Crustal Materials

Wendy Bohrson

Frank Spera

02/01/95-01/31/97 $81,000

National Science Foundation, NSF EAR 94-18720

Processes and rates of compositional zonation in crustal magma bodies: Constraints from high-precision U-Th disequilibria

U-Th isotope disequilibria studies have the potential to reveal the time scales and processes of silicic magma formation. In this study, Bohrson and Spera apply this technique to 2 young ash-flow tuffs located near Naples, Italy: the ~35,000 year old Campanian Ignimbrite and the ~12,000 year old Neapolitan Yellow Tuff. The compositional zoning evident in both deposits is interpreted to reflect zoning present in a crustal magma body just prior to eruption. The questions they are addressing include (1) what processes contribute to compositional zonation, and (2) how long does it take to form such a compositionally zoned magma body?

Their preliminary U-Th disequilibria results on the Campanian Ignimbrite indicate that the zoning process occurred in several thousand years. This is relatively rapid compared to apparent zoning times documented by other studies of this type; most silicic bodies zone over a period of at least 104 years. Data generated this year on the Neapolitan Yellow Tuff will provide an important point of comparison to the Campanian Ignimbrite. Additional compositional data currently being generated combined with their U-Th results will provide the needed constraints to understand how the process of zoning occurred in these deposits.

Frank Spera

12/15/94-12/14/96 $301,651

Department of Energy, DE-FG03-91ER 14211

Physical and Experimental Studies of Magma Rheology, Sedimentary Basins and Molecular Dynamics of Silicates

This collaborative project with D. A. Yuen at the University of Minnesota will improve 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 work-plan includes: (1) Construction of new rheological apparatus and laboratory measurements on melts and magmatic suspensions (2) Determination of the thermodynamical and transport properties of molten silicates by MD simulations (3) Three-dimensional modeling of salt diapirs including the effects of dehydration on salt rheology (4) Numerical modeling of magmatic underplating and the formation of granitic diapirs (5) Coupling between mantle convection with temperature-dependent and non-Newtonian rheology and mantle diapirs 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 modeling of heat and solute transport driven by thermal and salinity heterogeneities in geothermal systems.

Results cited below are for the UCSB part of this project. Additional results are given in the summary of activities by the University of Minnesota team lead by D. A. Yuen. Molecular Dynamics simulations on melts in the system NaAlSiO4-SiO4 at 3 GPa and high temperatures have been completed. Results have been published in American Mineralogist. The citations are: Molecular Dynamics Simulations of Liquids and Glasses in the system NaAlSiO4-Si02: Methodology and Melt Structures, American Mineralogist, vol 80, p 417-431 (1995). A second publication on this subject is: Molecular Dynamics Simulations of Liquids and Glasses in the System NaAlSiO4-Si02: Physical Properties and Transport Mechanisms, American Mineralogist, vol 81, p 284-302 (1996). In separate work, 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. Results are published in: Simulations of Convection with Crystallization in the System KAlSi2O6-CaMg Si2O6: Implications for Compositionally Zoned Magma Bodies, American Mineralogist, vol 80, p 1188-1207 (1995). A paper on anatexis driven by mafic magma underplating using this same approach is currently in preparation.

In additional work, the design, fabrication and assembly of a new high-precision concentric cylinder rheometer with capability in the range 10-3 to 3 Nm of torque and shear rates in the range 10-4 to 1 s-1 at 105 Pa and temperatures to 1600oC is almost complete. All major sub-systems have been procured and the final assembly is underway. They expect to be making measurements by early 1997.

Frank Spera

07/01/93-12/31/95 $60,204

National Science Foundation, NSF EAR93-03906

Magma Transport Phenomena: Microscopic to Macroscopic

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 included: 1) expansion of code capability to multicomponent natural systems using best thermodynamic database available, 2) analysis of the "crustal anatexis driven by basaltic underplating" paradigm, 3) study of the behavior of silicate mush piles, specifically the spontaneous development of melt channels within the mush during cumulate formation 4) modeling of radial-zonation of Sierran-type granitic plutons. At the microscopic scale, the method of Molecular Dynamics was 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, computation of the 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 was accomplished. Results compare well with laboratory measurements.

Frank Spera

07/15/93-12/31/96 $92,077

National Science Foundation, NSF OCE 93-02058

Convective Dynamics Beneath Crustal Oceanic Spreading Centers

Calculations on two major problems relevant to MOR dynamics have been recently completed. Spera 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 published in 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 pseudo fluid 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.