Chemical distinction between igneous and metamorphic orthopyroxenes especially those coexisting with Ca-rich clinopyroxenes: a re-evaluation

Abstract

AbstractA previously suggested simple relationship in a diagram of(Al2O3vs. MgO + FeO + Fe2O3)opx for the distinction between igneous and metamorphic orthopyroxenes is shown to be inadequate. A revised diagram is presented and its limitations are discussed. A diagram showing Fe2+/(Fe2+ + Mg) vs. 100Ca/(Fe2+ + Mg + Ca) for orthopyroxenes coexisting with Ca-rich clinopyroxenes proves to be useful for the discrimination of igneous and metamorphic orthopyroxenes and for evaluation of the extent and physical conditions of regional orthopyroxene re-equilibration.