Redistribution of Ca, Mg, and Fe during Pyroxene Exsolution; Potential Rate-of-Cooling Indicator

Abstract

The exsolution of pyroxene minerals forms an interesting example of a naturally occurring solid-state chemical reaction. During the past few years, the orientation, structure, and composition of exsolution lamellae in several igneous and metamorphic pyroxene crystals have been investigated by optical microscopy, X-ray diffraction, electron probe analysis, transmission electron microscopy, and analytical electron microscopy. One might suppose, therefore, that the reaction is now fully understood, but this is not so, for the more closely the crystals are examined, the more complex and wondrous they are found to be. The present study is concerned principally with the diffusion of Ca, Mg, and Fe that evidently takes place while the reaction is in progress, an aspect of the reaction that has received relatively little attention in the past. A previously derived equation for the augite slope of the solvus surface will be used to estimate the temperature at which ionic mobility within an augite crystal from the Skaergaard complex declined and the reaction came to a close. This temperature estimate may possibly provide an indication of the rate of cooling.