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We document the presence, composition, and number density (TND) of titanomagnetite nanolites and ultra‐nanolites in aphyric rhyolitic pumice, obsidian, and vesicular obsidian from the 1060 CE Glass Mountain volcanic eruption of Medicine Lake Volcano, California, using magnetic methods. Curie temperatures indicate compositions of Fe2.40Ti0.60O4 to Fe3O4. Rock‐magnetic parameters sensitive to domain state, which is dependent on grain volume, indicate a range of particle sizes spanning superparamagnetic (<50–80 nm) to multidomain (>10 μm) particles. Cylindrical cores drilled from the centers of individual pumice clasts display anisotropy of magnetic susceptibility with prolate fabrics, with the highest degree of anisotropy coinciding with the highest vesicularity. Fabrics within a pumice clast require particle alignment within a fluid, and are interpreted to result from the upward transport of magma driven by vesiculation, ensuing bubble growth, and shearing in the conduit. Titanomagnetite number density (TND) is calculated from titanomagnetite volume fraction, which is determined from ferromagnetic susceptibility. TND estimates for monospecific assemblages of 1,000 nm–10 nm cubes predict 10^12 to 10^20 m^−3 of solid material, respectively. TND estimates derived using a power law distribution of grain sizes predict 10^18 to 10^19 m^−3. These ranges agree well with TND determinations of 10^18 to 10^20 m^−3 made by McCartney et al. (2024), and are several orders of magnitude larger than the number density of bubbles in these materials. These observations are consistent with the hypothesis that titanomagnetite crystals already existed in extremely high number‐abundance at the time of magma ascent and bubble nucleation.
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Pumice clast analysis: Unveiling conduit dynamics in Utah’s Quaternary rhyolite volcanos.
Textural studies of pumice clasts have been used for decades to estimate eruption styles, particularly through bubble size distribution (BSD) analysis along with quantification of vesicle sizes, numbers, connectivity, and crystallization textures. In more recent years, studies have evolved to include finer-scale analysis by scanning electron microscopy (SEM) and large-scale digital elevation models (DEM). By integrating textural, geochemical, and SEM analysis of the vesicularity of pumice clasts, a comprehensive conduit model can be constructed, which can be further used to estimate volatile content and explosivity. In this contribution, we investigate pumice from two Quaternary deposits from the Black Rock Desert and Mineral Mountains located in west central Utah, a first-of-its kind study for these units. In hand sample, the 2.4 Ma crystal-poor Cudahy Mine pumice (SiO2 = 76.1 wt.%) appears characterized by small, uniform bubbles, whereas the <1 Ma Ranch Canyon pumice (SiO2 = 77.5%) contains ample K-feldspar phenocrysts and elongated fibrous vesicles. We employ X-ray tomographic and scanning electron microscopies to determine bubble sphericity and interconnectivity, crystal volumes, and density and vesicularity estimates. Our resulting data are used to establish a model for intra-conduit processes and eruption intensity of these two anorogenic, monogenetic rhyolite volcanoes. These imaging techniques provide a unique way to understand eruption dynamics for ancient volcanoes.
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- Award ID(s):
- 2242732
- PAR ID:
- 10523536
- Publisher / Repository:
- Geological Society of America Abstracts with Programs
- Date Published:
- Volume:
- 56
- Format(s):
- Medium: X
- Location:
- Springfield, Missouri
- Sponsoring Org:
- National Science Foundation
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