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

Abstract

AbstractClinopyroxene is a major rock forming mineral capable of incorporating diverse metal cations. As a consequence, clinopyroxene preserves valuable archives of magmatic processes. Understanding clinopyroxene is thus essential for understanding Earth’s wider chemical evolution. However, knowledge about the relative abundances of ferrous and ferric iron in magmatic clinopyroxene remains sparse 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 magmatic clinopyroxene crystals often containing appreciable quantities of ferric iron that affect its stability and behaviour in currently ill-constrained ways and limit our ability to exploit its chemistry to robustly reconstruct conditions of magma storage and evolution. Here we integrate optimised electron probe microanalysis and Mössbauer spectroscopy on endmember and single-crystal clinopyroxene samples to re-evaluate previously discredited approaches for estimating clinopyroxene ferric iron contents by stoichiometry. By ensuring that we measured all major and minor elements in clinopyroxene crystals with sufficient precision, we show that it is possible to readily obtain stoichiometric estimates of clinopyroxene ferric-to-total iron ratios with similar precisions to those derived from Mössbauer spectroscopy (1$${\sigma }$$ σ $$\sim$$ ∼ 3.5% absolute). Being able to robustly determine clinopyroxene ferric iron contents enables us to propose a new empirical scheme for assigning clinopyroxene components that explicitly accounts for ferric iron, which is primarily hosted within esseneite component (CaFe$$^{3+}$$ 3 + AlSiO$$_{6}$$ 6 ) in clinopyroxenes dominated by quadrilateral components and aegirine component (NaFe$$^{3+}$$ 3 + Si$$_{2}$$ 2 O$$_{6}$$ 6 ) in alkali clinopyroxenes. Our new scheme provides a framework for documenting the full spectrum of clinopyroxene compositions in both natural and experimental systems when analyses are performed with sufficient precision.