| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | November 21, 2006 |
| Latest Amendment Date: | November 21, 2006 |
| Award Number: | 0636011 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Benjamin R. Phillips
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | January 1, 2007 |
| End Date: | December 31, 2009 (Estimated) |
| Total Intended Award Amount: | $190,000.00 |
| Total Awarded Amount to Date: | $190,000.00 |
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| Recipient Sponsored Research Office: |
400 HARVEY MITCHELL PKWY S STE 300 COLLEGE STATION TX US 77845-4375 (979)862-6777 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
400 HARVEY MITCHELL PKY S STE 300 COLLEGE STATION TX US 77845-4375 |
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Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| NSF Program(s): | Geophysics |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
An experimental study is being conducted to determine the deformation mechanisms and fabric development of olivine aggregates deformed in a shear geometry at high pressures and known water contents. The deformation mechanisms and lattice preferred orientations of olivine control the physical properties and seismic anisotropy of the upper mantle. The deformation mechanisms and relative shear strengths of slip systems in olivine are well known under the dry conditions that characterize much of the oceanic lithosphere. Lattice preferred orientations of olivine produced in low pressure (300 MPa) deformation experiments in gas apparatus and modeled numerically correspond well to in situ seismic anisotropy of the upper mantle; shear on (010)[100] and (0kl)[100] leads to seismically fast crystallographic a axes aligned with the shear direction at oceanic spreading centers.
Recent results of shear experiments performed at high pressure (2 GPa) in a solid medium apparatus indicate that remarkably different olivine fabrics develop during deformation at wet conditions. Lattice preferred orientations developed at high pressure have been linked to dislocation glide dominantly on (010)[001] and (100)[001]; presumably, reductions in critical resolved shear stresses of these slip systems are brought about by interactions with hydrogen point defects that have high solubilities in olivine at pressures greater than 1 GPa. Based on these results, the seismically fast direction can be aligned perpendicular to the shear direction, with important consequences for geodynamic interpretations of seismic anisotropy. Similar fabrics have been observed in some naturally deformed peridotites, and these fabrics are thought to have been developed under wet conditions, such as in subduction zones where downgoing slabs release fluids. However, the high pressure olivine fabrics related to shear in the [001] direction observed in high pressure experiments using the solid medium assembly have not been reproduced in olivine aggregates sheared at 1.6 GPa using the molten salt assembly. The advantage of using the molten salt assembly is that stress can be measured with a resolution not attained using solid medium assemblies, so that accurate mechanical data can be collected and applied to geodynamic problems.
Another difference between the results obtained with the molten salt assembly and previous experimental work is that the strengths measured in the molten salt assembly are considerably lower than strengths of olivine deformed in the gas apparatus at the same temperature, strain rate and water content, but different confining pressures (1.6 vs. 0.3 GPa). To ensure that strengths measured at high pressure are accurate, the methods of measuring stress in the molten salt assembly are being refined and compared with measurements of shear stress in a high resolution triaxial gas apparatus using metals, whose strengths are not pressure dependent.
This research follows from experimental studies of Holyoke and Tullis, who performed high pressure (1.6 GPa) deformation experiments using a molten salt cell and obtained fabrics similar to those developed at lower pressure (300 MPa). These experiments are designed to bridge the gaps between disparate results of different experimental methods, to improve our understanding of the deformation mechanisms in the mantle, characterize those conditions that favor different lattice preferred orientations, and evaluate rheologies that govern the geodynamics of the mantle.
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