Earth’s mantle has a two-layer structure comprising an upper and a lower mantle, separated by a global seismic discontinuity at 660 km depth. The mantle transition zone (MTZ), which extends from the seismic discontinuity at 410 km depth to the base of the upper mantle plays a crucial role in mantle dynamics and geochemical cycling because minerals in the MTZ have as much as ten times the H2O storage capacity of minerals in the uppermost mantle or lower mantle. The principal aspect of this work is to contribute to a better understanding of the structure and composition of the deep Earth by determining the crystal structure of minerals that are stable at conditions of the MTZ, with emphasis on determining how hydrogen, ferric iron, and associated point defects influence their physical properties. In addition, I report the discovery and X-ray structure solution of a new sodium titanate mineral, nixonite, found in a garnet pyroxenite xenolith from the Darby kimberlite field in the west-central Rae Craton of Canada. Tying together the physical properties measurements on mantle minerals with geophysical observables, a new software tool was developed for calculating and visualizing the influence of hydration and iron content in olivine, wadsleyite and ringwoodite on the seismic velocity profiles in the MTZ. The influences of water and iron are included by how they change the elastic properties and density of the minerals, which are obtained by regression to published experimental thermoelastic data. By comparing calculated seismic velocity profiles derived from mineral physics to observed seismic velocity profiles from seismological studies, I show that the eastern U.S. low velocity anomaly, which may be associated with dehydration of the Farallon slab, is consistent with a mantle hydration to 20% of its water storage capacity. Hydration of wadsleyite leads to hydroxyl (OH-) groups and associated Mg or Fe point defects given by (Mg,Fe)2-xSiH2xO4. To investigate the influence of hydration on the structure and hydrogen bonds at the pressures of the MTZ, I carried out high-pressure, single-crystal synchrotron X-ray diffraction measurements of hydrated wadsleyite samples up to 35 GPa. Two different compositions with approximate mantle iron content (Fo90) with one containing ~0.25 weight percent (wt%) H2O and one with ~2.0 wt% H2O were studied. Structure refinements showed a change of crystal symmetry at 9 GPa, which I associate with compression mechanisms in the structure with cation vacancy ordering. The O–O interatomic distances of probable hydrogen bonding were monitored as a function of pressure and indicate the occurrence of pressure induced hydrogen bond symmetrization in the more hydrous wadsleyite sample above ~25 GPa. The implication of strong hydrogen bond in wadsleyite at high pressures for the partitioning of hydrogen isotopes in the mantle transition zone is discussed. Jeffbenite, with the ideal formula Mg3Al2Si3O12, is a newly characterized mineral similar to almandine garnet in composition but more related to zircon in crystal structure and contains significant amounts of ferric iron. I provide evidence that it might be a constituent mineral in the Earth’s lower transition zone. Single-crystal X-ray diffraction (XRD) and synchrotron Mössbauer spectroscopy (SMS) measurements were carried out as a function of pressure up to 30 GPa. No phase transitions were observed in jeffbenite during compression, although the high-pressure SMS data suggest that an electronic spin-pairing transition may occur in the Fe3+ at about 30 GPa.

Creator

• Wang, Fei

DOI

• https://doi.org/10.21985/n2-v3m2-by10

Subject

• Geophysics

Language

• en

Alternate Identifier

• etdadmin_upload_811541
• http://dissertations.umi.com/northwestern:15513

Date created

• 2021-01-01

Resource type

• Dissertation

Rights statement

• In Copyright