{"@context":"http://iiif.io/api/presentation/2/context.json","@id":"https://repo.library.stonybrook.edu/cantaloupe/iiif/2/manifest.json","@type":"sc:Manifest","label":"Elasticity of coesite and stishovite: implications for the Earth's mantle","metadata":[{"label":"dc.description.sponsorship","value":"This work is sponsored by the Stony Brook University Graduate School in compliance with the requirements for completion of degree."},{"label":"dc.format","value":"Monograph"},{"label":"dc.format.medium","value":"Electronic Resource"},{"label":"dc.identifier.uri","value":"http://hdl.handle.net/11401/78254"},{"label":"dc.language.iso","value":"en_US"},{"label":"dcterms.abstract","value":"Studies of the physical properties of the minerals contained in the subducted oceanic crust can provide constraints and insights of the thermal structure, composition and dynamics of the subducted plate. In particular, the elastic properties of the constituent mineral phases at high pressures and temperatures are needed for this goal. Seismic studies revealed the X-discontinuity with 3-8% impedance contrast in the depth range of 250-350 km at a variety of locations, including regions of subduction zones, mid-ocean ridges, continental cratons, and a few hot spots (e.g., Revenaugh and Jordan, 1991; Bagley and Revenaugh, 2008; Deuss and Woodhouse, 2002; Schmerr et al., 2013; Schmerr, 2015). The coesite\u2192stishovite phase transition has been proposed as a plausible candidate for the cause of this X-discontinuity. In this study, the compressional and shear wave velocities for coesite have been measured by the ultrasonic interferometry method (1) at pressures up to 12.6 GPa at room temperature, and (2) to 5.8 GPa and 1073 K with in situ synchrotron X-radiation. While the P wave velocity increases continuously with pressure, the S wave velocity decreases with pressure along all isotherms in both experiments. With these high P-T elastic data for coesite, together with literature data for stishovite, the velocity and impedance contrasts between coesite and stishovite were calculated to be ~34% and ~45% for P and S wave velocities, respectively, and ~64% and ~75% for their impedances at mantle conditions. These data are consistent with our room temperature measurements on coesite. The large velocity and impedance contrasts across the coesite\u2192stishovite transition imply that, to generate the velocity and impedance contrasts observed at the X-discontinuity, only a small amount of silica would be required. The velocity jump dependences for silica, d(lnVP)/d(SiO2) = 0.38 (wt%)-1 and d(lnVS)/d(SiO2) = 0.52 (wt%)-1, are utilized to place constraints on the amount of silica in the upper mantle, and provide a geophysical approach to track mantle eclogite materials and ancient subducted oceanic slabs.             The crystal structure and equation of state of coesite (C2/c) and its high-pressure polymorph coesite-II (P21/n) were studied using X-ray powder diffraction in a diamond-anvil-cell up to 31 GPa at room temperature together with first-principles calculations up to 45 GPa at 0 K. New diffraction peaks appear above 20 GPa, indicating the formation of coesite-II. The calculated enthalpies provide theoretical support for the pressure-induced phase transformation from coesite to coesite-II at ~21.4 GPa. Compared with coesite, the coesite-II structure is characterized by a \u201cdoubled\u201d b-axis and the breakdown of the linear Si1-O1-Si1 angle in coesite into two distinct angles: one is ~176\u02da, close to linear, whereas the other decreases by 22\u02da to 158\u02da. If coesite-II can exist at moderately low temperatures, then the phase transition of coesite\u2192coesite-II will change the elasticity as well as anisotropic properties of the subducted oceanic crust due to the dramatically different compressional behavior between coesite and coesite-II, as well as the appreciable amount of SiO2 in the MORB.             