The Endeavour Ridge is a segment of the Juan de Fuca Ridge, an active spreading centre which lies off western North America between the Pacific and Juan de Fuca plates. This segment is a bathymetric high and a site of hydrothermal activity—both characteristics suggest an underlying heat source such as an axial magma chamber which is associated with crustal generation. To investigate the creation and evolution of oceanic crust, a detailed refraction survey was carried out over the Endeavour Ridge in the fall of 1985. As a component of this survey, a diamond-shaped array consisting of eight OBS along a 20-km line across the ridge and two OBS placed along it at distances of 10 km on either side of the cross-ridge line was deployed to define the shallow crustal structure near and beneath the ridge, especially the possible existence of an axial magma chamber. Airgun shots at 0.2 km intervals along ~300 km of profiles provide conventional reversed and unreversed refraction lines as well as multiple full azimuthal coverage of the region. Travel-time and amplitude data from fifteen in-line airgun profiles recorded on the inner array were forward modelled using an algorithm based on asymptotic ray theory with a starting model obtained from a concurrent study. Two-dimensional models were constructed and then combined to obtain the three-dimensional structure of the region. These models consist of four layers, with the average model correlating well to the classic model of oceanic crust. Layer 2A averages 0.40 km in thickness and has velocities of 2.6 km/s and 2.8 km/s at the top and bottom of the layer, respectively. To achieve such a low velocity, Layer 2A must consist of highly fractured vesicular basalts. A sharp velocity increase to 4.8 km/s marks the transition to Layer 2B. This velocity discontinuity is also visible as a reflector on a. multichannel reflection line obtained through the centre of the study region and is caused by an abrupt decrease in porosity. Layer 2B averages 0.67 km in thickness, has a velocity of 5.4 km/s at its base and consists of less fractured pillow basalts and sheet flows. The Layer 2B-Layer 2C interface is a velocity increase to 5.8 km/s and is the pillow basalt-sheeted dike contact. A small velocity increase from 6.3 to 6.5 km/s delineates the base of the 0.95 km-thick Layer 2C which is the boundary between the sheeted dikes and cumulate gabbros in Layer 3. Layer 3 has the lowest velocity gradient (0.30 s⁻¹) and a velocity of 7.3 km/s at 4.65 km below the seafloor, the maximum depth constrained by the modelling. Lateral heterogeneities on the scale of 2-3 km are superimposed on this basic velocity structure. These heterogeneities are effects of porosity changes, differential pressure changes, and alteration caused by hydrothermal circulation. Layer 2A thins and increases in velocity away from the ridge; ridge-parallel cracks create a velocity anisotropy of ~10-25%, the faster direction parallel to the ridge. Velocities within Layers 2B and 2C also increase by 0.1 km/s away from the axis of the ridge. Layer 3 velocities decrease by 0.1 km/s for arrivals travelling under the ridge. Increased Layer 2 velocities at the ridge crest reveal high lateral velocity constrasts in very young crust, but within 0.03 Ma the oceanic crust at the ridge has matured to the off-ridge structure. No firm evidence exists for a large magma chamber under Endeavour Ridge. Although the bathymetric high and high-temperature hydrothermal discharges are evidence for a magma chamber, the lack of recent sheet flows at the ridge crest and the presence of a rift along the crest indicate the magma chamber is waning and must be of a size (<1 km in width) not resolvable by seismic refraction data. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

This dissertation contributes to our understanding of the use of the Li-isotopic composition of seawater as a tracer of the earth system with a focus on the role of low-temperature hydrothermal systems within the lava section of the ocean crust. Experiments were conducted to study the exchange coefficient (D(Li/Ca)) and isotopic fractionation factor (α; 1000ln(α)=Δ) for lithium between inorganic calcite and an aqueous solution as a function of solution chemistry. These experiments show that, under the conditions used, D(Li/Ca) negatively correlates with solution H+/Ca2+ ratio (and the solution pH) and Δ positively correlates with solution pH. The change in D(Li/Ca) with solution chemistry is interpreted as indicating that Li is incorporated into calcite as LiHCO3, and hence depends on solution H+/Ca2+. A series of diffusion experiments were performed to test whether changes in pH led to changes in the aqueous Li speciation that would lead to changes in the relative diffusivity of the two Li-isotopes, but no such changes were observed. It is proposed that the change in Δ with changing solution pH may either reflect a kinetic or equilibrium isotope fractionation associated with changing solution chemistry. These results have important implications for interpreting the Li content of calcite that has undergone any diagenetic modification. The Li-content and isotopic composition of rocks altered by low-temperature, off-axis hydrothermal systems in the upper oceanic crust were studied to better understand the role of these systems in controlling the Li-isotopic composition of seawater. Results of a detailed study of DSDP Holes 417A, 417D and 418A from 119 Myr Western North Atlantic Ocean basin show that the