The deep interior of the Earth is in some ways less accessible than the outermost reaches of the Solar System. We can send spacecraft to orbit the outer planets and their moons, but direct observations of Earth’s interior are limited by the depths of the deepest drill holes–about 12 kilometers, less than 0.2% of Earth’s radius. Fortunately, we have a diverse set of indirect probes that can be used to study the deep Earth. Together these have provided an increasingly detailed picture of Earth’s internal structure, and its evolution over time.
One suite of techniques for probing the Earth is geophysical, the most important being based on seismic waves. Another important source of information comes from the analysis of deep samples that have been brought to the surface by volcanic or tectonic activity. Similar samples are available from the Moon, Mars and other Solar System objects, and provide crucial information on their formation and internal structure.
To use geophysical and geochemical observations to understand the structure and evolution of Earth and other planets usually requires a fairly detailed understanding of the physical and chemical properties of their materials. This is the focus of the work in our research group at Case. We investigate fundamental material properties and processes at very high pressures and temperatures, using both experimental and computational methods. From an understanding of the material properties we build geochemical and/or geophysical models of large-scale processes in deep planetary interiors.
A large part of our work is focused on the diffusion of atoms through planetary materials, both solid and liquid. Diffusion plays a fundamental role in many important geochemical and geophysical processes, including creep deformation (the process by which Earth’s mantle convects), anelastic deformation (which plays an important role in the attenuation of seismic waves), chemical exchange between minerals and melt during mantle melting, bubble formation and degassing during volcanic eruptions, and resetting of isotopic ages of rocks and minerals at high temperatures. A detailed understanding of diffusion mechanisms at the atomic level is essential in order to apply laboratory data to Earth and planetary problems, and investigating these is a significant part of our effort.
Core-Mantle Chemical Exchange (funded by the National Science Foundation)
Whether there is significant material transfer across Earth’s core-mantle boundary is a question that has important implications for geochemistry and geodynamics. We are addressing this question in two ways.
1) Experiments to measure the partitioning of siderophile elements between solid and liquid iron alloys at high pressure. These data provide constraints on the chemical evolution of the outer core during inner core crystallization, which must be known in order to identify the characteristics of a chemical signal from the core (which might be brought to Earth’s surface by plumes originating at the core-mantle boundary).
2) Experiments and modeling to address the kinetics of chemical exchange across the core-mantle boundary.
Van Orman J.A., Li C., Crispin K.L. (2009) Aluminum diffusion and Al-vacancy association in periclase. Physics of the Earth and Planetary Interiors 172, 34-42.
Van Orman J.A., Keshav S., Fei Y. (2008) High-pressure solid/liquid partitioning of Os, Re and Pt in the Fe-S system. Earth and Planetary Science Letters 274, 250-257.
Van Orman J.A., Fei Y., Hauri E.H., Wang J. (2003) Diffusion in MgO at high pressures: Constraints on deformation mechanisms and chemical transport at the core-mantle boundary. Geophysical Research Letters 30, 1056-1059.
Planetary and Early Solar System Processes (funded by NASA)
We are involved in several collaborative projects on planetary materials (meteorites and Apollo lunar samples), most of which involve simulations of diffusion-controlled chemical differentiation processes. Active research projects include studies of volatiles in lunar ultramafic glasses (with Alberto Saal and Erik Hauri); experimental work to determine the closure temperatures of extinct radionuclide systems used for dating early solar system processes (with Leslie Hayden, Daniele Cherniak and others); and geochemical modeling of ureilite petrogenesis (with Cyrena Goodrich and others).
Wilson L., Goodrich C.A., Van Orman J.A. (2008) Thermal evolution and physics of melt extraction on the ureilite parent body. Geochimica et Cosmochimica Acta, in press.
Saal A.E., Hauri E.H., Lo Cascio M., Van Orman J.A., Rutherford M.C., Cooper R.F. (2008) Volatile content of lunar volcanic glasses and the presence of water in the Moon’s interior. Nature 454, 192-195.
Kleine T., Touboul M., Van Orman J.A., Bourdon B., Maden C., Mezger K., Halliday A. (2008) Hf-W thermochronometry: Closure temperature and constraints on the accretion and cooling history of the H chondrite parent body. Earth and Planetary Science Letters 270, 106-118.
