Whole-rock and mineral compositions of three eruptive deposits from the Twin Falls caldera, associated with the Yellowstone hotspot, provide a window into melt generation and evolution for hot, dry, A-type rhyolites. Three rhyolitic units were sampled via the Kimberly drill-core as a part of project HOTSPOT, a study focused on mantle plume and continental lithosphere interaction. Previous work has been done to collect high resolution U-Pb zircon ages, and Hf- and O-isotopic compositions. This study examined the geochemistry of apatite and zircon along with host rock compositions in the context of this previous work. The Kimberly core sampled the Shoshone Rhyolite (6.06 Ma, 120 m thick), Kimberly Member (7.70 Ma, 169 m thick), and Castleford Crossing Member (7.96 Ma, >1400 m thick). Apatite compositions more closely reflect the composition of their whole rock hosts than zircons. SiO2 content is higher in apatite of the Kimberly Member at (1.1 ± 0.75 wt.%), vs (0.72 ± 0.47 wt.%) for the Castleford Crossing and (0.84 ± 0.27 wt.%) for the Shoshone Rhyolite. REEs compensate for Si substitution in these apatites, with the Kimberly Member most enriched. Volatile contents in the apatites are typical of metaluminous A-type rhyolites, with very low Cl and high F concentrations. Average Ti-in-zircon crystallization temperatures were highest in the Castleford Crossing Member (847 ± 68°C), followed by the Shoshone Rhyolite (806 ± 78°C), and then the Kimberly Member (804 ± 70°C). Oxygen fugacity calculated from zircons has average ΔQFM values for the Shoshone (0.8), Kimberly (-0.2), and Castleford Crossing (0.2). Hf concentrations and Eu anomalies are comparable in zircons from all three units. REE patterns in zircons are also similar and concentrations of REEs in the Shoshone and Kimberly units are similar even though the whole rock compositions of all three units are distinct. Less than 15% of zircons in the Kimberly and Castleford Crossing rhyolites have CL-dark cores enriched in several REEs, U, and Th. These CL-dark cored zircons are likely xenocrysts entrained from chemically evolved granite and then overgrown with less enriched rims prior to eruption. There are several apatite grains with Si-LREE enriched rims in the Kimberly Member, which serves as further evidence of assimilation of silicic igneous rock by the Kimberly Member before eruption. Principal component analysis of the geochemical data distinguishes between the units using both whole-rock and apatite compositions. However, zircon compositions are not statistically distinguishable using PCA. A global comparison of Ti, U, Th, Yb, and Nb concentrations in zircons show that the zircons in the Central Snake River Plain are similar to zircons in Hawaiian basalts, while younger zircons from Yellowstone formed in cooler more differentiated magma. We propose that the zircon and apatite chemical patterns and trends confirm the A-type origin of Snake River Plain rhyolites and make it unlikely that they represent partial melts of felsic continental crust but are instead derived in large part from partial melts of young mafic crust--the midcrustal sill.

No Abstract / Thesis / Bachelor of Arts (BA)

<p dir="ltr">This research aims to examine the effect that mantle tractions have on surface deformation throughout the Pacific-North America plate boundary zone in Alaska, western Canada, and the Bering Sea region. We use 3-D geodynamic models to simulate the crust and upper mantle in order to investigate the tectonic force balance between plate boundary interactions, gravitational collapse, and basal tractions. We determine that mantle tractions with a magnitude of ~2.5-3.8 MPa, directed to the southeast, in conjunction with forces from the Yakutat flat slab, best fit the steady-state plate motion estimates in Alaska. We also show how these mantle tractions have likely aided in concentrating deformation to the northwest of incoming Yakutat oceanic plateau throughout the ~50 Ma evolution of flat slab subduction in this region. Finally, we conclude that mantle tractions also impact the broad zone of distributed deformation surrounding the Bering Sea. The confluence of basal forces and tectonic extrusion, due to the Yakutat flat slab, may lead to the evolution of a new plate boundary extending from northwest Alaska to the Kuril-Kamchatka subduction zone.</p>

