This dissertation investigates the spatial, temporal, geochemical, and petrologic development and evolution of mid-Miocene volcanic systems in the southeastern Oregon Plateau region of Oregon and Nevada. This integrated field and laboratory investigation conclusively demonstrates that flood basalt volcanism occurred on the Oregon Plateau over at least a 2 m.y. duration, and provides the first comprehensive view into the development of a mid-Miocene Oregon Plateau volcanic field and its relationship with regional flood basalt volcanism. The first portion of this study focuses on the geochemical and chronostratigraphic characteristics of flood basalt lava flows in the vicinity of Steens Mountain, Oregon. New 40Ar/39Ar ages and recalculated literature ages from the Steens Basalt type section illustrate that multiple magmatic centers were present locally, and that Oregon Plateau flood basalt activity was coeval with the main phase of Columbia River Basalt Group volcanism. The remainder of this study focuses on the Santa Rosa-Calico volcanic field (SC) of northern Nevada in order to better define and understand the link between mid-Miocene Oregon Plateau mafic and silicic volcanism. In the SC, mafic through silicic eruptive loci and shallow intrusive bodies are exposed along broadly north-south trending alignments, coincident with regional lithospheric structures. At least sixteen physically and compositionally distinct units are exposed in the SC representing approximately 2 m.y. of magma production. Local mafic volcanism was dominated by the eruption of Steens Basalt magmas. SC silicic magmas were produced by basaltic magma induced crustal melting of granitoid upper crust and erupted from diverse vent types and locations. At least four distinct intermediate (andesite-dacite) magmatic systems also are documented. Physical, chemical, and isotopic data indicate that open-system petrogenetic processes played a substantial role in the generation of these magmas and al (open full item for complete abstract)

Sediments recovered between ~354 and 765 mbsf (~15.9-18.4 Ma) in the ANDRILL AND-2A core contain dispersed accumulations of volcanic glass up to 50% by volume and are used to investigate the petrological evolution and influence of glaciations on volcanism in the McMurdo Sound region of Antarctica. Glass-rich sediments include muddy-to-fine sandstone and stratified diamictite. The glass varies in color, size, vesicularity, crystal content, angularity, and from fresh to moderately altered. Fresh glass with delicate cuspate forms suggests they were introduced into the basin as ash fall with minimal reworking. Altered glass has low total oxides (< 97 wt.%), low Ca/K ratios (< 2), high Alteration Index (> 40), and are typically more evolved than fresh glass. Pristine basaltic glasses (MgO 3-7 wt.%) are ne-normative (5-30 wt.%) and have restricted average SiO2 content (45.2 ± 0.8 wt.%). Overall the glass composition shows an increase in SiO2 content up-section. Fractional crystallization models indicate differentiation controlled by plagioclase, olivine, clinopyroxene and magnetite ± amphibole and apatite. Trace element concentrations are typical for Erebus Volcanic Province (EVP) volcanism. The data extends known composition of Mount Morning (18.7-11.4 Ma), the only known EVP Early-Middle Miocene source, to a mafic end, revealing a previously unknown phase of explosive, strongly alkaline, basaltic activity. The glass-rich sediments are part of larger sequences that record numerous glacial advances and retreats in the region. Sediments with high glass contents correspond to ice minimums and, geochemically, Ba/LREE ratios correlate to intervals of ice expansion (decreasing values) and contraction (increasing values) at multiple depths. Higher Ba/LREE may indicate tapping of more volatile-rich magmas. Within a single glacimarine cycle, glass angularity, vesicularity and composition also vary systematically. A model is supported where ice loading and unloading affects the st (open full item for complete abstract)

