Fe3+/FeT RATIOS IN AMPHIBOLE MINERALS CHARACTERIZED BY SINGLE-CRYSTAL SYNCHROTRON MÖSSBAUER SPECTROSCOPY: A TOOL TO UNDERSTAND THE REDOX STATE OF ARC MAGMAS

Amphibole is an important fractionating phase during calc-alkaline differentiation in arc magmas. Through the presence of both ferric and ferrous iron in its crystal structure, amphibole can provide constraints on variations in oxygen fugacity of the melt from which it crystallized. However, the relationship between Fe³⁺/Fe^T (Fe³⁺/[Fe²⁺ + Fe³⁺]) ratios and oxygen fugacity, Fe³⁺ incorporation into the structure, and amphibole chemistry are not well understood. To address these, we present major element compositions, hydrogen isotopes and H₂O contents, and Fe³⁺/Fe^T ratios (single-crystal synchrotron Mössbauer spectroscopy; SMS) of volcanic amphiboles, which crystallized from magmas recording a wide range of oxygen fugacity (ΔNNO = -1.0 to + 3.0, log units relative to the Ni-NiO buffer). These high spatial-resolution techniques are applied to the same area on each grain allowing to correlate data sets and avoid averaging inclusions, alteration and intra-grain compositional heterogeneity present in natural amphiboles.

We present SMS data of amphiboles collected in the time domain with two different set ups. A new set up involving two high-speed shutters^1,2 facilitates the collection of time spectra with a delay-time window that is significantly longer. In addition, the dual-shutter setup achieves high counting rates and significantly reduces measurement time (~1 hour) especially for natural samples, which contain low amounts of ⁵⁷Fe.

Combining H₂O contents and H isotopes (δD relative to VSMOW) we can distinguish between amphiboles that have experienced post-crystallization Fe oxidation through dehydrogenation (Fe²⁺ + OH = Fe³⁺ + O^2- + H⁺) versus those recording magmatic 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 1.53 to 0.17 wt.% and an increase in δD from -70 to +440‰ in a single hand sample. Magmatic amphibole Fe³⁺/Fe^T ratios vary between 0.14 ± 0.03 to 0.62 ± 0.03 and decrease with decreasing oxygen fugacity constrained by independent methods. This observation suggests that primary amphibole Fe³⁺/Fe^T values may be useful monitors of oxygen fugacity .

¹Toellner, T. S., et al., 2011, Journal of Synchrotron Radiation 18.2, 183-188.

²Ratschbacher, B. C., et al., 2023, American Mineralogist, 108(1), 70-86.