Shelby Isom - Academia.edu (original) (raw)

Papers by Shelby Isom

Structural and petrologic insights into the emplacement of effusive silicic lavas: Inyo Domes, Ca... more Structural and petrologic insights into the emplacement of effusive silicic lavas: Inyo Domes, California Shelby Lee Isom Dissertation submitted to the Eberly College of Arts and Sciences at West Virginia University in partial fulfillment of the requirements for the degree of

Multiple generations of ductile and brittle deformation recorded in the Obsidian Dome lava, Calif... more Multiple generations of ductile and brittle deformation recorded in the Obsidian Dome lava, California, inform on the rheological complexities of silicic lava emplacement.

Geological Society of America Abstracts with Programs, 2019

Earth and Planetary Science Letters, 2021

Folds on the surfaces of mafic lavas are among the most readily recognized geological structures ... more Folds on the surfaces of mafic lavas are among the most readily recognized geological structures and are used as first-order criteria for identifying ancient lavas on Earth and other planetary bodies. However, the presence of surface-folds on the surface of silicic lavas is contested in this study and we challenge the widely accepted interpretation that silicic lava surfaces contain folds using examples from the western United States and Sardinia, Italy. We interpret the ridges and troughs on their upper surfaces, typically referred to as 'ogives' or 'pressure ridges', as fracture-bound structures rather than folds. We report on the absence of large-scale, buckle-style folds and note instead the ubiquitous presence of multiple generations and scales of tensile fractures comparable to crevasses in glaciers and formed in ways similar to already recognized crease structures. We report viscosity data and results of stress analyses that preclude folding (ductile deformation in compression) of the upper surface of silicic lavas at timescales of emplacement (weeks to months). Therefore, analysis of fold geometry (wavelength, amplitude, etc.) is erroneous, and instead the signal produced reflects the strength and thickness of the brittle upper surface stretching over a ductile interior. The presence of ogives on the surfaces of lavas on other planetary bodies may help to elucidate their rheological properties and crustal thicknesses, but relating to their tensile strength, not viscosity.

Large-volume silicic ignimbrites erupt from reservoirs that vary in composition, temperature, vol... more Large-volume silicic ignimbrites erupt from reservoirs that vary in composition, temperature, volatile content and crystallinity. The 9.7 Ma Devine Canyon Tuff (DCT) of eastern Oregon is a large-volume (>250 km 3), compositionally zoned and variably welded ignimbrite. The ignimbrite exhibits heterogeneous trace element compositions, variable volatile content and crystallinity. These observations were utilized in the investigation into the generation, accumulation and evolution of the magmas composing the DCT. Building off previous research, pumices were selected from the range of trace element compositions and analyzed with respect to crystallinity, mineral abundances and assemblages. The DCT displays a gradational trace element enrichment and decrease in crystallinity from least evolved, dacite, at ~22% crystals to most evolved high-silica rhyolite at 3% crystals. Two distinct mineral populations of feldspar and clinopyroxene were identified in previous work, one belonging to the rhyolitic magma and the other to the dacitic magma. Volatile content derived from melt inclusion Fourier Transform Infrared (FTIR) spectrometer analysis revealed an increase in water content from 1.2 to 3.7 wt.% in the most evolved rhyolite. The DCT exhibits low and variable δ 18 O signatures, 4.52‰ to 5.76‰, based on δ 18 O values measured on quartz and sanidine. Low δ 18 O signatures of all DCT rhyolites suggest the incorporation of hydrothermally altered crust into the melt. Furthermore, quartz phenocrysts from all high-silica rhyolite groups display dark oscillatory zoned cores and Ti-rich bright rims. These data provide insight into how these magmas were generated and subsequently stored in the crust. Commonalities of petrographic and compositional ii features among rhyolites, especially the zoning characteristics of quartz phenocrysts, exclude the possibility of storage and evolution in multiple reservoirs. Envisioning a scenario where all magmas are stored within a single reservoir prior to eruption and assuming rhyolites A and D are the product of partial melting. The mixing of A and D rhyolites produced rhyolite B, and subsequent mixing of intermediate rhyolite B and endmember rhyolite D generated rhyolite C. However, some trace element inconsistencies, between mixing model and observed intermediate rhyolites suggest a secondary process. Post mixing, rhyolites B and C require some modification by fractional crystallization to account for LREE and other inconsistencies between mixed models and observed rhyolites. Finally, the origin of the dacite is likely through mixing of group D rhyolite and an intrusive fractionated basalt, which could have led to the eruption of the Devine Canyon Tuff.

