ALBERT

Description: The area of the 9.1-km-deep Continental Deep Drillhole (KTB) in Germany is used as a case study for a geothermal reservoir situated in folded and faulted metamorphic crystalline crust. The presented approach is based on the analysis of 3-D seismic reflection data combined with borehole data and hydrothermal numerical modelling. The KTB location exemplarily contains all elements that make seismic prospecting in crystalline environment often more difficult than in sedimentary units, basically complicated tectonics and fracturing and low-coherent strata. In a first step major rock units including two known nearly parallel fault zones are identified down to a depth of 12 km. These units form the basis of a gridded 3-D numerical model for investigating temperature and fluid flow. Conductive and advective heat transport takes place mainly in a metamorphic block composed of gneisses and metabasites that show considerable differences in thermal conductivity and heat production. Therefore, in a second step, the structure of this unit is investigated by seismic waveform modelling. The third step of interpretation consists of applying wavenumber filtering and log-Gabor-filtering for locating fractures. Since fracture networks are the major fluid pathways in the crystalline, we associate the fracture density distribution with distributions of relative porosity and permeability that can be calibrated by logging data and forward modelling of the temperature field. The resulting permeability distribution shows values between 10 –16 and 10 –19 m 2 and does not correlate with particular rock units. Once thermohydraulic rock properties are attributed to the numerical model, the differential equations for heat and fluid transport in porous media are solved numerically based on a finite difference approach. The hydraulic potential caused by topography and a heat flux of 54 mW m –2 were applied as boundary conditions at the top and bottom of the model. Fluid flow is generally slow and mainly occurring within the two fault zones. Thus, our model confirms the previous finding that diffusive heat transport is the dominant process at the KTB site. Fitting the observed temperature–depth profile requires a correction for palaeoclimate of about 4 K at 1 km depth. Modelled and observed temperature data fit well within 0.2 °C bounds. Whereas thermal conditions are suitable for geothermal energy production, hydraulic conditions are unfavourable without engineered stimulation.

Description: Following earlier studies, we present forward and inverse simulations of heat and fluid transport of the upper crust using a local 3-D model of the Kola area. We provide best estimates for palaeotemperatures and permeabilities, their errors and their dependencies. Our results allow discriminating between the two mentioned processes to a certain extent, partly resolving the non-uniqueness of the problem. We find clear indications for a significant contribution of advective heat transport, which, in turn, imply only slightly lower ground surface temperatures during the last glacial maximum relative to the present value. These findings are consistent with the general background knowledge of (i) the fracture zones and the corresponding fluid movements in the bedrock and (ii) the glacial history of the Kola area.

Description: In geodynamic models of mid-ocean ridges hydrothermal cooling processes are important to control the temperature and thus the rheological behaviour of the crust. However, the characteristic time scale of hydrothermal convection is considerably shorter than that of viscous flow of mantle material or cooling of the oceanic lithosphere and can hardly be addressed in a conjoined model. To overcome this problem we present an approach to mimic hydrothermal cooling by an equivalent, increased thermal conductivity. First the temperature and pressure dependence of crack related porosity and permeability are derived based on composite theory. A characteristic pore closure depth as a function of pressure, temperature and pore aspect ratio is defined. 2-D porous convection models are used to derive scaling laws for parameterized convection including a Rayleigh–Nusselt number relation for a permeability exponentially decreasing with depth. These relations are used to derive an equivalent thermal conductivity to account for consistently evolving hydrothermal heat transport in thermally evolving systems. We test our approach using a 1-D model for cooling of the oceanic lithosphere. Within the context of our modelling parameters we found a pronounced effect for young lithosphere (younger than 10 Ma) down to about 20 km. Significant deviations of the heat flux versus age from the 1/ $\sqrt t $ law may occur due to hydrothermal convection. For the bathymetry versus age curves slopes steeper than 1/ $\sqrt t $ slopes already occur for very young lithosphere. Hydrothermal convection leads to an increase of the total heat flux and heat loss with respect to the classical purely conductive cooling model. Comparison of the total heat flow and its conductive contribution with observations confirm previous suggestions that for young lithosphere heat flow measurements represent only the conductive part, while at older ages the total heat flow is observed. Within their scatter and uncertainties heat flow and bathymetry data are in general agreement with our hydrothermally enforced cooling model suggesting that hydrothermal convection may be important even up to high ages.

