In this thesis, I present a simple time-dependent model of heat transfer between a turbulently convecting and crystallizing magma body and the overlying hydrothermal circulation. Most of the known seafloor hydrothermal sites on faster-spreading ridges are dominated by basalt. The hydrothermal fields within parts of the Lau Basin in the Southwest Pacific are driven by andesitic magma. To determinate the different characteristics of magma-driven hydrothermal system, two types of magma material, basaltic and andesitic magma are considered. Two different crystallization scenarios are considered¡ªcrystals in suspension and crystals settling. In either case, I assume that large-scale convection within the magma chamber is homogenous. Also, the effect of crystallinity and water content-dependent magmatic viscosity is considered. Based on the proposed models, the total heat output from the upper surface of the magma chamber and the temperature in hydrothermal system are derived numerically. The simulation results show that without magma replenishment, the heat output and hydrothermal temperature decay rapidly within about ten years. For two different crystallization distribution cases, such rapid decay is not consistent with observations. The conflict between the simulation results and the field observations shows the need to develop more accurate magma convection models. Different from the existing modeling methods, I propose to model the magma convection with replenishment. The replenishment model can be classified into two categories in terms of status of magma chamber size. To replenish the magma system without changing the magma chamber size, the heat flux decaying rate is slowed down and hydrothermal system lifetime is extended for a little longer. Although this model is more accurate than existing ones in terms of slow decaying rate of heat flux, it does not achieve a steady state as is observed. This leads us to model replenishment with variant magma chamber size. I model the replenishment rate as a constant and exponential decay, respectively. Thus, I assume the magma chamber size is time-varying. Simulation results show that magma heat flux approaches a steady state over a time scale of decades. This result is consistent with the observations, which indicates the effectiveness of proposed modeling methods.

The Mid Ocean Ridge system with its volcanism and related hydrothermal activity has been a subject for many studies since the discovery of high temperature hydrothermal vents at the ridge surfaces in the 1970´s. This thesis focuses on deep sea hydrothermal activity on a superfast spreading ridge, the SouthernEast Pacific Rise (SEPR).The ridge is located in the South Pacific, off the coast of South America, and separates the Nazca Plate and the Pacific Plate. A fossil high temperature hydrothermal zone hosted by a fault was sampled 80 m below the lava/dike transition zone in the Pito Deep (a tectonic window intothe SEPR). Geochemical data from the fault zone indicates that cold (<150°C)and hot (<390°) fluids coexisted at the same time whilst the hydrothermal system was active. A numerical model (HYDROTHERM) developed by the USGS was used to recreate the geological settings in the SEPR in order to try to model the hydrothermal activity and fluid flow. The model solves two governingpartial differential equations numerically, the water component flow equation(Darcy law for flow in porous media) and the thermal energy transport equation(conservation of enthalpy for the water component and the porous media). The result of the modeling indicates that cold seawater can penetrate from the relatively permeable volcanic material into a highly permeable fault zone in the sheeted dike unit. The cooler seawater fluid flows down the fault zone,reheats and flows up again in a narrow upflow zone at the edge of the fracture/sheeted dike boundary. The result is a horizontal temperature gradient created in the fractured zone supporting the theory that hot and cold fluids can coexist in a fault hosted hydrothermal zone.

