Precise chronologies of climate events in the tropics are rare yet essential for understanding how tropical climate relates to global climate at millennial to longer time scales. An increasingly important area for understanding these interactions is the southern Bolivian Altiplano (15-22oS) which represents the waning and southeastern end of the South American Monsoon, a system that is, today, modulated by regional upper-air circulation anomalies under the influence of tropical Pacific sea-surface temperature gradients associated with El Niño/Southern Oscillation (ENSO). Mechanisms of summer rainfall variations on millennial and longer time scales are less well understood, despite well-established evidence for profound changes in hydrologic budgets on the southern Bolivian Altiplano. Large shifts in effective moisture on the southern Bolivian Altiplano produced deep lakes in the Poopo, Coipasa, and Uyuni basins, basins that are currently occupied by salt pans or very shallow lakes. We mapped shoreline stratigraphy and sampled carbonates for over 170 uranium-thorium (U-Th) and radiocarbon (14C) dates to refine paleolake history of the Southern Bolivian Altiplano. As part of this dissertation work, I helped assemble a U-Th dating facility at the University of Arizona and obtained over 90 uranium-thorium (U-Th) dates from paleolake carbonates. Carbonate textures were evaluated for potential diagenetic effects, but the principal consideration in dating such carbonates is the isotopic composition and quantity of initial Th incorporated into the carbonate. We establish criteria for statigraphically meaningful dates and strategies for successful U-Th dating of paleolake carbonates. The stable isotope, 87-strontium/86-strontium (87Sr/86Sr), and 234U/238U ratios of modern surface waters and of paleolake carbonates can be used as tracers of the region's various lake cycles and provides a test hydrologic models of these lake cycles.Volcanic tuffs provide important stratigraphic markers for paleolimnologic, geomorphic, and archeological studies. Despite the widespread occurrence of late Quaternary tuffs on the Bolivian Altiplano, few of these deposits have been previously recognized either from natural exposures or in paleolake sediment cores. We document the presence of 38 distal tuffs in Quaternary lacustrine and alluvial deposits, and determine the composition of glass and phenocrysts by electron microprobe analyses.

Gel-precipitated (U, Th) spheres produced at AERE Harwell, were investigated to study the development of the structure of the gel spheres at various stages of production. Various parameters including surface area, true (matrix) and geometric densities, porosity, crystallite size, compliance and related properties were investigated. A careful examination of whole and cleaved spheres by Scanning Electron Microscope was also carried out. The xerogel spheres were debonded in a tubular furnace in an atmosphere of CO[2] to remove or decompose the gelling agent (polymer) and other volatiles which might be present. The isothermal and constant-rate-of-heating (CRH) sintering behaviour of partially and fully debonded spheres were studied. In the latter work the shrinkage data of a batch of spheres were obtained from geometric (Hg) density measurements instead of the conventional dilatometric method used for pellets. The study has shown that ageing and heavy metal composition have a substantial influence on the structure of the debonded spheres, the rate of sintering, and the microstructure of the sintered products. Ageing and increasing amount of thorium generally retards sintering. Partially debonded (to 750°C) spheres sinter faster than fully debonded spheres and the former also develop larger grains on sintering. Gel-precipitation is a versatile technique which can be used to produce spheres which sinter to > 98% of the theoretical density at a comparatively low temperature (≈ 1400 C). Grain-boundary diffusion was found to be the probable dominant material transport mechanism in the sintering of (U, 30% Th)O[2].