Goodrich C.A., Van Orman J.A., Wilson L. (2007) Fractional melting and smelting on the ureilite parent body. Geochimica et Cosmochimica Acta 71, 2876-2895.
Van Orman J.A., Grove T.L. (2000) Origin of lunar high-Ti ultramafic glasses: Constraints from phase relations and dissolution kinetics of clinopyroxene-ilmenite cumulates. Meteoritics and Planetary Science 35, 783-794.
Diffusive Fractionation of Trace Elements and U-Series Isotopes During Mantle Melting
Incompatible trace elements and short-lived daughters in the uranium decay series provide key insights into the characteristics of the mantle and the processes and timescales of mantle melting. Ongoing experimental and numerical modeling studies are underway to examine the influence of solid-state diffusion on the trace element and isotopic compositions of basaltic magmas and the solids from which they are extracted. One of our goals is to provide new tools for investigating processes and rates of melting in Earth’s upper mantle and crust.
Image from T. Elliott (2005) Nature 437, 485-486
Bourdon B., Van Orman J.A. (2009) Melting of enriched mantle beneath Pitcairn seamounts: Unusual U-Th-Ra systematics provide insight into melt extraction processes. Earth and Planetary Science Letters, in press.
Van Orman J.A., Saal A.E., Bourdon B., Hauri E.H. (2006) Diffusive fractionation of U-series radionuclides during mantle melting and shallow level melt-cumulate interaction. Geochimica et Cosmochimica Acta 70, 4797-4812.
Saal A.E., Van Orman J.A. (2004) The 226Ra enrichment in oceanic basalts: Evidence for diffusive interaction proesses within the crust-mantle transition zone. Geochemistry, Geophysics, Geosystems 4, Art. No. Q02008.
Van Orman J.A., Grove T.L., Shimizu N. (2002) Diffusive fractionation of trace elements during production and transport of melt in Earth’s upper mantle. Earth and Planetary Science Letters 198, 93-112.
Rheology of Earth’s Deep Interior (funded by the National Science Foundation)
Constraining the viscous rheology of Earth’s deep mantle and core is a grand challenge in mineral physics. Our approach, which is complementary to high-pressure deformation experiments, is to determine atomic diffusion coefficients in the relevant materials at high pressures and temperatures. Diffusion is the rate-limiting step in creep of crystalline solids at high temperatures and low stresses, and the diffusion coefficient is usually the most important unknown in viscous flow laws. Ongoing experimental work on this topic is focused on diffusion in lower mantle mineral minerals, iron-nickel alloys, and zinc (an analog for hcp iron in Earth’s inner core).
Experiment on cation and oxygen self-diffusion in MgO at 25 GPa and 2273 K.
Yunker M.L., Van Orman J.A. (2007) Interdiffusion of solid iron and nickel at high pressure. Earth and Planetary Science Letters 254, 203-213.
Van Orman J.A. (2004) On the viscosity and creep mechanism of Earth’s inner core. Geophysical Research Letters 31, Art. No. L20606.
Van Orman J.A., Fei Y., Hauri E.H., Wang J. (2003) Diffusion in MgO at high pressures: Constraints on deformation mechanisms and chemical transport at the core-mantle boundary. Geophysical Research Letters 30, 1056-1059.
Molecular Dynamics Simulations of Silicate Liquids (funded by the National Science Foundation)
The thermodynamic and chemical transport properties of silicate liquids at very high pressures are important for understanding the early differentiation of the Earth into core and mantle, and the behavior of possible partially molten regions in the vicinity of Earth’s mantle transition zone and core-mantle boundary. In collaboration with Prof. Dan Lacks in the Department of Chemical Engineering at Case, we are investigating silicate melt properties and their relation to local melt structure using molecular dynamics methods. We are also investigating the structural environments, atomic radii, and solubilities of noble gases in silicate melts, and the mass dependence of atomic diffusion rates (i.e. the isotope effect).
Zhang L., Van Orman J.A., Lacks D.J. (2009) Effective radii of noble gas atoms in silicates from first principles molecular simulation. American Mineralogist, in press.
Lacks D.J., Van Orman J.A. (2007) Molecular dynamics investigation of viscosity, chemical diffusiviites and partial molar volumes of liquids along the MgO-SiO2 join as functions of pressure. Geochimica et Cosmochimica Acta 71, 1312-1323.