Compositional dependence of the coexisting pyroxene iron-magnesium distribution coefficient

Abstract

Iron-magnesium distribution coefficients for coexisting ortho- and clinopyroxene in 22 amphibolites from the New Jersey Precambrian Highlands range from 1.40 to 1.90. No systematic areal variation of the distribution coefficient is discernable within a 700 mi2 area. The distribution coefficient is, however, systematically related to pyroxene composition. The distribution coefficient tends to increase with increasing pyroxene weight % FeO (Fe as FeO) and decrease with increasing MgO and Al2O3. Data from other workers indicates that the distribution coefficient versus pyroxene composition trends found in the Highlands amphibolites are also present in both igneous and metamorphic rock suites from several other areas. Possible influence of pyroxene CaO on the distribution coefficient is also indicated. The Highlands amphibolite type trends are, however, directly opposite to those previously reported for Australian granulites. Both types of trends are apparently valid since both are present in at least one instance in metamorphic rocks from a relatively small area. The causes for the development of the two types of trends are imperfectly understood. Data presented indicates, however, that the New Jersey amphibolite type trends are apparently more characteristic of Fe-poor pyroxenes, whereas, the Australian granulite type trends are more characteristic of Fe-rich pyroxenes.