This study assesses the geological constraints on fluid flow within the main crater of Whakaari volcano (White Island) which is located in the Bay of Plenty, New Zealand. A review of the volcanological and morphological history, field mapping, and permeability experiments were used to propose a model for single-state (gas or liquid water) fluid flow in the volcano. Three structural scales were of most importance: (a) the elongate main crater (1.2 km by 0.5 km); which contains (b) three subcraters (~300-500 m in diameter); and (c) >14 historic eruption craters and crater complexes (30-300 m in diameter). A large (2.1x10⁸ m³) sector collapse formed the basic morphology and structure of the amphitheatre-like main crater ≤3.4 ka. Hot fluids are released from magma at ~1–2 km depth and circulated within a conduit-hosted volcano-hydrothermal system. The collapse event was likely to have removed low permeability cone lavas, significantly increasing meteoric water collection and lateral seawater infiltration within high permeability main crater fill above the magma conduit. It is proposed that this caused a susceptibility to ‘wet’ (i.e. phreatic and phreatomagmatic) eruptions which possibly formed three prehistoric subcraters and has been demonstrated in the last ~200 years of available historic record. The permeability of the remaining in-situ cone lavas is controlled by micro- (<1 mm) and macro- (>1 mm) cracks but despite these cracks, the cone lavas’ permeability is still sufficiently low to focus rising magmatic fluid flow through main crater fill. Low-to-high permeability lithified tuffs are inferred to fill the main crater at depth. Low permeability fine ash tuffs generally restrict vertical fluid flow put permit it when vertical trains of vesicles are present. Atmospheric steam and gas pluming is accommodated by a permeable zone of repeated and overlapping historic eruption crater-related discontinuities that extend to >250 m depth through highly permeable unlithified main crater fill in the west. It is likely to be this material into which the seawater infiltrates from the east. Throughout the main crater, fluid flow is focussed at subcrater margins due to steeply-dipping discontinuities between low permeability lava and low-to-high permeability crater fill deposits. The variable permeabilities of crater fill deposits are due to age-related factors of hydrothermal alteration, reworking/sorting, consolidation, and pore mineralisation. At shallow levels (<100 m depth), vertical fluid flow is diverted to historic eruption crater margins by very low permeability clay (reworked and altered tephra). High permeability coarse ash tuffs, Fe-rich lapilli tuffs, and surficial solfatara deposits do not appear to have much effect on the overall fluid flow system. The results of this study show that, within active volcanic craters, the spatial distributions of variably permeable lithologies are often related to discontinuous cratering structures. Together, these are significant geological constraints on fluid flow. Morphological changes to crater structure can directly impact the groundwater regime above the magma conduit and may strongly influence the occurrence of wet versus dry eruptions. This process is possibly a significant control on eruptive behaviour at volcanoes with similar fluid flow systems worldwide.

Hydrothermal systems at oceanic spreading centers play an important role in the composition of seawater, the formation of ore deposits, the support of microbial and macrofaunal ecosystems, and even for the development of life on early earth. These circulation systems are driven by heat transport from the underlying magma chamber, where latent heat of crystallization and sensible heat from cooling are transferred by vigorous, high Rayleigh number convection through a thin conductive boundary layer. The traditional study of magmatic-hydrothermal systems is primarily based on the time-series observation, which takes the form of repeat visits, continuous offline monitoring by autonomous instruments, or continuous online monitoring by instruments with satellite or cable links to shore. Although a number of studies have deployed autonomous monitoring instruments at vents and around mid-ocean ridges to investigate geophysical and hydrothermal processes, the data are still rather limited and a comprehensive understanding of magma-hydrothermal processes at oceanic spreading centers is lacking. Numerical modeling needs to be employed to elucidate the dynamic behavior of magmatic hydrothermal systems and for testing completing hypotheses in these complex, data-poor environments. In this dissertation, I develop a mathematical framework for investigating heat transport from a vigorously convecting, crystallizing, cooling, and replenished magma chamber to an overlying hydrothermal system at an oceanic spreading center. The resulting equations are solved numerically using MATLAB. The simulations proceed step-by-step to investigate several different aspects of the system. First, I consider a hydrothermal system driven by convection, cooling and crystallization in a ~ 100 m thick basaltic magma sill representing an axial magma chamber (AMC) at an oceanic spreading center. I investigate two different crystallization scenarios, crystal-suspended and crystal-settling, and consider both un-replenished and replenished AMCs. In cases without magma replenishment, the simulation results for crystals-suspended models show that heat output and the hydrothermal temperature decrease rapidly and crystallinity reaches 60% in less than ten years. In crystals-settling models, magma convection may last for decades, but decreasing heat output and hydrothermal temperatures still occur