Award Abstract # 0810209
Elasticity of Pyrope-Almandine-Grossular Garnet Solid Solution Series at High Pressure and Temperature using Ultrasonic Interferometry in Conjunction with Synchrotron Radiation.

NSF Org: EAR
Division Of Earth Sciences
Recipient: DELAWARE STATE UNIVERSITY
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 2008 = $349,066.00
FY 2012 = $30,000.00
History of Investigator:
  • Gabriel Gwanmesia (Principal Investigator)
    ggwanmesia@desu.edu
Recipient Sponsored Research Office: Delaware State University
1200 N DUPONT HWY
DOVER
DE  US  19901-2202
(302)857-6001
Sponsor Congressional District: 00
Primary Place of Performance: Delaware State University
1200 N DUPONT HWY
DOVER
DE  US  19901-2202
Primary Place of Performance
Congressional District:
00
Unique Entity Identifier (UEI): RZZ8BMQ47KX3
Parent UEI:
NSF Program(s): Petrology and Geochemistry,
Geophysics,
EDUCATION AND HUMAN RESOURCES,
EPSCoR Co-Funding
Primary Program Source: 01000809DB NSF RESEARCH & RELATED ACTIVIT
01001213DB NSF RESEARCH & RELATED ACTIVIT
Program Reference Code(s): 0000, 1031, OTHR, 9150
Program Element Code(s): 157300, 157400, 157500, 915000
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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Isaak, DG; Gwanmesia, GD; Davis, MG; Stafford, SC; Stafford, AM; Triplett, RS "The temperature dependence of the elasticity of Fe-bearing wadsleyite" PHYSICS OF THE EARTH AND PLANETARY INTERIORS , v.182 , 2010 , p.107 View record at Web of Science 10.1016/j.pepi.2010.06.01
Isaak D. G., Gwanmesia G. D., Falde D., Davis M. D., Stafford S. C., Stafford A. M., and Triplett R. S. (2010) "The Temperature Dependence of the Elasticity of Fe-Bearing Wadsleyite" Phys. Earth Planet In., , v.182 , 2010 , p.107?112.

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.

Some studies have suggested that the elastic properties, and in particular their variations with pressure and temperature of garnet, a major mantle mineral, could explain the steep velocity gradients between two major discontinuities at 410 km and 660-km depth (transition zone) in the Earth’s mantle, revealed by seismic studies of the Earth’s interior.

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
there are presently only three—1 PhD and 2 MS), is a well-documented issue. Delaware State University (DSU) is a historically black and predominantly undergraduate non-traditional research institution, and over the grant period nine (9) students (8 blacks, 1 white), seven (7) of whom are
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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