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- All Subjects: mantle
- All Subjects: ordinary chondrite
- Creators: Leinenweber, Kurt
Description
Shock metamorphism in meteorites constrains the impact histories of asteroids and planets. Shock-induced high-pressure (HP) minerals can provide more precise estimates of shock conditions than shock-induced deformation effects. In this research, I use shock features, particularly HP minerals, in ordinary-chondrite samples to constrain not only shock pressures but also the pressure-temperature-time (P-T-t) paths they experienced.
Highly shocked L5/6 chondrites Acfer 040, Mbale, NWA 091 and Chico and LL6 chondrite NWA 757 were used to investigate a variety of shock pressures and post-shock annealing histories. NWA 757 is the only highly shocked LL chondrite that includes abundant HP minerals. The assemblage of ringwoodite and majoritic garnet indicates an equilibration shock pressure of ~20 GPa, similar to many strongly shocked L chondrites. Acfer 040 is one of the only two chondrite samples with bridgmanite (silicate perovskite), suggesting equilibration pressure >25 GPa. The bridgmanite, which is unstable at low-pressure, was mostly vitrified during post-shock cooling. Mbale demonstrates an example of elevated post-shock temperature resulting in back-transformation of ringwoodite to olivine. In contrast, majoritic garnet in Mbale survives as unambiguous evidence of strong shock. In these two samples, HP minerals are exclusively associated with shock melt, indicating that elevated shock temperatures are required for rapid mineral transformations during the transient shock pulse. However, elevated post-shock temperatures can destroy HP minerals: in temperature sequence from bridgmanite to ringwoodite then garnet. NWA 091 and Chico are impact melt breccias with pervasive melting, blackening of silicates, recrystallization of host rock but no HP minerals. These features indicate near whole-rock-melting conditions. However, the elevated post-shock temperatures of these samples has annealed out HP signatures. The observed shock features result from a complex P-T-t path and may not directly reflect the peak shock pressure. Although HP minerals provide robust evidence of high pressure, their occurrence also requires high shock temperatures and rapid cooling during the shock pulse. The most highly shocked samples lack HP signatures but have abundant high-temperature features formed after pressure release.
Highly shocked L5/6 chondrites Acfer 040, Mbale, NWA 091 and Chico and LL6 chondrite NWA 757 were used to investigate a variety of shock pressures and post-shock annealing histories. NWA 757 is the only highly shocked LL chondrite that includes abundant HP minerals. The assemblage of ringwoodite and majoritic garnet indicates an equilibration shock pressure of ~20 GPa, similar to many strongly shocked L chondrites. Acfer 040 is one of the only two chondrite samples with bridgmanite (silicate perovskite), suggesting equilibration pressure >25 GPa. The bridgmanite, which is unstable at low-pressure, was mostly vitrified during post-shock cooling. Mbale demonstrates an example of elevated post-shock temperature resulting in back-transformation of ringwoodite to olivine. In contrast, majoritic garnet in Mbale survives as unambiguous evidence of strong shock. In these two samples, HP minerals are exclusively associated with shock melt, indicating that elevated shock temperatures are required for rapid mineral transformations during the transient shock pulse. However, elevated post-shock temperatures can destroy HP minerals: in temperature sequence from bridgmanite to ringwoodite then garnet. NWA 091 and Chico are impact melt breccias with pervasive melting, blackening of silicates, recrystallization of host rock but no HP minerals. These features indicate near whole-rock-melting conditions. However, the elevated post-shock temperatures of these samples has annealed out HP signatures. The observed shock features result from a complex P-T-t path and may not directly reflect the peak shock pressure. Although HP minerals provide robust evidence of high pressure, their occurrence also requires high shock temperatures and rapid cooling during the shock pulse. The most highly shocked samples lack HP signatures but have abundant high-temperature features formed after pressure release.