on decadal timescales. When magma replenishment is included, the magmatic heat flux approaches steady state on decadal timescales, while the magma body grows to double its original size. The rate of magma replenishment needed ranges between 5 x 10⁵ and 5 x 10⁶ m³/yr, which is somewhat faster than required for seafloor spreading, but less than fluxes to some terrestrial and subseafloor volcanoes on similar timescales. The heat output from a convecting, crystallizing, replenished magma body that is needed to drive observed high-temperature hydrothermal systems is consistent, with gabbro glacier models of crustal production at mid-ocean ridges. Secondly, I study the heat transfer model from a parametric perspective and examine the effects of both initial magma chamber thickness and magma replenishment rate on the hydrothermal heat output. The initial rate of convective heat transfer is independent of the initial sill thickness; but without magma replenishment, the rate of decay of the heat output varies linearly with thickness, resulting in short convective lifetimes and decaying hydrothermal temperatures for sills up to ~ 100m thick. When magma replenishment is included in crystals settling scenarios at constant or exponentially decreasing rates of ~ 10⁻⁸ m/s to the base of the sill, growth of the sill results in stabilized heat output and hydrothermal temperature on decadal timescales and a relatively constant to increasing thickness of the liquid layer. Sills initially ~ 10 m thick can grow, in principal, to ~ 10 times their initial size with stable heat output and a final melt thickness less than 100m. Seismic data provides evidence of AMC thickness, but it can not discriminate whether it denotes initial magma thickness or is a result of replenishment. These results suggest that magma replenishment might not be seismically detectable on decadal time scales. Periodic replenishment may also result in quasi-stable heat output, but the magnitude of the heat output may vary considerably in crystals suspended models at low frequencies; compared to crystals settling models. In these models the direct coupling between magmatic and hydrothermal heat output suggests that heat output fluctuations might be recorded in hydrothermal vents; but if damping effects of the basal conductive boundary layer and the upflow zone are taken into account, it seems unlikely that heat output fluctuations on a time scale of years would be recorded in hydrothermal vent temperatures or heat output. Thirdly, I extend the work to the binary system motivated by the fact that the real magmas are multi-component fluids. I focus on the extensively studied binary system, diopside-anorthite (Di-An), and investigate the effects of convection of a two-component magma system on the hydrothermal circulation system through the dynamic modeling of both temperature and heat output. I model the melt temperature and viscosity as a function of Di concentration, and incorporate these relations in the modeling of the heat flux. Simulations comparing the effects of different initial Di concentrations indicate that magmas with higher initial Di concentrations convect more vigorously, which results in faster heat transfer, more rapid removal of Di from the melt and growth of crystals on the floor. With magma replenishment, I assume that the magma chamber grows either horizontally or vertically. In either case magma replenishment at a constant rate of ~ 10⁻⁸ m³/a can maintain relatively stable heat output of 10⁷-10⁹ Watts and reasonable hydrothermal vent temperatures for decades. The final stabilized heat flux increases with increasing Di content of the added magma. Periodic replenishment with a 10 year period results in temperature perturbations within the magma that also increase as a function of increasing Di. With the simple magma model used here, one can not discern conclusively whether the decrease in magma temperature between the 1991/1992 and the 2005/2006 eruptions at EPR 9°50'N involved replenishment with more or less evolved magmas. Fourthly, I investigate a high-silica magma chamber as the hydrothermal circulation driver. I construct viscosity models for andesite and dacite melts as a function of temperature and water content and incorporate these expressions into a numerical model of thermal convective heat transport from a high Rayleigh number, well-mixed, crystallizing and replenished magma sill beneath a hydrothermal circulation system. Simulations comparing the time dependent heat flux from basalt, 0.1wt.% andesite, 3wt.% andesite, and 4wt.% dacite, indicate that higher viscosity magmas convect less vigorously, which results not only in lower heat transport and hydrothermal vent temperatures, but also in a lower decay rate of the vent temperature. Though somewhat colder, hydrothermal systems driven by unreplenished high-silica melts tend to have a longer lifetime than those driven by basalts, assuming a heat output cutoff of 10⁷ Watts. As in the basaltic case, magma replenishment at a rate of ~ 3 x 10⁵ - 3 x 10⁶ m³/a can maintain relatively stable heat output of 10⁷-10⁹ Watts and hydrothermal vent temperatures for decades. Idealized models of porous flow through the lower crust suggest such replenishment rates are not likely to occur, especially for high-viscosity magmas such as andesite and dacite. Long term stability of hydrothermal systems driven by these magmas requires an alternate means of magma replenishment. Finally, the dissertation concludes by discussing some avenues for future work. Most important of these are to: (1) couple magma convection with more realistic hydrothermal models and (2) link magma chamber processes to better physical models of replenishment and eruption.

