Matching Items (2)
Filtering by
- All Subjects: Mineralogy
- Creators: Desch, Steven
- Status: Published
Description
With the InSight mission deploying a seismometer , Martian bulk chemical compositional models are more important than ever. Three largely consistent models for the Martian mantle have been suggested over the past two decades. Of these three, two are fairly similar and one is dramatically different. Of these three, the EH70 (Sanloup et al., 1999) models have the systematically lower divalent cation to silicon ratios as compared to the other model, the DW85 (Dreibus and Wanke, 1985) model. However, impact of such a low (Mg+Fe+Ca)/Si ratio on mineralogy has not been experimentally investigated. Measurements have been made of the mineralogy of the EH70 bulk mantle composition (Sanloup et al., 1999)) through in-situ laser-heated diamond anvil cell (LHDAC) and large volume press (LVP). Majorite-garnet (Mj) dominated mineralogy has been observed up to 25 GPa. Bridgmanite (Bm) begins to appear from 25.2 GPa and continues in a mixed phase with Mj up to 27 GPa at which point only Bm and calcium perovskite (CaPv) remain. Akimotoite (Ak) is stable up to 1873 K, higher by ≈300 K compared to numerical calculations (Connolly, 2009). This may result in an Ak layer in the Martian mantle, something missing in Earth’s mantle. The overall ratio of pyroxene to olivine polymorphs by volume is high, approaching pure pyroxene. This agrees with numerical calculations. Additionally, ferropericlase (Fp) is stable at lower temperatures, suggesting a higher dependence on temperature for its stability, something that is different from Perple_X calculations which show a strong dependence on pressure. Furthermore, Mj, which make up a majority of the volume of EH70 mantles, was measured to increase in Fe content as pressure increases. The more oxidizing conditions coupled with the silicon-rich composition resulted in three times higher Fe3+ content in Mj as opposed to a pyrolite model. This increased Fe3+ meant our Mj composition approached that of skiagite (Ski,Fe2+ 3 Fe3+ 2 Si3O12) and this caused Mj to have a very low compressibility of only 152.8 GPa, lower than any other Mj compositions in literature. This result suggests that a mantle with EH70 bulk composition would have lower than predicted seismic wave velocities , lower than Perple_X predicts. The Al content of Mj was also found to suppress the first derivative of compressibility to 4.45, lower than that of Ski100 at 6.7. Such differences compared with pyrolitic composition are important to estimate the velocity profiles and to model the dynamics of the Martian mantle. This dataset of mineralogy and composition can also model terrestrial exoplanetary mantles. Current measurements of stellar abundances show a wide range of compositions, and especially compositions with (Mg+Fe+Ca)/Si ratios approaching 1 (Brewer and Fischer, 2016). This experimental study of EH70 composition can fill-in this gap.
ContributorsDolinschi, Jonathan David (Author) / Shim, Sang-Heon D. (Thesis advisor) / Desch, Steven (Committee member) / Lee, Mingming (Committee member) / Arizona State University (Publisher)
Created2019
Description
The mineralogy of the deep mantle is one of the key factors for the chemical evolution of the Earth. The constituent minerals of the mantle rock control the physical properties of the mantle, which have significant impacts on the large-scale processes occurring in the Earth's interior. In my PhD research, I adopted experimental approaches to investigate the mineralogy and the physical properties of the Earth's materials in the deep mantle by using the diamond anvil cells (DACs) combined with in-situ X-ray diffraction techniques.
First, I found that Ca-bearing bridgmanite can be stable in the deeper part of the Earth's lower mantle where temperature is sufficiently high. The dissolution of calcium into bridgmanite can change the physical properties of the mantle, such as compressibility and viscosity. This study suggests a new mineralogical model for the lower mantle, which is composed of the two layers depending on whether calcium dissolves in bridgmanite or forms CaSiO3 perovskite as a separate phase.
Second, I investigated the mineralogy and density of the subducting materials in the Archean at the P-T conditions near 670 km-depth. The experiments suggest that the major phases of Archean volcanic crust is majoritic garnet and ringwoodite in the P-T conditions of the deep transition zone, which become bridgmanite with increasing pressure. The density model showed that Archean volcanic crust would have been denser than the surrounding mantle, promoting sinking into the lower mantle regardless of the style of the transportation in the Archean.
Lastly, I further investigated the mineralogies and densities of the ancient volcanic crusts for the Archean and Proterozoic at the P-T conditions of the lower mantle. The experiments suggest that the mineralogy of the ancient volcanic crusts is composed mostly of bridgmanite, which is systemically denser than the surrounding lower mantle. This implies that the ancient volcanic crusts would have accumulated at the base of the mantle because of their large density and thickness. Therefore, the distinctive chemistry of the ancient volcanic crusts from the surrounding mantle would have given a rise to the chemical heterogeneities in the region for billions of years.
First, I found that Ca-bearing bridgmanite can be stable in the deeper part of the Earth's lower mantle where temperature is sufficiently high. The dissolution of calcium into bridgmanite can change the physical properties of the mantle, such as compressibility and viscosity. This study suggests a new mineralogical model for the lower mantle, which is composed of the two layers depending on whether calcium dissolves in bridgmanite or forms CaSiO3 perovskite as a separate phase.
Second, I investigated the mineralogy and density of the subducting materials in the Archean at the P-T conditions near 670 km-depth. The experiments suggest that the major phases of Archean volcanic crust is majoritic garnet and ringwoodite in the P-T conditions of the deep transition zone, which become bridgmanite with increasing pressure. The density model showed that Archean volcanic crust would have been denser than the surrounding mantle, promoting sinking into the lower mantle regardless of the style of the transportation in the Archean.
Lastly, I further investigated the mineralogies and densities of the ancient volcanic crusts for the Archean and Proterozoic at the P-T conditions of the lower mantle. The experiments suggest that the mineralogy of the ancient volcanic crusts is composed mostly of bridgmanite, which is systemically denser than the surrounding lower mantle. This implies that the ancient volcanic crusts would have accumulated at the base of the mantle because of their large density and thickness. Therefore, the distinctive chemistry of the ancient volcanic crusts from the surrounding mantle would have given a rise to the chemical heterogeneities in the region for billions of years.
ContributorsKo, Byeongkwan (Author) / Shim, Sang-Heon (Thesis advisor) / Garnero, Edward (Committee member) / Leinenweber, Kurt (Committee member) / Li, Mingming (Committee member) / Desch, Steven (Committee member) / Arizona State University (Publisher)
Created2020