Ultrasonic measurements on polycrystalline stishovite were conducted at pressures up to 13.6 GPa and 1073 K, in conjunction with synchrotron X-ray diffraction, and up to 9.4 GPa at room temperature. Anomalies were found in the pressure dependence of the shear wave velocity at both room temperature and high temperatures, which are consistent with the decreasing value of the shear elastic constant (C11-C12)/2 from our current DFT calculations, and also with previous theoretical calculations by Yang and Wu (2014). The softening in the shear-wave velocity of stishovite suggests that the phase transformation from stishovite to CaCl2-type silica initiates far below the transition pressure of ~46 GPa. With the current high P-T data for stishovite, the contrasts between coesite and stishovite were re-evaluated. The new values would only require alteration of the silica amount by -0.2% to -0.4 wt% to generate the 3-8% impedance contrast observed at the X-discontinuity at mantle conditions, and thus provides strong support for the coesite\u2192stishovite phase transformation to be the cause of the X-discontinuity.             Both coesite and stishovite can incorporate hydrogen in their structures under subduction pressure and temperature conditions. If the elasticities of coesite and stishovite change with the presence of water, the higher water solubility in stishovite as compared with coesite may alter the velocity and impedance contrasts between these two phases. In this study, I have successfully synthesized specimens of hydrous stishovite at the Geodynamics Research Center, Ehime University, Japan; several well-sintered Al-free and Al-bearing polycrystalline stishovite samples were obtained. Ultrasonic measurements with in situ X-ray diffraction were conducted on specimens from run OT1659, OT1682, OT1688. With future detailed analysis, these ultrasonic data can provide valuable insights into the effect of water on the elastic properties of stishovite."},{"label":"dcterms.available","value":"2018-06-21T13:38:44Z"},{"label":"dcterms.contributor","value":"Liebermann, Robert"},{"label":"dcterms.creator","value":"Chen, Ting"},{"label":"dcterms.dateAccepted","value":"2018-06-21T13:38:44Z"},{"label":"dcterms.dateSubmitted","value":"2018-06-21T13:38:44Z"},{"label":"dcterms.description","value":"Department of Geosciences"},{"label":"dcterms.extent","value":"176 pg."},{"label":"dcterms.format","value":"Monograph"},{"label":"dcterms.identifier","value":"http://hdl.handle.net/11401/78254"},{"label":"dcterms.issued","value":"2017-12-01"},{"label":"dcterms.language","value":"en_US"},{"label":"dcterms.provenance","value":"Made available in DSpace on 2018-06-21T13:38:44Z (GMT). No. of bitstreams: 1\nChen_grad.sunysb_0771E_13606.pdf: 8947510 bytes, checksum: af8eff756201c47193c520238bc26c1e (MD5)\n  Previous issue date:   12"},{"label":"dcterms.subject","value":"coesite"},{"label":"dcterms.title","value":"Elasticity of coesite and stishovite: implications for the Earth's mantle"},{"label":"dcterms.type","value":"Dissertation"},{"label":"dc.type","value":"Dissertation"}],"description":"This manifest was generated dynamically","viewingDirection":"left-to-right","sequences":[{"@type":"sc:Sequence","canvases":[{"@id":"https://repo.library.stonybrook.edu/cantaloupe/iiif/2/canvas/page-1.json","@type":"sc:Canvas","label":"Page 1","height":1650,"width":1275,"images":[{"@type":"oa:Annotation","motivation":"sc:painting","resource":{"@id":"https://repo.library.stonybrook.edu/cantaloupe/iiif/2/11%2F57%2F24%2F115724671146089707572841644431192309311/full/full/0/default.jpg","@type":"dctypes:Image","format":"image/jpeg","height":1650,"width":1275,"service":{"@context":"http://iiif.io/api/image/2/context.json","@id":"https://repo.library.stonybrook.edu/cantaloupe/iiif/2/11%2F57%2F24%2F115724671146089707572841644431192309311","profile":"http://iiif.io/api/image/2/level2.json"}},"on":"https://repo.library.stonybrook.edu/cantaloupe/iiif/2/canvas/page-1.json"}]}]}]}