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 (\(f_{\textrm{O}_{2}}\)). However, our knowledge of how and why \(f_{\textrm{O}_{2}}\) 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 Fe\(^{3+}\) contents and magmatic \(f_{\textrm{O}_{2}}\) conditions. Although many studies assume that all Fe in augitic clinopyroxene crystals from OIBs occurs as Fe\(^{2+}\), we find that up to half of the total Fe present can occur as Fe\(^{3+}\), with crystals from alkali systems typically containing more Fe\(^{3+}\) than those from tholeiitic systems. Thus, Fe\(^{3+}\) is a major if under-appreciated constituent of augitic clinopyroxene crystals erupted from ocean island volcanoes. Most Fe\(^{3+}\) in these crystals is hosted within esseneite component (CaFe\(^{3+}\)AlSiO\(_{6}\)), though some may be hosted in aegirine component (NaFe\(^{3+}\)Si\(_{2}\)O\(_{6}\)) in crystals from alkali systems. Observations from samples containing quenched matrix glasses suggest that the incorporation of Fe\(^{3+}\) is related to the abundance of tetrahedrally coordinated Al (\(\mathrm {^{IV}}\)Al), implying some steric constraints over Fe\(^{3+}\) partitioning between clinopyroxene and liquid (i.e., \(D\mathrm {^{{cpx-liq}}_{{Fe_{2}O_{3}}}}\) values), though this may not be an equilibrium relationship. For example, \(\mathrm {^{IV}}\)Al-rich \(\{hk0\}\) prism sectors of sector-zoned crystals contain more Fe\(^{3+}\) than \(\mathrm {^{IV}}\)Al-poor \(\{\bar{1}11\}\) hourglass sectors. Moreover, \(\mathrm {^{IV}}\)Al-rich compositions formed during disequilibrium crystallisation are enriched in Fe\(^{3+}\). Apparent clinopyroxene-liquid Fe\(^{2+}\)–Mg exchange equilibria (i.e., \(K\mathrm{{_{D, {Fe^{2+}-Mg}}^{cpx-liq}}}\) 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 \(K\mathrm{{_{D, {Fe^{2+}-Mg}}^{cpx-liq}}}\) values lies closer to experimentally reported values of 0.24\(-\)0.26 than values of \(\sim\)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 \(f_{\textrm{O}_{2}}\) conditions about one log unit above fayalite-magnetite-quartz (FMQ) equilibrium (i.e., \(\sim\)FMQ+1) and our glassy Azorean sample recording evolution under significantly more oxidising conditions (\(\ge\)FMQ+2.5) before experiencing syn-eruptive reduction, likely as a result of SO\(_{2}\) degassing; our other glassy sample from Iceland was also affected by reductive SO\(_{2}\) 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 \(f_{\textrm{O}_{2}}\) conditions on clinopyroxene-liquid equilibria involving Fe\(^{2+}\) and Fe\(^{3+}\).