Description: 〈span〉〈div〉SUMMARY〈/div〉The physics of magmatic systems within continental crust is poorly understood. We developed a thermomechanical compositional two-phase flow formulation based on the conservation equations of mass, momentum and energy for melt and solid, including compaction of the solid matrix, melting, melt segregation, melt ascent and freezing. We use a simplified melting law to track the enrichment or depletion in SiO〈sub〉2〈/sub〉 of the advected silicic melt and solid. The nonlinear viscoplastic rheology includes the effect of melt porosity. 2-D models with different heat input are carried out for cases without and with differential melt-matrix flow. The retention number, 〈span〉R〈/span〉〈sub〉t〈/sub〉, as a measure of melt mobility is varied between 1 and infinity. In the case of no melt segregation (large 〈span〉R〈/span〉〈sub〉t〈/sub〉) our models show transient oscillatory behaviour followed by stationary convection in the lower crust enforced by a solid–melt phase transition. In the case of two-phase flow (i.e. small 〈span〉R〈/span〉〈sub〉t〈/sub〉) melt separates from the solid matrix and accumulates in high melt porosity magma bodies within 10 s ka. We find a new melt ascent mechanism, termed CATMA, for〈strong〉〈span〉C〈/span〉〈/strong〉〈span〉ompaction/decompaction〈/span〉〈strong〉〈span〉A〈/span〉〈/strong〉〈span〉ssisted〈/span〉〈strong〉〈span〉T〈/span〉〈/strong〉〈span〉wo-phase flow〈/span〉〈strong〉〈span〉M〈/span〉〈/strong〉〈span〉elt〈/span〉〈strong〉〈span〉A〈/span〉〈/strong〉〈span〉scent〈/span〉. This is a combination of compaction and decompaction of the contact zones between accumulated magma and solid rock that dislodges solid material from the roof that sinks through and partly dissolves in the magma. This process can be seen as an efficient microstoping mechanism and is associated with the formation of melt rich and chemically enriched channels within the magma body. The emplacement depths of magma change from 〉20 km for low heat flows to 〈10 km for high heat flows. In most models with high degrees of melting, two stacked SiO〈sub〉2〈/sub〉-enriched magmatic zones form interpreted as granitic layers. Models with stronger crustal rheology show porosity waves on a few km scale. Deviatoric stresses immediately above the evolving magma bodies are of the order of a few MPa, too small to overcome brittle or plastic yield stresses. The models predict significant chemical separation of depleted versus enriched composition, resulting in significant chemical stratification of the crust with spatial variations in solidus temperatures, and in a dual melt porosity distribution with crystal-poor magma bodies (〉60 per cent melt) on top of low melt fraction mushes (〈20 per cent). Comparison with the Altiplano–Puna magma body shows that the best agreement with observational data is obtained for a moderate (85–90 mW m〈sup〉−2〈/sup〉) heat flux and retention number of the order of 3 to 30.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉The physics of magmatic systems within continental crust is poorly understood. We developed a thermo-mechanical-compositional two-phase flow formulation based on the conservation equations of mass, momentum, and energy for melt and solid, including compaction of the solid matrix, melting, melt segregation, melt ascent and freezing. We use a simplified melting law to track the enrichment or depletion in SiO〈sub〉2〈/sub〉 of the advected silicic melt and solid. The non-linear visco-plastic rheology includes the effect of melt porosity. 2D-models with different heat input are carried out for cases without and with differential melt-matrix flow. The retention number, 〈span〉Rt〈/span〉, as a measure of melt mobility, is varied between 1 and infinity. In the case of no melt segregation (large 〈span〉Rt〈/span〉) our models show transient oscillatory behavior followed by stationary convection in the lower crust enforced by a solid—melt phase transition. In the case of two-phase flow (i.e. small 〈span〉Rt〈/span〉) melt separates from the solid matrix and accumulates in high melt porosity magma bodies within 10 s ka. We find a new melt ascent mechanism, termed CATMA, for〈strong〉〈span〉C〈/span〉〈/strong〉〈span〉ompaction/decompaction〈/span〉〈strong〉〈span〉A〈/span〉〈/strong〉〈span〉ssisted〈/span〉〈strong〉〈span〉T〈/span〉〈/strong〉〈span〉wo-phase flow〈/span〉〈strong〉〈span〉M〈/span〉〈/strong〉〈span〉elt〈/span〉〈strong〉〈span〉A〈/span〉〈/strong〉〈span〉scent.〈/span〉 This is a combination of compaction and decompaction of the contact zones between accumulated magma and solid rock that dislodges solid material from the roof which sinks through and partly dissolves in the magma. This process can be seen as an efficient microstoping mechanism and is associated with the formation of melt rich and chemically enriched channels within the magma body. The emplacement depths of magma change from 〉 20 km for low heat flows, to 〈 10 km for high heat flows. In most models with high degrees of melting, two stacked SiO〈sub〉2〈/sub〉-enriched magmatic zones form, interpreted as granitic layers. Models with stronger crustal rheology show porosity waves on a few km-scale. Deviatoric stresses immediately above the evolving magma bodies are of the order of a few MPa, too small to overcome brittle or plastic yield stresses. The models predict significant chemical separation of depleted versus enriched composition, resulting in significant chemical stratification of the crust with spatial variations in solidus temperatures, and in a dual melt porosity distribution with crystal-poor magma bodies (〉 60 per cent melt) on top of low melt fraction mushes (〈 20 per cent). Comparison with the Altiplano-Puna magma body shows that the best agreement with observational data is obtained for a moderate (85 mW m〈sup〉−2〈/sup〉 to 90 mW m〈sup〉−2〈/sup〉) heat flux and retention number of order 3 to 30.〈/span〉