Structural and petrologic insights into the emplacement of effusive silicic lavas: Inyo Domes, Ca... more Structural and petrologic insights into the emplacement of effusive silicic lavas: Inyo Domes, California Shelby Lee Isom Dissertation submitted to the Eberly College of Arts and Sciences at West Virginia University in partial fulfillment of the requirements for the degree of

Multiple generations of ductile and brittle deformation recorded in the Obsidian Dome lava, Calif... more Multiple generations of ductile and brittle deformation recorded in the Obsidian Dome lava, California, inform on the rheological complexities of silicic lava emplacement.

Geological Society of America Abstracts with Programs, 2019

Earth and Planetary Science Letters, 2021

Folds on the surfaces of mafic lavas are among the most readily recognized geological structures ... more Folds on the surfaces of mafic lavas are among the most readily recognized geological structures and are used as first-order criteria for identifying ancient lavas on Earth and other planetary bodies. However, the presence of surface-folds on the surface of silicic lavas is contested in this study and we challenge the widely accepted interpretation that silicic lava surfaces contain folds using examples from the western United States and Sardinia, Italy. We interpret the ridges and troughs on their upper surfaces, typically referred to as 'ogives' or 'pressure ridges', as fracture-bound structures rather than folds. We report on the absence of large-scale, buckle-style folds and note instead the ubiquitous presence of multiple generations and scales of tensile fractures comparable to crevasses in glaciers and formed in ways similar to already recognized crease structures. We report viscosity data and results of stress analyses that preclude folding (ductile deformation in compression) of the upper surface of silicic lavas at timescales of emplacement (weeks to months). Therefore, analysis of fold geometry (wavelength, amplitude, etc.) is erroneous, and instead the signal produced reflects the strength and thickness of the brittle upper surface stretching over a ductile interior. The presence of ogives on the surfaces of lavas on other planetary bodies may help to elucidate their rheological properties and crustal thicknesses, but relating to their tensile strength, not viscosity.

Large-volume silicic ignimbrites erupt from reservoirs that vary in composition, temperature, vol... more Large-volume silicic ignimbrites erupt from reservoirs that vary in composition, temperature, volatile content and crystallinity. The 9.7 Ma Devine Canyon Tuff (DCT) of eastern Oregon is a large-volume (>250 km 3), compositionally zoned and variably welded ignimbrite. The ignimbrite exhibits heterogeneous trace element compositions, variable volatile content and crystallinity. These observations were utilized in the investigation into the generation, accumulation and evolution of the magmas composing the DCT. Building off previous research, pumices were selected from the range of trace element compositions and analyzed with respect to crystallinity, mineral abundances and assemblages. The DCT displays a gradational trace element enrichment and decrease in crystallinity from least evolved, dacite, at ~22% crystals to most evolved high-silica rhyolite at 3% crystals. Two distinct mineral populations of feldspar and clinopyroxene were identified in previous work, one belonging to the rhyolitic magma and the other to the dacitic magma. Volatile content derived from melt inclusion Fourier Transform Infrared (FTIR) spectrometer analysis revealed an increase in water content from 1.2 to 3.7 wt.% in the most evolved rhyolite. The DCT exhibits low and variable δ 18 O signatures, 4.52‰ to 5.76‰, based on δ 18 O values measured on quartz and sanidine. Low δ 18 O signatures of all DCT rhyolites suggest the incorporation of hydrothermally altered crust into the melt. Furthermore, quartz phenocrysts from all high-silica rhyolite groups display dark oscillatory zoned cores and Ti-rich bright rims. These data provide insight into how these magmas were generated and subsequently stored in the crust. Commonalities of petrographic and compositional ii features among rhyolites, especially the zoning characteristics of quartz phenocrysts, exclude the possibility of storage and evolution in multiple reservoirs. Envisioning a scenario where all magmas are stored within a single reservoir prior to eruption and assuming rhyolites A and D are the product of partial melting. The mixing of A and D rhyolites produced rhyolite B, and subsequent mixing of intermediate rhyolite B and endmember rhyolite D generated rhyolite C. However, some trace element inconsistencies, between mixing model and observed intermediate rhyolites suggest a secondary process. Post mixing, rhyolites B and C require some modification by fractional crystallization to account for LREE and other inconsistencies between mixed models and observed rhyolites. Finally, the origin of the dacite is likely through mixing of group D rhyolite and an intrusive fractionated basalt, which could have led to the eruption of the Devine Canyon Tuff.