To better understand how the evolution of Cenozoic deep-water circulation related to changes in global climate and ocean basin configuration, we generated Nd isotope records from Ocean Drilling Program sites in the southeastern Atlantic to track deep water mass composition through time. We used fossil fish debris from ODP Sites 1262-1264 (Leg 208), spanning present-day water depths of 2500-4750 m, to reconstruct the isotopic signature of deep waters over the past ~53 Ma. The data indicate an initial transition from relatively non-radiogenic values (??Nd=~-10) at 53 Ma to more radiogenic values (~-8.5) at ~32 Ma. From ~32 Ma to 3.85 Ma, the Nd signal becomes more nonradiogenic, ~-12.3 at the top of the record. Comparison of our data with Nd isotopic records derived from a North Atlantic Fe-Mn crust show similar non-radiogenic values (~-10.5) in the 53??32 Ma interval and a trend toward more non-radiogenic values beginning at ~20 Ma. The data likely reflect an overall shift from a Southern Ocean deep water source to the ultimate incursion of deep waters from the North Atlantic. The non-radiogenic values at the base of the record reflect a Southern Ocean source of deep water. The shift toward more radiogenic values indicates an increased contribution of Pacific waters to the Southern Ocean source as the tectonic gateways changed after ~35-33 Ma. The subsequent trend toward more non-radiogenic Nd isotope values is approximately concurrent with the increase of benthic foraminiferal ??18O values, based on comparison with a compilation of global data. Thus, changes in oceanic gateway configuration in addition to overall cooling and the build-up of continental ice on Antarctica may have altered the Nd isotope character of Southern Ocean deep waters during the early Oligocene.

<p> Nd isotope analysis is well suited for mapping major tectonic boundaries in highly metamorphosed orogenic belts. In this study, approximately 80 samples have been analyzed to map 2 such boundaries in the Central Gneiss Belt of the Grenville Province of Ontario. In Central Ontario, lithotectonic terranes with mapped outcrops of gneisses intruded by eclogites and/or coronitic metagabbro have Nd model ages less than 1.8 Ga are interpreted as components of the allochthonous polycylic belt. More northerly terranes are comprised of similar gneissic materials, but have different types of mafic intrusives and have model ages greater than 1. 8 Ga. These terranes are interpreted as fragments of the parautochthonous belt. These two belts are divided by a major thrust, termed the Allochthon Boundary Thrust (ABT) (Rivers, et. al., 1989). Continuing to the north, another step in the Nd model ages has been used to identify and map a cryptic suture between Archean and early Proterozoic crustal materials (Dickin & McNutt, 1989). </p> <p> Along the Georgian Bay coastline, between Pointe Au Baril and Parry Sound, the Shawanaga Shear Zone has been interpreted as the location of the ABT (Culshaw, et. al., 1994). Analysis of over 50 samples are used to map the crustal formation ages in this region and have confirmed this interpretation. Orthogneisses of the Britt Domain have Nd model ages in the range 1.8- 1.9 Ga. Reworking of the original crust has given these rocks U-Pb crystallization ages of~ 1.45 Ga, which means that these rocks have been metamorphosed prior to the Grenvillian event. Crossing the ABT, the orthogneisses of the Shawanaga Domain have a younger range of crustal formation ages, 1.4 - 1.7 Ga. The U-Pb crystallization ages of these rocks are ~ 1.36 Ga, and they lack signs pre-Grenvillian metamorphism. To the south of Franklin Island, the location of the ABT is difficult to map, as outcrop lies beneath the waters of Georgian Bay. Results of the Nd isotope analyses suggest that the ABT passes through the western edge of the Snake Islands, rather than to their east, as previously interpreted (Culshaw, et. al., 1994). </p> <p> Approximately 15 Nd isotope analyses were used to investigate a recently proposed location of the ABT (Ketchum & Davidson, 2000) in the vicinity of the Powassan Batholith. Results from near Arnstein, Restoule and Magnetewan agreed with the existing location of the ABT. To the east of the Powassan Batholith, 3 Nd model ages coupled with a lack of mappable eclogites and/or coronitic metagabbros suggest that earlier interpretations of the position of the ABT may be correct and that further studies in this region are necessary. </p> <p> A cryptic suture identified by crustal formation ages has been the focus of several previous studies (Dickin & McNutt, 1989, 1990; Holmden & Dickin, 1995; Dickin, 1998; Guo & Dickin, 1996). This suture has been mapped from the Georgian Bay coast through Lake Nipissing to the Ontario-Quebec border. New Nd isotope analyses and studies of the regional magnetics have identified a thrust slice between the Grenville Front tectonic zone (GFTZ) and the parautochthonous belt. The cryptic suture appears to coincide with a previously undescribed tectonic boundary west of the Key River. To the west of this boundary, straight orthogneisses within the thrust slice have Nd model ages greater than 2.2 Ga. These differ from the orthogneisses and metaplutonic tonalites to the east of this boundary, which exhibit kilometer-scale isoclinal folds and crustal formation ages between 1.8- 2.0 Ga, the previously identified range for the Britt Domain. </p> <p> Major steps in the depleted mantle model ages are observed in all three regions, allowing mapping of the ABT and the Penokean Suture. It is concluded that, in metamorphic orogenic belts, such as the Grenville Province, detailed mapping of major tectonic boundaries is greatly enhanced by the use of Nd isotope analysis. </p> / Thesis / Master of Science (MSc)

