 55 kb, 1400°C 72 kb and 1500°C 90 kb. Ultrabasic xenoliths in Basutoland kimberlites have similar pressure-temperature lines. Lavas, including alkali basalts, basanites, andesites and rhyolites give temperatures from 1025°C to 890° C.