James Scoates

Ocean island basalts are indirect records of Earth’s mantle chemistry and are fundamentalfor constraining its evolution through geological time. Volcanoes of the ~6000 km-longHawaiian-Emperor chain represent an extraordinary record of Pacific mantle chemistry over thepast ~80 Ma, formed as the Pacific tectonic plate passed over the deeply sourced Hawaiianmantle plume. Hawaiian volcanoes are divided into southwestern (Loa) and northeastern (Kea)geographic and geochemical trends that reflect the chemical structure of the underlying plumeand its deep mantle sources. This dissertation addresses a change in the chemistry of eruptedlavas between the Northwest Hawaiian Ridge and the younger Hawaiian volcanoes using a newdatabase from the northernmost island of Kaua‘i that includes high-precision radiogenic isotopes(Pb, Sr, Nd, Hf), major element oxides, trace element concentrations, and ⁴⁰Ar/³⁹Argeochronology. Western and eastern Kaua‘i erupted at different times and from distinct mantlesources. The distribution of Loa and Kea compositions on Kaua‘i is opposite to the youngerHawaiian Islands and is explained by a more westward trend of plate motion relative to the Loa-Kea compositional boundary in the plume. Loa compositions dominated the geochemistry oferupted lavas for over 2 Ma before the onset of the bilateral Loa and Kea geochemical trends.Loa compositions are isotopically enriched and may be influenced by subducted materialsrecycled in the deep mantle source of the Hawaiian plume that were stored within the thermallyand chemically distinct Pacific large low shear wave velocity province (LLSVP). Thalliumisotopes were used to test whether these materials originally consisted of pelagic sediment andwere measured in 33 samples from the entire geochemical range of Hawaiian lavas. Resultsshow evidence of recycled materials on the Kea side of the plume, implying that the lowermantle is heterogeneous both within and outside of the LLSVP. This dissertation providesimportant insight into the timing and source of two major geochemical components along themost active hotspot system on Earth and expands our knowledge on the chemical structure of theHawaiian plume, its evolution in space and time, and the nature of its deep mantle sources.

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The formation of cratonic lithosphere and its participation in continental collision are first-order processes in global tectonics. The Western Gneiss Complex (WGC) of southwestern Norway is a fragment of continental crust that uniquely preserves a complete record of its burial and exhumation during collisional orogeny along with rare fragments of sub-continental lithospheric mantle that were entrained into the terrane during its residence in the mantle. Despite the importance of the WGC for characterising processes operating in the deep crust and mantle during continent-continent collision, its rate and style of burial and exhumation have not been comprehensively studied and the protracted evolution of the included peridotite bodies remains unclear. Lu-Hf garnet and micro-analytical U-Pb rutile geochronology are two powerful tools for lithosphere and tectonics research as they can be used to link ages to conditions of equilibration of rock-forming assemblages. Using these techniques applied to eclogites in the WGC, I constrained the burial rate for continental crust during collisional orogeny to ~5 mm yr-¹, developed a quantitative framework for evaluating geodynamic changes during continental collision, and proposed that deeply buried continental crust is exhumed largely as a flat-slab in the mid-crust, possibly due to erosion of a paleo-plateau in the upper plate. Using Lu-Hf garnet geochronology applied to ultrahigh-pressure (UHP) enstatite-bearing eclogites in the WGC, I provided well-constrained empirical evidence for non-lithostatic eclogitisation, a process that explains the localised occurrence of anomalously-high pressures conditions in deeply buried continental crust. When these research outcomes are compared to the lower plate in the India-Asia collision zone, they demonstrate consistency in the rate and depth of burial and the style of exhumation of continental crust during collisional orogeny. Using Lu-Hf garnet geochronology applied to included peridotite bodies in the WGC, I provided the first well-constrained geochronological evidence for the stabilisation of a buoyant cratonic sub-continental lithospheric mantle in the Archean that melted and recrystallised in concert with major supercontinent break-up intervals. The techniques used herein could be applied to other collisional settings and to other mantle peridotite suites to better constrain the emergence and evolution of global plate tectonics cycles.

