Oldenburg, C. M.

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.

The Phlegrean Fields is an active caldera structure, located on the periphery of Naples (Italy. After the last eruptive event (the Monte Nuovo eruption in 1538), periodic episodes of unrest have characterized the evolution of this volcanic district, involving seismic activity and slow ground motion (bradyseism). During these episodes of unrest, some remarkable changes have also affected the composition of the hydrothermal fluids discharged at La Solfatara fumarolic field. These unrest phenomena result from the complex interaction between magma chamber, hydrothermal fluid circulation, and country rocks undergoing thermal and mechanical stresses. In order to make an effective hazard assessment in such a densely populated area as the Phlegrean Fields, we must first reach a better understanding of the mechanism driving bradyseismic activity and determine the relation between ground deformation and hydrothermal fluid circulation. In this work, we present some results of numerical modeling of both the hydrothermal fluid circulation at La Solfatara, and of its effects on rock deformation. The modeling results show that periods of intensified magmatic degassing can explain many features of the recent crises of unrest at Phlegrean Fields.

Upward displacement of brine from deep reservoirs driven by pressure increases resulting fromCO2 injection for geologic carbon sequestrationmay occur through improperly sealed abandoned wells, through permeable faults, or through permeable channels between pinch-outs of shale formations. The concern about upward brine flow is that, upon intrusion into aquifers containing groundwater resources, the brinemay degrade groundwater. Because both salinity and temperature increase with depth in sedimentary basins, upward displacement of brine involves lifting fluid that is saline but also warm into shallower regions that contain fresher, cooler water. We have carried out dynamic simulations using TOUGH2/EOS7 of upward displacement of warm, salty water into cooler, fresher aquifers in a highly idealized two-dimensional model consisting of a vertical conduit (representing a well or permeable fault) connecting a deep and a shallow reservoir. Our simulations show that for small pressure increases and/or high-salinity-gradient cases, brine is pushed up the conduit to a new static steady-state equilibrium. On the other hand, if the pressure rise is large enough that brine is pushed up the conduit and into the overlying upper aquifer, flow may be sustained if the dense brine is allowed to spread laterally. In this scenario, dense brine only contacts the lower-most region of the upper aquifer. In a hypothetical case in which strong cooling of the dense brine occurs in the upper reservoir, the brine becomes sufficiently dense that it flows back down into the deeper reservoir from where it came. The brine then heats again in the lower aquifer and moves back up the conduit to repeat the cycle. Parameter studies delineate steady-state (static) and oscillatory solutions and reveal the character and period of oscillatory solutions. Such oscillatory solutions aremostly a curiosity rather than an expected natural phenomenon because in nature the geothermal gradient prevents the cooling in the upper aquifer that occurs in the model. The expected effect of upward brine displacement is either establishment of a new hydrostatic equilibrium or sustained upward flux into the bottom-most region of the upper aquifer.