Re-evaluating stoichiometric estimates of iron valence in magmatic clinopyroxene crystals

Abstract

Clinopyroxene is a major rock forming mineral capable of incorporating a wide range of metal cations. As a consequence, clinopyroxene preserves valuable archives of magmatic processes and understanding clinopyroxene is essential for understanding Earth’s broader chemical evolution. However, knowledge about the relative abundances of ferrous and ferric iron in magmatic clinopyroxenes remains poor because it is not currently possible to routinely measure the valence of iron in clinopyroxene crystals without either separating single crystals for bulk analysis or securing access to Mössbauer spectroscopy or a synchrotron radiation source to perform in-situ microanalysis. This is despite many magmatic clinopyroxene crystals seemingly containing appreciable quantities of ferric iron, highlighting the potential for developing widely applicable clinopyroxene-based oxybarometers. Here we integrate optimised electron probe microanalysis and Mössbauer spectroscopy on endmember and single-crystal clinopyroxenes to re-evaluate previously discredited approaches for estimating clinopyroxene ferric iron contents by stoichiometry. By ensuring that all major and minor elements in clinopyroxene crystals are measured with sufficient precision, we show that it is possible to readily obtain stoichiometric estimates of clinopyroxene ferric to total iron contents with similar precisions to those derived from Mössbauer spectroscopy (1σ∼ 3.5% absolute). Being able to robustly determine clinopyroxene ferric iron contexts enables us to propose a new scheme for assigning clinopyroxene components that explicitly accounts for ferric iron, which is primarily hosted within esseneite component (CaFe3+AlSiO6) in clinopyroxenes dominated by quadrilateral components and aegirine component (NaFe3+Si2O6) in alkali clinopyroxenes. Our new scheme provides a framework for documenting the full spectrum of clinopyroxene compositions in studies on both natural and experimental systems.