Thesis advisor: Christopher J. Hepburn / The Nashoba terrane in eastern Massachusetts comprises Cambrian-Ordovician mafic to felsic metavolcanic rocks and interlayered sediments metamorphosed during the mid-Paleozoic and intruded by a series of dioritic to granitic plutons during the Silurian to earliest Carboniferous. This work comprises two parts discussing the Sm-Nd isotope characteristics and major and trace element geochemistry of the Nashoba terrane: the first discusses the Cambrian-Ordovician metamorphosed units, the second discusses the Silurian-Carboniferous plutons. Part I: The Nashoba terrane in eastern Massachusetts lies between rocks of Ganderian affinity to the northwest and Avalonian affinity to the southeast. Its relationship to either domain was unclear and has been investigated. Major and trace element geochemical data indicate a mix of arc, MORB, and alkaline rift related signatures consistent with an origin of the terrane as a primitive volcanic arc-backarc complex built on thinned continental crust. Newly determined Sm-Nd isotopic data clarifies the original tectonic setting. Amphibolites of the Marlboro and Nashoba Formations have high εNd values (+4 to +7.5) consistent with formation in a primitive volcanic arc with minimal interaction between arc magmas and crust. Intermediate and felsic gneisses have moderate εNd values between +1.2 and –0.75 indicating a mixture of juvenile arc magmas and an evolved (likely basement) source. Depleted mantle model ages of 1.2 to 1.6 Ga indicate a Mesoproterozoic or older age for this source. Metasedimentary rocks have negative εNd values between –6 and –8.3 indicating derivation primarily from an isotopically evolved source (or sources). The model ages of these metasedimentary rocks (1.6 to 1.8 Ga) indicate a source area of Paleoproterozoic or older age. The εNd values and model ages of the intermediate and felsic rocks and metasedimentary rocks indicates that the basement to the Nashoba terrane is Ganderian rather than Avalonian. The Nashoba terrane therefore represents a southward continuation of Ganderian arc-backarc activity as typified by the Penobscot and/or Popelogan-Victoria arc systems and the Tetagouche-Exploits backarc basin in the northern Appalachians. Part II: Between 430 and 350 Ma the Nashoba terrane experienced episodic dioritic and granitic plutonism. Previous workers have suggested a supra-subduction zone setting for this magmatism based on the calc-alkaline nature of the diorites. Previously determined major and trace element geochemical data along with newly determined Sm-Nd isotopic data indicate that a subduction zone was active beneath the Nashoba terrane during the majority of the 430 to ca. 350 Ma magmatism (and likely throughout). Trace element geochemistry indicates a strong arc component in all magmas and suggests that the various Silurian to Carboniferous plutonic rocks of the Nashoba terrane could all have been derived by modification of a slightly enriched NMORB-type source via subduction zone input and crustal contamination. Most of the rocks from this period have intermediate εNd values consistent with contamination of juvenile magmas by an evolved source. The late Proterozoic model ages for most of these rocks suggest the Ganderian basement of the Nashoba terrane as the source of evolved material. One rhyolite from the nearby Newbury Volcanic Complex (of unknown affinity) has a moderately negative εNd value consistent with derivation by partial melting of Cambrian-Ordovician metasedimentary rocks of the Nashoba terrane. This suggests that the Newbury Volcanic Complex formed as the surface expression of mid-Paleozoic Nashoba terrane plutonism. Geochemical and isotopic similarities between the plutonic rocks of the Nashoba terrane and widespread contemporary Ganderian plutonism suggest that the Nashoba terrane remained a part of Ganderia during its transit and accretion to the Laurentian margin. Significantly younger model ages in the youngest granitic rocks indicate that Avalonian crust may have underthrust the Nashoba terrane after 400 Ma and contributed to the generation of these granites. / Thesis (MS) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

>Magister Scientiae - MSc / The Platreef, is a contact-type pyroxenitic reef in the Northern Limb of the Bushveld Complex, enriched in platinum group elements (PGE) and base metal sulfides (BMS). Relatively subdued mining in the Platreef, compared to RLS, has been attributed to limited knowledge regarding irregular distribution, complex style and genesis of PGE mineralisation in the Platreef. This study was, therefore, aimed at investigating the petrogenesis of the Platreef, particularly to evaluate whether the formation of the ore reefs resulted from a single or multiple sill-like magma intrusions. The study also sought to unveil the interplay of fractional crystallisation, hydrothermal fluid activities, floor rock and crustal contamination on the formation of Platreef types, PGE mineralisation and the magma source.