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Mafic layered intrusions in the Earth’s crust are natural laboratories for evaluating differentiation processes of mantle-derived magma. The Mesoproterozoic Kiglapait intrusion (Labrador, Canada) represents a remarkable case study of how these intrusions form under closed-system crystallization of basaltic magma. The Kiglapait intrusion is the largest and youngest troctolitic intrusion contained within the vast Nain Plutonic Suite, one of the best-preserved examples of Proterozoic anorthosite massifs that distinguish magmatic activity in the Earth’s crust from ~1.8 to 0.9 billion years ago. The radiogenic isotope ratios of Pb-Sr-Nd-Hf are powerful geochemical tools for identifying magma sources, detecting contamination, and tracing mixing processes in igneous rocks. To measure accurate and precise isotopic ratios by MC-ICP-MS, the analysis of sample–matrix-matched reference materials is required. A new comprehensive isotope database of mafic to ultramafic reference materials is provided to assess the accuracy of Pb-Sr-Nd-Hf isotopes in Kiglapait samples and to be used as a reference dataset for the isotopic study of other terrestrial, and extraterrestrial, mafic-ultramafic rocks. Integration of Pb-Sr-Nd-Hf isotope and trace element geochemistry of whole rocks and mineral separates allows for definition of the Kiglapait source and parent magma composition. An event of post-crystallization addition of radiogenic Pb is distinguished, the effects of which are effectively leached from plagioclase during sample pre-treatment. An in situ LA-ICP-MS technique is also developed for measuring Pb isotope ratios at high spatial resolution in minerals with very low Pb concentrations, such as plagioclase and clinopyroxene. Combined, the solution-based Pb-Sr-Nd-Hf and in situ Pb isotopic results demonstrate that the primary Kiglapait magma was mantle-derived, with minor assimilation of lower crust during ponding and ascent, and that assimilation of local country rocks, as recorded primarily in Sr isotopic variations, was limited to the uppermost gabbros and ferrosyenites during the final stages of crystallization. This multi-isotopic and trace element geochemical framework developed for the Kiglapait intrusion, and at a larger scale for the entire Nain Plutonic Suite, can be adapted to layered intrusions and Proterozoic anorthosite plutonic suites worldwide to better constrain a wide range of geological issues from mantle heterogeneity to crustal differentiation to Proterozoic geodynamics.

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The Neoarchean Stillwater Complex (Montana, USA) and the Paleoproterozoic Bushveld Complex (South Africa), two of the world’s largest layered intrusions, have been cornerstones for the study of magmatic processes in the Earth’s crust. Mafic layered intrusions are natural laboratories for assessing the emplacement, crystallization, and cooling mechanisms of mantle-derived basaltic magmas. Most layered intrusions do not yet have robust geochronological frameworks from the base to the top of their stratigraphic successions. Zircon is recognized as a relatively common accessory mineral in the Stillwater and Bushveld intrusions and crystallized from highly fractionated interstitial melt at near-solidus temperatures (980-720°C). High-precision geochronologic frameworks established for both intrusions by U-Pb zircon dating, combined with trace element and hafnium isotope compositions of zircon, reveal extended durations of magmatism (3-5 million years) and non-stratigraphic or out-of-sequence ages for both intrusions. Dating of platinum group element deposits in both intrusions (J-M Reef, Stillwater; Merensky Reef, Bushveld) indicates that they are intrusion-wide time markers that crystallized synchronously over large distances (>300 km, Bushveld). The recognition that zircon can be successfully extracted from mafic-ultramafic rocks associated with magmatic ore deposits provides new opportunities for assessing the timing and duration of mineralization processes in layered intrusions worldwide. Zircon from a thick anorthosite horizon in the Stillwater Complex has been identified as a reference material for U-Pb geochronology of Archean rocks (>2.5 Ga) and fills an important gap in the geologic timescale for the application of precise and accurate U-Pb geochronology. Collectively, the dating results indicate that both the Stillwater Complex and Bushveld Complex do not represent the products of progressively crystallized magma chambers but instead formed as stacks of amalgamated sills representing repeated injections of magma at different stratigraphic levels. These conclusions call into question current concepts regarding the origin of layered intrusions and challenge us to rethink our understanding of the timescales of magma processes throughout Earth history.

