京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18802号 / 理博第4060号 / 新制||理||1584(附属図書館) / 31753 / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 竹村 惠二, 教授 平原 和朗, 教授 大沢 信二 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

In the current study we developed an internal state variable (ISV) model based on the Bammann inelasticity internal state variable model (BIISV) to include damage, recrystallization, and texture development, which we then implemented into a mantle convection code, TERRA2D, to incorporate higher fidelity material behavior into mantle convection simulations. With experimental stress strain data found in the literature model constants for the BIISV model were determined for a number of geologic materials. The BIISV model was shown to be far superior to the steady state power law model currently used by the geologic community to capture the deformation of geologic materials. Once implemented and verified in TERRA2D the BIISV model revealed locations of hardened material that behaved like diverters in the cold thermal boundary layer that the power law model could never produce. These hardened regions could be a plausible reason for the current subduction zones present on the earth. We then altered the BIISV model equation to include the effects of damage, recrystallization, and texture development in order to model possible weakening mechanisms in the cold thermal boundary layer of the mantle. Inclusion of damage and recrystallization allowed the cold thermal boundary layer to mobilize and plunge downward into the hotter region below. Texture development increased the intensity of rotational flow within the hotter zone as cold boundary material plunged downward which aided in destabilizing the cold upper thermal boundary layer. The inclusion of an internal state variable model with damage, recrystallization, and texture development represents a significant advancement in handling deformational physics for mantle phenomena in a comprehensive, unified, and automatic manner.

Reflectivity images of the lower crust and uppermost mantle beneath the Illinois basin have been derived from reprocessing of several hundred kilometers of industry seismic reflection data using extended vibroseis recorrelation. The recorrelation was based on extending an originally 4-s correlated record, acquired with a 16-s sweep from 14 to 126 Hz, to the absolute limit of the full 20 s (~70 km) listening travel time. The reconstructed bandwidth includes frequency components suitable for imaging structures from signals received from both sedimentary basin reflectors and those received from reflectors in the deep crust and upper mantle. Mantle and sub-Moho reflectors are imaged down to 18 s two-way travel time (~62 km) and are observed on intersecting profiles generally dipping to the southwest and striking northwest-southeast. Occasional Moho reflections are also observed across the profiles (~12 s or ~38 km) while reflectivity in the lower crust is generally marked by intermittent horizontal packages and short, gently dipping reflections and diffraction segments. The presence of newly observed mantle reflectivity beneath the Illinois basin indicates significant upper mantle heterogeneity, relative to other parts of the USA studied using reflection methods. The relatively isolated occurrence of mantle reflections beneath the basin makes it difficult to uniquely infer their origin. However, available geologic and geophysical constraints, especially from geochemical and geochronological studies of drilled basement rocks, effectively limit the possibilities to: (1) remnants or "scars" of sub-crustal processes associated with lithospheric extension or delamination related to the melting of the Proterozoic crust that led to the emplacement of the granite--rhyolite province that underlies much of USA Midcontinent; or (2) deformation caused by plate subduction associated with the hypothetical accretion of a juvenile arc to the pre-1.6 Ga southern margin of the Laurentian continent.

