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.
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 – Black/Female, Stony Brook University; Adaire Heady – Black/Female, Stony Brook University; Richard Triplett – White/Male, Delaware State University; Mark King - Black/Male, Delaware State University; Franz Delima - Black/Male, MS, Delaware State University). Adaire Heady is co-author on a grossular publication (Gwanmesia et al.,2013) resulting from the project, while Richard Triplett, is currently preparing a manuscript for submission to Physics of Earth and Planetary Interiors (PEPI) based on elasticity data from his MS thesis research. Both Adaire Heady and Ashley Thompson from DSU, together with Melisa Sims from the University of South Carolina are pioneers in a MS Geosciences Instrumentation program at Stony Brook University, under a National Science Foundation diversity initiative entitled: "A Career Path for African-American Students from HBCUs to National Laboratories". The program was launched in 2011, by Professor Robert C. Liebermann (PI) at Stony Brook University (SBU), and with Prof. Lars Ehm (SBU) and Gabriel Gwanmesia (DSU) as co-PIs, to recruit undergraduate science and engineering students from underrepresented groups into the graduate program in the Department of Geosciences at SBU, to educate them via formal courses and research projects to the M. S. degree in geosciences, and to position them for employment as science associates in national user facilities of the U. S. Department of Energy such as the National Synchrotron Light Source (NSLS) at the Brookhaven National Laboratory (BNL). The research projects of these students are supervised by Prof. Ehm and involve work both on SBU campus and at BNL, and upon graduating, the students will represent the first females in mineral physics, joining two African-American males who have received MS degrees in Geophysics, including Gabriel Gwanmesia with a PhD in Geophysics.
