Frank Spera

12/15/97 - 12/14/98

Department of Energy, DE-FG03-91ER14211

Collaborative Research: Magma Rheology, Mixing of Rheological Fluids, Molecular Dynamics Simulation, and Lithospheric Dynamics

Our aims are (1) experimental rheological measurements of magma including melts, melt-vapor emulsions and crystal-melt suspensions (2) Molecular Dynamics (MD) simulations of molten and glassy silicate and aluminosilicate geomaterials with emphasis on understanding the connection between atomic structure and properties at temperatures and pressures characteristic of continental lithosphere (3) high resolution simulation of thermohaline convection in low-porosity geomedia (4) development of geochemical models for the evolution of crustal magma bodies undergoing simultaneous assimilation, fractional crystallization, periodic recharge and periodic eruption that incorporate energy conservation in addition to total and species mass conservation.

This collaborative project with D. A. Yuen at the University of Minnesota, Minneapolis will promote our understanding of the thermal, chemical, mechanical and rheological states of the continental crust and subcrustal lithosphere with particular emphasis on the nonlinear interactions among the various subsystems including hydrothermal and magmatic components. Our workplan encompasses the following (1) determination of the structure and property of melts by MD simulations of multicomponent systems. (2) Molecular Dynamics of mesoscale fluid-dynamical phenomenon in the range of tens of microns, specifically for two-fluid and chemically reacting systems (3) mixing processes of rheological fluids in convection and visualization of complex processes (4) shear zones in faulting from grain-size rheology (5) development of stress fields and faulting (6) numerical modeling of heat and mass transport driven by thermal and chemical heterogeneity's in hydrothermal-magmatic systems (7) experimental rheometric measurements of magma at high temperatures and development of non-linear constitutive relations.

The results reported below are the UC Santa Barbara part of this project. Additional results can be found in the summary of activities by the University of Minnesota team led by D. A. Yuen. Construction of a novel custom high temperature rheometer based on a concentric cylinder design has been completed. This device can investigate the rheology of magma, a complex mixture of solids, melt and vapor bubbles, at temperatures from 700 °C to 1375 °C for shear rates in the range 10-5 to 1 s-1. The instrument has been calibrated and certified using NBS standard borosilicate and lead silicate compositions; we can measure the viscosity of these melts to within 0. 02 log10 units. This 2-sigma uncertainty is better than the NBS quoted value of 0. 029 log10 units. A paper is in press in the journal Review of Scientific Instruments that details the salient features of the design, fabrication and certification of this device. Experiments are presently underway exploring the relative viscosity, defined as the ratio of the viscosity of the mixture to the viscosity of single phase melt for a rhyolite from the DOE sponsored drilling site at Obsidian Dome, near Long Valley California as well as melt-vapor emulsions of molten albite and molten orthoclase compositions. There are essentially no previous measurements for the viscosities of these magmatic emulsions with vapor contents in the range 0 to circa 50 volume percent. Preliminary results suggest the presence of a percolation threshold of about 10 to 20 volume percent beyond which the mixture (relative) viscosity depends upon vapor content rather strongly and (2) a critical shear rate exists such that for g > g crit the relative viscosity decreases with increasing bubble loading whereas at shear rates less than the critical value the relative viscosity increases with bubble content. The goal of this work is to develop comprehensive non-linear constitutive relations for melt-vapor emulsions in the range of shear rates and temperatures relevant to crustal magmas. We also plan to investigate solid-melt mixtures. Other work completed this year includes an extensive set of MD simulations applicable to molten and glassy CaAl2Si2O8 at temperatures and pressures characteristic of continental lithosphere. Subducting crust is rich in anorthite component and the goal of these studies has been to explore the atomic structure of this melt especially the relationship between its changing structure and its macroscopic thermodynamic and transport properties. This work is currently in press in the journal American Mineralogist . Other MD simulations for compositions in the system NaAlO2-SiO2 have also been carried out; we have shown that MD results may be used in combination with the analytic theory of Adams-Gibbs-DiMarzio (AGD) to predict the variation of oxygen self-diffusion as a function of pressure, temperature and composition in this model crustal system. A third sub-project that is in progress is the development of a geochemical model that takes energy conservation into explicit account to forward model the geochemical evolution of crustal magma bodies that are undergoing simultaneous assimilation, fractional crystallization, recharge and eruption. This work is now being written up and two papers detailing it will be submitted by December 1998. Results have already been presented at the AGU Fall 1997 meeting. Finally, high-resolution simulations of the dynamics of thermohaline convection in saturated porous media have been completed. We have confirmed the earlier work of Spera and Yuen that in low porosity geologic media that a doubly-advective instability sets in that leads to chaotic behavior for Darcy Rayleigh numbers and buoyancy ratios in the range found in nature. These simulations take account of the temperature and composition dependence of fluid properties (H2O +NaCl) and allow for the importance of dispersion (rather than simple molecular diffusion) of solute. This doubly-advective instability has application to diagenesis, the formation of crustal ore deposits and the evolution of metamorphic terrains. This work is being written up for submission to Earth and Planetary Science Letters and Science and we will present some results at the Fall 1998 AGU meeting in San Francisco.

 

Frank Spera

Alain Trial

7/1/96 - 12/30/98

National Science Foundation, EAR-9627800

Collaborative Research: Role of Shear Heating in the Generation and Ascent of Granitic, Basaltic, and Komatiitic Magma

A model has been set up to look at the geochemical evolution of magma bodies undergoing simultaneous assimilation, fractional crystallization, magma recharge and magma venting or eruption. This model explicitly incorporates energy conservation and provides information regarding the path magma follows as thermal equilibration is approached. The model itself is a set of 5 coupled non-linear ordinary differential equations solution of which gives the magma temperature, assimilant temperature, fraction of magma, mass of cumulates and the isotopic and trace element composition of the magma including oxygen isotopic ratios as equilibrium is approached.