The northeast African continental margin of the Western Desert of Egypt is host to a complexly deformed series of Phanerozoic basins. Substantial sedimentary deposition (~5 km) and basin formation resulted from regional deformation due to continental collision and repeated rifting and inversion cycles. Limited sedimentary exposure and exploration has prevented elucidation of Phanerozoic basin evolution, particularly in the Paleozoic. Previous studies of the region have largely relied upon sedimentary analysis, gravity, and 2D/3D seismic data. This study, in contrast, has employed extensive use of detrital zircon (U-Th)/He thermochronology (n=1004) from 17 wells in conjunction with 3D seismic, well log correlation, and heat flow data to elucidate a spatiotemporally comprehensive tectonic and stratigraphic model. The detrital zircon thermochronometric data provides new evidence that the lower Paleozoic, Carboniferous, and Mesozoic stratigraphic sequences of the Western Desert represent thermally distinct, tectonically controlled sequences with independent thermal evolutions. The lower Paleozoic sequence has been partially thermally reset, reaching temperatures of ~140-170 ̊C. Partial resetting is noted throughout the region and reached its thermal maximum in the Permo-Triassic, synchronous with onset of Neotethyan rifting. The Carboniferous sequence has not been thermally reset, with exposure to temperatures no greater than ~140 ̊C and reaching thermal maximum presently. Carboniferous (U-Th)/He ages have dominant input from short-lag-time zircons (exhumation to deposition) and indicate the stratigraphic sequence was proximally sourced. The proximal sourcing is likely from transmitted stress and fault reactivation in Egypt during the Hercynian Orogeny that caused fault block exhumation and erosional unroofing. Sediment was shed from uplifted fault blocks that formed the eastern boundary of the Carboniferous sequence. The Mesozoic sequence has not been thermally reset, reaching temperatures no greater than ~120 ̊C and presently reaching thermal maximum. Localized areas with stacking of lower Paleozoic, Carboniferous, and Mesozoic sequences likely bury the lower Paleozoic to abnormally deep depths (~7 km) and elevated temperatures of ~200 ̊C. Evidence from faulting relationships, basin controlling structures, and heat flow data indicate that N-S trending basement structures may define a region of crustal transition between the Archean-Paleoproterozoic Saharan Metacraton and the juvenile Arabian-Nubian Shield. / text

Craton cores far from plate boundaries have traditionally been viewed as stable features that experience minimal vertical motion over 100-1000Ma time scales. Here we show that the Fennoscandian Shield in southeastern Sweden experienced several episodes of burial and exhumation from similar to 1800Ma to the present. Apatite, titanite, and zircon (U-Th)/He ages from surface samples and drill cores constrain the long-term, low-temperature history of the Laxemar region. Single grain titanite and zircon (U-Th)/He ages are negatively correlated (104-838Ma for zircon and 160-945Ma for titanite) with effective uranium (eU=U+0.235xTh), a measurement proportional to radiation damage. Apatite ages are 102-258Ma and are positively correlated with eU. These correlations are interpreted with damage-diffusivity models, and the modeled zircon He age-eU correlations constrain multiple episodes of heating and cooling from 1800Ma to the present, which we interpret in the context of foreland basin systems related to the Neoproterozoic Sveconorwegian and Paleozoic Caledonian orogens. Inverse time-temperature models constrain an average burial temperature of similar to 217 degrees C during the Sveconorwegian, achieved between 944Ma and 851Ma, and similar to 154 degrees C during the Caledonian, achieved between 366Ma and 224Ma. Subsequent cooling to near-surface temperatures in both cases could be related to long-term exhumation caused by either postorogenic collapse or mantle dynamics related to the final assembly of Rodinia and Pangaea. Our titanite He age-eU correlations cannot currently be interpreted in the same fashion; however, this study represents one of the first examples of a damage-diffusivity relationship in this system, which deserves further research attention.

Thesis advisor: Jeremy D. Shakun / Arctic permafrost contains a substantial stock of carbon that could be released to the atmosphere as CH4 and CO2 upon thawing, making it a potentially powerful amplifier of future warming. The sensitivity of permafrost to climate change is uncertain, however, and occurs on time scales longer than those captured by the instrumental record. Speleothems – cave precipitates deposited from flowing or dripping water – in currently frozen regions record past episodes of thaw, which can be used to assess the response of permafrost to long-term warmth. Here, we present 90 uranium-thorium ages on speleothems from across the North American Arctic, sub-Arctic and northern alpine regions to reconstruct a 600-kyr permafrost history. Widespread speleothem growth supports an episode of extensive permafrost thaw during the Marine Isotope Stage 11 interglacial about 400 ka, when global temperature was only slightly warmer than pre-industrial conditions. Additional growth is evident during MIS 13, curiously, a smaller magnitude interglacial. Ice-core records of atmospheric greenhouse gases do not show elevated concentrations at these times, perhaps suggesting that the permafrost carbon pool was smaller than today or released gradually enough to be buffered by other reservoirs. / Thesis (MS) — Boston College, 2017. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