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The isotopic compositions of oceanic island basalts erupted at the Earth’s surface can be used to directly constrain the isotopic signatures of their deep mantle sources. Basaltic rocks are highly susceptible to seawater alteration, which can significantly modify elemental contents and potentially hinder the use of radiogenic isotopes as geochemical tracers. In this study, multi-isotopic (e.g., Pb-Hf-Sr-Nd) analyses on the same acid-leached sample aliquot are shown to produce reliable results for use in the discrimination of mantle source components of oceanic basalts. Application to basalts from the Ninetyeast Ridge in the Indian Ocean and from Mauna Kea volcano on Hawaii in the Pacific Ocean allows for an enhanced resolution of their source components and distribution in the deep mantle. Isotopic geochemistry of the Ninetyeast Ridge sampled during Ocean Drilling Project Leg 121 reveals a Kerguelen and Amsterdam-St. Paul mantle plume origin with the presence of at least three source components and no contribution from an Indian mid-ocean ridge basalt source. The isotopic characteristics of the Ninetyeast Ridge basalts are typical of the Dupal isotopic domain and consistent with recycling of altered oceanic crust and a mixture of pelagic sediments and lower continental crust into their mantle source. This supports a deep origin for the enriched mantle “EM-1”-like Dupal signatures encountered in Indian Ocean island basalts. In the Pacific Ocean, isotopic heterogeneity in basalts from the deepest parts of the Hawai’i Scientific Drilling Project (HSDP2) core on Mauna Kea, combined with prior results from overlying flows, indicate that shield basalts can be explained by mixing of variable proportions of four isotopically distinct components intrinsic to the Hawaiian mantle plume. The “Kea” component is the prevailing composition in Mauna Kea basalts and throughout volcanic activity of the Hawaiian hotspot. The relatively depleted isotopic compositions of this “Kea” component are shared by other Pacific oceanic island basalt groups and are very similar to those of the common mantle component “C”. This suggests that “Kea” may be a common and widespread composition within the deep mantle beneath the Pacific Ocean basin.

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The Saint-Urbain and Big Island rutile-bearing ilmenite Fe-Ti oxide deposits are locatedin the composite 450 km² Saint-Urbain anorthosite (1055-1046 Ma, U-Pb zircon) and inthe Lac Allard intrusion (1057-1062 Ma, U-Pb zircon) of the 11,000 km² Havre-SaintPierre anorthosite suite, respectively, in the Grenville Province of Eastern Canada. Slowcooling rates of 3-4°C/m.y. are estimated for both anorthosites, based on combined U-Pbzircon/rutile/apatite and ⁴⁰Ar/³⁹ Ar biotite/plagioclase geochronology, and resulted fromemplacement during the active Ottawan Orogeny. Slow cooling facilitated (1) diffusionof Zr from ilmenite and rutile, producing thin (10-100 microns) zircon rims on theseminerals, and (2) formation of sapphirine via sub-so lidus reactions of the type: spinel +orthopyroxene + rutile ± corundum → sapphirine + ilmenite. New chemical andanalytical methods were developed to determine the trace element concentrations and Hfisotopic compositions of Ti-based oxides. Rutile is a magmatic phase in the depositswith minimum crystallization temperatures of 781°C to 1016°C, calculated by Zr-inrutile thermometry. Ilmenite present in rutile-free samples has higher Xhem (hematiteproportion in ilmenite), higher high field strength element concentrations (Xhem = 30-17;Nb = 16.1-30.5 ppm; Ta 1.28-1.70 ppm), and crystallized at higher temperatures thanilmenite with more fractionated compositions (Xhem = 21-11; Nb = 1.36-3.11 ppm; Ta =
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The Wrangellia flood basalts are parts of an oceanic plateau that formed in theeastern Panthalassic Ocean (ca. 230-225 Ma). The volcanic stratigraphy presently extends>2300 km in British Columbia, Yukon, and Alaska. The field relationships, age, andgeochemistry have been examined to provide constraints on the construction of oceanicplateaus, duration of volcanism, source of magmas, and the conditions of melting andmagmatic evolution for the volcanic stratigraphy.Wrangellia basalts on Vancouver Island (Karmutsen Formation) form anemergent sequence consisting of basal sills, submarine flows (>3 km), pillow breccia andhyaloclastite (1.5 km). Karmutsen stratigraphy overliesDevonian to Permian volcanic arc (~380-355 Ma) and sedimentary sequences and isoverlain by Late Triassic limestone. The Karmutsen basalts are predominantlyhomogeneous tholeiitic basalt (6-8 wt% MgO); however, the submarine part of thestratigraphy, on northern Vancouver Island, contains picritic pillow basalts (9-20 wt%MgO). Both lava groups have overlapping initial EHf and ENd, indicating a common, oceanisland basalt (OIB)-type Pacific mantle source similar to the source of basalts from theOntong Java and Caribbean Plateaus. The major-element chemistry of picrites indicatesextensive melting (23-27%) of anomalously hot mantle (~1500°C), which is consistentwith an origin from a mantle plume head.Wrangellia basalts extend ~450 km across southern Alaska (Wrangell Mountainsand Alaska Range) and through southwest Yukon where
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