Evaluation of Coexisting Garnet-Biotite, Garnet-Clinopyroxene, and Other Mg-Fe Exchange Thermometers in Adirondack Granulites

INTRODUCTION

Of the many thermometers that have been proposed for metamorphic and igneous rocks, those that depend on the distribution of a pair of cations or groups of cations between two coexisting phases are perhaps the most widely appled and yet the most poorly calibrated for natural compositions. The most attractive feature of these systems is the apparent simplicity of their application, requiring only an analysis of the coexisting phases. Unfortunately, the partitioning of cations between co-exiting phases may depend on bulk compositions as well as temperature. In some systems—for example, Mg-Fe²⁺ exchange between olivine and orthopyroxene—bulk composition has a more important effect than temperature on the distribution co-efficient (Ramberg and DeVore, 1951; Medaris, 1969; Matsui and Nishizawa, 1974). Consequently, it is important to test the effect of bulk composition as well as temperature when applying cation exchange (K_D) thermometry. (K_D is defined as the ratio of a pair of cations between co-existing phases. For example, K_D(Fe-Mg)^(garn-cpx) = (Fe garnet/Mg garnet) (Mg cpx/Fe cpx).)

The effect of pressure, on the other hand, is generally quite small, because the δV is usually very small for a given reaction of the type A (in phase X) + B (in phase Y) = B (in phase X) + A (in phase Y). Most often, the pressure dependence of K_D is ignored for extrapolations within the crust—that is, for δP of less than 10 kb.