285-3 A revised calibration for Al-in-amphibole geobarometry based on the abundance of octahedral Al

Abstract:

Constraining the pressure of magma storage in the crust is of primary importance for our understanding of the architecture of the crust and magmatism within it. Amphibole is common in many granitoid and rhyolitic rocks, and while Al-in-hornblende geobarometry is a well-established method, the need to disentangle the effects of temperature and pressure has hampered our ability to use it in detail. Médard & Le Pennec (2022) propose an alternative approach based on the concentration of octahedral Al – rather total Al – in amphibole. While this approach is theoretically sound, the abundance of octahedral Al is strongly affected by the choice of amphibole mineral formula calculation procedure. Médard & Le Pennec (2022) assume that 20% of the total Fe is Fe³⁺, which is generally too inaccurate for natural amphiboles. We derive an alternative calibration using the same experimental dataset but employing the Schumacher_Max amphibole formula calculation procedure proposed by Gualda & Vlach (2005), derived from the method of Schumacher (1991). The pressures obtained using the new calibration for amphibole from Peach Spring Tuff (SW USA) high-silica rhyolite show excellent agreement with rhyolite-MELTS pre-eruptive storage pressures calculated using the composition of glass in contact with the same amphibole crystals. Al-in-amphibole pressures for the Searchlight Pluton (NV) show a systematic increase with inferred depth, consistent with the stratigraphic position of the samples within the pluton, lending support to the new calibration. Finally, pressures calculated for amphiboles from the Graciosa A-type Province (southern Brazil), which vary from calcic to sodic in composition, show the same pressure range irrespective of amphibole group – this suggests that Al-in-amphibole geobarometry using this approach is not limited to hornblende compositions, and can be applied more generally. We recommend using the Schumacher_Max amphibole formula recalculation procedure and our new calibration to estimate pressures for amphiboles commonly observed in granitoids and rhyolites. This approach also allows pressure determination for non-rim compositions, opening opportunities to investigate the depth-history of magmas over time. Robust determination of amphibole crystallization pressures using only amphibole compositions that can be determined using electron-beam techniques for a broad range of amphibole group minerals greatly enhances the possibilities of assessing the conditions of emplacement of silicic plutons and of pre-eruptive storage of rhyolitic systems, providing useful insight into the architecture of silicic magmatic systems.

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