Li content of the lavas decreases with depth in the upper ~30 m below sediments while the Li-isotopic composition increases from a low value and then does not show systematic variation in deeper sections. No evidence was found to support a role for a change in mineralogy of alteration products to explain the observed variation in Li composition of lavas. There is also no evidence for the modification of the composition of hydrothermal fluid due to a mixture with sediment pore-fluid. Simple one-dimensional fluid flow and fluid-rock reaction models also cannot explain the observed variation. Instead, a model of free-circulation of seawater through the upper few tens of meters of the lavas, and leakage of modified fluid into the deeper portion of the lava pile seems to explain the observed variations best. To investigate the role of low-temperature off-axis hydrothermal systems on the Li content and isotopic composition of seawater from the Cretaceous to modern era, five DSDP/ODP holes with crustal age spanning from 13.6 to 95 Myr were studied in combination with results from previous studies. Results suggest that the average amount of Li added to the upper oceanic crust decreases, while its average Li-isotopic composition increases, from the Cretaceous to the modern. The simplest explanation for these variations may be a decrease in Li concentration and an increase in Li-isotopic composition, of seawater over this time interval. / Graduate

Les étoiles à neutrons ont été largement étudiées depuis que Baade and Zwicky ont postulé leur existence en 1934. Ces études ont été et sont réalisées à l'interface de différents domaines de la physique tels que : l'astrophysique en rayons X, l'observation des pulsars, la relativité générale et plus dernièrement les ondes gravitationnelles, la physique du solide, ainsi que la physique nucléaire. Dans cette thèse nous nous concentrerons sur la description des étoiles à neutrons dans le cadre de la physique nucléaire et précisément de la croûte interne de l'étoile. Ces étoiles sont caractérisées par une masse importante de l'ordre d'une à deux masses solaires dans un rayon de 10 km. Quant à leur structure interne elle peut être décrite en trois strates : la croûte externe, la croûte interne et le cœur. La croûte externe correspond à un réseau cristallin de noyaux atomiques et un gaz d'électrons relativistes. Vient ensuite la croûte interne, définie lorsque les noyaux de la croûte externe sont si riches en neutrons qu'ils les libèrent dans le milieu pour former un gaz. Ici, nous ne parlons plus de noyaux mais d'agrégats car tous les nucléons qui les composent ne sont plus systématiquement liés. Cette structure complexe et sa composition est à l'origine de nombreuses propriétés caractéristiques des étoiles à neutrons.C'est ainsi que nous construirons notre étude en trois parties. Tout d'abord nous commencerons par traiter le gaz de neutrons entourant les agrégats. Le gaz de neutrons que nous considérons uniforme ici est superfluide et devrait donc présenter un mode de Goldstone. Cette description sera effectuée à l'aide de la QRPA. Puis nous en viendrons à la description des agrégats. Dans ces conditions on s'attend à observer des cristaux de sphères, des cylindres et des plaques de matière nucléaire, que nous décrirons grâce à l'approximation ETF. Puis nous terminerons par la description de l'interaction entre les agrégats et le gaz au niveau dynamique, et ce dans le cadre de la théorie hydrodynamique. Ces résultats seront appliqués à l'astrophysique et en particuliers aux glitches. / Neutron stars have been extensively studied since Baade and Zwicky have proposed their existence in 1934. Their description is at the interface of numerous domains of physics, e.g., X-ray astrophysics, pulsar signal observation, general relativity and nowadays gravitational waves, solid state physics, and also nuclear physics. In this thesis we will concentrate on the nuclear physics description, especially of the inner crust. These stars are charaterized by their large mass from one to two solar masses, in a radius of 10 km. Their inner structure can be divded in three major layers: the outer crust, the inner crust and the core. The outer crust consists of nuclei coexisting with an electron gas to ensure charge neutrality. If one goes deeper into the crust, the ratio of neutrons with respect to the total nucleon number increases. Eventually, the excess of neutrons in the nuclei gets so high that they drip out from the nuclei and create a dilute neutron gas. From now on, we will speak of nuclear clusters instead of nuclei. This phenomenon defines the limit between the outer crust and the inner crust. This complicated structure and composition is at the origin of many characteristic properties of neutron stars. Hence, we will construct our work in three major parts. First, we start to account for the neutron gas surrounding the clusters, which we treat as uniform. Here, the neutron gas is assumed to be superfluid, and one can expect a Goldstone mode. This description will be done in the framework of QRPA. Second, we will focus on the study of properties of the clusters contained in the inner crust. Under these conditions we expect to see cystal of spheres, rods and plates of bound nucleons, that we will describe with the help of the ETF approximation. Third, we will finish by treating the interaction between the clusters and the gas with hydrodynamics. The results will be applied to astrophysics and in particular to glitches.

