RAMAN BAROMETRY FOR MAGMATIC SYSTEMS USING FELDSPAR AND PYROXENE

Magmatic storage conditions are important variables that exert control over the evolution and eruptive behavior of volcanic systems. Temperature conditions for most magmas can be accurately determined using major and trace-element compositions of minerals. The pressures, and storage depths, of magmas are less readily constrained. Dissolved volatile contents of melt inclusions provide one method for estimating magmatic pressures, but they have limitations caused by leaking and assumptions regarding saturation. In recent years, mantle and metamorphic petrologists have established the use of Raman spectroscopy as a tool for accurate barometry of peak conditions. Briefly, mineral inclusions in crystal hosts preserve residual pressures related to entrapment. Entrapment conditions are then estimated by comparing the relative spectral shift of Raman peaks between the pressurized inclusion and the same phase at ambient conditions. Despite the success of this technique, it has never been applied to igneous systems, until now. Here, we measure the Raman spectra of igneous feldspars and clinopyroxene from 0 to 2.2 GPa using the Raman system in the Mineral Physics Laboratory at the University of Texas at Austin. Overall, results show a near perfect linear shift of Raman peaks with increasing pressure (r²>0.90). Sanidine (~Or₆₃), from the early Bishop Tuff Fall deposit F1, has a single prominent peak located at wavenumber 515 cm^-1 at ambient conditions, which shifts to 523 cm^-1 at 2.2 GPa. Labradorite, from an unknown locality, has prominent peaks at 509 and 790 cm^-1 at ambient conditions. The 509 cm^-1 peak shifts linearly to 517 cm^-1 at 2.2 GPa, whereas the 790 cm^-1 peak does not systematically respond to pressure. Clinopyroxene (En₁₃Fs₄₆Wo₄₁), from Yellowstone Caldera, has 3 prominent peaks at ambient conditions located at 382 cm^-1, 662 cm^-1, and 1011 cm^-1, which shift to 389, 670, and 1022 cm^-1, respectively, at 2.2 GPa. The slope of the linear peak shift with increasing pressure for the minerals is 3.5±0.2 cm^-1 GPa^-1, which is indistinguishable from the linear pressure dependency of many minerals (Kohn 2014). To test our experimental calibration, we have designed experiments to analyze feldspar and pyroxene inclusions in crystal hosts from diverse volcanic systems with well-known storage conditions.