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