Iron valence systematics in clinopyroxene crystals from ocean island basalts

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 and 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., D^cpx−liq_Fe2O3 values), though this may not be an equilibrium relationship. For example, ^IVAl-rich {hk0} prism sectors of sector-zoned crystals contain more Fe³⁺ than ^IVAl-poor {111 ̄ } hourglass sectors. Moreover, ^IVAl-rich compositions formed during disequilibrium crystallisation are enriched in Fe³⁺. Apparent clinopyroxene-liquid Fe²⁺–Mg exchange equilibria (i.e., K^cpx−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 K^cpx−liq _D,Fe²⁺_−Mg values lies closer to experimentally reported values of 0.24−0.26 than values of ∼0.28 returned from a general 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 was also affected by reductive SO₂ degassing. Overall, our findings demonstrate that the Fe valence systematics of clinopyroxene crystals can record information about the conditions under which OIBs evolve, but that further experimental work is required to properly disentangle the effects of magma composition, disequilibrium and fo₂ conditions on clinopyroxene-liquid equilibria involving Fe²⁺ and Fe³⁺