X-ray diffraction (XRD) was used to measure the unit cell parameters of pyrope (Mg3Al2Si3O12), grossular (Ca3Al2Si3O12) and four intermediate solid solutions synthesized at ~6 GPa and 1400°C in multi-anvil (MA) apparatus. Intermediate garnet solid solutions on this join show regular asymmetric positive excess volumes, but roughly 2-3 times bigger than previous reports from garnet hydrothermally synthesized in piston cylinder apparatus. The binary Margules equation used to fit this excess volume data gives parameters = 2.0±0.1 cm3/mol, which is about half as big as = 4.3±0.1 cm3/mol. Garnets synthesized in diamond anvil cells (DAC) with wider XRD peaks than those synthesized in MA have more nearly symmetrical excess volumes about 3 times larger than those previous piston cylinder syntheses. The large size difference between divalent Mg and Ca is the key reason for the non-ideal mixing properties of pyrope-grossular garnet solid solution. Partial order/disorder of Ca-Mg in the X site may be responsible for the interesting, variable mixing phenomena among different synthesis methods. The large excess volumes we measured from MA-synthesized garnets imply that the solvus of pyrope-grossular garnet will become experimentally accessible at high pressures, perhaps less than 10 GPa, unlike the expectations derived from the much smaller excess volumes of hydrothermally grown garnets. X-ray diffraction (XRD) methods and diamond anvil cells were used to measure the unit cell parameters of multi-anvil (MA) grown garnets: pyrope, grossular, and four intermediate solid solutions up to ~600°C and ~10 GPa. The unit cell parameters of these synthetic garnets increase with temperature and decrease with pressure. Thermal expansion coefficients of garnets in this series were calculated from the unit cell volumes' change with temperature. They are all in the range from ~2.0-2.8*10-5 K-1, and uniformly increase with temperature but differ with garnet compositions. At high temperature, the calculated thermal expansion coefficients of end-member pyrope and grossular in this study are larger than those reported by Skinner (1956). The compressional properties of these MA-synthesized pyrope and grossular are comparable with previous reports (Finger 1978; O'Neill et al., 1989; Zhang et al., 1998 and 1999; Pavese et al., 2001; Jiang et al., 2004). The Birch-Murnaghan EOS yields Κ0=165.4±1.8GPa, with Κ0’ fixed to be 5.92 for grossular and Κ0=172.5±2.0GPa for pyrope, with Κ0’ fixed to be 4.4. The bulk moduli of garnet with intermediate composition are all 155~160GPa, smaller than the end-members, showing no significant compositional dependence, as is consistent with the fact that garnets on this join have large positive excess volume, which makes them more compressible at high pressure. Application of these results indicates that the excess volumes in the pyrope-grossular series remain high even at high P and T. Multi-anvil (MA) technique was used to study the grossular-pyrope garnet solid solution for conditions of pressure and temperature stability against exsolution. Two garnet phases Py90Gr10 and Py40Gr60 with wt.% composition ratio 1:1, the expected consolute composition, were heated at 6GPa and different temperatures. XRD measurement results showed that these two garnet phases converged completely to one phase with composition ~Py65Gr35 at 1200°C, indicating that the critical temperature of pyrope-grossular garnet solvus is lower than 1200°C, lower than some literature modeling results (Haselton and Newton, 1980). At 8GPa, long term heating experiments for both convergence and divergence showed that two garnet phases with composition around ~Py82Gr18 and ~Py62Gr38 were equilibrated with each other at 1200°C. These garnet pairs represent the positions of the pyrope-grossular garnet solvus' two limbs at 1200°C and 8 GPa. Convergence experiments at 1100 °C and 8 GPa also showed changing composition of a widely-separated compositionally different garnet pair. However the equilibrium composition at 1100 °C and 8 GPa failed to be constrained by divergence heating experiments because the relatively low temperature and much slow diffusion rate of Mg/Ca cation exchange in garnet. Observations of garnet immiscibility at < 10 GPa reported here suggest that the MA-garnet excess volumes represent internal equilibrium values. Deduction from our new two phase equilibrium experiments shows that pyrope-grossular solvus has a higher critical temperature in the range 800-900°C at 1 bar compared to previous thermodynamic models (T < 600°C) (Ganguly et al., 1996), suggesting that at pressure as high as 2GPa, exsolution in garnet can happen at a higher temperature than previous thought, which is strongly supported by the high temperature (800-860ºC) exsolution in garnet samples found from natural metagabbro, South Harris (Cressey,1978), and an immiscible garnet pair in pyrope-rich garnet crystal collected from Garnet Ridge, Arizona was reported by Wang et al. (2000).

