Iron valence systematics in clinopyroxene crystals from ocean island basalts
Authors: Neave David , Stewart Alexander , Hartley Margaret , Namur Olivier 
Published: June 2023

The valence state of Fe plays a vital role in setting and recording the oxidation state of magmas, commonly expressed in terms of oxygen fugacity (fO2 ). However, our knowledge of how and why fO2 varies within and between magmatic systems remains patchy because of diverse challenges associated with estimating the valence state of Fe in glasses and minerals routinely. Here we investigate Fe valence systematics in magmatic clinopyroxene crystals from ocean island basalts (OIBs) erupted in Iceland the Azores to explore whether they record information about magma Fe3+ contents and magmatic fO2 conditions. Although many studies assume that all Fe in augitic clinopyroxene crystals from OIBs occurs as Fe2+, we find that up to half of the total Fe present can occur as Fe3+, with crystals from alkali systems typically containing more Fe3+ than those from tholeiitic systems. Thus, Fe3+ is a major if under-appreciated constituent of augitic clinopyroxene crystals erupted from ocean island volcanoes. Most Fe3+ in these crystals is hosted within esseneite component (CaFe3+AlSiO6), though some may be hosted in aegirine component (NaFe3+Si2O6) in crystals from alkali systems. Observations from samples containing quenched matrix glasses suggest that the incorporation of Fe3+ is related to the abundance of tetrahedrally coordinated Al (IVAl), implying some steric constraints over Fe3+ partitioning between clinopyroxene and liquid (i.e., Dcpx-liqFe2O3 values), though this may not be an equilibrium relationship. For example, IVAl-rich {hk0} prism sectors of&nbsp;sector-zoned crystals contain more Fe3+ than IVAl-poor {-111} hourglass sectors. Moreover, IVAl-rich compositions formed during disequilibrium crystallisation are also enriched in Fe3+. Apparent clinopyroxene-liquid Fe2+&ndash;Mg exchange equilibria (i.e., Kcpx-liq D;Fe2+-Mg values) are similarly affected by disequilibrium crystallisation in our samples. Nonetheless, it is possible to reconcile our observed clinopyroxene compositions with glass Fe valence systematics estimated from olivine-liquid equilibria if we assume that Kcpx-liq D;Fe2+-Mg values lies closer to experimentally reported values of 0.24&ndash;0.26 than values of ~0.28 returned from a general predictive model. In this case, olivine-liquid and clinopyroxene-liquid equilibria record equivalent narratives, with one of our glassy samples from Iceland recording evolution under fO2 conditions about one log unit above fayalite-magnetite-quartz (FMQ) equilibrium (i.e., ~FMQ+1) and our glassy Azorean sample recording evolution under significantly more oxidising conditions (~FMQ+2.5) before experiencing syn-eruptive reduction, likely as a result of SO2 degassing; our other glassy sample from Iceland is also affected by reductive SO2 degassing. Overall, our findings demonstrate that the Fe valence systematics of clinopyroxene crystals can record important information&nbsp;about the conditions under which OIBs evolve, but that further experimental work is&nbsp;required to properly disentangle the effects of magma composition, disequilibrium and&nbsp;fO2 conditions on clinopyroxene-liquid equilibria involving Fe2+ and Fe3+.