Louise H. Kellogg

The role of plumes in mantle helium fluxes

In a process for producing olefin polymers, a hydrocarbylaluminum hydrocarbyloxide such as diethylaluminum ethoxide is introduced into the reaction zone in a stream separate from the catalyst to reduce fouling.

EFFECT OF MANTLE PLUMES ON THE GROWTH OF D BY REACTION BETWEEN THE CORE AND MANTLE

The core-mantle boundary is a fundamental compositional discontinuity in the Earth, where molten iron alloy from the core meets solid silicate minerals from the mantle. Heat flow from the core to the mantle creates a thermal boundary layer at the base of the mantle. At the same time, chemical reactions may create a layer of different composition and density than the overlying mantle. Using a finite element model of convection in a spherical shell that includes formation of dense material at the base of the shell, we investigate how a layer forms at the base of the mantle. Development of a layer at the base of the mantle by this method depends both on the composition of the material forming at the core-mantle boundary and on the rate at which the material diffuses into the mantle. If the material is less than 3–6% denser than the overlying mantle, assuming reasonable choices of lower mantle thermodynamic parameters, the reactant material will be swept away by upwelling plumes. More dense material, on the other hand, forms an internally convecting layer that entrains material from above. The varying thickness and composition of the layer is consistent with geophysical observations of the characteristics of the deep mantle.

Numerical models of a dense layer at the base of the mantle and implications for the geodynamics of D

To investigate the dynamics of the mantles D″ layer, we explore numerical models of mantle convection which feature a dense basal boundary layer. We use a double-diffusive finite element convection scheme and vary thermal and chemical Rayleigh numbers and properties including viscosity, thermal conductivity, and internal heating. For isoviscous models with heating only from below, the thermal Rayleigh number (Ra) is set at either 106 or 107. The negative chemical buoyancy of a dense layer produces a stable boundary layer when the ratio of chemical to thermal buoyancy (the buoyancy number B) is around 1. For B = 0.5 the layer is unstable, while B = 0.6 may produce a layer which remains stable for long periods, depending on other factors (such as Rayleigh number, layer thickness, and thermal diffusivity of the layer). At high enough Ra, small-scale convection can occur within the layer. Using Ra = 107, we look at changes in layer thickness from 100 to 300 km at two values of B(0.6 and 1). Convection within the layer takes place most easily when the layer has a greater initial thickness. For B = 0.6, the layer is not always continuous along the lower boundary, so convection within the layer does not occur unless the initial thickness is fairly high (300 km). Increasing the thermal diffusivity of the layer (to simulate enrichment in metals) enhances heat conduction across the layer and can suppress internal convection within D″. Enhanced conduction also leads to higher plume temperatures. Including internal heating mainly increases the overall heat flow through the mantle, leading to higher surface heat flux. Finally, we examine models with temperature-dependent viscosity and pressure-dependent viscosity using Rayleigh numbers in the range of 107. Convection within the dense layer is enhanced by the relative reduction in viscosity. Our models exhibit only a modest decrease in viscosity (an order of magnitude) but illustrate how a dense, low-viscosity layer interacts with cold, viscous downwellings.

Sinking Mafic Body in a Reactivated Lower Crust: A Mechanism for Stress Concentration at the New Madrid Seismic Zone

We propose a geodynamic model for stress concentration in the New Madrid seismic zone (NMSZ). The model postulates that a high-density (mafic) body situated in the deep crust directly beneath the most seismically active part of the NMSZ began sinking several thousands of years ago when the lower crust was suddenly weakened. Based on the fact that deformation rates in the NMSZ have accelerated over the past 9 k.y., we envision the source of this perturbation to be related to the last North American deglaciation. Excess mass of the mafic body exerts a downward pull on the elastic upper crust, leading to a cycle of primary thrust faulting with secondary strike-slip faulting, after which continued sinking of the mafic body reloads the upper crust and renews the process. This model is consistent with the youth of activity, the generation of a sequence of earthquakes, and the velocity evolution during interseismic periods, which depend upon the density contrast of the mafic body with respect to the surrounding crust, its volume, and the viscosity of the lower crust.

The effect of temperature dependent viscosity on the structure of new plumes in the mantle: Results of a finite element model in a spherical, axisymmetric shell

Abstract We have developed a finite element model of convection in a spherical, axisymmetric shell that we use to simulate upwelling thermal plumes in the mantle. The finite element method provides the flexibility to include realistic properties such as temperature-dependent viscosity, the focus of this paper. We used this model to investigate the effect of temperature-dependent viscosity on the structure of new plumes originating at the core-mantle boundary. Because the way in which mantle viscosity varies with temperature is not well constrained, we determined the plume structure using a variety of viscosity laws. We focus on 3 different viscosity laws: (1) constant viscosity; (2) weakly temperature-dependent viscosity, in which the viscosity increases by a factor of 10 between the hottest and the coldest material; and (3) strongly temperature-dependent viscosity, in which the viscosity varies by a factor of 1000. In a constant viscosity fluid, the plume exhibited a spout structure without a distinctive head. The plume head and tail consisted largely of material from the hot thermal boundary layer at the base of the spherical shell that represented the mantle. When the viscosity was strongly temperature-dependent, starting plumes developed a mushroom structure with a large, slow-moving head, followed by a narrow, faster moving tail. Material from the overlying shell was assimilated into the plume head during formation of the upwelling in models with strongly temperature-dependent viscosity, while the plume tail showed little entrainment. Constant viscosity models and models with weakly temperature-dependent viscosity showed almost no entrainment in the head or tail. The large plume head that formed in models with strongly temperature-dependent viscosity created and then shed “blobs” of material from the deep mantle that did not arrive at the surface near the plume but instead were deposited elsewhere in the upper mantle.

