Crustal Materials

Personnel: W. Bohrson, J. Bryce, F. Spera*, D. Stein, A. Trial (*Agenda coordinator)

Processes and Rates of Compositional Zonation in Crustal Magma Bodies: Constraints from High-precision U-Th Disequilibria (NSF EAR94-18720) (Wendy Bohrson, Frank J. Spera)

Samples (8-10) which span the compositional range of the Campanian Ignimbrite (~35 ka), located near Naples, Italy, are being processed for U-Th isotopic analyses; measurements will take place in November, 1995. An additional 30 samples from a single exposure of the Campanian Ignimbrite have been collected and are being processed for major and trace element analyses. U-Th work on a subset of these will begin in early 1996. In addition, final separation of minerals from the ignimbrite samples is underway; mineral separate U-Th work is planned for Spring 1996. Together, these data will be used to quantify time scales and processes of silicic magma genesis.

Physical and Experimental Studies of Magma Rheology, Sedimentary Basins and Molecular Dynamics of Silicates (DOE DE-FG03-91ER 14211) (Frank J. Spera)

This is a three-year collaborative project involving both laboratory and numerical work in three areas of relevance to the geoscience mission of the DOE. The three areas encompass: (1) experimental studies on the rheology of magmas and Molecular Dynamics simulations of molten silicates of relevance to crustal processes (2) modelling of thermal-mechanical processes and porous media convection in sedimentary basins and (3) analysis of the causes and consequences of magmatic underplating in sedimentary basins including crustal anatexis. This interdisciplinary project draws diverse talents and will improve our understanding of the thermal, chemical, dynamical and mechanical state of the continental crust and subcrustal lithosphere with particular focus on the interactions between the various subsystems. The detailed work-plan includes: (1) Construction of new rheological apparatus and laboratory measurements on melts and magmatic suspensions (2) The determination of the thermodynamical and transport properties of molten silicates of crustal relevance by MD simulations (3) Three-dimensional modelling of salt diapirs including the effects of dehydration on salt rheology (4) Numerical modelling of magmatic underplating and the formation of granitic diapirs (5) Coupling between mantle convection with temperature-dependent and non-Newtonian rheology and, in particular, mantle diapirs or plumes, on the thermal regime and subsidence curves of rift-related basins (6) The dynamical influences of lithospheric phase transitions on the thermal-mechanical evolution of sedimentary basins (7) The development of stress fields and criteria for faulting in the crust and finally (8) Numerical modelling of heat and solute transport driven by thermal and salinity heterogeneity in low-porosity fractured and/or granular geologic media with applications to sedimentary basins and geothermal systems.

Magma Transport Phenomena: Microscopic to Macroscopic (NSF EAR93-03906) (Frank J. Spera)

This project involves research in magma transport phenomena at both the macroscopic and microscopic scale. Work at the macroscopic scale utilizes a sophisticated computer code that faithfully captures details of convection in two-component melts undergoing phase change. The work includes: 1) up-grading code to 3-dimensions with some technical improvements including more realistic two-phase non-Newtonian rheology, 2) expansion of code capability to multicomponent natural systems using the best thermodynamic database available, 3) analysis of the crustal anatexis driven by basaltic underplating paradigm, 4) study of the behavior of silicate mush piles, specifically the spontaneous development of melt channels within the mush during cumulate formation 5) modeling of radial-zonation of Sierran-type granitic plutons. At the microscopic scale, the method of Molecular Dynamics will be used to study the transport properties (trace and chemical diffusion and melt viscosity) of melts in the systems Na2O-SiO2 and NaAlSiO4-SiO2 at high temperatures and pressures. In particular, we will attempt to compute the full (n-1)2 diffusion matrix for chemical diffusion in the system Na2O2-Al2O3-SiO2 at geologically relevant conditions of temperature and pressure using the linear response theory embodied in the Green-Kubo relations.

Connective Dynamics Beneath Crustal Oceanic Spreading Centers (NSF OCE-9302058) (Frank J. Spera)

Calculations on two major problems relevant to MOR dynamics have been recently completed. We have studied the role of salinity buoyancy on the style and evolution of hydrothermal circulation in low-permeability anisotropic materials such as fractured oceanic crust. Unlike the situation when convection is driven solely by thermal buoyancy, when salinity contributes to buoyancy, flows become chaotic (but deterministic) and transiently layered. The need to resolve very thin chemical boundary layers necessitates great care in the choice of the spatial and time resolution scales for these simulations.

A second set of calculations using the SAC code to investigate the dynamics of convection within a binary CaMgSi2O6-CaAl2Si2O8 melt that is undergoing crystallization has been completed and submitted to American Mineralogist. 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, percolative flow of melt through mush and the variation of system enthalpy with composition, temperature and solid fraction. 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 in terms of a mixture enthalpy equation with subsidiary expressions, based on thermochemical data and phase relations, that relate the mixture enthalpy to temperature, composition and phase abundance at each location. Species conservation is written in terms of the low-density component and allows for advection and diffusion as well as the relative motion between solid and melt.

Systematic simulations were performed in order 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 due to non-linear coupling between the momentum, energy and species conservation equations. A consequence of the coupling includes the spontaneous development of compositional heterogeneity in terms of the modal abundance 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 heat extraction rate due to bursts of crystallization and concomitant buoyancy generation are also found. 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 heterogeneity develop. The rate of formation of the compositional stratification is highest for the sill-like body due to its high cooling rate. Compositional zonation in a fully solidified body 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 homogenous and slightly superheated initial states assuming local equilibrium prevails during the course of phase change.

For the next year we will continue to study MOR melt dynamics. Specifically, we are looking into mixing dynamics in regions that span the solidus to liquidus temperatures. These are regions where the flow changes from a Darcy percolative flow (solid dominated) to clear - liquid viscous flow (melt dominated). The goal is to better understand the boundary between a mostly solid mush and a liquid - dominated melt lens. Additional calculations on the role of the rheological two-phase (solid/melt) convective flows are also in progress. The rheological properties of mush are well-known to be non-Newtonian. We are studying how such behavior affects magma evolution. In mush that consists of about 30 to 60 volume percent solid, a power-law rheology is appropriate. The release of compositional buoyancy upon crystallization of non-Newtonian magma can lead to significant viscous heating.