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Mars has an intriguing topographic and crustal dichotomy that divides the planet into a Northern Hemisphere and a Southern Hemisphere. The biggest question is to understand how this dichotomy was formed and what are the implications to the interior evolution of the planet. This study uses topography and gravity data obtained from the Mars Global Surveyor satellite observations in a crustal thickness program written by Mark Wieczorek. This program uses an algorithm that incorporates spherical harmonic coefficients of topography and gravity and calculates the Moho topography given a set of user-input parameters; it also generates crustal thickness maps. I modified the code to produce gravity misfit maps for interpretation of crustal and mantle structures. Experiments were performed to test various parameters in the program and observe their effects on the resulting crustal thickness. The algorithm assumes a uniform density for the crust and mantle. A crustal density of 2900 kg/m3 and a mantle density of 3500 kg/m3 were used to represent a basaltic crust and an olivine mantle, derived from petrologic studies of meteorites assumed to have originated from Mars. As the crustal density is held constant while varying the mantle density (and vice versa), it is the total density contrast that changes the crustal thickness; the larger the density contrast, the smaller the crustal thickness. Higher-order terms in the algorithm equation were also tested. Results show that the higher-order terms do not affect the overall crustal thickness by much thus are negligible. In modeling the crustal thickness, a minimum crustal thickness of 5 km was used to anchor the topography so that the resulting crust do not have negative or zero thickness. By using this minimum thickness, there is about 40 km thickness difference between the Northern Lowlands and the Southern Highlands. Crustal thickness and gravity misfit maps show a plume-like track from the South Pole to the Tharsis Rise region, consistent with the theory proposed by Zhong (2010) on a mantle plume track. A ring-like feature in the misfit maps resembles a regional-scale impact crater that may have created the Borealis Basin in the Northern Lowlands, also consistent with current hypotheses about the formation of the Northern Lowlands. A result of Airy isostacy (with only the linear terms in topography) is that the Moho topography will be related to the surface topography with a scale factor depending on the crust-mantle density contrast. Isostatic studies here reveal that Mars may be more or less isostatic at spherical harmonic degree-1 wavelength scale where as at spherical harmonic degree-2, an impact-like structure is revealed just east of the Tharsis Rise region, suggesting a possible impact. Due to the non-uniqueness of gravity, these models have a limited constraint on the crustal thickness and further studies involving a seismometer are needed to get a more precise look into Mars' interior. / Master of Science

Mantle plumes are anomalously hot, narrow upwellings of mantle material that originate at the core-mantle boundary. As plumes rise they may form volumetrically large "heads" (~1000 km in diameter) with narrower (~100 km) "tails." Plume head melting is thought to form Large Igneous Provinces (LIPs), vast outpourings of basaltic lava (~106 km3), while plume tail melting forms linear chains of ocean island basalts (OIBs) similar the Emperor-Hawaii Seamount chain. Mantle plume derived melts indicate that these structures sample deep Earth geochemical and lithological heterogeneities. Studying plume-derived lavas can clarify important planetary-scale questions relating to the accretion of the Earth, primordial geochemical reservoirs, the fate of subducted materials, planetary differentiation, and convective mixing. / Ph. D. / Mantle plumes are hot, narrow upwellings of plastically flowing mantle material. These structures are thought to originate at the core-mantle boundary. Because mantle plumes originate in the deep interior of the planet, they are though to sample both primitive materials that are remnants of Earth’s formation as well as recycled crustal materials that have been subducted from the surface into the deep interior of the planet. When mantle plumes near the surface of the planet they begin to partially melt during a process known as adiabatic decompression melting. When these melts cool, they crystallize to form basalts. These rocks and their associated minerals can be studied to determine lava temperatures and pressures of formation. The geologic record suggests that relatively recently mantle plumes cool and eventually become magmatically inactive. In this project, we used the Galapagos plume as a case study to investigate why it has systematically cooled over the last 90 Ma. The Galapagos mantle plume is possibly the oldest active plume and records a 90 Ma volcanic evolution. We studied Galapagos-related lavas and olivine crystals across the entire 90 Ma evolution of the plume to better understand the life-death cycle of mantle plumes. Our data suggest that the plume may be cooling due to an increase in the amount of recycled oceanic crust. Alternatively, a recycled oceanic crust component could have always been present in the source of the plume, though was diluted during high degrees of partial melting when it was hottest at 90 Ma.

