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No description available.

No description available.

No description available.

In this thesis, I present one- and three-dimensional numerical solutions to a two-phase fluid flow problem. The context of these investigations is the evolution of a viscous permeable matrix with a small fraction of melt that is representative of partial melt in the Earth's mantle. The matrix compacts under gravity as melt moves upward. In addition to the simple compaction solution, a range of solutions representing stably propagating waves are possible. I first present a coherent mathematical development of the governing equations for the three-dimensional problem. I then describe a one-dimensional numerical algorithm (1D2PF) that solves the second-order inhomogeneous P.D.E. for the velocity of the viscous matrix, V, for arbitrary melt fraction distribution, φ (the volume fraction occupied by melt). Combined with a time-stepping algorithm which advances the melt fraction in time, fully time-dependent 1D solutions are obtained. With an initial constant base melt fraction φ0 with a superposed localised concentration of melt, I explore the evolution and formation of solitary compaction waves. Using (1D2PF) I investigate the width, amplitude and phase velocity of stable solitary waves, and examine how these parameters depend on the initial conditions, permeability coefficient (k0) and melt and matrix viscosities (ηf and ηm). I demonstrate the existence of a threshold initial width above which secondary solitary waves form, with larger widths producing longer wave trains and smaller widths producing a small-amplitude oscillatory disturbance to the background melt fraction. Experiments with k0, ηf and ηm reveal that the width of the stable solitary wave is simply proportional to the compaction length parameter δ=√k0ηm/ ηf and its velocity varies as δ16/ 9/ηm . I also show that the width of the solitary waves varies as λS=4.6δ and the amplitude follows the relation AS≃89/δ . For initial melt fractions whose distribution is wider than the threshold width, secondary waves are produced with progressively smaller amplitude, and hence slower propagation velocity. I demonstrate that smaller values of δ result in the same volume of melt being partitioned over increasing numbers of relatively thinner solitary waves. The amplitude of the initial perturbation to the background melt fraction however is shown to have no effect on the number of solitary waves produced. A train of such waves arriving at the surface could provide an explanation of intermittentvolcanic activity above a region of partial melt. In a preliminary study of two-phase flow in three-dimensions I have also made significant progress toward the development of a three-dimensional two-phase flow simulation program. To do so, I have adapted the three-dimensional viscous fluid convection program (TDCON) by Houseman (1990). The new program TD2PF depends on a potential-function formulation similar to that of Spiegelman (1993a), in which the divergence of the matrix velocity field, D=∇·V, and the vector potential, A, are the primary variables. I have introduced new functionality to a significantly expanded threedimensional Poisson solver (program TDPOTS) but lack of time prevented a successful conclusion to the development of a general 3D solver for the divergence field D.

Evidence for chemical heterogeneity in the mantle is widespread in oceanic basalts, yet its consequences for basalt petrogenesis are little understood. A significant unknown is the effect that heterogeneity has on the dynamics of magma flow in the mantle. Observations of oceanic crust and the upper mantle suggest that magma migrates to the surface through a network of high porosity channels. In this thesis, I use computational models of coupled magma/mantle dynamics beneath mid-ocean ridges to question whether a physical connection exists between channelized flow and mantle heterogeneity. The models are intialized with simple, hypothetical patterns of heterogeneity that cause the fusiblity of the model mantle to vary. The principal result is that channelized melt flow is a consequence of melting in a heterogeneous mantle. Magma from preferentially melted heterogeneities nucleates high porosity, high permeability channels that grow by a feedback between magma flux and dissolution. Using the models in various configurations, I explore the dynamics of channel formation and investigate how the topology of mantle heterogeneity affects melt segregation and focusing be- neath ridge axes. Additionally, I use the models to predict the speed and time scale of melt migration. A simple model of equilibrium partitioning is used to cast the results in terms of 230Th disequilibria. Comparisons of the modelled geochemistry against global measurements indicate that the models presented here provide a reasonable, first-order description of the dynamics of magma flow beneath ridges. I also explore a systematic connection between plate kinematics and global pat- terns of mid-ocean ridge bathymetry with three dimensional models of solid mantle flow beneath transform faults. The results provide new constraints on the scale of melt focusing and melt redistribution at ridge axes, and pose questions for future 3D studies of melt migration beneath ridges .

