Papers by Gregory Shofner
AGU Fall Meeting Abstracts, Dec 1, 2010
ABSTRACT The behavior of siderophile elements during metal-silicate segregation, and their result... more ABSTRACT The behavior of siderophile elements during metal-silicate segregation, and their resulting distributions provide insight into core formation processes. Determination of partition coefficients allows the calculation of element distributions that can be compared to established values of element abundances in the silicate (mantle) and metallic (core) portions of the Earth. Moderately siderophile elements, including W, are particularly useful in constraining core formation conditions because they are sensitive to variations in T, P, oxygen fugacity (fO2), and silicate composition. To constrain the effect of pressure on W metal/silicate partitioning, we performed experiments at high pressures and temperatures using a multi anvil press (MAP) at NASA Johnson Space Center and laser-heated diamond anvil cells (LHDAC) at the University of Maryland. Starting materials consisted of natural peridotite mixed with Fe and W metals. Pressure conditions in the MAP experiments ranged from 10 to 16 GPa at 2400 K. Pressures in the LHDAC experiments ranged from 26 to 58 GPa, and peak temperatures ranged up to 5000 K. LHDAC experimental run products were sectioned by focused ion beam (FIB) at NASA JSC. Run products were analyzed by electron microprobe using wavelength dispersive spectroscopy. Liquid metal/liquid silicate partition coefficients for W were calculated from element abundances determined by microprobe analyses, and corrected to a common fO2 condition of IW-2 assuming +4 valence for W. Within analytical uncertainties, W partitioning shows a flat trend with increasing pressure from 10 to 16 GPa. At higher pressures, W becomes more siderophile, with an increase in partition coefficient of approximately 0.5 log units.
Lunar and Planetary Science Conference, Mar 1, 2010
Lunar and Planetary Science Conference, Mar 17, 2014
The behavior of siderophile elements during metal-silicate segregation, and their resulting distr... more The behavior of siderophile elements during metal-silicate segregation, and their resulting distributions provide insight into core formation processes. Determination of partition coefficients allows the calculation of element distributions that can be compared to established values of element abundances in the silicate (mantle) and metallic (core) portions of the Earth. Moderately siderophile elements, including W, are particularly useful in constraining core formation conditions because they are sensitive to variations in T, P, oxygen fugacity (fO 2), and silicate composition.
New high-P-T experimental data (50 GPa, ~4000 K) on W metal-silicate partitioning improves constr... more New high-P-T experimental data (50 GPa, ~4000 K) on W metal-silicate partitioning improves constraints on core formation conditions and timing (Hf-W).
Base map compiled by the Illinois State Geological Survey from digital data (Raster Feature Separ... more Base map compiled by the Illinois State Geological Survey from digital data (Raster Feature Separates) provided by the United States Geological Survey. Topography compiled from imagery dated 1968. Field checked 1970. North American Datum of 1927 (NAD 27) Projection: Transverse Mercator 10,000-foot ticks: Illinois State Plane Coordinate system, west zone (Transverse Mercator) 1,000-meter ticks: Universal Transverse Mercator grid system, zone 15 Recommended citation: Grimley, D.A. and G.A. Shofner, 2008, Surficial Geology of Ames Quadrangle, Monroe and Randolph Counties, Illinois: Illinois State Geological Survey, Illinois Geologic Quadrangle Map, IGQ Ames-SG, 2 sheets, 1:24,000. Geology based on field work and data compilation by David A. Grimley 2006–2007 and Greg A. Shofner, 2002–2004. Digital cartography by Jennifer E. Carrell, Zahra Golshani, and Jane E.J. Domier, Illinois State Geological Survey. This map was funded in part by the U.S. Geological Survey, National Cooperative Geo...