Immersive Visualization and Analysis of LiDAR Data

We describe an immersive visualization application for point cloud data gathered by LiDAR (Light Detection And Ranging) scanners. LiDAR is used by geophysicists and engineers to make highly accurate measurements of the landscape for study of natural hazards such as floods and earthquakes. The large point cloud data sets provided by LiDAR scans create a significant technical challenge for visualizing, assessing, and interpreting these data. Our system uses an out-of-core view-dependent multiresolution rendering scheme that supports rendering of data sets containing billions of 3D points at the frame rates required for immersion (48---60 fps). The visualization system is the foundation for several interactive analysis tools for quality control, extraction of survey measurements, and the extraction of isolated point cloud features. The software is used extensively by researchers at the UC Davis Department of Geology and the U.S. Geological Survey, who report that it offers several significant advantages over other analysis methods for the same type of data, especially when used in an immersive visualization environment such as a CAVE.

Homogenization of the mantle by convective mixing and diffusion

Abstract Mantle convection stirs and homogenizes the subducted oceanic lithosphere with the convecting mantle. Convective mixing stretches and thins the subducted oceanic crust from an original thickness of 6 km to a thickness of 2 cm or less. The thinned, subducted oceanic crust can be observed as pyroxenite bands in high-temperature peridotite massifs. On the scale of centimeters, the bands are destroyed by diffusive processes. In this paper, the homogenization of the subducted oceanic crust with the depleted mantle is modeled by considering the combined problem of thinning and diffusion at a stagnation point. A layer of different composition from the surrounding material is thinned by normal strain until its identity is destroyed by diffusive processes. Thinning dominates the destruction of a layer if ɛa 2 /D , where ɛis the strain rate, 2a is the initial layer thickness, and D is the diffusivity. Diffusion dominates if ɛa 2 /D . Our results indicate that the mantle is homogeneous at the centimeter scale. This conclusion is insensitive to variations in the strain rate and the diffusivity, and it is supported by isotopic studies of high-temperature peridotite massifs. Variations in isotope ratios in MORB can be attributed to the imperfect homogenization of the MORB source region.

Numerical modeling of chemically buoyant mantle plumes at spreading ridges

The geometry of spreading of plumes beneath midoceanic ridges is investigated by three-dimensional numerical modeling, with the goal of characterizing the width of the plume along the ridge, or the “waist width”. Chemically buoyant plumes are modeled, in order to compare to previously reported laboratory tank experiments. The plume is generated near the bottom of the box and rises and forms a mushroom-head with some entrainment of surrounding fluid. The head flattens at a considerable depth beneath the ridge before rising to the surface. Once the plume reaches the surface the head is quickly divided by the diverging plates, and the waist width is found to have reached a steady-state value, W. The results show that W is proportional to the square root of the volumetric flux of the plume divided by the diverging plate speed, which is consistent with previously reported experimental data. Our scaling law gives an independent method for estimating the volumetric flux of mantle plumes. The calculated plume fluxes are two to four times larger than previous estimates. If plume buoyancy is purely of thermal origin, then excess temperatures can be estimated from the fluxes. For Iceland, Azores and the Galapagos, the calculated excess temperatures are 140, 57, and 51°C respectively, in agreement with recent, independent estimates from modeling of gravity and bathymetry.

Enabling scientific workflows in virtual reality

To advance research and improve the scientific return on data collection and interpretation efforts in the geosciences, we have developed methods of interactive visualization, with a special focus on immersive virtual reality (VR) environments. Earth sciences employ a strongly visual approach to the measurement and analysis of geologic data due to the spatial and temporal scales over which such data ranges. As observations and simulations increase in size and complexity, the Earth sciences are challenged to manage and interpret increasing amounts of data. Reaping the full intellectual benefits of immersive VR requires us to tailor exploratory approaches to scientific problems. These applications build on the visualization methods strengths, using both 3D perception and interaction with data and models, to take advantage of the skills and training of the geological scientists exploring their data in the VR environment. This interactive approach has enabled us to develop a suite of tools that are adaptable to a range of problems in the geosciences and beyond.

From outcrop to flow simulation: Constructing discrete fracture models from a LIDAR survey

Terrestrial light detection and ranging (LIDAR) surveys offer potential enrichment of outcrop-based research efforts to characterize fracture networks and assess their impact on subsurface fluid flow. Here, we explore two methods to extract the three-dimensional (3-D) positions of natural fractures from a LIDAR survey collected at a roadcut through the Cretaceous Austin Chalk: (1) a manual method using the University of California, Davis, Keck Center for Active Visualization in the Earth Sciences and (2) a semiautomated method based on mean normal and Gaussian curvature surface classification. Each extraction method captures the characteristic frequencies and orientations of the primary fracture sets that we identified in the field, yet they extract secondary fracture sets with varying ability. After making assumptions regarding fracture lengths and apertures, the extracted fractures served as a basis to construct a discrete fracture network (DFN) that agrees with field observations and a priori knowledge of fracture network systems. Using this DFN, we performed flow simulations for two hypothetical scenarios: with and without secondary fracture sets. The results of these two scenarios indicate that for this particular fracture network, secondary fracture sets marginally impact (10% change) the breakthrough time of water injected into an oil-filled reservoir. Our work provides a prototype workflow that links outcrop fracture observations to 3-D DFN model flow simulations using LIDAR data, an approach that offers some improvement over traditional field-based DFN constructions. In addition, the techniques we used to extract fractures may prove applicable to other outcrop studies with different research goals.