The Bohemian Massif was consolidated during the Variscan orogeny (~400-300 Ma), which involved several oceanic subductions and collisions of continental micro-plates. The central part of the Bohemian Massif, the Moldanubian domain, shows a large accumulation of felsic high-pressure metamorphs. We present a numerical model of exhumation of these rocks due to continental collision and underthrusting. The key feature of the model is a felsic (light, rheologically weak and rich in radioactive elements) material in the lower crust of one of the colliding blocks. We examine the influence of the rate of convergence of the two blocks, radiogenic heating in the felsic lower crust and efficiency of erosion, on the model evolution and pressure-temperature conditions in the lower-crustal material. The models where the material is sufficiently weakened due to radiogenic heating show formation of an orogenic plateau, sedimentation in a foreland basin, and crustal thickening accompanied by gravity-driven exhumation of the lower crust and subsequent sub-horizontal flow in the middle crust. In colder and/or faster models, the thickening is dominated by folding. We correlate the tectonic style of these two types of models with differences between the high-grade rocks in the southern (Moldanubian) and northern (Sudetic) parts...

The region of West Bohemia is one of the most geodynamically active areas of Central Europe. The activity is characterized by many natural phenomena such as recurrent seismic swarms. The crustal structure of the region was subject of many geophysical investigations that revealed increased reflectivity of the lower crust. In this thesis, we adapt the multi-azimuthal approach for the detection of crustal discontinuities developed by Hrubcov'a et al. (2013). We apply the ap- proach on waveforms of the 2000 and 2008 swarms that occurred in the Nov'y Kos- tel focal area registered by several German stations. Analysis of the data of more distant German stations allowed expanding of the information about the depth and character of the crust/mantle transition in a broader area of the West Bo- hemia region. The results show a thickening of the zone in Germany with in- creasing distance from the epicentral area. Such findings are in agreement with previous active and passive seismic experiments.

Thesis advisor: Seth C. Kruckenberg / Quantitatively describing the deformational behavior (i.e. the rheology) of lower crustal materials has proven challenging due to the highly variable nature of structural and compositional fabrics in the lower crust. Further, many flow laws describing the rheology of monophase aggregates are experimentally derived and do not necessarily apply to polyphase materials, such as gabbro, that dominate the lower crust. Here, we present the results of integrated microstructural analysis and electron backscatter diffraction (EBSD) textural analysis from exhumed lower crustal granulites in the Mount Hay block of central Australia. The preservation of heterogeneous mafic and felsic granulites containing monophase and/or polyphase mixtures of anorthite, pyroxene, and quartz (interlayered on the mm- to m-scale) make this region uniquely suited for advancing our knowledge of the processes that affect deformation and the rheology of the lower crust. Forty-two samples from distinct structural and compositional domains were chosen to compare the microstructural record of deformation, the development of crystallographic textures, and to provide estimates of lower crustal rheology and deformation conditions. Full thin-section maps of crystallographic texture were produced using EBSD methods. The resultant orientation maps were processed to characterize crystallographic textures in all constituent phases, and all other quantifiable aspects of the rock microstructure (e.g., grain size, grain shape, misorientation axes). The EBSD analysis reveals the presence of strong crystallographic preferred orientations (CPO) in nearly all constituent phases, suggesting deformation dominated by dislocation creep. Differential stresses during deformation are calculated using grain size piezometry for all major phases, and range between 34-54 MPa in quartz within monophase layers. Two-pyroxene geothermometry was used to constrain deformation temperatures to ca. 780-810 C. Based on the estimated CPO patterns, stress, and temperature, we quantify strain rates and effective viscosities of all major phases through application of monophase flow laws. Monophase strain rates range from 2.10 x 10-12 s-1 to 1.56 x 10-11 s-1 for quartz, 4.68 x 10-15 s-1 to 2.48 x 10-13 s-1 for plagioclase feldspar, 1.56 x 10-18 s-1 to 1.64 x 10-16 s-1 for enstatite, and 5.66 x 10-16 s-1 to 1.00 x 10-14 s-1 for diopside. The determined flow law variables used for monophase calculations were subsequently applied to two different models – the Minimized Power Geometric model of Huet et al. (2014) and the Asymptotic Expansion Homogenization (AEH) method of Cook (2006) – in order to calculate a bulk aggregate viscosity of the polyphase material. At a strain rate of 10-14 s-1, polyphase effective viscosities for our samples range from 3.07 x 1020 to 2.74 x 1021 Pa·s. We find that the bulk viscosity of heterogeneous, gabbroic lower crust in the Mount Hay region lies between that of monophase plagioclase and monophase quartz, and varies as a function of composition. These results are consistent with past modeling studies and geophysical estimates. / Thesis (MS) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

