75-5 Modeling the Viscosity of Flow-banded Rhyolite using Raman Spectroscopy

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session (Posters)

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Abstract:

Viscosity is one of the most important physical properties governing magma ascent and eruption dynamics. Most predictive models for the temperature-dependence of melt viscosity are based on chemical composition with no explicit link to underlying structural organization of the melt. Raman spectroscopy provides a means to explore this link. We present Raman data for rhyolitic glass from the basal shear zone of the Minyon Falls Rhyolite, Eastern Australia. Flow bands within the basal obsidian are comprised of alternating dark and light bands of compositionally homogeneous brown and colorless glass. Raman spectra were collected with a HORIBA XploRa Plus spectrometer equipped with a 532-nm Nd laser at 50% intensity focused through a 50x objective. Over 270 spectra were acquired using a 300-micrometer confocal pinhole, 200-micrometer slit, and a grating of 1800 grooves per millimeter. Exposure times were 3 x 30 seconds. All spectra show two broad bands related to melt structure between 200-700 cm^-1 and 800-1300 cm^-1, commonly referred to low wavenumber (LW) and high wavenumber (HW) bands, respectively. We calculated melt viscosity based on the ratio of the intensity of LW and HW bands, normalized to an interlaboratory glass standard from Newberry Volcano, following Giordano et al. (2019) A calibrated database of Raman spectra for natural silicate glasses: implications for modelling melt physical properties, J. Raman Spectroscopy, 51, 1822-1838. Repeat analysis of the remelted Newberry glass yielded a reproducibility of log(η) = 9.09 ± 0.08 Pa-s (2σ, n=50), for a magma temperature of 850°C. In contrast, Raman-derived viscosities for the texturally heterogeneous Minyon Falls Rhyolite vary by glass type. Brown glasses from dark flow bands yield a mean viscosity of 9.7 ± 0.3 log Pa-s (n=144), whereas colorless glasses from light flow bands yield a mean viscosity 9.2 ± 0.2 log Pa-s (n=127). Our results indicate that the HW band exerts greater systematic control on calculated viscosity. This band is associated with tetrahedral units with a variable number (n) of bridging oxygens, referred to as Qⁿ units. Our ongoing efforts are focused on deconvolution of the HW region in our spectra to further investigate the relationship between the melt structure, as expressed in Qⁿ units, and melt viscosity. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11212

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