Fe3+/FeT ratios in volcanic amphiboles determined by single-crystal synchrotron Mössbauer spectroscopy as a function of mineral composition, magmatic oxygen fugacity and post-crystallization processes.

The oxygen fugacity (f_O2) of arc magmas contributes to generate the calc-alkaline compositional trend that characterizes continental crust and controls the composition of magmatic fluids and transport of redox-sensitive elements during ore formation. In igneous rocks, magmatic f_O2 is determined either through Fe-Ti oxybarometry, glass Fe₂O₃/FeO ratios, and/or equilibrium among coexisting ferromagnesian phases. However, subsolidus re-equilibration, secondary alteration, and the rare occurrence of these minerals can make it problematic to constrain f_O2 in arc magmatic rocks.

Amphibole, through the incorporation of Fe²⁺ and Fe³⁺ into its crystal structure and its common occurrence in arc magmas, has the potential to provide constraints on variations in magmatic f_O2. However, the relationship between Fe³⁺/Fe^T ratios and f_O2, Fe³⁺ incorporation into the structure, and amphibole chemistry are not well understood. To address these, we present major element compositions (EMPA), hydrogen isotopes and H₂O contents (SIMS), and Fe³⁺/Fe^T ratios (single-crystal synchrotron Mössbauer spectroscopy) of volcanic amphiboles, which crystallized from magmas recording a wide range of magmatic f_O2 (ΔNNO = -1.0 to + 3.0, log units relative to the Ni-NiO buffer).

Preliminary results show no correlation in the amount of octahedral Al³⁺ (Al³⁺ _C-site = Fe³⁺_C-site) nor A site vacancy (Na,K⁺ _A-site + Fe²⁺ _C-site = ☐ _A-site + Fe³⁺ _C-site) with Fe³⁺/Fe^T in amphiboles, which have been proposed to be involved in Fe³⁺ accommodation. Combining H₂O contents and H isotopes (δD relative to VSMOW) can distinguish between amphiboles that have experienced post-crystallization Fe oxidation versus those recording magmatic Fe³⁺/Fe^T ratios. We observe that both non-dehydrogenated and dehydrogenated amphiboles exist in a single hand sample. For example, post-crystallization dehydrogenation resulted in a decrease of H₂O contents from 1.48 to 0.17 wt.% and an increase in δD from -91 to +293‰ in a single hand sample. Dehydrogenation is associated with an increase in Fe³⁺/Fe^T ratios. Amphiboles that have not experienced significant dehydrogenation (> 1.3 wt. % H₂O and δD values of -110 to -60 ‰), exhibit a decrease in Fe³⁺/Fe_T ratios with decreasing f_O2, suggesting that primary amphibole Fe³⁺/Fe^T values may be useful monitors of magmatic f_O2.