京都大学 / 0048 / 新制・課程博士 / 博士(人間・環境学) / 甲第21186号 / 人博第858号 / 新制||人||204(附属図書館) / 29||人博||858(吉田南総合図書館) / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 小木曽 哲, 教授 石川 尚人, 教授 酒井 敏, 准教授 飯塚 毅 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DGAM

To explore whether the dispersion of sediments in the North Atlantic can be related to modern and past Atlantic Meridional Overturning Circulation (AMOC) flow speed, particle size distributions (weight%, Sortable Silt mean grain size) and grain-size separated (0–4, 4–10, 10–20, 20–30, 30–40 and 40–63 μm) Sm-Nd isotopes and trace element concentrations are measured on 12 cores along the flow-path of Western Boundary Undercurrent and in the central North Atlantic since the Last glacial Maximum (LGM). North Atlantic is a useful place to explore how size-specific sediment provenance is related to sedimentary inputs and deep-current advection because mantle-derived materials in Iceland is a unique sedimentary source compared to crustal-derived terranes in Europe, Greenland and North America. The four main processes transporting sediments from continents to the North Atlantic (bottom currents, turbidity currents, ice-rafting events, airborne inputs) can be well distinguished through the size-specific physical and geochemical records. When primarily advected by the bottom currents, Holocene sediments show that the finer-sized fractions (0–4, 4–10, 10–20 μm) were transported further, and the coarser size fraction (40–63 μm) matched local inputs. In the deep coretops (> 2700 m) proximal to southern Greenland, fine-slit size fraction (10–20 μm) instead of clay size fraction (0–4 μm) observed more Icelandic-material contribution. In the past, the 20–30, 30–40 and 40–63 μm particles in the shallower Iceland-proximal core (1249 m) reflect Icelandic composition variation due to the abrupt volcanic eruption around 13–9 ka; while in the deeper Iceland-proximal core (2303 m) they were sensitive to the changing bottom flow speed. Downstream in cores proximal to southern Greenland (> 2272 m) and eastern North America (3555 m), composition of the 20–63 μm sediments could be used as an indicator for the retreating of the Greenland and Laurentide Ice Sheets which affect the sediment accessibility of the covered geological terranes; while the 0–4, 4–10 and 10–20 μm particles were more sensitive towards the changing direction (northern-sourced or southern-sourced) and velocity of the bottom current. In the open North Atlantic, the composition of the 0–10 μm particles were less variable between the cold and warm climate intervals compared to the 10–63 μm particles, and the 30–40 and 40–63 μm size fractions were sensitive towards both ice-rafting events and bottom flow direction. During LGM, shallower and vigorous northern-sourced water (NSW) was observed overlaying the deeper southern-sourced water (SSW), with the boundary between 2133 to 2303 m in southern Iceland, and ~ 2272 m in southern Greenland. Reduced NSW occurred during Heinrich Stadial 1, until AMOC above ~ 3500 m recovered to vigorous modern-like version no later than ~ 13.5 ka. Sluggish overflow was observed in North Atlantic between 12.2–11.7 ka above ~ 3500 m. Reduction of Iceland-Scotland Overflow Water occurred around 9.7 ka, and started recovering to its modern vigorous no later than ~ 8.6 ka. These relative past AMOC strength variations (vigorous/sluggish) are firstly converted to actual bottom-current speed (in cm/s) using laminar advection model in this work: vertical settling velocity of particle having the most Icelandic contribution is calculated by Stokes’ Law, and the lateral deep-sea current speed is calculated when the vertical settling depth and the lateral advection distance of the particle traveled before settling are constrained. Primary modelling errors originating from temperature/salinity variations in past deep seawater, winnowing process in fine particles, basaltic-signature dilution by crustal input, and lateral advection pathways of Icelandic-material are further discussed, indicating relatively low modelling error (< ~ 10–20 %). The modelling results agree well with modern deep-sea current speed measurements and backtrack-trajectory eddy resolving model (Ocean model for the Earth Simulator, OFES18), indicating reasonable quantifications of past AMOC flow speeds.