Title of Document: HIGH PRESSURE REDOX GEOCHEMISTRY OF TUNGSTEN IN METAL–SILICATE SYSTEMS: IMPLIC... more Title of Document: HIGH PRESSURE REDOX GEOCHEMISTRY OF TUNGSTEN IN METAL–SILICATE SYSTEMS: IMPLICATIONS FOR CORE FORMATION IN THE EARTH Gregory Allen Shofner, Ph.D., 2011 Directed By: Adjunct Associate Professor, Andrew J. Campbell, Department of Geology Geochemical models of core formation are commonly based on core and mantle abundances of siderophile elements. Because of the affinity of these elements for metallic phases, they are thought to be highly concentrated in Earth's core. Tungsten is a moderately siderophile element that may provide constraints on the pressure, temperature, composition, and oxygen fugacity conditions, and on the timing of core formation in the Earth. Previous experimental studies suggest that pressure exerts little influence over tungsten metal/silicate partitioning up to 20 gigapascals (GPa). But, core formation models, based on W, predict metal–silicate equilibration pressures outside the available experimental pressure range, thus, requiring extra...
We here introduce a readily determined index of surface karstification, termed “sinkhole” index, ... more We here introduce a readily determined index of surface karstification, termed “sinkhole” index, based on the mean spacing of closed contours in a given area. The index shows a high correlation with total sinkhole area and a moderate correlation with total volume. The index was measured in 5056 blocks with dimensions of 2.5’ of latitude by 2.5’ of longitude, covering much of Tennessee. A new map showing the distribution of this index in the state is similar to one previously published karst map of Tennessee, but shows the variation of karstification in a more detailed manner. The sinkhole index was also used to compare the distribution of sinkholes and caves in Tennessee, using cave data compiled by the Tennessee Cave Survey. Maps of the sinkhole index and the number and total length of caves in each 2.5’ x 2.5’ block show strong regional similarities. However, there are dramatic exceptions. In addition, using blocks as the basic unit of analysis, the correlation coefficients betwee...
American Mineralogist, 2016
Geochemical models of core formation are commonly based on core and mantle abundances of sideroph... more Geochemical models of core formation are commonly based on core and mantle abundances of siderophile elements. Because of the affinity of these elements for metallic phases, they are thought to be highly concentrated in Earth's core. Tungsten is a moderately siderophile element that may provide constraints on the pressure, temperature, composition, and oxygen fugacity conditions, and on the timing of core formation in the Earth. Previous experimental studies suggest that pressure exerts little influence over tungsten metal/silicate partitioning up to 20 gigapascals (GPa). But, core formation models, based on W, predict metal-silicate equilibration pressures outside the available experimental pressure range, thus, requiring extrapolation. Therefore, higher pressure experimental data on tungsten were needed to constrain this important parameter. High pressure melting experiments were conducted to 50 GPa and 4400 K using a diamond anvil cell, and to 26 GPa and 2500 K using a multi-anvil press. Diamond anvil cell samples were sectioned using a focused ion beam instrument. The W-WO 2 oxygen fugacity buffer was characterized to high pressure, also using diamond anvil cells and a multi-anvil press, combined with synchrotron x-ray diffraction. Combining the high pressure W-WO 2 oxygen fugacity buffer and the database of metal/silicate partitioning data, a new approach was taken to model the FeW exchange reaction. Compared to the common linear method of parameterizing metalsilicate partitioning data, this approach captures the non-linear pressure dependence on oxygen fugacity, and allows for modeling of the excess Gibbs energy of mixing based on the activity ratios of Fe, FeO, W, and WO 2. Applying this non-linear parameterization to the problem of core formation in the Earth, a pressure-temperature solution of 38 GPa and 3100 K in a peridotite silicate composition for a single-stage, magma ocean core formation model was determined that constrains equilibrium core formation conditions in the Earth. This solution was
We here introduce a readily determined index of surface karstification, termed "sinkhole&quo... more We here introduce a readily determined index of surface karstification, termed "sinkhole" index, based on the mean spacing of closed contours in a given area. The index shows a high correlation with total sinkhole area and a moderate correlation with total volume. The index was measured in 5056 blocks with dimensions of 2.5' of latitude by 2.5' of longitude, covering much of Tennessee. A new map showing the distribution of this index in the state is similar to one previously published karst map of Tennessee, but shows the variation of karstification in a more detailed manner. The sinkhole index was also used to compare the distribution of sink- holes and caves in Tennessee, using cave data compiled by the Tennessee Cave Survey. Maps of the sinkhole index and the number and total length of caves in each 2.5' x 2.5' block show strong regional similarities. However, there are dramatic exceptions. In addition, using blocks as the basic unit of analysis, the...
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Papers by Gregory Shofner