This dissertation attempts to determine three non-traditional stable isotope compositions, lithium (Li), magnesium (Mg) and molybdenum (Mo), of Earth's mantle and investigate the possible processes that cause the mass-dependent fractionation of the samples and apply it to trace diffusion and recycling processes al high temperature. Analytical techniques for Li and Mg isotope analysis are based on the approach of James and Palmer (2000) and Pogge ven Strandmann el a1. (2011). Tbe precision of Mg isotope analysis has been improved 10 < 0.03%0. This dissenation has built up a successful approach for the separation and precise isotope analyses of Mo in mantle basaliS (25D < 0.06%0) based on the approach of (Archer and Vance, 2008) and modified it for igneous rocks with low Mo concentration.

The lowermost mantle-also known as D"-comprises the few hundred kilometres above the core-mantle bound- ary, and is known to show significant seismic anisotropy. In this thesis I attempt to use observations of shear wave splitting to constrain flow in this region. In order to accurately measure splitting in D", it is necessary to constrain that present at the top of the mantle. Using the method of source-side shear wave splitting, over 100 novel measurements of anisotropy in the mantle beneath mid-ocean ridges are made across the globe. Splitting mostly increases away from the axis on ridges themselves, and fast directions become increasingly parallel to the spreading direction. This is consistent with the alignment of olivine a-axes being parallel to flow. However, models based purely on lattice preferred orientation of olivine cannot predict the observed difference between splitting in SKS waves by previous authors and the new S wave measurements; one possible explanation is the presence or horizontal pockets or melt at ~80-150 km depth. These new measurements allow us to vastly improve our understanding of both ridges processes and the lowermost mantle, as they provide the means to use mid-ocean ridge earthquakes to probe regions of D" inaccessible with only deep events. Over 700 measurements of differential S-ScS splitting are then made, using corrections obtained for the upper mantle, beneath North and Central America. Fast orientations along paths from South American earthquakes are consistent with previous observations (showing horizontally-polarised shear waves travel faster than vertically- polarised ones), but measurements made along paths from mid-ocean ridge earthquakes constrain the possible symmetries of anisotropy in the lowermost mantle. They show that radial anisotropy is not an adequate approximation to the style of anisotropy present in D" under the Caribbean. Using elastic constants obtained from ab initio calculations and deformation experiments for a variety of candidate lowermost mantle phases, the shear planes and directions which are compatible with the observations are shown. Assuming horizontal shear, slip on (001) planes in post-perovskite seems the likeliest mechanism to produce the observed splitting. These and earlier measurements of shear wave splitting are then tested against recent models of mantle flow derived from seismic and other geophysical constraints. The flow model is used to derive elastic constants using three different candidate sets of slip systems in post-perovskite. A method to forward-model the shear wave splitting in an arbitrarily anisotropic Earth is developed and used to show that the plasticity model which favours slip on (010) in post-perovskite produces fast orientations best compatible with observations. However, not all observations can be accurately reproduced, suggesting that deformation-induced texturing in post-perovskite may not be the only mechanism producing anisotropy in DU I suggest possible routes towards addressing our current lack of understanding of lowermost mantle processes, some of which are developed in the current work.

No description available.