This dissertation contains two studies that investigate the Mesozoic and Cenozoic tectonics of the western USA. The first study investigates shortening in the Sevier thrust belt of northeast Utah and southwest Wyoming. Cross section restoration suggests that there was ∼8-14 km of pre-Absaroka-thrust shortening above the Jurassic Preuss salt detachment (PSD), but not below it, in the hanging wall of the Absaroka thrust. Reflection seismic data show that the Crawford thrust is not offset along the PSD, indicating that the additional shortening on the Absaroka plate was transferred east before main movement on the Crawford thrust. Integration of surface and subsurface geology suggests slip from the Willard or Lost Creek thrust was transferred several tens of kilometers east along the PSD between ∼102-90 Ma.The second study investigates the low-temperature thermochronology of the Laramide Ranges. We dated 91 borehole and surface samples from basement-cored uplifts of the Rocky Mountain foreland (Wind River, Beartooth, Bighorn and Laramie Ranges), and the Uncompahgre Uplift, using the apatite (U-Th)/He system. (U-Th)/He ages generally increase with increasing elevation. Most samples show age dispersion of tens to hundreds of Myr. Several samples show correlations between (U-Th)/He age and effective U concentration (eU = [U] + 0.235[Th]), indicating that radiation damage has affected (U-Th)/He age. Many surface and near-surface samples have (U-Th)/He ages that are older than apatite fission-track ages.Forward and inverse modeling using a radiation damage diffusion model showed that (U-Th)/He ages may be widely dispersed, and may be older than apatite fission-track ages within a fossil partial retention zone. Most samples, however, do not exhibit the predicted (U-Th)/He age-eU correlation. We show that the effects of grain size can obscure (U-Th)/He age-eU correlations. Best-fit thermal histories from the inversion of age-eU pairs were extrapolated to other elevations to create model age-elevation plots. "Too-old" (U-Th)/He ages that are not within a fossil partial retention zone are likely due to He implantation from high-eU phases. Inverse modeling of (U-Th)/He age data suggests that rapid exhumation within the Laramide province began earlier in the Bighorn Mountains (before ∼71 Ma) than the Beartooth Range (before ∼58 Ma).

Uplift of the Al Hajar Mountains in Oman has been related to either Late Cretaceous ophiolite obduction or the Neogene Zagros collision. To test these hypotheses, the cooling of the central Al Hajar Mountains is constrained by 10 apatite (U-Th)/He (AHe), 15 fission track (AFT), and four zircon (U-Th)/He (ZHe) sample ages. These data show differential cooling between the two major structural culminations of the mountains. In the 3km high Jabal Akhdar culmination AHe single-grain ages range between 392 Ma and 101 Ma (2 sigma errors), AFT ages range from 518 Ma to 324 Ma, and ZHe single-grain ages range from 62 +/- 3Ma to 39 +/- 2 Ma. In the 2 km high Saih Hatat culmination AHe ages range from 26 +/- 4 to 12 +/- 4 Ma, AFT ages from 73 +/- 19Ma to 57 +/- 8 Ma, and ZHe single-grain ages from 81 +/- 4 Ma to 58 +/- 3 Ma. Thermal modeling demonstrates that cooling associated with uplift and erosion initiated at 40 Ma, indicating that uplift occurred 30 Myr after ophiolite obduction and at least 10 Myr before the Zagros collision. Therefore, this uplift cannot be related to either event. We propose that crustal thickening supporting the topography of the Al Hajar Mountains was caused by a slowdown of Makran subduction and that north Oman took up the residual fraction of N-S convergence between Arabia and Eurasia.

The integration of low-temperature thermochronometers, including apatite and zircon (U-Th)/He (AHe, ZHe) and apatite fission-track (AFT) methods, allows for a quantification of the thermal history experienced by rocks as they heat and cool through upper crustal temperature regimes (<200°C). Whereas these methods are practical in geologic terranes that have undergone rapid cooling, application to strata with protracted cooling histories is complicated by the enhanced role of grain-specific parameters (volume, chemistry, radiation damage) on the kinetics of helium diffusion and fission track annealing. The effects of these variables are most prevalent in sedimentary samples, where natural variance in detrital accessory mineral populations results in a broad range of diffusion kinetics and great dispersion in corresponding cooling dates. This thesis integrates contemporary thermochronometer diffusion and annealing kinetics to investigate the burial and exhumation history of two natural laboratories. In the Mackenzie Mountains and Plain of the Northwest Territories, long-term radiation damage accumulation in zircon from Neoproterozoic siliciclastic units produces ZHe dates that track Albian to Paleocene burial and exhumation in front of the foreland-propagating fold-thrust belt. For the Phanerozoic stratigraphic section, AFT annealing kinetics are calculated from Devonian and Cretaceous samples, and are incorporated into multi-kinetic AFT modeling. These kinetics also constrain AHe date-radiation damage trends, and when combined allow for an estimation on the magnitude of eroded sediment across regional pre-Albian and post-Paleocene unconformities. Finally, conodont (U-Th)/He data from Anticosti Island, Québec in the Gulf of the St. Lawrence are compared with ZHe, AHe and AFT data to test their utility as a thermochronometer for carbonate basin analysis. These data evince a Mesozoic thermal history previously unattributed to the region. Ultimately, this thesis provides a novel assessment on the ways in which thermochronometer date dispersion can be quantified to assess the thermal evolution of sedimentary basins from burial through to inversion.

