In the Earth's upper crust, where aqueous fluids can circulate freely, most mineral transformations are controlled by the coupling between the dissolution of a mineral that releases chemical species into the fluid...In the Earth's upper crust, where aqueous fluids can circulate freely, most mineral transformations are controlled by the coupling between the dissolution of a mineral that releases chemical species into the fluid and precipitation of new minerals that contain some of the released species in their crystal structure, the coupled process being driven by a reduction of the total free-energy of the system. Such coupled dissolution-precipitation processes occur at the fluid-mineral interface where the chemical gradients are highest and heterogeneous nucleation can be promoted, therefore controlling the growth kinetics of the new minerals. Time-lapse nanoscale imaging using Atomic Force Microscopy(AFM) can monitor the whole coupled process under in situ conditions and allow identifying the time scales involved and the controlling parameters. We have performed a series of experiments on carbonate minerals(calcite, siderite, dolomite and magnesite) where dissolution of the carbonate and precipitation of a new mineral was imaged and followed through time. In the presence of various species in the reacting fluid(e. g. antimony, selenium, arsenic, phosphate), the calcium released during calcite dissolution binds with these species to form new minerals that sequester these hazardous species in the form of a stable solid phase. For siderite, the coupling involves the release of Fe^(2+) ions that subsequently become oxidized and then precipitate in the form of FeIIIoxyhydroxides. For dolomite and magnesite,dissolution in the presence of pure water(undersaturated with any possible phase) results in the immediate precipitation of hydrated Mg-carbonate phases. In all these systems, dissolution and precipitation are coupled and occur directly in a boundary layer at the carbonate surface. Scaling arguments demonstrate that the thickness of this boundary layer is controlled by the rate of carbonate dissolution,the equilibrium concentration of the precipitates and the kinetics of diffusion of species in a boundary layer. From these parameters a characteristic time scale and a characteristic length scale of the boundary layer can be derived. This boundary layer grows with time and never reaches a steady state thickness as long as dissolution of the carbonate is faster than precipitation of the new mineral. At ambient temperature, the surface reactions of these dissolving carbonates occur on time-scales of the order of seconds to minutes, indicating the rapid surface rearrangement of carbonates in the presence of aqueous fluids. As a consequence, many carbonate-fluid reactions in low temperature environments are controlled by local thermodynamic equilibria rather than by the global equilibrium in the whole system.展开更多
The nature and formation time of the Xinghuadukou complex in Northeast China are important for determining the tectonic evolution of the Precambrian geological evolution of the Erguna massif. In this study, we present...The nature and formation time of the Xinghuadukou complex in Northeast China are important for determining the tectonic evolution of the Precambrian geological evolution of the Erguna massif. In this study, we present the results of zircon U-Pb dating of two metapelites from the complex. Detrital and metamorphic zircons from the metapelites yield a depositional age of -601 Ma and a metamorphic age of 496-509 Ma, indicating that the supracrustal rocks formed during the Neoproterozoic and recorded pan-African metamorphic events in the Erguna massif. Garnet porphyroblasts in SiI-Grt-Bt-Ms paragneiss show diffusion zoning, implying a decreasing P-T trend. Based on mineral transformation and P-T estimates using conventional geothermobarometers and phase equilibria modeling, three metamorphic stages were determined, including an early prograde metamorphic stage, a near peak upper amphibolite facies metamorphic stage, and a near-isobaric cooling stage with P-T conditions of 6.1 kb, 645 ℃, 5-6 kb, 710-740 ℃, and 4.4 kb, 625℃, respectively. The metamorphic history of the Xinghuadukou complex is thus defined by a clockwise P-T trajectory, which implies that the metamorphism of the metapelites documented the subduction, subsequent uplift and post collision process.展开更多
Cavities and fractures significantly affect the flow paths in carbonate reservoirs and should be accurately accounted for in numerical models.Herein,we consider the problem of computing the effective permeability of r...Cavities and fractures significantly affect the flow paths in carbonate reservoirs and should be accurately accounted for in numerical models.Herein,we consider the problem of computing the effective permeability of rock samples based on high-resolution 3DCT scans containingmillions of voxels.We use the Stokes-Brinkman equations in the entire domain,covering regions of free flow governed by the Stokes equations,porous Darcy flow,and transitions between them.The presence of different length scales and large(ten orders of magnitude)contrasts in permeability leads to highly ill-conditioned linear systems of equations,which are difficult to solve.To obtain a problem that is computationally tractable,we first analyze the relative importance of the Stokes and Darcy terms for a set of idealized 2D models.We find that,in terms of effective permeability,the Stokes-Brinkman equations are only applicable for a special parameter set where the effective free-flow permeability is less than four orders of magnitude different from the matrix permeability.All other cases can be accurately modeled with either the Stokes or the Darcy end-member flows,depending on if there do or do not exist percolating free-flow regions.The insights obtained are used to perform a direct computation of the effective permeability of a rock sample model with more than 8 million cells.展开更多
基金CVP acknowledges funding through the Marie Curie ITN GrantNo. PITN-GA-2012-317235 (CO2React)The present study received funding from the European Union’s Horizon 2020 Research and Innovation Programme under the ERC Advanced Grant Agreement No. 669972 (Disequilibrium Metamorphism) to AR.