Intraplate magmatism provides important constraints on the evolution, dynamics and composition of the Earth's mantle. Uncertainties remain, however, in understanding the mantle sources related to intraplate magmatism. For example, the influence or existence of mantle stratification, core-mantle interaction and recycling of subducted components are poorly understood. The extent to which geochemical signatures of intraplate magmas are modified during melt transportation, eruption and emplacement also requires better definition. This thesis employs He, O and Re-Os isotope systematics on selected volcanic and intrusive rocks from a number of oceanic and continental intraplate settings to address some of these key issues. New constraints on the effect of syn-eruptive degassing and post-emplacement mobility processes on intraplate magmas are presented. Ocean Island Basalts (OIB) from the Western Canary Islands and Coppermine Continental Flood Basalts (CFB) suggest that Re degassing for both modem and ancient alkaline and tholeiitic sub-aerially empted lavas are similar in extent. Crustal contamination is shown to explain some low δ(^18)O (≤5%(_0) in central Icelandic lavas and low (^3)He/'(^4)He (<8R(_A)) in Western Canary Island and central Iceland lavas. Study of the Mackenzie large igneous province (LIP) also reveals that relatively limited (2-4%) crustal contamination can generate large (^187)Os/(^188)Os isotopic variation (≥80 γOs units) in high Os concentration cumulate rocks. This finding provides direct support for the role of crustal contamination in the formation of stratiform chromitite reefs in the Muskox Intrusion. By inference, contamination by ancient (≥0.5Ga) continental crust might also explain coupled (^186)Os-(^187)Os enrichments in some continental intraplate magmas. He isotope ratios of intraplate magmas have been used to infer mantle stratification with both shallow and deep mande origins proposed for a long-lived, primordial, high (^3)He/(^4)He reservoir. In this work the continental lithospheric mantle, especially Archaean peridotite and eclogite, are shown to possess low (^3)He/(^4)He (<4R(_A)). This observation, combined with low (^3)He/(^4)He (6±1R(_A)) of continental intraplate alkaline volcanics (CIAV), derived from the lithosphere-asthenosphere boundary, provides evidence that a long-lived primordial He component does not exist in the shallow upper mantle. Low δ(^18)O (< 0.9%(_0)) and high (^187)Os/(^188)Os (γOs = +8 to +37) for high Os concentration (> 50 ppt) Western Canary OIB indicate that recycled oceanic crust forms part of the mantle source for these basalts, a theme also common to low δ(^18)O measured in central Icelandic glasses with (^3)He/(^4)He ratios similar to those of mid ocean ridge basalts. Decoupling of lithophile (Sr, Nd, Pb) and siderophile (Os) isotope tracers from He in both settings provides evidence for (^3)He-recharge with (^3)He/(^4)He up to 11.8R(_A) for the Western Canary Islands and 34.3R(_A) for central Icelandic lavas. There is evidence in Western Canary Island lavas, and in OIB globally, that a common or 'universal' mantle component is present with a near-chondritic (^187)Os/(^188)Os (≥0.127), mantle-like δ(^18)O (~5.2%(_0)) and elevated (^206)Pb/(^204)Pb (≥19.1) with respect to the depleted mantle; the so-called FOZO (Focus Zone), PHEM (Primary He mantle), or C (Common) components. Coppermine Re-Os isochron ages (1284 ± 13 Ma) are within error of the accepted 1270 ± 4 Ma U-Pb age for the Muskox Intrusion. γOs initials derived from both the Coppermine CFB (+2.0) and Muskox Intmsion (+1.3) lie on an enstatite or ordinary chondritic Os isotopic evolution curve for the Earth's mantle indicating derivation from a source similar to the universal component recognised in modem-day OIB. These results can be interpreted in the context of a veined or banded peridotite-pyroxenite mantle. Small amounts of partial melting of a heterogeneous mantle source leads to great isotopic variability whilst larger degrees of partial melting will lead to more homogeneous mantle-derived compositions. Pyroxenite sources might also explain the correlated isotopic and elemental compositions, including the possibility of (^186)Os-(^187)Os enrichment in OIB through high Re/Os and Pt/Os ratios. Ultimately, a universal mantle component appears to be present in all intraplate magmatism. This universal component is not always related to high (^3)He/(^4)He and appears to provide an endmember component to all intraplate mantle melting signatures from the Archaean to the present-day.