This thesis focuses on calculation of finite and progressive deformation in different tectonic scales using 2D numerical models with application to natural cases. Essentially, two major tectonic areas have been covered: a) salt tectonics and b) upper mantle deformation due to interaction between the lithosphere and asthenosphere. The focus in salt tectonics lies on deformation within down-built diapirs consisting of a source layer feeding a vertical stem. Three deformation regimes have been identified within the salt: (I) a squeezing channel flow underneath the overburden, (II) a corner flow underneath the stem, and (III) a pure channel flow within the stem. The results of the model show that the deformation pattern within the stem of a diapir (e.g. symmetric or asymmetric) can reveal information on different rates of salt supplies from the source layer (e.g. observed in Klodowa-diapir, Poland). Composite rock salt rheology results in strong localization and amplification of the strain along the salt layer boundaries in comparison to Newtonian rock salt. Flow and fold structures of passive marker lines are directly correlated to natural folds within a salt diapir. In case of the upper mantle, focus lies on deformation and resulting lattice preferred orientation (LPO) underneath an oceanic plate. Sensitivity of deformation and seismic anisotropy on rheology, grain size (d), temperature (T), and kinematics (v) has been investigated. The results of the model show that the mechanical lithosphere-asthenosphere boundary is strongly controlled by T and less so by v or d. A higher strain concentration within the asthenosphere (e.g. for smaller potential mantle temperatures, higher plate velocities, or smaller d) indicates a weaker coupling between the plate and the underlying mantle, which becomes stronger with the age of the plate. A Poiseuille flow within the asthenosphere, significantly affects the deformation and LPO in the upper mantle. The results of the model show, that deformation in the upper mantle at a certain distance away from the ridge depends on the absolute velocity in the asthenosphere. However, only in cases of a driving upper mantle base does the seismic anisotropy and delay times reach values within the range of natural data.

Processos de fusão parcial e metassomatismo foram identificados em xenólitos mantélicos da Província Ígnea Alto Paranaíba (PIAP), na região de Coromandel (Minas Gerais), pela combinação de técnicas de análise textural (petrografia, microscopia eletrônica de varredura), análises químicas minerais pontuais de elementos maiores (por Microssonda Eletrônica), traços (por LA-Q-ICPMS) e isótopos (Sr em clinopiroxênio por LA-MC-ICPMS). Os xenólitos, oriundos dos kimberlitos Indaiá 1, Limeira 1 e Forca, foram divididos em três grupos exibindo texturas, mineralogia e química distintas: peridotitos com espinélio ou Cr- espinélio, interpretados como fragmentos de fácies espinélio no manto litosférico raso; xenólitos com flogopita e/ou clinopiroxênio secundário representando porções do manto litosférico que sofreram metassomatismo por interação com fluidos ou magmas; e dunitos e harzburgitos com textura mosaico porfiroclástica a fluidal compatíveis com porções que sofreram efeitos de um fluxo térmico maior, possivelmente associadas a astenosfera ou a zonas de maior permeabilidade na litosfera. De modo geral os xenólitos mantélicos da Província Ígnea do Alto Paranaíba foram afetados por intensa fusão parcial, indicada especialmente pelo alto valor de mg# em olivina e ortopiroxênio, e pelo empobrecimento de Al em clinopiroxênio e minerais do grupo do espinélio. Boa parte desses xenólitos foi modificada por metassomatismo modal e/ou críptico, que atingiu inclusive níveis rasos do manto litosférico (fácies espinélio). Os harzburgitos e dunitos com textura porfiroclástica com mosaico têm baixos valores de mg# em olivina e ortopiroxênio, e altas concentrações de de Ti, Ca e Na em relação aos demais grupos de amostras sugerindo um processo de metassomatismo críptico de alta temperatura (astenosfera, ou zonas permeáveis da litosfera). Estimativas com base em razões \'Y IND.N\' e \'Yb IND.N\' (normalizadas por Manto PrimitivoPM) em clinopiroxênio, indicam que os peridotitos com Cr-espinélio sofreram uma porcentagem maior de fusão parcial (15-20%), em relação aos peridotitos com espinélio (5-8%), e preferencialmente fracionada. Em