Browsing by Author "Farough, Aida"
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- Characteristics of magma-driven hydrothermal systems at oceanic spreading centersLowell, Robert P.; Farough, Aida; Hoover, Joshua; Cummings, Kylin (American Geophysical Union, 2013-06)We use one- and two-limb single-pass models to de termine vent field characteristics such as mass flow rate Q, bulk permeability in the discharge zone k(d), thickness of the conductive boundary layer at the base of the system , magma replenishment rate, and residence time in the discharge zone. Data on vent temperature, vent field area, heat output, and the surface area and depth of the subaxial magma chamber (AMC) constrain the models. The results give Q similar to 100kg/s, k(d)similar to 10(-13)m(2), and similar to 10m, essentially independent of spreading rate, and detailed characteristics of the AMC. In addition, we find no correlation between heat output at individual vent fields and spreading rate or depth to the AMC. We conclude that high-temperature hydrothermal systems are driven by local magma supply rates in excess of that needed for steady state crustal production and that crustal permeability enables hydrothermal circulation to tap magmatic heat regardless of AMC depth. Using data on partitioning of heat flow between focused and diffuse flow, we find that 80-90% of the hydrothermal heat output is derived from high-temperature fluid, even though much of the heat output discharges as low-temperature fluid. In some cases, diffuse flow fluids may exhibit considerable conductive cooling or heating. By assuming conservative mixing of diffuse flow fluids at East Pacific Rise 9 degrees 50N, we find that most transport of metals such as Fe and Mn occurs in diffuse flow and that CO2, H-2, and CH4 are taken up by microbial activity.
- An experimental study on characterization of physical properties of ultramafic rocks and controls on evolution of fracture permeability during serpentinization at hydrothermal conditionsFarough, Aida (Virginia Tech, 2015-09-28)Serpentinization is a complex set of hydration reactions, where olivine and pyroxene are replaced by serpentine, magnetite, brucite, talc and carbonate minerals. Serpentinization reactions alter chemical, mechanical, magnetic, seismic, and hydraulic properties of the crust. To understand the complicated nature of serpentinization and the linkages between physical and chemical changes during the reactions, I performed flow-through laboratory experiments on cylindrically cored samples of ultramafic rocks. Each core had a well-mated through-going tensile fracture, to investigate evolution of fracture permeability during serpentinization. The samples were tested in a triaxial loading machine at an effective pressure of 30 MPa, and temperature of 260°C, simulating a depth of 2 km under hydrostatic conditions. Fracture permeability decreased by one to two orders of magnitude during the 200 to 340 hour experiments. Electron microprobe and SEM data indicated the formation of needle-shaped crystals of serpentine composition along the walls of the fracture, and chemical analyses of sampled pore fluids were consistent with dissolution of ferro-magnesian minerals. The rate of transformation of olivine to serpentine in a tensile fracture is calculated using the data on evolution of fracture permeability assuming the fracture permeability could be represented by parallel plates. Assuming the dissolution and precipitation reactions occur simultaneously; the rate of transformation at the beginning of the experiments was ~ 10-8-10-9 (mol/m2s) and decreased monotonically by about an order of magnitude towards the end of the experiment. Results show that dissolution and precipitation is the main mechanism contributing to the reduction in fracture aperture. The experimental results suggest that the fracture network in long-lived hydrothermal circulation systems may be sealed rapidly as a result of mineral precipitation, and generation of new permeability resulting from a combination of tectonic and crystallization-induced stresses may be required to maintain fluid circulation. Another set of flow through experiments were performed on intact samples of ultramafic rocks at room temperature and effective pressures of 10, 20 and 30 MPa to estimate the pressure dependency of intact permeability. Porosity and density measurements were also performed with the purpose of characterizing these properties of ultramafic rocks. The pressure dependency of the coefficient of matrix permeability of the ultramafic rock samples fell in the range of 0.05-0.14 MPa-1. Using porosity and permeability measurements, the ratio of interconnected porosity to total porosity was estimated to be small and the permeability of the samples was dominantly controlled by microcracks. Using the density and porosity measurements, the degree of alteration of samples was estimated. Samples with high density and pressure dependent permeability had a smaller degree of alteration than those with lower density and pressure dependency.
- A Parameterized Approach to Partitioning Between Focused and Diffuse Heat Output and Modeling Hydrothermal Recharge at The East Pacific Rise 9°50´NFarough, Aida (Virginia Tech, 2011-12-02)Ever since the discovery of seafloor hydrothermal systems at mid ocean ridges, scientists have been trying to understand the complex dynamic processes by which thermal energy is transported advectively by chemically reactive aqueous fluids from Earth's interior to the surface. Hydrothermal systems are generally assumed to consist of a heat source and a fluid circulation system. Understanding the interconnected physical, chemical, biological, and geological processes at oceanic spreading centers is important because these processes affect the global energy and biogeochemical budgets of the Earth system. Despite two decades of focused study of hydrothermal systems, several key questions remain concerning the behavior and evolution of hydrothermal vent systems. Among these are: (a) the partitioning of heat transport between focused and diffuse flow, and (b) the spatial extent and distribution of hydrothermal recharge. These are the main topics of investigation in this thesis. To address these issues, I first use a single-pass modeling approach using a variety of observational data in a simple parametric scale analysis of a hydrothermal vent field to determine fundamental parameters associated with the circulation and magmatic heat transfer for a number of seafloor hydrothermal systems for which the constraining data are available. To investigate the partitioning of heat flux between focused high temperature and diffuse flow I extend the one-limb single pass model to incorporate two single-pass limbs to represent deep and shallow circulation pathways. As a result, I find that 90% of the heat output is from high temperature fluid circulating in the deep limb even though much of the heat loss appears at the seafloor as low-temperature diffuse flow. Next, I use the parametric description of hydrothermal circulation to investigate hydrothermal recharge at the East Pacific Rise 9°50′ N hydrothermal site. Using a 1-D model of recharge through an area of 10⁵ m² elucidated by microseismicity in the oceanic crust I find that anhydrite precipitation is likely to result in rapid sealing of pore space in the recharge zone. This would lead to rapid decay of hydrothermal venting, which is contrary to observations. Then I consider two-dimensional numerical models of hydrothermal circulation in a porous box heated from below. The preliminary results of these models suggests that the anhydrite precipitation zone will be more diffuse, but additional work is needed to test whether anhydrite precipitation will seal the pore space.