Basaltic magmatism is a fundamental process through which rocky objects across the Solar System differentiate and evolve. Basaltic partial melts generated within planetary mantles provide a record of the geologic evolution of mantle source regions, magma storage and ascent dynamics, and the processes through which primary and secondary crusts are established. In terrestrial basaltic systems, comprehensive macro- and microscale investigations that integrate the physical (e.g. crystal size and shape) and chemical (e.g. elemental stratigraphy) properties of crystals have often led to the identification of multiple, petrogenetically distinct crystal populations throughout one mineral phase. This has led to a so-called “paradigm shift” in the field of terrestrial igneous petrology: magmatic systems are “open” in nature. However, the extent to which open system processes exist on other rocky, differentiated planetary objects remains largely unconstrained. Thus, this work targets a suite of basaltic samples collected during the lunar Apollo missions, representing the only direct sampling of basaltic materials from spatially constrained locations on another inner Solar System object. Through a detailed textural, mineralogical, and geochemical investigation this work provides new insights into the emplacement of lava on the lunar surface and new constraints on the evolution of magmatic systems within (and on) the lunar crust. First, X-ray computed tomography (XCT) datasets from a lithologically diverse suite of Apollo basaltic rock chips captured and quantified lunar basalt petrofabrics in 3D. From study of mineral (e.g., ilmenite), and vesicle distributions, resulting textures were found to be broadly consistent with terrestrial pahoehoe lava flow stratigraphy. Second, petrographic study and in-situ geochemical analysis of major and minor silicate phases in thin sections of the same samples indicated the presence of distinct crystal populations within multiple phases. This (open full item for complete abstract)

The Trans-Mexican Volcanic Belt is host to several active volcanic fields, within which are clusters of monogenetic volcanoes that erupt in close temporal and spatial proximity to one another. These fields provide a unique opportunity to investigate the petrologic and geochemical evolution associated with monogenetic magmatism at a single location over a well-constrained timeframe. The Holocene Tacambaro cluster, located in the Michoacan-Guanajuato volcanic field in central Mexico, comprises four monogenetic volcanoes. These centers erupted basaltic andesite to andesite over ~3,300 years, starting with an explosive eruption and evolving to effusive eruptions, with increasing SiO2 and decreasing crystal contents. We have performed petrography, elemental and isotopic analyses, and geochemical modeling of Tacambaro cluster samples to assess the potential roles of fractional crystallization, crustal assimilation, and mantle source enrichment contributing to their petrogenesis. Major elements are generally consistent with variable degrees of fractional crystallization, but trace elements and isotopes are indicative of mantle source enrichment. The temporal-compositional variations are interpreted to derive from multiple melts of a heterogeneous, slab fluid-enriched mantle source, which undergo variable crustal storage and extents of fractional crystallization.

Magmatism at Earth's subduction zones produce vast tracts of plutons that coalesce to form batholiths and other large intrusive complexes. This activity also results in the production of volcanic rocks, including the explosive production of airborne tephra that can be distributed across hundreds or even thousands of square kilometers. Erupted volcanic ash is an instantaneously generated index of source magma geochemistry and geochronology, allowing for detailed reconstructions of the processes that influence magma generation, evolution, and emplacement, and the subduction tectonics driving the magmatic system. Tephra is often preserved distally from the volcanic sources in the stratigraphic record of marine basins, providing a near-complete geochronological record of magmatic activity. Bentonites form from the devitrification of accumulations of volcanic ash. In submarine conditions, volcanic glass readily alters into smectitic clay, resulting in the repurposing of major elements into the crystalline clay structure, and the mobility of large ion lithophile elements, which are easily lost due to their high solubility. Nevertheless, many elemental signatures are conserved through the devitrification process, including many of the high field strength elements such as the actinides, lanthanides, high charge metals, and isotopic ratios. The extensive tephragenic record of Cretaceous volcanism preserved from Wyoming to South Dakota provides an opportunity to test the hypothesis that bentonite whole-rock geochemistry, particularly trace elements and Sr and Nd isotopes, preserves a record of arc magmatism. In addition, the Sr and Nd isotopic signatures of tephra can be used to test correlations of ash deposits with source intrusive rocks. The long record of these Cretaceous ash beds — nearly 40 million years — also provide a unique opportunity to see how ash bed chemistry changes through time, and to test models of regional tectonics related to changes in the subduction (open full item for complete abstract)