algumas amostras nos peridotitos com Crespinélio ainda são encontrados padrões enriquecidos de \'ETRL IND.N\', indicando processos de metassomatismo críptico. Os cristais de clinopiroxênio das amostras de xenólitos metassomatisados têm padrão de distribuição de \'ETR.IND.N\' semelhantes entre si, com padrão de \'ETRL IND.N\' convexo e enriquecido em relação aos \'ETRP IND.N\' (\'La IND.N\' e \'Lu IND.N\', respectivamente 5-14xPM e 0,3-1xPM). Uma amostra de harzburgito com bolsões de clinopiroxênio (In1-12), tem o clinopiroxênio com as maiores razões de \'ETR IND.N\' do conjunto, mantendo o padrão de \'ETRL IND.N\' convexo e enriquecido em relação aos \'ETRP IND.N\' \'(La IND.N\' e \'Lu IND.N\', respectivamente 2543xPM e 1-2,5xPM) dos peridotitos metassomatisados. Razões \'ANTPOT.87 Sr\'/ANTPOT. 86 Sr\' de clinopiroxênio desta última amostra (0,706-0,714) e de flogopita ilmenita wehrlito (0,706-7,10) sugerem que o metassomatismo seria derivado de porção da astenosfera ou litosfera com material proveniente de subducção de crosta. Um valor mais baixo de razão \'ANTPOT. 87 Sr\'/\'ANTPOT.86 Sr\' para clinopiroxênio de peridotito com espinélio (0,703),indica, juntamente com aspectos texturais e de química de maiores e traços, pouca ou nenhuma, influencia do metassomatismo nesta amostra. / Partial melting processes and metasomatism were identified in mantle xenoliths of the Alto Paranaíba Igneous Province (APIP), next Coromandel (Minas Gerais), by combining textural analysis techniques (petrography, scanning electron microscopy) and specific mineral chemical analyzes of major elements (for Electronic Microprobe), trace elements (LA-Q-ICPMS) and isotopes (Sr in clinopyroxene by LA-MC-ICPMS). The xenoliths which are from Indaiá 1, Limeira 1 and Forca kimberlite were divided into three groups displaying different textures, mineralogy and chemistry: peridotites with spinel or Cr-spinel, were interpreted as fragments in the shallow lithospheric mantle; xenoliths with phlogopite and/or secondary clinopyroxene representing the lithospheric mantle portions that suffered metasomatism by interaction with fluids or magmas; and dunites and harzburgites with porphyroclastic texture with mosaic are compatible with portions that have suffered the effects of a higher heat flow, possibly associated with asthenosphere or zones of greater permeability in the lithosphere. Mantle xenoliths from APIP were affected by intense partial melting, suitable for high-value mg# of olivine and orthopyroxene, and the depletion of Al in clinopyroxene and spinel minerals from the group. Many of these xenoliths were modified by modal and/or cryptic metasomatism, which reached even shallow levels of the lithospheric mantle (facies spinel). Harzburgites and dunites with prophyroclastic texture with mosaic have low values of mg# and high concentrations of Ti, Ca and Na in olivine and orthopyroxene compared to other groups of samples suggesting a cryptic metasomatism process of high temperature. \'Y IND.N\' and \'Yb IND.N\' (normalized by Primitive Mantle-PM) ratios in clinopyroxene indicate that the peridotites with Cr-spinel suffered a higher percentage of partial melting (15-20%) compared to the peridotites with spinel (5-8%), and preferably fractional. Some samples in peridotites with Cr-spinel are still found \'LREE IND.N\' enriched ratios patterns, indicating cryptic metasomatism processes. The clinopyroxene crystals of metasomatized xenoliths samples have similar \'REE IND.N\' ratios pattern, with convex \'LREE IND.N\' pattern with higher ratios compared to \'HREE IND.N\' (LaN:5-14xPM and LuN 0,3-1xPM). A harzburgite with insterticial pockted clinopyroxene (In1-12), has the clinopyroxene with the highest \'REE IND.N\' of the set, the convex and enriched \'LREE IND.N\' pattern as well (\'La IND.N\': 25-43xPM and \'Lu IND.N\' 1-2.5XPM) of metasomatized peridotites. Clinopyroxene \'ANTPOT.87 Sr\'/\'ANTPOT.86 Sr\' ratios of the latter sample (0,706 to 0,714) and from phlogopite wehrlite ilmenite sample (0,706 to 0,710) suggest that metassomatism fluids from astenosphere or lithosphere were derivative from crust subduction processes. A lower value ratio \'ANTPOT.87 Sr\'/ANTPOT.86 Sr\' for clinopyroxene in peridotite with spinel (0,703), is in agreement with textural aspects, major and trace elements information, that this group of xenolith little, or no influence of metasomatism.