Micol Todesco

Simulations of convection with crystallization in the system KAlSi 2 O 6 -CaMgSi 2 O 6 ; implications for compositionally zoned magma bodies

Abstract A model has been developed and applied to study the origin of compositional and phase heterogeneity in magma bodies undergoing simultaneous convection and phase change. The simulator is applied to binary-component solidification of an initially superheated and homogeneous batch of magma. The model accounts for solidified, mushy (two- or three-phase), and all-liquid regions self-consistently, including latent heat effects, perco- lative flow of melt through mush, and the variation of system enthalpy with composition, temperature, and solid fraction. Phase Equilibria and thermochemical and transport data for the system KAlSi2O6-CaMgSi2O6 were utilized to address the origin of compositional zonation in model peralkaline magmatic systems. Momentum transport is accomplished by Darcy percolation in solid-dominated regions and by internal viscous stress diffusion in melt-dominated regions within which relative motion between solid and melt is not allowed. Otherwise, the mixture advects as a pseudofluid with a viscosity that depends on the local crystallinity. Energy conservation is written as a mixture-enthalpy equation with subsidiary expressions that are based on thermochemical data and phase relations that relate the mixture enthalpy to temperature, composition, and phase abundances at each location. Species conservation is written as the low-density component (KAlSi2O6) and allows for advection and diffusion as well as the relative motion between solid and melt. Systematic simulations were performed to assess the role of thermal boundary conditions, solidification rates, and magma-body shape on the crystallization history. Examination of animations showing the spatial development of the bulk (mixture) composition (C) melt composition (C1), temperature (T), solid fraction (fs), mixture enthalpy (h), and velocity (V), reveals the unsteady and complex nature of convective solidification owing to nonlinear coupling among the momentum, energy, and species conservation equations. A consequence of the coupling includes the spontaneous development of compositional heterogeneity in terms of the modal abundances of diopside and leucite in the all-solid parts of the domain (i.e., modal mineralógical heterogeneity) as well as spatial variations in melt composition particularly within mushy regions where phase relations strongly couple compositional and thermal fields. Temporal changes in the rate of heat extraction because of bursts of crystallization and concommitant buoyancy generation are also found. Crystallization of diopside, the liquidus phase in all cases, enriches residual melt in low- density K-rich liquid. The upward flow of this material near the mush-liquid interface leads to the development of a strong vertical compositional gradient. The main effect of magma-body shape and different thermal boundary conditions is in changing the rate of solidification; in all cases compositional heterogeneities develop. The rate of formation of the compositional stratification is highest for the sill-like body because of its high cooling rate. Compositional zonation in a fully solidified body is found to be both radial and vertical. The most salient feature of this simple model is the spontaneous development of large-scale magma heterogeneity from homogeneous and slightly superheated initial states, assuming local equilibrium prevails during the course of phase change.

Effects of atmospheric conditions on surface diffuse degassing

[1]xa0Diffuse degassing through the soil is commonly observed in volcanic areas and monitoring of carbon dioxide flux at the surface can provide a safe and effective way to infer the state of activity of the volcanic system. Continuous measurement stations are often installed on active volcanoes such as Furnas (Azores archipelago), which features low temperature fumaroles, hot and cold CO2 rich springs, and several diffuse degassing areas. As in other volcanoes, fluxes measured at Furnas are often correlated with environmental variables, such as air temperature or barometric pressure, with daily and seasonal cycles that become more evident when gas emission is low. In this work, we study how changes in air temperature and barometric pressure may affect the gas emission through the soil. The TOUGH2 geothermal simulator was used to simulate the gas propagation through the soil as a function of fluctuating atmospheric conditions. Then, a dual parameters study was performed to assess how the rock permeability and the gas source properties affect the resulting fluxes. Numerical results are in good agreement with the observed data at Furnas, and show that atmospheric variables may cause the observed daily cycles in CO2 fluxes. The observed changes depend on soil permeability and on the pressure driving the upward flux.

Ground heating and methane oxidation processes at shallow depth in Terre Calde di Medolla (Italy): Numerical modeling

The area known as Terre Calde (literally “hot lands”) in the plain of the Po River (Italy) is well known for unusual ground temperatures, and up to now, the cause o/f the heating has not been fully investigated. These higher-than-average temperatures are commonly associated with diffuse methane seepage. A detailed study of shallow stratigraphy, temperature profile, and associated gas concentrations and flow rates recently suggested that the observed anomaly could be related to the exothermic oxidation of biogenic methane, possibly rising from a shallow peat layer. In this work, a porous media flow simulator (Transport of Unsaturated Groundwater and Heat 2) was applied to verify a conceptual model of this phenomenon. The model describes a layered system, with a shallow unsaturated zone, where methane is continuously supplied along the base and heat is generated as a result of its oxidation above the water table. To mimic the oxidation process, heat sources are placed within the layer where oxidation takes place, and the heat generation is computed as a function of methane flux entering the layer. Numerical simulations were carried out imposing different methane flow rates along the base of the model. The simulations also explored the efficiency of methane oxidation, considering different heat generation rates and accounting for seasonal effects. The good match between observed and simulated temperature profiles suggests that the main features of the process are captured by the model and that the conceptual model devised on the base of available data is plausible from a physical point of view.