 

Frank Spera

Daniel Stein

3/1/97 - 2/28/99

National Science Foundation, EAR-9614376

Experimental Rheometry of Magmatic Multiphase Suspensions

During 1998 this project entered its operational phase as the new computer-controlled magma rheometer produced its initial results. Measurements conducted on a high-temperature viscosity standard material have verified that the instrument recovers the published values for the reference standard within the published precision. A full description of the instrument and its component systems, as well as the results for the standard, has been published in "The Review of Scientific Instruments". Additional measurements of a synthetic liquis material of composition near that of the mineral phase orthoclase (microcline) feldspar have also been conducted and the results are consistent with those produced in other laboratories.

Significant progress has been realized in the quest to provide the first high-precision rheological measurements on concentrated silicate magmatic gas-liquid emulsions at geologically-realistic shearing rates. Reliable techniques for preparing magmatic emulsions with as much as 60 volume percent of vapor phase have been developed. The liquid phases to be used consist of 1) A natural rhylolite obsidian from the Mediterranean Island of Lipari and 2) Liquids near in composition.

In the experiments to be conducted, silicate liquids containing suspensions of vapor bubbles held at high temperatures are sheared between the concentric, counter-rotating cylinders of the rheometer's sample assembly. Viscosity in these multi-phase materials is expected to vary with the experiment temperature, the shearing rate, and volume fraction of bubbles, perhaps in an intricate manner.

Data obtained in this study will lead to significant progress in understanding the complex rheological behavior of magmatic systems and their consequent natural hazards.

 

Wendy Bohrson

Frank Spera

2/1/95 - 1/31/99

National Science Foundation, EAR-9418720

Processes and Rates of Compositional Zonation in Crustal Magma Bodies: Constraints from High-precision U-Th Disequilibria

The Campanian ignimbrite is an 80 km 3 , moderately welded alkali trachyte located near Naples, Italy. The petrology and U-Th disequilibria characteristics of this explosively-generated deposit have been studied through major and trace element analyses (~15) as well as high-precision Th isotopic analyses (~12). These data indicate that this young deposit derived from a zoned magma body that experienced high-level crustal contamination; the most likely contaminant is hydrothermally-altered volcanic basement. Additional Th work, in the form of mineral isochrons, is underway to determine the timing and extent of this contamination process. In addition, the first high-precision 40 Ar/ 39 Ar ages of the deposit have been generated, dating the deposit at ~39,000 years. Additional Ar analyses are now underway to address questions raised by recent field work: an older ignimbrite, separated from the "classic" Campanian by a soil, has been discovered, and the new Ar analyses will assess the age of this unit. The results could be very important because this older unit may represent a previously unidentified explosive eruption in the Naples area; such information would be crucial to accurate assessments of volcanic hazard in this densely populated region of Italy.

This grant also supported theoretical work by Bohrson and Spera about the nature of magma-wallrock interaction, specifically the process of assimilation-fractional crystallization (AFC). AFC is governed by the laws of conservation of mass, energy, and species. Prior to the work Bohrson and Spera are currently doing, assessment of this process had been limited to treatments that considered conservation of mass and species. The addition of conservation of energy has dramatically changed the theoretical chemical trends associated with the process of AFC; a review of the literature, currently underway, suggests these findings may explain data sets that were previously only poorly explained by traditional AFC modeling. In addition, some of the long-held assumptions about the process of AFC have been shown to be incorrect by this new formulation (Spera and Bohrson, 1997; Bohrson and Spera, 1997).

 

 

Frank Spera

Wendy Bohrson

3/15/97 - 2/28/99

National Science Foundation, EAR-9614381

Isotopic and Petrological Constraints on Magma Dynamics at Mt. Etna

Previous work has established that there are large variations in chemical and isotopic compositions in historical and ancient eruptions from Mt. Etna, Italy. We are addressing this matter in more detail using state-of-the-art geochemical and isotopic techniques.

The first phase of the work is now complete: study of the 500 ka to 100 ka record (Bryce et al. , 1997, Bryce et al. , 1998, Bryce, 1998). Results indicate that Etna changes from a dominantly tholeiitic fissure volcano to a mildly alkalic composite volcano where central eruptions dominate. The change occurred over a 100 ka interval, starting around 200 ka and ending around 85 ka. Detailed modeling indicates that the geochemical and isotopic characteristics of the mantle source region from which Etnean magmas were extracted changed discontinuously. The data suggest that the source region may have experienced carbonatite metasomatism, that extensive fractional crystallization affected the magmas, and that some limited alkali enrichment, perhaps due to assimilation of crustal material, may have also occurred. This study represents the Ph. D. dissertation of Julia Bryce, who completed her degree in August 1998. She is now preparing the data and interpretations for submission to peer-reviewed journals.

Historical lavas from Mt. Etna record distinct changes in particular geochemical characteristics with time. Interpretations of the causes of these changes range from mantle heterogeneity to crustal contamination. We have undertaken a detailed characterization of the size distribution, chemical and isotopic composition of phenocrysts and microphenocrysts from a number of key historical eruptions. Petrographic descriptions, electron microprobe work and crystal-size distribution analyses are currently underway. These will be followed by detailed microsampling of individual phenocrysts for Sr and Pb isotopes. Our results will provide constraints on mantle-dominated vs. crust-dominated models for the chemical and isotopic variations, and on processes of magma formation, storage and ascent through the continental crust.