Fe3+/FeT ratios in amphibole minerals determined 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 (Fe³⁺) and ferrous (Fe²⁺) iron in its crystal structure, amphibole has the potential to provide constraints on variations in Fe³⁺/Fe^T ratios and thus magmatic oxygen fugacity (f_O2) of the melt from which it crystallized.

We present major element compositions (electron microprobe), hydrogen isotopes and H₂O contents (secondary ion mass spectrometry), and Fe³⁺/Fe^T ratios (single-crystal synchrotron Mössbauer spectroscopy; SMS) of volcanic amphiboles, which crystallized from magmas recording a wide range of f_O2 (ΔNNO = -1.0 to + 3.0, log units relative to the Ni-NiO buffer). These high spatial-resolution analytical techniques are applied to the same area on each grain thus allowing to correlate data sets and avoid averaging inclusions and intra-grain compositional heterogeneity present in many natural amphiboles.

Amphibole Fe³⁺/Fe^T ratios increase with increasing f_O2 varying from 0.14 to 0.67 over the investigated f_O2 range thus amphiboles may be useful monitors of magmatic oxygen fugacity. However, individual volcanic amphiboles can experience extreme dehydrogenation and associated sub-solidus Fe oxidation (e.g., with Fe³⁺/Fe^T ratios of up to 0.86, low H₂O: 0.17±0.01 wt. % and extreme δD: 421±4 ‰ relative to SMOW), which likely occurred during shallow crustal storage. It is therefore important to combine the aforementioned high-spatial resolution and high-precision analytical techniques to distinguish primary magmatic from secondary amphibole Fe³⁺/Fe^T ratios to interpret Fe³⁺/Fe^T ratios correctly in the context of magmatic f_O2 .