David Axford Neave , Alexander G Stewart, Margaret Hartley, Olivier Namur

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 understanding of the nature and causes of fO2 variability within and between magmatic systems remains patchy because of the challenges associated with estimating Fe valence in glasses and minerals robustly and routinely. Here we present analyses of clinopyroxene crystals from OIB samples erupted in Iceland and the Azores performed to investigate their Fe valence systematics and explore their potential for recording information about magma Fe3+ contents and fO2 conditions. Although many studies assume that all Fe in magmatic clinopyroxene crystals occurs as Fe2+, we find that up to half of the total Fe present in magmatic clinopyroxene crystals may 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 clinopyroxene crystals erupted from ocean island volcanoes. Most Fe3+ in these crystals is hosted within esseneite component (CaFe3+AlSiO6), though some can 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 mediated by the abundance of tetrahedrally coordinated Al (IVAl), implying strong steric controls over Fe3+ partitioning between clinopyroxene and liquid (i.e., Dcpx-liq Fe2O3 values). For example, IVAl-rich prism sectors of sector-zoned crystals contain more Fe3+ than hourglass sectors. Moreover, IVAl-rich compositions formed during disequilibrium crystallisation at high degrees of undercooling are also enriched in Fe3+. Apparent clinopyroxene-liquid Fe2+–Mg exchange equilibria (i.e., KDcpx-liqFe2+-Mg values) are similarly affected by disequilibrium crystallisation. Nonetheless, it is possible to reconcile our observed clinopyroxene compositions with glass Fe valence systematics estimated from olivine-liquid equilibria if we assume that equilibrium was best recorded by the lowest-Fe3+ compositions in clinopyroxene crystals affected by disequilibrium crystallisation. In this case, olivine-liquid and clinopyroxene-liquid equilibria record equivalent narratives, with 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. Overall, our findings demonstrate that the Fe valence systematics of clinopyroxene crystals record important 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+.