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Thesis advisor: Ethan Baxter / Serpentinites assume a critical role in geochemical and geophysical cycles, from recycling fluid into the sub-arc mantle to facilitating exhumation within subduction zones. Rodingite dikes can be used as a lens to investigate the hydration of the oceanic lithosphere as their development is synchronous with serpentinization, and while serpentinites lack sufficient mineral phases for geochronology, rodingite dikes are rich in andradite and grossular garnet which are potentially amenable for geochronology. This research seeks to constrain the timescales and duration of hydration of the oceanic lithosphere within the Alpine Tethys ocean basin, and associated serpentinization, by examining Apennines rodingites from the Internal Ligurides (Italy). These rodingites experienced seafloor hydrothermal alteration and were obducted onto the continental margin during Alpine orogenesis. As a result, they are ideal for studying seafloor metasomatism as they were not affected by prograde subduction zone metamorphism and dehydration. Sr isotopic and trace element profiles were constructed across two rodingite-serpentinite transects, revealing a complex, multi-stage hydration history consisting of 1) Widespread serpentinization, 2) Gabbroic intrusions, 3) Rodingitization, and 4) Localized, late-stage advective fluid flow. Serpentinizing fluids locally display strong continental crustal isotopic signatures, while rodingitization fluids are characterized by seawater-like values. U-Pb geochronology on rodingite garnets produced an age of 96.1 ± 8.9 Ma, which could represent either the main rodingitization phase or the late-stage advective alteration. / Thesis (MS) — Boston College, 2023. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

<p>  </p> <p>Reconstructing past glacier and ice-sheet extents is important to better understand how glacial systems have responded to past climate changes in hope of constraining predictions of their responses to ongoing anthropogenic climate warming. As such, the most recent period of climatic variations, from the Last Glacial Maximum (LGM, ca. 21 ka) through today, is of great interest as a prominent example of how ice has reacted to past climatic warming events. Surface exposure dating utilizing cosmogenic nuclides can directly constrain when past ice deglaciated in current and former glacial landscapes. Numerous studies have utilized long-lived cosmogenic radionuclides (i.e., 10Be, 26Al) in polar regions to reconstruct glacial systems. However, due to prevalent non-erosive cold-based ice, prior nuclides from pre-LGM can be preserved. </p> <p>The research described in this dissertation applies <em>in situ </em>cosmogenic 14C (<em>in situ</em> 14C), an emerging geochronometer, to polar glacial landscapes in Sweden and Antarctica to constrain the timing and rate of glacial ice retreat. <em>In situ </em>14C more closely reflects the post-LGM deglacial signal in polar regions because it is less likely to preserve prior nuclides (inheritance) under minimally erosive ice. Our cosmogenic 10Be–26Al–14C concentrations near the Riukojietna ice cap, the last remaining ice cap in Sweden, combined with a sedimentary record from a proximal proglacial lake, indicate the ice cap likely survived during a warm period in the Holocene, but was less extensive than today. The <em>in situ</em> 14C exposure data from nunataks in western Dronning Maud Land (DML), East Antarctica indicate significant coastal thickening (up to 850 m) not predicted by models to date. In addition, this work dates the timing of post-LGM ice surface lowering in two drainage basins in western DML. These results demonstrate the significant contribution of <em>in situ</em> 14C in polar regions.</p> <p>In addition to applications of <em>in situ</em> 14C in polar regions, this work also describes the development of a compositionally dependent <em>in situ</em> 14C production rate calculator. The ability to extract <em>in situ</em> 14C from samples which quartz cannot be separated (either quartz-poor or fine-grained) would allow new avenues of research. The computational framework will be a useful tool in efforts to broaden the utility of <em>in situ</em> 14C to quartz-poor and fine-grained rock types. </p>

<p> A detailed petrographic, geochemical and strontium isotope study of the rocks representing the variations observed across the Coe Hill granite, Grenville Province, southeastern Ontario provides the basis for the determination of the age and possible origin of this pluton. This study also provides insight into the relationships between similar granitoid plutons in the immediate vicinity.</p> <p> The Coe Hill granite is a medium to coarse grained hypidiomorphic to allotriomorphic, leucocratic quartz monzonite with subordinate isolated inclusions of dioritic and gabbroic gneiss. Variations upon this otherwise homogeneous granitoid occur in discordant aplite dykes, assimilating mafic xenoliths and along brecciated contact boundaries.</p> <p> Generally the rocks of this pluton are more basic than the average for similar granitoids in the area (12.4% vs. 6.0% mafics) as observed in both thin section and major element oxide diagrams. Besides this trend major element diagrams have uniform distributions. A Rb-Sr isochron was determined for the rocks of the pluton proper which gave an age representative of the emplacement of these rocks (t = 1063 ± 21 M.a.; Ri = 0.7040 ± 5). Trace element Rb shows a value similar to the norm for granitoids yet is enriched in comparison to values acquired from similar granitoids in the area (144 ppm vs. 63 ppm).</p> <p> Through a synthesis of the available data, and that which was acquired from this study on the Coe Hill granite, a comparison with the available data on the Loon Lake quartz monzonite can be made. This comparison illustrates a great deal of strikingly similar trends which have been taken to represent a lower crustal, or upper mantle origin, cogenetic relationship between these two granitoids.</p> / Thesis / Bachelor of Science (BSc)

