Beam Time at the Stanford Linear Accelerator Awarded to USGS Scientist


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Beam Time at the Stanford Linear Accelerator Awarded to USGS Scientist

By Jim Hein and Helen Gibbons
May 2007

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Above: Large sample (1.5 by 0.9 by 0.3 m) of cobalt-rich crust and substrate rock. [larger version]

Above: Same sample of cobalt-rich crust and substrate rock shown above, cut through the long axis. The crust, which began to grow on the substrate rock about 70 million years ago, shows distinct growth layers. A mudstone cobble (light tan) occurs within the crust on the right side. [larger version]

Above: Crust pavement (approx 3 by 4 m) on the upper flank of Horizon Seamount, central Pacific Ocean; 2,000-m water depth. [larger version]

Above: Schematic representation of the surface reactions that take place during sorption of Te(IV) on the FeOOH surface and its subsequent oxidation to Te(VI).

U.S. Geological Survey (USGS) geologist Jim Hein was recently awarded coveted beam time at the Stanford Synchrotron Radiation Laboratory (SSRL), part of the Stanford Linear Accelerator Center (SLAC) in Palo Alto, California. On the basis of peer-reviewed proposals, Hein was awarded 120 hours of beam time to study processes involved in the acquisition of metals from seawater by marine iron-manganese oxide crusts (Fe-Mn crusts). Synchrotron radiation consists of high-intensity (extremely bright) X-rays or light produced by electrons circulating at nearly the speed of light in a storage ring. The analyses to be completed at SSRL will include:

X-ray absorption near-edge spectroscopy (XANES), a technique for resolving the chemical forms of metals. The XANES data will yield information about the metals' reactivity and environment of formation.
extended X-ray absorption fine-structure spectroscopy (EXAFS), the only viable technique for determining the local structure around metals at concentrations as low as 50 parts per million (ppm). The EXAFS data will show whether the metals are associated with the Fe minerals or the Mn minerals that compose the crusts, and how the metals are bound to these minerals, such as whether the bonds are weak or strong.

Fe-Mn crusts are important for two reasons: They record as much as 70 million years (m.y.) of changes in ocean chemistry linked to changes in the Earth's climate, and they are potential sources of the valuable metals that they concentrate from seawater. Fe-Mn crusts form pavement-like deposits, as much as 25 cm thick, on hard-rock substrates throughout the ocean basins. They form by direct precipitation from cold ambient bottom waters onto the flanks and summits of seamounts, ridges, and plateaus where the rocks have been swept clean of sediment at least intermittently for millions of years. The crusts form at water depths of about 400 to 4,000 m, with the thickest and most metal-rich crusts occurring at depths of about 800 to 2,500 m. Crusts have an extremely high mean surface area (300 m²/g) and remarkably slow growth rates (1-6 mm/m.y.), which allow for the adsorption of abundant elements from seawater.

Fe-Mn crusts contain approximately equal proportions of Fe and Mn and are especially enriched in tellurium (Te), Mn, cobalt (Co), lead (Pb), bismuth (Bi), and platinum (Pt) relative to those metals' concentrations in the Earth's lithosphere and in seawater. Fe-Mn crusts may have an economic potential for Co, Mn, Pt, Te, titanium (Ti), nickel (Ni), rare-earth elements, thallium (Tl), and other elements.

Te is the metal most strongly (50,000 times) enriched in Fe-Mn crusts and will be used as a model compound in these initial experiments at SSRL. Hein and his partners in the project hypothesize that the profound enrichment of Te is due to its adsorption as Te(IV) and subsequent oxidation to Te(VI) on the surface of the Fe oxyhydroxide (FeOOH) part of Fe-Mn crusts. (Te(IV) is a form of Te that can bond to anions with a total of 4 negative charges; Te(VI) is a more stable form that can bond to anions with a total of 6 negative charges.) Another reason the scientists chose Te as a model compound is that the adsorption of Te onto Fe-Mn crusts likely controls Te's concentration and dominant chemical form—Te(IV) or Te(VI)—in the global ocean. However, little is known about the mechanisms by which Te is sequestered by Fe-Mn oxides and Fe-Mn colloids (very tiny suspended particles) in the water column, and then stabilized in Fe-Mn crusts—questions that will be answered by this research.

Te has many important industrial uses, arguably the most important of which is the newly emerging, cutting-edge solar-cell technologies for which Te is a critical component. According to Ken Zweibel of the National Renewable Energy Laboratory: "Finding enough tellurium for CdTe [cadmium-telluride, a compound whose physical characteristics make it an ideal material for the production of solar cells] is the largest barrier to the multi-terawatt use of CdTe for electricity. It is widely regarded as the lowest cost photovoltaic technology with the greatest potential…. This is actually important to the United States and the world."

In addition, understanding the surface geochemical reactions of Te (and other metals) on naturally occurring Fe-Mn crusts has far-reaching applications, from improved understanding of global-ocean chemical balances to the development of new techniques for extracting metals from ores. Furthermore, Te is a geochemical analog to selenium (Se), both having the same range of reaction states in natural systems. Se is known to be an essential nutrient over a small concentration range and toxic at higher concentrations, whereas Te seems to have no important biological function. Understanding the processes that control the concentrations and ratios of these two elements in natural systems may have significant environmental applications.

Partners in the project include Andrea Koschinsky of the International University of Bremen, Germany, John Bargar of SSRL, and Alex Halliday of Oxford University, UK.

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Geologists and Biologists Endeavor to Understand Seamount Environments Off California
January 2004