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Controls on Fe3+/FeT ratios in volcanic amphiboles across silicic magmatic systems: effects of magmatic redox, crystallization temperature, and dehydrogenation
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DOI：10.1007/s00410-026-02321-0.png)
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Abstract
En 中文
Amphiboles record crystallization temperatures, pressures, and melt compositions through their major- and minor-element chemistry. Their Fe3+/FeT (Fe3+/(Fe3+ + Fe2+)) ratios have also been proposed to record melt Fe3+/FeT ratios, and, by extension, magmatic oxygen fugacity ( $${f}_{{O}_{2}}$$ ). To test this, we analyzed Fe3+/FeT ratios in volcanic amphiboles from five silicic volcanic systems using single-crystal synchrotron-Mössbauer spectroscopy and combined these data with major-element chemistry, H2O concentrations, and H isotope compositions measured from the same area of individual grains. The studied amphiboles crystallized under independently constrained magmatic $${f}_{{O}_{2}}$$ conditions spanning ΔNNO = −0.6 to + 2.2 (log units below and above the Ni-NiO buffer). The data show that Fe3+/FeT ratios are influenced by both primary magmatic processes and post-crystallization modifications. Three of the five volcanic systems show evidence for complex amphibole growth histories manifested in low- and high-Al amphibole compositions. Across the studied samples, Fe3+ is primarily accommodated through a ferri-Tschermak-type substitution (Si4+(IV) + Fe2+(VI) = Al3+(IV) + Fe3+(VI)) and amphibole Fe3+/FeT ratios generally increase with increasing $${f}_{{O}_{2}}$$ ranging from 0.14 ± 0.03 to 0.68 ± 0.01. However, two processes shift Fe3+/FeT ratios relative to values expected from independently constrained $${f}_{{O}_{2}}$$ : post-crystallization dehydrogenation and temperature-dependent Fe2+-Fe3+ exchange. Although these processes modify Fe3+/FeT ratios, they do not change amphibole Fe# (FeTotal/(FeTotal + Mg)), which varies more consistently with independently constrained $${f}_{{O}_{2}}.$$ Amphibole Fe3+/FeT ratios therefore provide the most reliable record of magmatic $${f}_{{O}_{2}}$$ in systems that show no evidence for dehydrogenation and don’t have complex thermal and/or mixing histories. In more complex volcanic systems, amphibole Fe# provides a useful complementary metric for evaluating redox systematics.
Keywords:
Volcanic amphibole
Oxygen fugacity
Mössbauer spectroscopy
Hydrogen isotopes
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