Partition of Strontium between Coexisting K-Feldspar and Plagioclase in Some Metamorphic Rocks

Abstract

Trace quantities of strontium in forty-four pairs of coexisting natural potassium and plagioclase feldspars extracted from metamorphic rocks were analyzed using an X-ray spectrographic technique. The main objective of this work has been to evaluate the usefulness of strontium distribution coefficients as indicators of metamorphic grade. Rocks from six different areas were studied in detail, most areas including two zones of metamorphism; in this way a range of metamorphic grade from middle amphibolite facies to various grades of granulite facies was investigated. Strontium partitioning between coexisting feldspar phases in isofacial rocks can be described by the Nernst distribution law and is of the type (mol per cent Sr-feldspar in K-feldspar)/(mol per cent Sr-feldspar in plagioclase) = constant. Data from most areas show systematic and significant changes in strontium partitioning with gross changes in metamorphic grade. With increasing grade, strontium is preferentially taken into the K-feldspar phase....