<p> The Thelon Tectonic Zone is a major structural, metamorphic and lithological feature of the Precambrian Shield in the Northwest Territories. Situated within this Zone is a 30 square kilometre plutonic body. Rubidium- Strontium whole rock geochronology yields an age range of 1650 - 2200 Ma for the emplacement of this pluton. Field and petrographic evidence indicates that the pluton has suffered post-emplacement deformation, representative of a late Proterozoic metamorphic event. Major and trace element geochemistry suggests that this high Ca, ferrogranodiorite-tonalite intrusive body is associated with a subduction-related, continental margin tectonic environment. </p> / Thesis / Bachelor of Science (BSc)

<p> The northwestern Melville Peninsula in the area of 69°34'N/84°50'W consists of Precambrian Shield rocks that have suffered multiple metamorphic events, including a high grade metamorphic event in the late Archean, ranging from upper amphibolite to granulite grade locally. The oldest unit present is a tonalite-granodiorite-granite suite showing both foliation and lineation and a somewhat migmatic texture. This biotite-hornblende orthogneiss has produced a Rb-Sr whole rock age date of 2.55 +/- 0.2 Ga. Field relationships indicate that this is a metamorphically induced age rather than an emplacement age. A coarse grained granitic pegmatite associated with the orthogneiss has produced a Rb-Sr isochron indicating an age of 1.83 +/- 0.06 Ga. Petrographic and field evidence indicate that this represents the emplacement age of the pegmatite. </p> / Thesis / Bachelor of Science (BSc)

<p> The Hawke River Terrane of the Grenville Province in eastern Labrador is dominated by a coronitic gabbronorite to anorthosite body plus associated amphibolitic to granodioritic bodies known as the White Bear Arm Complex (WBAC). Petrographic and geochemical evidence supports a constant volume sub-solidus diffusion origin for double coronas of orthopyroxene and amphibole around olivine. Geochemical evidence suggests that corona formation was not completely isochemical, requiring an influx of potassium, iron and magnesium, but on a whole-rock scale was probably closed to the latter two elements. Nd and Sr isotopic evidence indicates that the coronas did not form as a response to a much later (eg. Grenville aged) metamorphic event, and probably. developed during post-crystallisational cooling or during a metamorphism shortly thereafter. </p> <p> The WBAC is cut by a monzonitic to granodioritic body mineralogically and chemically similar to the WBAC 'monzonite', known as the Paradise Arm Pluton (PAP). Petrographic, geochemical, and field relationship evidence indicates an igneous origin for the PAP and the WBAC monzonite. Both of these units have been subjected to low grade (greenschist facies) metamorphism. The WBAC and the PAP are hosted by the Paradise Metasedimentary Gneiss Belt, an aluminous paragneiss which has been raised to middle to upper amphibolite grade metamorphism, except immediately adjacent to the contact with the WBAC, where granulite grade has been obtained, forming high-temperature aluminosilicates such as osumilite and sapphirine. The contact zone has also been contaminated by fluids from the gabbronorites, reflected by petrography, elemental and isotopic geochemistry. </p> <p> Field relationships and geochemistry are ambiguous regarding whether the monzonitic units were derived by fractional crystallisation of the gabbronorite or partial melting of the paragneisses, but trace element variation favours the latter model. The PAP is a product of physical, geochemical and Sr isotopic mixing between a granodioritic end-member and a paragneissic one. </p> <p> Sm-Nd isotope systematics on whole-rocks do not provide a precise emplacement age for the WBAC gabbronorite, but indicate a Labradorian age (ca. 1.65 Ga old). Rb-Sr wholerock systematics show a Labradorian-aged trend and a steeply sloping trend interpreted as a mixing line with an unspecified crustal component, possibly corresponding to the potassium metasomatism indicated earlier. Rb-Sr on minerals from two rocks produces a precise age of 1675 ± 15 Ma and a similar, imprecise age. </p> <p> The WBAC monzonite and the Paradise Arm Pluton give Rb-Sr ages of 1621 ± 11 Ma CMSWD=24) and 1573 % 18 Ma (MSWD=9) respectively, with low initial 87Sr/ 86Sr ratios around 0.7040. These are interpreted as a minimum emplacement age and a mixing line, respectively. The PMGB paragneisses give a Rb-Sr age of 1630 ± 16 Ma CMSWD=35), with an initial 87Sr/ 86Sr composition around 0.704. All ages are equal within expanded 2σ error of each other (ie. MSWD=1), and give Labradorian ages. There is no evidence of Grenville-aged (ca. 1.0 Ga old) metamorphism. </p> <p> Rb-Sr isotopic and E Nd -E Br variations suggest crustal contamination of the gabbronorites, although the contaminant cannot be identified. Two separate mechanisms are required to generate the observed trends: a fluid metasomatism and a bulk-rock crustal contamination, or else contamination by currently unidentified low Sr crustal material. The WBAC gabbronorites provide evidence for 'extra'-LREE-depleted mantle beneath eastern Labrador at 1.65 Ga, with E Nd of at least +7.6. </p> / Thesis / Master of Science (MSc)

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