Stability of a chemically layered upper mantle

The possibility that the upper mantle at depths less than 670 km is chemically as well as mineralogically layered has been extensively discussed. One idea posits that sublithospheric upper mantle (d < 400 km) is dominantly harzburgitic and of low intrinsic density compared with majoritic and clinopyroxene-rich piclogite which occupies the seismic transition region at depths between 400–670 km. The gravitational stability of the ‘harzburgite over piclogite’ arrangement (light above heavy) when heated from below is investigated here in order to better understand the dynamics of mixing. The calculations neglect the effects of plates, compressibility, viscous dissipation, phase change and radiogenic heating and focus on the role played by Δρρ0, the intrinsic density difference between the layers during mixing at fixed Rayleigh number. The dimensionless parameters of this problem include the thermal Rayleigh number based on the heat flux q0 into the basal piclogitic layers (Rq = α gq0d4k κν), the ratio of chemical to thermal buoyancy Rρ = (Δρρ0)kαq0d, and the thickness ratio of the two layers Δ. Here α, g, d, κ, k, ν, Δρ, ρ0 represent the expansitivity, gravity, total depth, thermal diffusivity, thermal conductivity, kinematic viscosity, isothermal difference in density between the two layers (i.e. the intrinsic density difference) and density of the piclogitic bottom layer, respectively. A constant-viscosity Newtonian rheology is assumed for the sublithospheric upper mantle between 100–670 km. n nThree measures of the extent and thoroughness of mixing are used to quantify mixing; these include the variance of the compositional field, the two-point spatial correlation function for composition and the average composition within each layer. The spatial correlation enables one to define a dominant length scale characteristic of the size of the chemical anomalies (L∗). The adimensional variance, sometimes called the mixing intensity, may be used to define a mixing time. Simulations at fixed Rq but with Δρρ0 = 0, 2, 4, 8% have been carried out for periods of time equivalent to the age of the Earth. The critical Rρ that separates well-mixed states from poorly mixed ones is Rρ ≈ 15 for Rq = 2 × 105. For nominal upper-mantle parameters this implies a critical intrinsic density difference Δρρ0 ≈ 3%. The style of mixing is grossly different depending on whether Δρρ0 is less than, or greater than, the critical value. Plume penetration with rapid changes in the average size of chemical heterogeneities is the dominant mechanism at low Δρρ0 whereas for high Δρρ0 viscous entrainment and the stretching of tendrils along the layer interface is the dominant mixing style. For density ratios near the critical value, the fraction of fertile (easily fused) peridotite within the dominantly harzburgitic upper mantle above the top of the transition region varies quasiperiodically with period ≈ 0.6 Ga, roughly equal to the supercontinent cycle time. Intermittent periods of increased lower-layer transport across the top of the transition zone may correlate with spikes in the volumetric rate of magma generation due to decompression melting of ascending fertile peridotite.

How Steep Is My Seep? Seepage in Volcanic Lakes, Hints from Numerical Simulations

The existence and survival of volcanic lakes require the accomplishment of a delicate balance between meteoric recharge, evaporation, and water loss by infiltration within the volcanic edifice, commonly referred to as seepage. A deep-seated, volcanic component may participate to a variable extent to the lake’s evolution, depending on volcanic activity. In this work, we apply a numerical model of hydrothermal fluid circulation to study the interaction between the hot volcanic gases and the shallow lake water. We focus on the conceptual model developed for Poas volcano (Costa Rica), where a shallow magma intrusion drives the hydrothermal activity underneath and around the crater lake. Numerical simulations are carried out to assess the role of relevant system properties, including rock permeability, reservoir conditions, lake geometry, and meteoric recharge. Our results suggest that vertical seepage can be severely hindered by the ascent of volcanic gases, whereas horizontal infiltration through the vertical lake walls may ensure a long-term water loss. Our simulations also show that the permeability distribution, especially around the lake, determines the overall pattern of circulation affecting the development and spatial distribution of hot springs and fumaroles, and ultimately controlling the evolution of the lake.