Orientador: Sebastião Gomes de Carvalho / Banca: Aroldo Misi / Banca: Yociteru Hasui / Banca: Jorge Silva Bettencourt / Banca: Antenor Zanardo / O presente trabalho reúne 3 artigos científicos, já formalizados para publicação em revistas internacionais, foi confeccionado para ser apresentado como tese de doutorado / Artigos em inglês, introdução e resumo em português / Resumo: O depósito de Fortaleza de Minas vem sendo estudado a mais de 20 anos desde a sua descoberta em 1983 e apresenta similaridades com outros depósitos komatiíticos arqueanos descritos na literatura. Sua configuração atual reflete ação de processos metamórficos e deformacionais diversos gerando novos tipos de minério sem, no entanto, perder sua identidade primária komatiítica e permitindo reconstruir sua faciologia vulcanológica. Contexto regional A região de Fortaleza de Minas corresponde a um bloco cratônico arqueano retrabalhado na Faixa Móvel Brasília de idade neoproterozóica. Os terrenos granito-greenstone afloram em meio à metasedimentos supracrustais proterozóicos dos grupos Araxá e Canastra que compõe a nappe de Passos. O bloco arqueano corresponde ao limite sul do antigo Craton do Paramirim fazendo contato com o cinturão granulítico de Alfenas a Sul. Estudos geológicos e geofísicos recentes demonstram que esta área corresponde a uma zona de junção entre dois blocos crustais, denominados Brasília e São Paulo, que representam duas paleo-placas separadas pela zona de sutura de Alterosa. No bloco Brasília ocorrem os greenstone belts de Morro do Ferro e Pihum-i. As idades obtidas para estes greenstone belts estão distribuídas entre 2,8 e 2,9 Ga e 2,9 e 3,1 Ga respectivamente e são discutidas no capítulo 3. O greenstone belt do Morro do Ferro ocorre como faixas descontínuas e estreitas estruturadas em quilhas sinclinais representando as raízes desta seqüência greenstone. Estão fortemente deformadas e cortadas pelo sistema sinistral de falhas transcorrentes Campo do Meio responsável pela estruturação sigmoidal da região (capítulo 2) (Fig. 1, pg. 33 e Fig. 1, pg.4). O Greenstone Belt Morro do Ferro é representado por derrames komatiíticos e mais restritamente toleíticos com intercalações subordinadas de sedimentos químicos exalativos...(Resumo completo, clicar acesso eletrônico abaixo) / Abstract: After 20 years of exploration and mining in the Fortaleza de Minas nickel deposit (formerly known as O'Toole) a better understanding of the geological framework and the volcanic environment was achieved. The different ore types observed in the deposit and its distribution revealed the original volcanic setting for the ore formation and also the geological evolution and transformation of the different ore types. The nickel mineralization is classified as a Type 1 deposit (Lesher and Keays, 2002). It is associated to an open trough structure of lava pathway. Later metamorphism and deformation obliterated most of the original volcanic textures and promoted a strong stretching of the ore zone and remobilization of the massive breccia ore along a major shear zone installed at the base of an upper fractionated host flow unit in contact with a footwall BIF. In the Neoproterozoic Brasiliano Cycle (0.6 Ga) a new ore type was formed of hydrothermal origin with extremely high nickel grades and PGE nuggests...(Complete abstract, click electronic address below) / Doutor

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Rich gold-telluride lodes (steeply dipping and flatly dipping) and minor gold-quartz stockwork mineralization characterize the Kalgoorlie gold-field. The origin of these gold deposits, the relationship between deposits and then nature of the host rocks are the major problems considered in this thesis. Extensive diamond drilling at the essentially unmineralized southern end of the field provided excellent material for stratigraphic studies and for country rock analysis whilst ore samples were obtained from both mines and drill core.

Rich gold-telluride lodes (steeply dipping and flatly dipping) and minor gold-quartz stockwork mineralization characterize the Kalgoorlie gold-field. The origin of these gold deposits, the relationship between deposits and then nature of the host rocks are the major problems considered in this thesis. Extensive diamond drilling at the essentially unmineralized southern end of the field provided excellent material for stratigraphic studies and for country rock analysis whilst ore samples were obtained from both mines and drill core.

The clay mineral composition of sediments deposited in the last six oxygen isotope stages in the Gulf of Mexico continental slope was characterized. Smectite and illite were found to be the two major clay minerals of the clay fraction while kaolinite, chlorite and quartz were present in the clay fraction but in less proportions. Variations in clay mineral abundances, especially in the relative abundances of smectite and illite, were identified in relation to climate changes. Smectite was the most abundant mineral in sediments of the current (stage 1) and last interglacial maxima (stage 5) while illite dominates the clay min-eralogy of sediments from the last glacial maximum (stage 2). Relationships between clay mineralogy and physical properties were investigated as well. Significant positive correla-tions were found between Atterberg limits with the smectite content of the bulk sediment and with clay content. However, the relationship with smectite yielded a significantly higher correlation coefficient. Smectite and clay content also affect the natural water con-tent of sediments and its changes with depth.

