75-6 Nano-scale Characterization of Rhyolite Lava using Raman Microscopy

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

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Abstract:

Flow bands in silicic lava provide insight into the dynamics of magma ascent and emplacement. While glassy bands are typically defined by distribution of micrometer-sized crystals (microlites), vesicles, and/or glass color, there is growing recognition of textural heterogeneity at the nanoscale. We are utilizing Raman microscopy to investigate the nature and origin of nanometer-sized crystals (nanolites) in flow-banded rhyolite from the Tweed volcano in Eastern Australia. Flow bands within the basal obsidian of the 1.25-km³ rhyolite are comprised of alternating dark and light bands of compositionally homogeneous glass of varying color and microlite abundance. Dark bands contain brown glass with low microlite contents, whereas light bands contain colorless glass extending to higher microlite concentrations.

Raman spectra were collected with a HORIBA XploRa Plus spectrometer and a 532-nm Nd laser at 50% intensity focused through a 50x objective. Each spectrum was acquired using exposure times of 3 x 30s, a grating of 1800 grooves per millimeter, 200-micrometer slit, and 300-micrometer confocal pinhole. Nanolite numbers (N#) were determined from the ratio of the intensity of the baseline-corrected Raman peak associated with magnetite at 670cm^-1 and the Fe⁺³ portion of the high-wavenumber band of silicate glass centered around 970cm^-1, following Di Genova et al., 2018, Measuring the degree of “nanotilization” of volcanic glasses: Understanding syn-eruptive processes recorded in melt inclusions. Lithos, 318, 209-218. Although both glass types span most of the range of measured nanolite numbers, their median values are distinct: brown, microlite-poor glass yields N# = 3.84 ± 1.17 (1σ, n=139) and clear, microlite-rich glass gives N# = 0.74 ± 1.06 (1σ, n=126). Given the nano-scale heterogeneity observed among flow bands within samples that experienced a shared cooling history, nanolite formation during lava emplacement is unlikely. Instead, we favor a model in which nanolites form during shallow magma ascent. We suggest that the degree of nanolitization reflects proximity to sights of degassing. In this model, melt immediately adjacent to bubble walls or melt fractures, where volatile loss is greatest, experiences larger undercooling and potentially Fe oxidation. These conditions favor high degrees of nanolitization. In contrast, diminished volatile loss further from degassing sights leads to greater crystallization of microlites relative to nanolites.

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

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