Assessing magmatic volatile equilibria through FTIR spectroscopy of unexposed melt inclusions and their host quartz: a new technique and application to the Mesa Falls Tuff, Yellowstone

Abstract

Fourier transform infrared spectroscopy (FTIR) is a precise, non-destructive, and highly sensitive means of measuring volatile species in melt inclusions. Existing methods of applying FTIR to quartz-hosted melt inclusions, however, require challenging physical manipulation of the host quartz crystals, which can present a significant limitation. Here, we describe a technique for measuring the H2O and CO2 concentrations of melt inclusions fully enclosed in quartz crystals using transmission FTIR, where melt inclusion thickness is calculated using the host quartz silicate overtones. The greater thickness of quartz crystals permitted by this technique allows an additional assessment of volatile equilibrium by measuring structural OH− within the quartz host itself. To demonstrate the advantages of this technique, we applied it to quartz crystals from the Mesa Falls Tuff, Yellowstone. The majority of melt inclusions consist of transparent or brown glassy melt inclusions and record concentrations of 2.9–3.3 wt% H2O and 181–561 ppm CO2, with precision similar to that obtained by standard double-exposed FTIR. Based on volatile saturation models, such concentrations reflect a largely un-degassed magma body equilibrated at pre-eruptive pressures of 100–150 MPa. These H2O concentrations and equilibrium conditions are supported by independent assessments using mineral–mineral and mineral-melt thermohygrometry and thermodynamic modelling using rhyolite-MELTS. The preservation of pre-eruptive volatile concentrations is further corroborated by measurements of OH− in the quartz hosts. A clear decrease in hydroxyl concentrations from core to rim matches closely the zoning in Al, which, combined with the position of quartz infrared bands, most likely reflects variations in Al-moderated OH− solubility rather than diffusive loss. The outermost 200 µm rims, however, show a sharper decline in OH− and corresponding increase in Li+ which is interpreted to record minor diffusive H+ loss and incorporation of Li+ to maintain charge balance. Our FTIR-based technique is simple, effective, and widely applicable to other quartz-bearing magmatic systems, opening up new possibilities to trace pre- syn- and post-eruptive volatile behaviour.