The clay mineral composition of sediments deposited in the last six oxygen isotope stages in the Gulf of Mexico continental slope was characterized. Smectite and illite were found to be the two major clay minerals of the clay fraction while kaolinite, chlorite and quartz were present in the clay fraction but in less proportions. Variations in clay mineral abundances, especially in the relative abundances of smectite and illite, were identified in relation to climate changes. Smectite was the most abundant mineral in sediments of the current (stage 1) and last interglacial maxima (stage 5) while illite dominates the clay min-eralogy of sediments from the last glacial maximum (stage 2). Relationships between clay mineralogy and physical properties were investigated as well. Significant positive correla-tions were found between Atterberg limits with the smectite content of the bulk sediment and with clay content. However, the relationship with smectite yielded a significantly higher correlation coefficient. Smectite and clay content also affect the natural water con-tent of sediments and its changes with depth.

Thèse : Sciences : Paris, Faculté des sciences : 1812. / Titre provenant de l'écran-titre.

Thèse : Sciences physiques : Paris, Faculté des sciences : 1857. / Titre provenant de l'écran-titre.

Diagenesis is the sum of those processes whereby an originally sedimentary assemblage attempts to reach and maintain an equilibrium with its environment. Numerous factors influence and affect the diagenesis of sedimentary elastic assemblages - fundamentally unstable when deposited but never reaching equilibrium ith their environment. The interrelationship of these factors, however, precludes the identification of any single factor as wholly controlling the diagenesis of elastic sediments. The multifactorial nature of diagenesis is illustrated here by reference to the Middle Jurassic Dogger Formation and Ravenscar Group which outcrop on the North Yorkshire Moors and the Brent Group from Wells 3/3-1, -2, and -3 in the Ninian Field, and 210/15-2, also in the Northern North Sea.The Ravenscar Group is interpreted as being deposited in a fluvio-deltaic complex. Here, attempted equilibration of non-marine sediments with their pore waters resulted in a variety of diagenetic modifications. These are interpreted as being influenced strongly by bacterial degradation of organic matter, which lowered pH and then reduced Eh. This reduction of pH caused feldspar dissolution and muscovite neomorphosis to kandites throughout. Similarly, in the texturally and inineralogically mature sandbodies, quartz overgrowths and veriniform kandites precipitated from oxygenated pore waters, whilst chlorite and overgrowths formed in anoxic pore waters. This resulted in complete reduction of porosity in, and cementation of, finer and texturally less mature overbank sands. Conversely, channel sands were cemented into rigid but porous quartzose frameworks. In addition, soil horizons of sphaerosiderite developed as standing water on the floodplains stagnated. Bacterial ferric iron reduction throughout the water table then raised the pH and extensive siderite cements were precipitated. During burial, calcium carbonate saturated formation waters migrated into the remaining porous sandbodies and precipitated replacive ferroan calcite. In marine elastics, meanwhile, illite and potassium-feldspar overgrowths precipitated before in situ bacterial processes lowered pH here also. This resulted in dissolution of feldspars, muscovite neomorphism and precipitation of vermiform kandites. Subsequently, pH rose and ferroan calcite cementation occurred.It is suggested that "aggressive fluids" migrated into the larger connected sandbodies during burial. They dissolved the carbonate cements and precipitated dense pockets of blocky kandites. These sediments were little affected during continued burial. However, their Recent weathering may have dissolved carbonates and feldspars as well as neomorphosing chlorite to vermiculite.The reservoir rocks of the Ninian Field have formed from mature quartz-rich elastic sediments which accumulated in a transgressive sequence above Liassic mudrocks which was subsequently incised into by a fluvial system. Diagenetic modifications of the original sediments are similar to those of the Ravenscar Group, although neither chlorite nor soil horizons were observed in the wholly non-marine sediments. Porosity in marine sediments is occluded by extensive authigenic illite within a generally quartz framework. The effects of the fresh water table are also seen in the marine sediments over which the delta prograded. Then, during burial, ferroan calcite cementation and subsequent leaching and blocky kandite precipitation occurred here also. Hydrocarbon maturation and migration into the reservoir was preceded by alteration of pre-existing carbonates to ankerite, and. minor illitisation of blocky kandites. However, the only effort which can be related to oil emplacement is the widespread pyritisation around the oil-water contact.The sandstones from 210/15-2 are interpreted as formed from coarse elastic sediments which accumulated in a shallow-marine nearshore environment, possibly incised into by distributary channels. Initial marine connate water precipitated potassium-feldspar overgrowths before bacterial processes lowered pH and caused widespread kandite formation. Subsequently, these sediments were affected by ferroan calcite cementation, then leaching and blocky kandite precipitation. Although oil has migrated into these sediments, no other effects were observed.In addition to the factors which have been proposed previously as influencing diagenesis, I should like to propose that the climate of both the source area and of the depositional basin was of fundamental importance to diagenesis and many of the features observed in these rocks may be related to the original tropical climate. Moreover, as a result of the fundamental stability of the quartzose frameworks established during eogenesis, this climatic "finger print" may be recognised in all these sediments despite their subsequent diverse history.