<p> Lead isotope ratios have been determined by a VG.354 thermal ionization mass spectrometer on Sudbury Igneous Complex sulphide ores. The isotopic ratios are contrasted with the lead isotope profile of selected country rocks in the vicinity of the complex. South Range data form a linear array whose slope indicates an age of approximately 1.85 Ga, the published age of the igneous complex. They also indicate North Range data a magmatic origin for lead in the ore. North Range data describe a parallel isochron, with lower 206Pb/204Pb and 207Pb/204Pb ratios. The South and North Range data are similar to the lead-isotope composition of country rocks close to them, and indicate rather different crustal-source rocks for sulphide ores in these regions of the complex. This fact suggests that the Sudbury Igneous Complex was generated by a meteorite impact which occurred at the edge of the Huronian succession overlying the Southeast edge of the Superior Province. </p> / Thesis / Master of Science (MSc)

Due to Earth’s efficient crustal recycling through plate tectonics, the remaining physical record of Earth’s first two billion years consists of mineral fragments and heavily metamorphosed rocks in isolated Archean cratons. Characterization of Earth’s earliest tectonic processes requires investigation of all available records; the mineral garnet has been largely overlooked. The major element chemistry and samarium-neodymium (Sm-Nd) isotope ratios preserved in fragmented detrital garnet and Archean metamorphic garnet record the timing and conditions of early tectonic events. This work presents detailed methodology for a new detrital garnet geochronometer unlocking age information from previously undateable detrital garnet surviving recycling in sediment, sedimentary rocks, and metasedimentary rocks. The new method’s utility is demonstrated by dating garnet from a Scottish sedimentary rock and nineteen individual garnet grains from a tributary to the French Broad River in the southern Appalachians. In the southern Appalachians, garnet and existing monazite ages overlap (though the mean garnet age is slightly younger) to record the most recent metamorphic event and both are younger than inherited zircon ages. Proof-of-concept testing demonstrating protocol development for blank-correction and routine analysis of samples smaller than 1 ng advances small Sm-Nd analysis. Additionally, this work applies existing Sm-Nd garnet geochronology methods to search for garnet older than 2.5 Ga and provide age constraints on the complicated metamorphic histories of two Archean cratons. A search for detrital garnet in a sample from the Jack Hills metasedimentary belt of Western Australia hosting the Earth’s oldest known terrestrial materials (ca. 4.3 Ga) failed to produce garnet. Instead, two samples collected ~4 km south of the Jack Hills belt in the Narryer Terrane were dated to confirm Narryer regional metamorphism at ca. 2.6 Ga. The Acasta gneiss of northern Canada, arguably Earth’s oldest known cohesive rock outcrop (ca. 4.0 Ga), produced one of the Earth’s oldest known garnet ages. Garnet ages of ca. 2.95 Ga constrain the timing of Archean metamorphism and the data also indicate potential for preservation of even older garnet. Finally, a compilation of published garnet ages in the literature is presented to summarize the community’s progress in the search for Earth’s oldest garnet. / 2017-12-06T00:00:00Z

<p dir="ltr">To better predict volcanic behavior, we must understand the processes that occur in the underlying magma reservoirs. This thesis contains three chapters that work together to better understand processes that occur in sub-volcanic reservoirs. Chapter 2 is a study of an ancient, coupled volcanic-plutonic system to determine the link between the volcanic and plutonic parts of the system. The IXL-Job Canyon magmatic system is an ~28-29 Ma system, which shows a rapid transition between eruption of tuffs and lava flows to construction of an upper-crustal pluton, via incremental emplacement. The system experienced an eruptive hiatus during and after pluton construction, until the eruption of a newly identified, younger, rhyolitic tuff. This work suggests that the absence of volcanic activity at the surface does not mean that the underlying magmatic plumbing system is also inactive. Chapter 3 compiles existing U-Pb zircon ID-TIMS data for upper-crustal, silicic magmatic systems, to determine the size and frequency of magmatic increments that accumulate to build up these systems. A Monte Carlo-based model is used to investigate the underlying distributions of the increment size and time between increments, and results in sizes and inter-event times that follow an exponential distribution. This work helps guide how we can try to introduce broadly generalizable complexities into thermal models of such systems. Chapter 4 focuses on organic interferences, a common issue that impacts the speed and quality of U-Pb and Pb-Pb data collected on TIMS instruments. We share two techniques used at the Purdue Radiogenic Isotope Geology Lab to first reduce and then avoid any residual organic interferences. These techniques help shorten analytical times, increasing throughput, and provide a means to reduce uncertainties on our measurements, since the presence of organic interferences can bias and increase the uncertainties on U-Pb dates.</p>

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