
NSF Org: |
EAR Division Of Earth Sciences |
Recipient: |
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Initial Amendment Date: | August 8, 2008 |
Latest Amendment Date: | July 11, 2012 |
Award Number: | 0810209 |
Award Instrument: | Standard Grant |
Program Manager: |
Robin Reichlin
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
Start Date: | August 1, 2008 |
End Date: | July 31, 2013 (Estimated) |
Total Intended Award Amount: | $349,066.00 |
Total Awarded Amount to Date: | $379,066.00 |
Funds Obligated to Date: |
FY 2012 = $30,000.00 |
History of Investigator: |
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Recipient Sponsored Research Office: |
1200 N DUPONT HWY DOVER DE US 19901-2202 (302)857-6001 |
Sponsor Congressional District: |
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Primary Place of Performance: |
1200 N DUPONT HWY DOVER DE US 19901-2202 |
Primary Place of
Performance Congressional District: |
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Unique Entity Identifier (UEI): |
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Parent UEI: |
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NSF Program(s): |
Petrology and Geochemistry, Geophysics, EDUCATION AND HUMAN RESOURCES, EPSCoR Co-Funding |
Primary Program Source: |
01001213DB NSF RESEARCH & RELATED ACTIVIT |
Program Reference Code(s): |
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Program Element Code(s): |
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Award Agency Code: | 4900 |
Fund Agency Code: | 4900 |
Assistance Listing Number(s): | 47.050 |
ABSTRACT
The deepest drilling into the interior of the Earth is limited to less than 15 km. Consequently, the bulk of its interior is inaccessible to direct sampling, and its structure and mineralogical composition can only be inferred using indirect evidence. Efforts to understand the mineralogy of the Earth?s mantle have combined information from several different fields including seismology, petrology and geochemistry, but the most direct information about the physical structure of the Earth?s deep interior comes from seismology which provides information about variation of the elastic wave velocity and density as a function of depth.
Seismic studies have provided models that show rapid increases (velocity jumps or discontinuities) in the seismic waves at 410 to 660 km depths in the Earth and unusually steep velocity gradients in the region between the discontinuities, called the transition zone. Experimental petrological studies of minerals of the upper mantle show that they transform to high-pressure forms at the pressure and temperature conditions of the transition zone.
In particular pyroxene, one of the dominant upper mantle minerals in basaltic and peridotitic rocks transforms into the garnet structure at pressure and temperature conditions of the lower regions of the upper mantle and transition zone.
In this research program, we propose to systematically measure the pressure and temperature dependence of the elastic properties of the pyrope (Mg3Al2Si3O12?Py) - almandine (Fe3Al2Si3O12-Alm)-grossular (Ca3Al2Si3O12-Gr) solid solutions. We will conduct these experimental studies using specimens synthesized in the Stony Brook High Pressure Laboratory, the techniques of ultrasonic interferometry, and in conjunction with the synchrotron X-ray facilities at the National Synchrotron Light Source of the Brookhaven National Laboratory.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Previous measurements of the elastic properties of garnet compositions along the pyrope (Py: Mg3Al2Si3O12)-majorite (Mj-Mg4Si4O12) solid solution, revealed that the pressure derivatives of the elastic moduli are unaffected by substitution of Si for Mg and Al in the garnet structure. Secondly, the derivatives are normal and comparable to those of wadsleyite (β-Mg2SiO4) and ringwoodite (γ-Mg2SiO4) that are also major transition zone phases, and thus produce velocity gradients that are shallower than the observed seismic velocity gradients. As a result, other studies have attributed the high seismic velocity gradients in the transition zone to mineral assemblages containing Ca and Na.
In this project, we have measured the elastic wave velocities of grossular (Grs: Ca3Al2Si3O12) Almandine (Alm: Fe3Al2Si3O12), a natural garnet (Py70Alm17Grs13) and a complex synthetic garnet containing different cations as a function of pressure (up to 10 GPa) and high temperatures (up to 1000oC) by ultrasonic interferometry techniques inside a D-DIA type cubic anvil high pressure apparatus, interfaced with synchrotron X-radiation at the
superconducting wiggler beam line, at the National Synchrotron Light Source
(NSLS) of the Brookhaven National Laboratory.
The results from the current study are in general, consistent with data for other pyrope-almandine-grossular compositions. The pressure dependence of the elastic bulk modulus determined for the garnets are independent of composition, with values falling between 4 and 5, within mutual uncertainty in the measurements. Secondly, we find the adiabatic bulk modulus (Kso = 171 GPa) for grossular to be comparable to those for pyrope-majorite garnets. In contrast, the shear modulus (G) of about 90 GPa for pyrope and pyrope-majorite compositions is about 19% lower than 107.4 (2) GPa obtained for
grossular in this study, and at least 14% lower than values for most grossular-rich garnets. Thus, while replacing Mg by Ca in the dodecahedral site has minimum effect on the compressibility of garnet, it does significantly affect its shear modulus, and as a consequence garnet with significantly higher Ca concentration than that of pyrolitic garnet will increase the shear wave velocity, but not the compressional wave velocity of the mantle transition zone. Thus, the most recent elasticity results for grossular garnet should be incorporated into revised calculations for pyrolitic and piclogitic compositions for the Earth’s mantle when comparing with seismic models.
The underrepresentation of African-American students in the Geosciences fields, and in particular in the field of mineral physics (a discipline in which
men, and three (3) females, have received valuable research training through
the grant support, and of the trainees, 5 have gone on to graduate school(Ashley Thompson –...
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