The amount of light an exoplanet reflects and emits towards an observer waxes and wanes as the planet orbits through its phases. The amplitude and profile of reflection phase curves constrain the albedo of planetary surfaces and atmospheres, while the thermal amplitude and profile reveal temperature distributions and heat transport efficiencies, all providing valuable insight into the nature of exoplanet surfaces and atmospheres. In this dissertation I highlight the usefulness of utilizing full orbital phase curves in addition to occultation measurements, which provides a higher sensitivity to planetary photons at the expense of a more challenging data reduction. In the first few chapters of this dissertation, I introduce a novel non-parametric algorithm to produce clean, robust exoplanet phase curves, and apply it to separate ensembles of 115 Neptunian and 50 Terran exoplanets observed by the Kepler satellite to measure an upper limit on the average albedo of Kepler’s Neptunian planets, and make the first constraint on the average albedo of Terran worlds. In the fourth chapter, I present the full orbital phase curve and occultation of the ultra-hot Jupiter WASP-100b observed by the Transiting Exoplanet Survey Satellite (TESS), and with the use of Bayesian methods, present the first measurement of a phase shift of the thermal maximum among the phase curves observed by TESS, the degree of which challenges the predicted efficiency of heat transport in the atmospheres of ultra-hot Jupiters. In the final chapter, I present an example of how the NASA ROCKE-3D general circulation model can be used to explore the physical mechanisms that influence the habitability of terrestrial exoplanets, and then show how I generated phase curves from the 3-dimensional models to study the signals produced by simulated TRAPPIST-1 habitable-zone worlds. The work in this dissertation contributes valuable new information to the astronomical literature and provides avenues for further research on the nature of short-period exoplanets.

The oceanic lithosphere makes up approximately two-thirds of the surface of the earth. Oceanic crust, which is underlain by lithospheric mantle, is formed at mid-ocean ridges and is shaped by a combination of igneous accretionary processes at and near the ridge axis, and post-emplacement tectonic and hydrothermal processes as it evolves. Through time the crust is covered by sediments, sealing it from the overlying ocean, which influences hydrothermal circulation and cooling in the lithosphere below. Finally, oceanic lithosphere is subsumed at subduction zones. In this thesis I utilize seismic data to investigate the oceanic lithosphere from formation to near subduction using seismic datasets from the East Pacific Rise (EPR) and the Juan de Fuca (JdF) plate. In my first chapter I investigate the hypothesis that eustatic sea level fluctuations induced by the glacial cycles of the Pleistocene influence mantle-melting at mid-ocean ridges (MORs) using a unique bathymetry and crustal thickness dataset derived from a 3D multi-channel seismic (MCS) investigation of the East Pacific Rise from 9°42’ to 57’N. The results of this study show variations in crustal thickness and bathymetry at timescales associated with Pleistocene glacial cycles, supporting the inference that mantle melt supply to MOR may be modulated by sea level variations. Further investigations of the hypothesis that sea level variations may influence MOR dynamics are presented in appendices one and two. In appendix one I explore whether variations at the timescales of glacial cycles are apparent in MCS datasets from the intermediate spreading JdF ridge as well as bathymetry data from the fast spreading EPR. In appendix two I present a case study in which I re-examine the crustal thickness and bathymetry data from the northern EPR presented in chapter one in order to assess how fine-scale segmentation of the ridge axis appears in data, and compare different methodological approaches to describing MOR generated topography. In my second chapter I present results from a wide-angle controlled source seismic experiment conducted along a transect crossing the JdF plate from ~20 km east of the axis at the Endeavour segment of the JdF ridge to the Cascadia margin off of Washington state. I utilize a joint refraction-reflection traveltime inversion to generate a two-dimensional tomographic Vp model of the sediments, crust and upper mantle. Analysis of this Vp model, along with characterization of the basement topography along the transect, reveals three intervals (spanning millions of years) of distinct crust and upper mantle properties indicating a spatially heterogeneous JdF plate which is interpreted as inherited from changes in the mode of accretion at the paleo-JdF ridge, differences in plate interior processes, and deformation near the subduction zone. In my third chapter I present results of a MCS study of the sediment section conducted along a transect spanning ~350 km along the Cascadia margin from offshore southern Oregon to offshore Washington state. In this study I utilize prestack depth migrated MCS data to describe the reflectivity of the sediment section and invert for impedance and density. I also present results of amplitude variation with angle of incidence analysis conducted using pre-stack seismic gathers. Results indicate along margin variations in the characteristics of the sediments as well as complex changes in the stress state along the Cascadia margin. Synthesis of these analyses provides an in-depth assessment of patterns of sedimentation and properties of the sediment section as it experiences the effects of the onset of subduction.