In order to evaluate the electron microprobe (EMP) method for chemical dating of monazite, we chemically analyzed selected suites of monazite grains that were previously dated by standard U/Pb isotopic methods at three laboratories each equipped with a sensitive high resolution ion microprobe (SHRIMP). Representing diverse igneous and metamorphic lithologies, these grains yielded conventional isotopic ages ranging in age from Neoarchean to Devonian. Chemical dating was performed at Virginia Tech using a Cameca SX-50 EMP in which the analytical routines and settings were specifically optimized for monazite geochronology, including correction of analytical peaks for all major spectral interferences and correction of peak intensities for local background emission. Placement of cross-grain analytical traverses was based on backscattered electron (BSE) images together with wavelength-dispersive (WD) generated X-ray maps for Y, Th, U, and Ca, which revealed the internal compositional complexity of each grain. Shorter EMP traverses were selected adjacent to each SHRIMP pit in order to provide the best possible comparison of ages obtained by the two dating methods. Synthesis protocol for key elemental measurements (Y, Th, U, and Pb) was developed utilizing the 1Ï elemental errors associated with individual analyses, providing an objective approach for data synthesis. Analytical dates were either accepted or excluded based on analytical and spatial justifications. Isotopic dating techniques utilize three independent age calculations, provided the sample is old enough to have accumulated sufficient 207Pb (i.e., ≥~1000 Ma). Similarly, the chemical dating method can utilize two independent age calculations (i.e., Th/Pb and U/Pb) and a U-Th-Pbtotal centroid age in Th/Pb vs. U/Pb space, verified independently against the calculated Th* or U* CHIME ages. Across the entire 2,200 m.y. age range represented by the sample set, the chemical ages calculated from the EMP data chemical ages are internally consistent (within 2Ï error) with the previously measured SHRIMP isotopic ages, except in one case where bulk mixing of discrete age domains within an ablation pit led to an isotopically discordant apparent age. Overall, this study illustrates that EMP chemical dating (1) represents both an accurate and precise primary method for dating monazite from igneous and polymetamorphic terrains; (2) provides superior spatial resolution for obtaining meaningful ages from small and/or irregular domains of discrete age that may be irresolvable or misinterpreted by other dating techniques that sample larger volumes; and (3) illuminates the geological meaning of isotopically discordant monazite ages obtained using conventional methods with lower spatial resolution (e.g., SHRIMP). / Ph. D.