Optimal generation scheduling attempts to minimise the cost of power production while satisfying the various operation constraints and physical limitations on the power system components. The thermal generation scheduling problem can be considered as a power system control problem acting over different time frames. The unit commitment phase determines the optimum pattern for starting up and shutting down the generating units over the designated scheduling period, while the economic dispatch phase is concerned with allocation of the load demand among the on-line generators. In a hydrothermal system the optimal scheduling of generation involves the allocation of generation among the hydro electric and thermal plants so as to minimise total operation costs of thermal plants while satisfying the various constraints on the hydraulic and power system network. This thesis reports on the development of genetic algorithm computation techniques for the solution of the short term generation scheduling problem for power systems having both thermal and hydro units. A comprehensive genetic algorithm modelling framework for thermal and hydrothermal scheduling problems using two genetic algorithm models, a canonical genetic algorithm and a deterministic crowding genetic algorithm, is presented. The thermal scheduling modelling framework incorporates unit minimum up and down times, demand and reserve constraints, cooling time dependent start up costs, unit ramp rates, and multiple unit operating states, while constraints such as multiple cascade hydraulic networks, river transport delays and variable head hydro plants, are accounted for in the hydraulic system modelling. These basic genetic algorithm models have been enhanced, using quasi problem decomposition, and hybridisation techniques, resulting in efficient generation scheduling algorithms. The results of the performance of the algorithms on small, medium and large scale power system problems is presented and compared with other conventional scheduling techniques.

Made available in DSpace on 2014-06-11T19:22:33Z (GMT). No. of bitstreams: 0 Previous issue date: 2012-02-17Bitstream added on 2014-06-13T20:09:45Z : No. of bitstreams: 1 paredesquinones_m_me_ilha.pdf: 1414308 bytes, checksum: 2da97bcc0006bca0f620b6c44bf8177e (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Neste trabalho foi desenvolvido um modelo baseado em programação linear inteira mista para determinar o custo mínimo da operação de um sistema hidrotérmico de longo prazo em um mercado elétrico competitivo. O modelo é capaz de encontrar os valores ótimos de operação para um período futuro nas usinas térmicas e hidroelétricas, considerando os limites normais e de emergência e as perdas de potência ativa nas linhas de transmissão, para diferentes níveis de carregamento, com múltiplos reservatórios de acumulação e de regulação. Adicionalmente, a previsão da hidrologia foi modelada mediante modelo SARIMA para o período de análise, considerando o histórico dos dados hidrológicos. O modelo foi implementado na linguagem de modelagem matemática GMPL. O problema de programação linear inteira mista foi resolvido usando o solver GLPSOL. As predições das hidrologias foram feitas na linguagem estatística R. Foi desenvolvida uma interface gráfica para gerenciar o modelo em GMPL, a previsão hidrológica em R e o solver GLPSOL na linguagem Qt. Desta forma é possível adicionar novos modelos, alterar dados do modelo e visualizar os resultados. Todas as linguagens e softwares usados são livres de distribuição e modificação. Para o modelo desenvolvido foram analisados os seguintes casos: sistema de 6 barras; sistema uninodal brasileiro e sistema interligado peruano. Os resultados obtidos foram comparados com os resultados encontrados na literatura com o objetivo de validar a proposta deste trabalho / In this work developed a model based on mixed integer linear programming to determine the minimum cost of operation of a hydrothermal system in a long-term competitive electricity market. The model is able to find the optimal values of operation for a future period in thermal and hydroelectric power plants, considering the normal and emergency limits and the active power losses in transmission lines, for different loading levels, with multiple storage reservoirs and dams. In addition, predicting the hydrology was modeled by the SARIMA model for the period of analysis, considering the hydrological historical data. The model was