Indiana University-Purdue University Indianapolis (IUPUI) / This dissertation has three main focuses: (1) identify the distribution of a new rock type (Mg-spinel lithology) on the Moon and explore the likely petrogenesis of Mg-spinel; (2) investigate the presence of olivine in the crater central peaks and analyze the sources of olivine; (3) determine the compositional variations of lunar crust with depth, and establish a new model to describe the structure of the lunar crust.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Ocean Engineering; and the Woods Hole Oceanographic Institution), 1991. / Includes bibliographical references (p. 247-253). / Sr, Nd and Pb isotopic analyses of 477 samples representing 30 islands or island groups, 3 seamounts or seamount chains, 2 oceanic ridges and 1 oceanic plateau [for a total of 36 geographic features] are compiled to form a comprehensive oceanic island basalt [OIB] data set. These samples are supplemented by 90 selected mid-ocean ridge basalt [MORB] samples to give adequate representation to MORB as an oceanic basalt end-member. This comprehensive data set is used to infer information about the Earth's mantle. Principal component analysis of the OIB+MORB data set shows that the first three principal components account for 97.5% of the variance of the data. Thus, only four mantle end-member components [EMI, EMII, HIMU and DMM I are required to completely encompass the range of known isotopic values. Each sample is expressed in terms of percentages of the four mantle components, assuming linear mixing. There is significant correlation between location and isotopic signature within geographic features, but not between them, so discrimination analysis of the viability of separating the oceanic islands into those lying inside and outside Hart's (1984, 1988) DUPAL belt is performed on the feature level and yields positive results. A "continuous layer model" is applied to the mantle component percentage data to solve for the spherical harmonic coefficients using approximation methods. Only the degrees 0-5 coefficients can be solved for since there are only 36 features. The EMI and HIMU percentage data sets must be filtered to avoid aliasing. Due to the nature of the data, the coefficients must be solved for using singular value decomposition [SVD], versus the least squares method. The F-test provides an objective way to estimate the number of singular values to retain when solving with SVD. With respect to the behavior of geophysics control data sets, only the degree 2 spherical harmonic coefficients for the mantle components can be estimated with a reasonable level of confidence with this method. Applying a "delta-function model" removes the problem of aliasing and simplifies the spherical harmonic coefficient solutions from integration on the globe to summation over the geographic features due to the properties of deltafunctions. With respect to the behavior of geophysics control data sets, at least the degree 2 spherical harmonic coefficients for the mantle components can be estimated with confidence, if not the degrees 3 and 4 as well. Delta-function model solutions are, to some extent, controlled by the nonuniform feature distribution, while the continuous layer model solutions are not. The mantle component amplitude spectra, for both models, show power at all degrees, with no one degree dominating. The DUPAL components [EMI, EMII and HIMU], for both models, correlate well with the degree 2 geoid, indicating a deep origin for the components since the degrees 2-3 geoid is inferred to result from topography at the core-mantle boundary. The DUPAL and DMM components, for both models, correlate well [and negatively] at degree 3 with the velocity anomalies of the Clayton-Comer seismic tomography model in the 2500-2900 km depth range [immediately above the core mantle boundary]. The EMII component correlates well [and positively] at degree 5 with the velocity anomalies of the Clayton-Comer model in the 700-1290 km depth range, indicating a subduction related origin. Similar positive correlations for the geoid in the upper lower mantle indicate that subducted slabs extend beyond the 670 km seismic discontinuity and support a whole-mantle convection model. / Lynn A. Oschmann. / Ph.D.