La datation des carbonates est un enjeu important en géosciences car ces minéraux sont présents dans tous les grands cycles superficiels terrestres. Actuellement, l’âge de formation des cristaux de calcite de failles peut être déterminé par la méthode de déséquilibre U-Th ou par la méthode U-Pb. La potentialité de la méthode (U-Th-Sm)/He a été envisagée à partir des premiers résultats de diffusion de l’hélium dans les carbonates (Copeland et al. 2007) qui montrent que l’hélium est rétentif à basse température, inférieure à 70°C. Le développement de cette méthode nécessite une identification des propriétés cristallochimiques et du comportement de la calcite afin de discuter la validité des âges obtenus. Une zone de déformation au contact d’une faille normale du fossé d’effondrement de Gondrecourt à Augeville (Est du bassin de Paris) et des cristaux de calcite géodique des argilites de Tournemire (Aveyron) ont été étudiés. Une étude pétrographique et géochimique (analyse des isotopes stables δ18O et δ13C et des éléments mineurs et traces) a permis d’identifier les phases de remplissage de la zone de déformation d’Augeville. Sur les phases de remplissages une étude cristallographique par DRX (diffraction des rayons X) et ATG (analyse thermogravimétrique) a permis d’identifier les propriétés mécaniques de la calcite à différentes températures. Des expériences de diffusion de l’hélium ont été conduites selon un protocole différent de celui de Copeland et al. (2007) : temps de chauffage inférieur à 12 heures au lieu de paliers de chauffage de plusieurs heures à semaines. Des datations (U-Th-Sm)/He ont été réalisées sur toutes les phases de remplissage de la zone de déformation ainsi que sur les cristaux géodiques de Tournemire. Les résultats pour Gondrecourt mettent en évidence une zone de déformation à remplissage calcitique pluriphasée qui marque chaque étape de l’évolution tectonique de la zone. Il s’agit majoritairement de remplissages de brèches hydrauliques à brèches chaotiques avec, pour finir, la mise en place de filons. Sur ces minéralisations les résultats cristallochimiques montrent que lorsqu’ils sont soumis à une température croissante, de température ambiante à 400°C, ou sur le long terme à 200°C, il y a un accroissement du nombre de microfissures qui aboutit à la formation de clivage. L’évolution cristallochimique sous l’effet de la température présente des modifications structurales pour chacune des phases étudiées. Les expériences de diffusion dépendent de ces microfissures et de cette évolution microstructurale. Ces résultats mettent en évidence les mécanismes de diffusion de l’hélium dans la calcite à travers de multiples domaines de diffusion différents de la taille du grain. Ces domaines de diffusion dépendent de l’état microstructural de l’échantillon. Les résultats de datation (U-Th-Sm)/He présentent une large dispersion de 0,1 Ma à 35 Ma pour la zone de déformation de Gondrecourt et de 0,4 Ma à 20 Ma pour les cristaux géodiques de Tournemire. La variation des âges est en partie liée à la position des cristaux de calcite dans la succession paragénétique qui entraine un continuum de fracturation qui génère de multiples domaines de diffusion dans les cristaux de calcite. / Carbonate dating is a major issue in geosciences due to their widespread terrestrial environments presence . Precipitation ages determination are actually provide by U-Th disequilibrium series for young <1 Ma carbonates crystal and/or U-Pb method for older samples. In addition, Copeland et al. (2007) have proposed the development of (U-Th-Sm)/He dating method based on retentive helium behavior at surface temperature, conclusion issue of their first diffusion coefficients determined on carbonates samples. In this thesis work, the intention was to get an identification of crystallo-chemical properties and behavior of helium in calcite to date samples by (U-Th-Sm)/He and finally discuss the validity of this method. Deformation zones in contact to the normal fault of the Gondrecourt trench close to Augeville (eastern side of Basin of Paris) and calcite crystals geodic Tournemire argillites (Aveyron) were studied. Petrographic and geochemical couples studies (analysis of stable isotopes δ18O and δ13C and minor and trace elements) have allow the filling phases identification of the Augeville deformation zone. Based on the filling phases results, a crystallographic study by XRD (X-ray diffraction) and TGA (Thermogravimetric analysis) have been performed to identify the mechanical properties of calcite at different temperatures. Then, diffusion experiments on natural helium were conducted using a protocol differing than Copeland et al. (2007): heating time less than 12 hours instead of heating stages from hours to weeks. (U-Th-Sm)/He analyses were performed on all filling phases of the deformation zone and on the crystal geodes of Tournemire. Results for Gondrecourt samples show a deformation zone that marks several calcitic fillings corresponding to each stage of the tectonic evolution of the area. Calcite filling mainly corresponds to breccia and/or vein fillings which have precipitated approximatively in the same time. Crystallochemical results of these samples, show an increased number of microcracks leading to the formation of cleavage when samples are subjected to temperature increasing, from room temperature to 400 °C, or several days the long term at 200 °C. The crystallochemical evolution under the effect of temperature exhibits structural modifications for each studied phase. Diffusion experiments depend on these microcracks abundance and the microstructural evolution. These results emphasize the mechanisms of helium diffusion in calcite across multiple diffusion domains at different grain size. These multiple diffusion domains are linked to microstructural state of the sample. The results of dating (U-Th-Sm)/He ages show a wide dispersion of 0.1 to 35 Ma for the deformation zone Gondrecourt and from 0.4 to 20 Ma for Tournemire crystal geodes. Ages variation are partly related to the calcite crystals position in the paragenetic sequence. When a mineral undergoes deformations it becomes less retentive.