implemented in the language of mathematical modeling GMPL. The mixed linear integer programming problem was solved using solver GLPSOL. The predictions in the hydrology using the statistical language R. It was developed a graphical interface to manage the GMPL model, the hydrologic forecasting, and solver GLPSOL in Qt graphical language. By this way, is possible to add new models, change the data of the model and view the results. All languages and software used are free to distribute and modify. For the model developed the following cases were analyzed: 6 bar system; Brazilian uninodal system and Peruvian interconnected system. The results were compared with results in the literature in order to validate the proposal of this work

Orientador: Rubén Augusto Romero Lázaro / Coorientador: Marcos Julio Rider Flores / Banca: Jose Roberto Sanches Mantovani / Banca: Secundino Soares Filho / Resumo: Neste trabalho foi desenvolvido um modelo baseado em programação linear inteira mista para determinar o custo mínimo da operação de um sistema hidrotérmico de longo prazo em um mercado elétrico competitivo. O modelo é capaz de encontrar os valores ótimos de operação para um período futuro nas usinas térmicas e hidroelétricas, considerando os limites normais e de emergência e as perdas de potência ativa nas linhas de transmissão, para diferentes níveis de carregamento, com múltiplos reservatórios de acumulação e de regulação. Adicionalmente, a previsão da hidrologia foi modelada mediante modelo SARIMA para o período de análise, considerando o histórico dos dados hidrológicos. O modelo foi implementado na linguagem de modelagem matemática GMPL. O problema de programação linear inteira mista foi resolvido usando o solver GLPSOL. As predições das hidrologias foram feitas na linguagem estatística R. Foi desenvolvida uma interface gráfica para gerenciar o modelo em GMPL, a previsão hidrológica em R e o solver GLPSOL na linguagem Qt. Desta forma é possível adicionar novos modelos, alterar dados do modelo e visualizar os resultados. Todas as linguagens e softwares usados são livres de distribuição e modificação. Para o modelo desenvolvido foram analisados os seguintes casos: sistema de 6 barras; sistema uninodal brasileiro e sistema interligado peruano. Os resultados obtidos foram comparados com os resultados encontrados na literatura com o objetivo de validar a proposta deste trabalho / Abstract: In this work developed a model based on mixed integer linear programming to determine the minimum cost of operation of a hydrothermal system in a long-term competitive electricity market. The model is able to find the optimal values of operation for a future period in thermal and hydroelectric power plants, considering the normal and emergency limits and the active power losses in transmission lines, for different loading levels, with multiple storage reservoirs and dams. In addition, predicting the hydrology was modeled by the SARIMA model for the period of analysis, considering the hydrological historical data. The model was implemented in the language of mathematical modeling GMPL. The mixed linear integer programming problem was solved using solver GLPSOL. The predictions in the hydrology using the statistical language R. It was developed a graphical interface to manage the GMPL model, the hydrologic forecasting, and solver GLPSOL in Qt graphical language. By this way, is possible to add new models, change the data of the model and view the results. All languages and software used are free to distribute and modify. For the model developed the following cases were analyzed: 6 bar system; Brazilian uninodal system and Peruvian interconnected system. The results were compared with results in the literature in order to validate the proposal of this work / Mestre

Hydrothermal systems on active volcanoes can be studied through characterization of the emitted fluids and surface discharges using major element and isotopic compositions of fumaroles and thermal springs within the volcanic area. This thesis aims at understanding the geochemistry of the existing hydrothermal systems of Kanlaon, Biliran and Bulusan volcanoes in the Philippines and the contribution of magma degassing in the formation of fluids circulating within the hydrothermal system. This study also aims at improving the geochemical monitoring program of PHIVOLCS by suggesting parameters to use in evaluating the volcano’s activity and their evolution that may lead to volcanic unrest.Kanlaon