A close relationship between lithospheric extension and mantle plumes is commonly assumed as the driving force for evolving divergent plate boundaries that manifest as a rift in either oceanic or continental settings. While lithospheric extension is robustly supported by geophysical and geochemical evidence, the role of mantle plumes is often less clear. In fact, in many cases, ongoing integrated geochemical and geophysical work suggest that heterogenous asthenospheric and sub-continental lithospheric mantle components or complex interactions between those two reservoirs (i.e., variable asthenospheric erosion or delamination of lithospheric mantle, especially the SCLM associated with continental rifts) may more robustly account for geological, geochemical, and geophysical observations at many of the prominent rifts, including each of the three rifts explored as part of this thesis. Herein, we integrate trace element and gas geochemistry from three prominent rift zones, including: 1) the Equatorial Mid-Atlantic Ridge (MAR) (5°N to 7°S), 2) the West Antarctic Rift System (WARS), and 3) the East African Rift System (EARS), to evaluate magmatic sources and processes that influence their evolution. Basalts from the Equatorial MAR provide an ideal setting to study the compositional variations in the upper mantle related to mixing and investigate interactions between mantle plumes and ridges without the risks of crustal contamination associated with working in continental settings. MORBs from the northern Equatorial MAR (5°N to 3°S) display trace element, radiogenic isotope, and helium isotopic characteristics indicative of mixing between the depleted mantle (DM) and a HIMU plume while the southern Equatorial MAR (3°S to 7°S) exhibit extremely depleted MORB compositions. The DM basalts have significantly lower (factor of 2) volatile (H2O and Cl)/Pb ratios in samples with the extremely non-radiogenic Pb isotope ratios, but vary by less than ~50% when volatile concentratio (open full item for complete abstract)

Mt. Early and Sheridan Bluff are two Miocene volcanoes located near 87°S in the southern Transantarctic Mountains on the East Antarctic craton, ~1000 km away from any other exposed Cenozoic volcano associated with the West Antarctic Rift System (WARS). The basalts are unique in comparison with WARS, in that subalkaline (tholeiite) to alkaline (hawaiite and mugearite) magmas were nearly simultaneously generated during the Early Miocene (20 to 19 Ma). Close examination of isotopic measurements provide insight into mantle sources and cause of melting beneath the East Antarctic craton. Enriched isotopic signatures such as mildly elevated δ18O in olivine (average =5.55 ± 0.19 ‰) and whole rock 87Sr/86Sr ratios (0.7038-0.7049) indicate contamination by crustal material. However, the correlations between isotopes and major and trace elements (e.g. SiO2, MgO, Sr, and Zr) are either not seen or opposite of what would be expected for models for combined assimilation and fractional crystallization. Alternatively, the higher Sr and O and lower Nd and Pb isotopic values of Mt. Early and Sheridan Bluff samples compared to WARS basalts likely reflect crustal material within their mantle source (e.g., an EMII-like component). The isotopic variations shown by alkaline and tholeiitic basalts could be explained by different degrees of partial melting of a heterogeneous mantle containing crustal materials. Possible mechanisms to add crust is lithospheric delamination which occurred beneath this portion of the Transantarctic Mountains or recycled by past subduction zone processes.

In Antarctica, where much of the continent is covered by ice, the use of remotely sensed geophysical data is a valuable tool for reconstructing geologic history. Data from the submarine continental shelf are fundamental for determining the structural deformation and geomorphological history of the Antarctic plate. The western Ross Sea contains a segment of the West Antarctic Rift System known as the Terror Rift. The rift lies entirely below sea level and stretches between the two volcanic provinces of Mount Melbourne and Mount Erebus. High-resolution seismic and bathymetry data from the western Ross Sea are used to analyze the structure, kinematics, and deformation history of the Terror Rift. Recent glacial history of the western Ross Sea has also been identified. A revised fault and associated volcanic edifice map of the Terror Rift in the western Ross Sea is provided. The eastern limit of faulting associated with the Terror Rift is redefined by this study. The first balanced cross sections and extension values have been calculated for the Terror Rift. Outcomes of this study provide definite constraint on the magnitude of extension since the onset of rifting in the late Neogene and define the mode of rifting.