Iron valence systematics in clinopyroxene crystals from ocean island basalts

• Main Authors: Neave, David, Stewart, Alexander, Hartley, Margaret, Namur, Olivier
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2024
• Subjects: Clinopyroxene; Ferric iron; Ocean island basalt; Iron valence; Partitioning; Iceland; Ocean Island
• Online Access: https://research.manchester.ac.uk/en/publications/5efcd13e-de98-4327-b689-2bf8ae453f7b; https://doi.org/10.31223/X5N959; https://pure.manchester.ac.uk/ws/files/316272342/s00410-024-02144-x.pdf

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 ( fo 2 ). However, our knowledge of how and why fo 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 fo 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 ( IV Al), implying some steric constraints over Fe 3+ partitioning between clinopyroxene and liquid (i.e., D cpx−liq Fe2O3 values), though this may not be an equilibrium relationship. For example, IV Al-rich {hk0} prism sectors of sector-zoned crystals contain more Fe 3+ than IV Al-poor {111 ̄ } hourglass sectors. Moreover, IV Al-rich compositions formed during disequilibrium crystallisation are enriched in Fe 3+ . Apparent clinopyroxene-liquid Fe 2+ –Mg exchange equilibria (i.e., K cpx−liq D,Fe 2+ −Mg values) are similarly affected by disequilibrium crystallisation in our samples. Nonetheless, it is possible to reconcile our observed clinopyroxene compositions with glass ...