文摘In the Earth's upper crust, where aqueous fluids can circulate freely, most mineral transformations are controlled by the coupling between the dissolution of a mineral that releases chemical species into the fluid and precipitation of new minerals that contain some of the released species in their crystal structure, the coupled process being driven by a reduction of the total free-energy of the system. Such coupled dissolution-precipitation processes occur at the fluid-mineral interface where the chemical gradients are highest and heterogeneous nucleation can be promoted, therefore controlling the growth kinetics of the new minerals. Time-lapse nanoscale imaging using Atomic Force Microscopy(AFM) can monitor the whole coupled process under in situ conditions and allow identifying the time scales involved and the controlling parameters. We have performed a series of experiments on carbonate minerals(calcite, siderite, dolomite and magnesite) where dissolution of the carbonate and precipitation of a new mineral was imaged and followed through time. In the presence of various species in the reacting fluid(e. g. antimony, selenium, arsenic, phosphate), the calcium released during calcite dissolution binds with these species to form new minerals that sequester these hazardous species in the form of a stable solid phase. For siderite, the coupling involves the release of Fe^(2+) ions that subsequently become oxidized and then precipitate in the form of FeIIIoxyhydroxides. For dolomite and magnesite,dissolution in the presence of pure water(undersaturated with any possible phase) results in the immediate precipitation of hydrated Mg-carbonate phases. In all these systems, dissolution and precipitation are coupled and occur directly in a boundary layer at the carbonate surface. Scaling arguments demonstrate that the thickness of this boundary layer is controlled by the rate of carbonate dissolution,the equilibrium concentration of the precipitates and the kinetics of diffusion of species in a boundary layer. From these parameters a characteristic time scale and a characteristic length scale of the boundary layer can be derived. This boundary layer grows with time and never reaches a steady state thickness as long as dissolution of the carbonate is faster than precipitation of the new mineral. At ambient temperature, the surface reactions of these dissolving carbonates occur on time-scales of the order of seconds to minutes, indicating the rapid surface rearrangement of carbonates in the presence of aqueous fluids. As a consequence, many carbonate-fluid reactions in low temperature environments are controlled by local thermodynamic equilibria rather than by the global equilibrium in the whole system.
基金supported by the National Natural Science Foundation of China(No.41472164)the MADE-IN-EARTH ERC starting grant of Switzerland(No.33577)
文摘The nature and formation time of the Xinghuadukou complex in Northeast China are important for determining the tectonic evolution of the Precambrian geological evolution of the Erguna massif. In this study, we present the results of zircon U-Pb dating of two metapelites from the complex. Detrital and metamorphic zircons from the metapelites yield a depositional age of -601 Ma and a metamorphic age of 496-509 Ma, indicating that the supracrustal rocks formed during the Neoproterozoic and recorded pan-African metamorphic events in the Erguna massif. Garnet porphyroblasts in SiI-Grt-Bt-Ms paragneiss show diffusion zoning, implying a decreasing P-T trend. Based on mineral transformation and P-T estimates using conventional geothermobarometers and phase equilibria modeling, three metamorphic stages were determined, including an early prograde metamorphic stage, a near peak upper amphibolite facies metamorphic stage, and a near-isobaric cooling stage with P-T conditions of 6.1 kb, 645 ℃, 5-6 kb, 710-740 ℃, and 4.4 kb, 625℃, respectively. The metamorphic history of the Xinghuadukou complex is thus defined by a clockwise P-T trajectory, which implies that the metamorphism of the metapelites documented the subduction, subsequent uplift and post collision process.
基金funded in part by Shell Norge AS and the Research Council of Norway through grants No.175962 and 186935Lie also acknowledges partial funding from the Center of Mathematics for Applications,University of Oslo.The Pipe Creek CT-scan data was originally collected by the Bureau of Economic Geology at The University of Texas at Austin with funding from the Industrial Associates of the Reservoir Characterization Research Laboratory.The authors are grateful to Bob Loucks,Chris Zahm,and Jim Jennings for assistance in accessing the data.
文摘Cavities and fractures significantly affect the flow paths in carbonate reservoirs and should be accurately accounted for in numerical models.Herein,we consider the problem of computing the effective permeability of rock samples based on high-resolution 3DCT scans containingmillions of voxels.We use the Stokes-Brinkman equations in the entire domain,covering regions of free flow governed by the Stokes equations,porous Darcy flow,and transitions between them.The presence of different length scales and large(ten orders of magnitude)contrasts in permeability leads to highly ill-conditioned linear systems of equations,which are difficult to solve.To obtain a problem that is computationally tractable,we first analyze the relative importance of the Stokes and Darcy terms for a set of idealized 2D models.We find that,in terms of effective permeability,the Stokes-Brinkman equations are only applicable for a special parameter set where the effective free-flow permeability is less than four orders of magnitude different from the matrix permeability.All other cases can be accurately modeled with either the Stokes or the Darcy end-member flows,depending on if there do or do not exist percolating free-flow regions.The insights obtained are used to perform a direct computation of the effective permeability of a rock sample model with more than 8 million cells.