volcano’s extensive hydrothermal system evolved into two distinct hydrothermal systems independent of each other. A mature hydrothermal system represented by Pataan thermal area is characterized by neutral Na + K chloride (bicarbonate) fluids and an immature system, represented by Hagdan is characterized by the presence of acid-sulfate waters. Chemical and isotopic analyses were performed on thermals waters to classify the samples that are linked to the existence of these two hydrothermal systems. Using Cl-SO4-HCO3 relative abundances, Kanlaon’s thermal waters are classified as acid sulfate, acid sulfate-chloride, neutral chloride, and neutral bicarbonate waters. The linear trend formed by Na + K and Cl of Pataan and Mambucal samples can be explained by groundwater/meteoric dilution. This is consistent with the light sulfur isotopic signatures between δ34S = -3.4 ‰ and +1.2 ‰ of the mature hydrothermal system. This implies that the origin of sulfur is linked to the surficial oxidation of H2S. In contrast, the immature hydrothermal system shows significantly heavier sulfur isotopic composition (δ34S = +8.2 ‰), which indicates that sulfur may have originated from the disproportionation of magmatic SO2 or from the fractionation between hydrothermal sulfate and sulfide (SO42-/H2S) pairs that have achieved isotopic equilibrium.On Biliran Volcano, the area of Vulcan thermal grounds exhibits the greatest thermal activity. Thermal waters of Biliran are classified into 6 types based on their geochemistry and location on the ground. Location 1 is composed of summit springs and location 2-6 are springs found along the periphery of the volcano with varying distances from the summit springs. Immature waters are discharging from the springs located at the summit. These are the acid sulfate-chloride waters. The high concentration of SO42- and Cl- is a clear indication of the presence of magmatic HCl, H2S, and SO2. The high δ34S (+14.7 ‰ to +26.6 ‰) values suggest that these fluids were formed from the disproportionation of magmatic SO2. The acidity of the summit springs is coming from the HCl which is a contribution from the degassed magma at depth. Mature neutral (SO42- - HCO3-) Cl springs are found away from the summit moving towards the margins of the neutralization zone. Neutral Cl fluids evolved from progressive neutralization of previously acidic fluid by water-rock interaction that migrated laterally and emerged as bicarbonate waters in the periphery of the volcano. Mt. Bulusan hydrothermal system is complex and tends to show the signature of a deep neutral Cl fluid. Based on Cl and SO42-, there are two groups of springs and these are found in two different locations. Type I springs are located on Mt. Bulusan close to the crater. The predominance of HCO3- and SO42- can be associated with shallow interactions and processes (i.e. boiling of hydrothermal fluids producing steam) that modify the fluid of meteoric in origin. Major gases such as CO2(g) and H2S(g) are incorporated in the groundwater via condensation. Type II springs are located on the periphery of the volcano, far from the location of Type I springs. These springs are characterized by the presence of Cl- and HCO3- ions at concentration levels greater than SO42- concentrations. The low solubility of CO2 allows the gas phase to be transported over long distances and converted to HCO3-. The origin of Type II fluids can either be through adsorption of CO2-bearing gases, or condensation of CO2-rich geothermal steam. The proximity of one Type II spring to sea level can have a bearing on the origin of Cl- in the fluids, but the fact that all Cl- composition of these springs are quite uniform, this means that the chloride must have come from one source and that it is highly unlikely to receive any contribution from seawater. This is also supported by the Cl/B and Cl/Li composition of Type II springs. Mt. Bulusan does not have ‘pure’ neutral chloride water signature but rather a mixture of neutral Cl waters and HCO3-rich waters. / Le travail de thèse est consacré à l’étude de systèmes hydrothermaux de volcans actifs qui ont été caractérisés grâce à l’étude géochimique (éléments majeurs et composition isotopique) des fluides hydrothermaux émis en surface de zones volcaniques. L’objectif principal de la thèse est l’interprétation de la composition géochimique des fluides hydrothermaux présents dans 3 systèmes actifs :les volcans Kanlaon, Biliran et Bulusan aux Philippines. Cette