Natural climate variability over the past hundreds of thousands of years provides a uniquewindow into the drivers and processes that connect different parts of our climate system. This thesis investigates interactions between Earth’s mantle, its oceans, and ice sheets over the Quaternary. The dominant process that connects these spheres is glacial isostatic adjustment (GIA), which is the deformation of Earth’s mantle (and consequently its surface, gravity field, and sea level) in response to changes in ice and ocean mass loading. This dissertation focuses on time periods during which surface temperatures were warming or warmer than today to understand how these warm intervals affected ice sheets, permafrost, and sea level. I put my results in the context of current and future warming to improve predictions of future change and compare natural to anthropogenic variability. The thesis opens with an investigation of relative (i.e., local) sea level around Norway overthe last 16 thousand years (ka). Postglacial Norwegian sea level, though dominated by postglacial rebound and associated sea-level fall, is punctuated by two periods of sea-level rise. The causes of these episodes, named the ‘Tapes’ and ‘Younger Dryas’ transgressions, remain debated despite more than a century of study. I produce the first standardized and quality-controlled compilation of Norwegian sea-level data, then employ an ensemble of empirical Bayesian hierarchical statis- tical models to estimate relative sea level along the Norwegian coastline. The resulting model enables an examination of the relative contributions of isostatic rebound and global mean sea-level (GMSL) rise to the Tapes transgression, and lays the foundation for future applications such as in- version of sea-level data for Fennoscandian ice-sheet volume and the comparison of modern rates of Norwegian sea-level rise to pre-industrial rates. Chapter Two aims to better understand sea-level and Antarctic ice-sheet variability during the Holocene, which is the last time global temperatures may have exceeded early industrial (1850 CE) values. Both the Greenland and Antarctic ice sheets likely retreated inland of their present- day extents during the Holocene, yet previous GMSL reconstructions suggest that Holocene GMSL never surpassed early industrial levels. I use relative sea-level observations, GIA predictions, and new estimates of postglacial thermosteric sea-level and mountain glacier evolution to show that the available evidence is consistent with GMSL that exceeded early industrial levels in the mid- Holocene (8-4 ka) and an Antarctic Ice Sheet that was smaller than present at some time in the last 6000 years. I also demonstrate that Antarctic ice retreat lags Antarctic temperature by 250 years, which highlights the vulnerability of the future Antarctic ice sheet to 20th and 21st century warming. Comparing our reconstruction to projections for the future indicates that GMSL rise in the next 125 years will very likely (?>0.9) be faster than at any time in the last 5000 years, and that by 2080 GMSL will more likely than not be the highest of any time in the past 115,000 years. In Chapter Three, I explore the effect of GIA on subsea permafrost. Subsea permafrost forms when sea-level rise submerges terrestrial permafrost. Subsea permafrost underlies ∼1.8 million km² of Arctic continental shelf, with thicknesses in places exceeding 700 m. Sea-level variations over glacial–interglacial cycles control subsea permafrost distribution and thickness, yet no permafrost model has accounted for GIA, which leads to deviations of local sea level from the global mean. I incorporate GIA into a pan-Arctic model of subsea permafrost over the last 400,000 years. Including GIA significantly reduces estimates of present-day subsea permafrost thickness, chiefly because of hydro-isostatic effects and deformation related to Northern Hemisphere ice sheets. Additionally, I extend the simulation 1000 years into the future for emissions scenarios outlined in the Intergovernmental Panel on Climate Change’s sixth assessment report. I find that subsea permafrost is preserved under a low-emissions scenario but mostly disappears under a high-emissions scenario. In the final chapter, I turn to the Last Interglacial (LIG, 129–116 ka), a time interval considered a partial analogue for future warming due to its elevated temperatures. Observations of oscillations in LIG local sea level, combined with an assumption that the Laurentide Ice Sheet collapsed prior to the LIG, have been used to infer Antarctic and Greenland ice-sheet melt histories as well as oscillations in LIG global mean sea level. However, evidence of a Laurentide Ice Sheet outburst flood at ∼125 ka suggests that Laurentide Ice Sheet remnants may have persisted longer into the LIG than typically thought. Here we explore the effect on LIG sea level of a Laurentide collapse that occurred during rather than prior to the LIG and a West Antarctic Ice Sheet that collapsed in the early LIG. We find that due to GIA, this asynchronous ice-sheet evolution produces a global pattern of sea-level oscillations that is similar to field observations. We demonstrate that the oscillation pattern can be produced by the combination of ongoing GIA from the penultimate deglaciation with the fingerprint of West Antarctic collapse. By showing that LIG Laurentide persistence would lead to an RSL oscillation that accords with field evidence, we highlight the need for LIG climate simulations to consider Laurentide ice-sheet dynamics and for more constraints on the LIG history of the Laurentide Ice Sheet.

This dissertation presents a numerical model constructed to investigate the dynamics and structures of the Earth’s mantle. Deformation of the Earth’s mantle, which is composed of solid silicate minerals, is strongly governed by the constitutive relation-ship among multiple length-scale structures and properties. To explain the realistic consti-tutive behavior of the silicate mantle, an Internal State Variable (ISV) theory that is an advanced and novel constitutive approach for history-dependent elastoviscoplasticity was applied. The ISV constitutive model was, in turn, implemented into a three-dimensional geodynamic code, TERRA3D, which uses the Finite Element method developed for the mantle convection problem. The sequential studies performed in this dissertation are presented in the follow-ing order: i) a comprehensive summary of the mantle material structures (compositions and microstructural features) and its mechanical properties (elasticity and rheology), ii) a development of a recrystallization and grain size dependent ISV constitutive model for the polycrystalline materials such as minerals and metals, which explains comprehensive mineral physics occurring under the conditions of pressure, temperature, and strain rate within the mantle and their history dependence, and iii) an application of the recrystalli-zation and grain size dependent ISV model to the Earth’s mantle convection problem us-ing the TERRA3D for an investigation of the grain size and dynamic recrystallization efect on the mantle dynamics. The applied ISV constitutive model within the TERRA3D Finite Element frame-work captures the subscale dynamics (dislocation density evolution, dynamic and static recrystallization, grain growth, and grain refinement) and their effect on the large-scale rheology and dynamics of the Earth’s mantle. The numerical investigations reveal that the potential for the mechanical instability and weakening within the mantle arises from the kinetics of grain size and recrystallization and their rheological effect. This mechanical instability leads to the mantle convection entering the episodic overturn regime. The TERRA3D-ISV mantle convection model herein also provides some insightful discover-ies regarding the dynamics and structures within the mantle, explaining its complex rhe-ology caused by the kinetics of recrystallization, grain size, hardening, dislocation recov-ery, and diffusion in the geological settings.

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