Abstract

Experimentally determined Ca-Mg-Fe pyroxene phase relations at 800-1200°C and from less than one atmosphere up to 15 kbar are combined with calculated phase equilibria for the diopside-enstatite and hedenbergite-ferrosilite joins to produce a graphical two-pyroxene thermometer applicable to a wide variety of rocks from the earth, moon, and meteorites. Samples with appreciable contents of “others” components require special projection onto the Di-En-Hd-Fs pyroxene quadrilateral; Wo, En, and Fs components as normally calculated will not yield correct temperatures. The projection approximates the activities of those components in natural pyroxenes but is largely empirical, and may not be appropriate for large contents of nonquadrilateral components. Therefore, the thermometer should be used only for pyroxenes having Wo + En + Fs ≥ 90%. The effects of pressure are ≤8°C/kbar; graphs are presented for relations at one atmosphere and at 5, 10, and 15 kbar, along with approximate formulas for interpolating between these pressures. The occurrence of granule exsolution—the coalescence of exsolved pyroxene, with possible migration to grain boundaries—can complicate the use of the thermometer for slowly cooled rocks. The primary pyroxene compositions must be reconstructed from textural evidence before correct igneous or peak-metamorphic temperatures can be inferred.

The experimental data and inferred temperatures for three-pyroxene assemblages permit calibration of an improved pigeonite thermometer.

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