étude a également pour but d’améliorer le programme de monitoring de l’activité volcanique du PHIVOLCS (Philippine Institute of Volcanology and Seismology) en proposant des paramètres géochimiques utiles à l’évaluation de l’état d’activité d’un volcan et qui peuvent également fournir de signaux précurseurs d’une activité éruptive. Dans le cas du Volcan Kanlaon, la présence de deux systèmes hydrothermaux distincts a été mise en évidence grâce à l’analyse géochimique et isotopique des eaux thermales présentes dans le massif volcanique. Un système hydrothermal « mature » caractérisé par des fluides neutres chlorures (Na+K/Cl) est présent sous la zone hydrothermale de Pataan. Le deuxième système hydrothermal, situé dans la zone de Hagdan, présente au contraire des propriétés d’un système « immature » dominé par des fluides de type acide sulfate. En comparant les abondances relatives en Cl-SO4-HCO3, différents types de composition de sources thermales sont observées :acide sulfate, acide sulfate-chlorure, neutre chlorure et neutre bicarbonate. La corrélation linéaire qui existe entre les alcalins (Na+K) et les chlorures dans les échantillons de Pataan et de Mambucal suggère une origine identique et un processus de simple dilution par des eaux d’origine météoritique. D’autre part, la signature isotopique des sulfates à Mambucal avec une gamme de valeurs de δ34S entre -3.4 ‰ et +1.2 ‰ est typique de l’oxydation à proximité de la surface de l’H2S et tend à confirmer le caractère « mature » du système hydrothermal. La signature isotopique contrastée des sulfates de Hagdan avec un δ34S = +8.2 ‰ suggère que l’origine du soufre dans ce système « immature » pourrait être liée soit à la disproportionation du SO2 d’origine magmatique soit résulté d’un fractionnement isotopique à l’équilibre au niveau du sytème hydrothermal de la paire SO42-/H2S.Dans le cas du volcan Biliran, l’activité hydrothermal principale est située dans la zone sommitale de Vulcan. 6 types de composition géochimique différents ont été mis en évidence. Dans la zone sommitale de l’édifice volcanique, des eaux « immatures » de type acide sulfate-chlorure ont été identifiées. Les concentrations élevées en SO42- et Cl- suggèrent une contribution magmatique et la présence de HCl, H2S and SO2 émis par le dégazage d’une intrusion magmatique superficielle. Les valeurs élevées en δ34S (+14.7 ‰ à +26.6 ‰) suggèrent clairement que les sulfates proviennent de la réaction de disproportionation de SO2 d’origine magmatique. D’autre part, les valeurs d’acidité de ces sources sont nettement corrélées à l’abondance des chlorures et donc à la contribution d’HCl gazeux d’origine magmatique. En périphérie de la zone sommitale, les sources chaudes sont caractérisées par des eaux « matures » de type (SO42- - HCO3-) + Cl- dont l’acidité est largement neutralisée. Des fluides neutre chlorures provenant de la neutralisation progressive de fluides acides par interaction avec les roches encaissantes et enrichis en bicarbonates sont également présents dans les zones périphériques du volcan Biliran.Le système hydrothermal du Mt. Bulusan est complexe mais tend à révéler la présence d’un réservoir profond de composition neutre chlorure. Deux groupes de sources chaudes distincts sont présents dans deux zones distinctes de l’édifice volcanique. Sur base des compositions en Cl et SO42-, deux groupes distincts de sources chaudes ont été observées. Le type I, localisé à proximité du cratère du Bulusan, est caractérisé par la prédominance de HCO3- et SO42- et pourrait résulter de la condensation et dissolution de vapeurs (H20(g), CO2(g) et H2S(g)) essentiellement hydrothermales par des eaux superficielles d’origine météoritique. Le type II, observé beaucoup plus en périphérie de l’édifice volcanique, se distingue par l’abondance des ions Cl- et HCO3- qui dominent largement les concentrations en SO42-. L’origine des sources chaudes de type II est liée à la condensation/dissolution de vapeurs hydrothermales riches en gaz carbonique, le CO2 ayant, en raison de sa plus faible réactivité chimique, la faculté de diffuser latéralement sur de longue distance au sein d’un édifice volcanique. La proximité de certaines sources de type II avec la mer tend à suggérer une origine marine pour le chlore. Cependant, le même type II est également observé à grande distance de la mer où une contribution marine est difficilement envisageable. D’autre part, les compositions relatives en Cl/B and Cl/Li des sources de type II ne semblent pas compatibles avec une origine marine. Aucune composition de fluide « mature » de type neutre chlorure n’a été observée, l’origine des fluides de type II pourrait cependant résulter d’un mélange entre des eaux enrichies en HCO3- et des eaux neutres chlorures. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Seafloor hydrothermal systems have been intensively studied for the past few decades; however, the location of recharge zones and details of fluid circulation patterns are still largely uncertain. To better understand the effects of anhydrite precipitation on hydrothermal flow paths, we conduct 2-D numerical simulations of hydrothermal circulation at a mid-ocean ridge using a NaCl-H2O numerical code. The simulations focus on East Pacific Rise hydrothermal system at 950N due to availability of key observational data to constrain the models. Seismicity data that is available suggests that fluid flow is primarily along axis and that recharge is focused into a small zone near a 4th order discontinuity in the ridge axis. Simulations are carried out in an open-top square box 1500 m on a side maintained at a surface pressure of 25 MPa, and nominal seawater temperature of 10 C. The sides of the box are assumed to be impermeable and insulated. A constant temperature distribution is maintained along the bottom of the box consisting of a 1000 m long central-heated region maintained at 450 C to represent the axial magma chamber and ensure P-T conditions for phase separation; a linearly decreasing temperature profile from 450 to 300 C is maintained along the 250 m long segments adjacent to the heated region to delineate the recharge zone. We constructed a homogeneous model with a uniform cell size of 25 m with a permeability of 10-13 m2 and a similar model with a 200 m thick layer 2A region with a permeability of 10-12 m2. For the homogeneous model the simulations were run for 100 years to approximate steady state conditions and the model with layer 2A was run for 50 years. Assuming that anhydrite precipitation resulted from the decrease in solubility with increasing temperature as downwelling fluid gets heated, the rate of porosity decrease and sealing time was calculated at 50 and 100 years. The results showed that sealing occurred most rapidly at the bottom of the recharge areas near the base of the high-temperature plumes, where complete sealing occurred after ~55-625 years for an initial porosity of 0.1. The simulations also suggested that sealing would occur more slowly at the margins of the ascending plumes, with times ranging between ~ 80 and 5000 years. The sealing times in the deep recharge zone determined in these simulations are considerably greater than estimated from 1D analytical calculations, suggesting that with a 2D model, focused recharge at the EPR 950N site may occur, at least on a decadal time scale. More detailed analyses are needed to determine whether such focused recharge can be maintained for longer times. / Master of Science

Hydrothermal processes at oceanic spreading centers are largely influenced by changing magmatic heat input. I use the FISHES code to investigate the evolution of surface temperature and salinity as a function of time-varying heat flux at the base of a two-phase, vapor-brine hydrothermal system. I consider a two-dimensional rectangular box that is 1.5 km deep and 4 km long with homogeneous permeability. Impermeable, insulated conditions are imposed on the left and right hand boundaries. To simulate time-varying heat flux from a sub-axial magma chamber of 500 m long half-width, I consider a variety of basal boundary conditions: (1) a constant heat flux with an value of 130 W/m2; (2) a sinusoidal heat flux with a period of 6 years and an amplitude ranging between 100 and 50 W/m2; (3) step, random, and exponential heat fluxes ranging between 200 and 15 W/m2; and (4) an analytical function of temporally decaying heat flux resulting from a simulated cooling, crystallizing magmatic sill. As a result of the investigation I find: (1) changes in bottom temperature and salinity closely follow the temporal variations in magmatic heat inputs; (2) the surface temperature response is severely damped and high frequency variations in heat flow are not detected; (3) in regions where phase separation of vapor and brine occurs, surface salinity variations may be recorded in response to changing conditions at depth, but these are smaller in amplitude. / Master of Science