ContributorsHu, Jinping (Author) / Sharp, Thomas G. (Thesis advisor) / Tyburczy, James A. (Committee member) / Leinenweber, Kurt (Committee member) / Wadhwa, Meenakshi (Committee member) / Zolotov, Mikhail (Committee member) / Arizona State University (Publisher)
Created2016
Description
The transport of hydrogen to the Earth’s deep interior remains uncertain. The upper mantle minerals have very low hydrogen solubilities (hundreds of ppm). The hydrogen storage capability in the transition zone minerals (2 wt%) is high compared to those of the upper mantle. The hydrogen storage in the lower mantle is not well known. The main minerals in the lower mantle bridgmanite and ferropericlase have very low hydrogen storage capacities (less than 20 ppm). In order to further understand the hydrogen storage in the lower mantle, a series of experiments had been conducted to simulate the environment similar to the Earth’s mantle. The experiments with hydrous Mg2SiO4 ringwoodite (Rw) show that it converts to crystalline dense hydrous silica, stishovite (Stv) or CaCl2-type SiO2(mStv), containing ∼1 wt% H2O together with bridgmanite (Brd) and MgO at the pressure-temperature conditions expected for lower mantle depths between approximately 660 to 1600 km. Brd would break down partially to dense hydrous silica (6–25 mol%) and(Mg,Fe)O in mid-mantle regions with 0.05–0.27 wt% H2O. The hydrous stishovite has a CaCl2 structure, which is common among hydrous minerals in the lower mantle. Based on this observation, I hypothesize the existence of hydrous phases in the lower mantle. The experiments found a new hexagonal iron hydroxide (η-Fe12O18+x/2Hx) between the stability fields of the epsilon and pyrite-type FeOOH at 60–80 GPa and high temperature. The new phase contains less H2O, limiting the H2O transport from the shallow to the deep mantle in the Fe–O–H system. Possible hydrogen storage in Ca-perovskite was studied. CaPv could contain 0.5–1 wt% water and the water in CaPv could distort the crystal structure of CaPv from cubic to tetragonal structure. In conclusion, hydrogen can be stored in hydrous stishovite in the shallower depth of the lower mantle. At greater depth, the new η phase and pyrite-type phase would take over the hydrogen storage. The role of CaPv in deep water storage needs to be considered in future studies.
ContributorsChen, Huawei (Author) / Shim, Sang-Heon (Thesis advisor) / Garnero, Edward (Committee member) / Bose, Maitrayee (Committee member) / Li, Mingming (Committee member) / Leinenweber, Kurt (Committee member) / Arizona State University (Publisher)
Created2019
Description
The redox conditions of Earth have been changing since proto-Earth’s accretion from the solar nebula. These changes have influenced the distribution and partitioning of volatile elements between the atmosphere and the mantle (Righter et al., 2020; Stagno and Fei, 2020. Though oxygen fugacity fO2 is arguably not the main factor for phase stability at certain pressure-temperature conditions (McCammon, 2005), it can influence which phases are stable, especially within a closed system such as the ones presented in this study. Despite the importance of controlling fO2 for interpreting the history of planetary bodies, there have been no methods to control the redox conditions in the laser-heated diamond anvil cell (LHDAC). This thesis has examined the feasibility for controlling redox conditions in the LHDAC using a mixture of Ar and H2 for insulation media. The experiments of this study were carried out at the GSECARS sector of the Advanced Photon Source at Argonne National Laboratory. In this study, α-Fe2O3 (hematite), ε-FeOOH (CaCl2-type), and Fe3O4 (magnetite) starting materials were used for probing changes of redox conditions. Experiments were also conducted with a pure Ar-medium for ε-FeOOH at the same pressure-temperature conditions of the hydrogen-bearing medium in order to provide a reference point for data which has uncontrolled redox conditions for an initially Fe(2+)-free material. The results for the ε-FeOOH starting material in Ar show transformation to ι-Fe2O3 (Rh2O3(II)-type) at 30.0 GPa and 1900 K, while in Ar + H2 it transformed to Fe5O7 with minor FeH (dhcp) at 30.0 GPa and 1850 K. For α-Fe2O3 in Ar + H2, it was found to convert to ε-FeOOH, Fe5O7, Fe5O6, and FeH (dhcp) at 36.5 GPa and 1800 K. For Fe3O4 in Ar + H2, it was found to convert to Fe4O5 (CaFe3O5-type), Fe5O6, and minor FeH (fcc) at 26.0 GPa and 1800 K. These results demonstrate that H in an Ar medium can promote the conversion of some Fe(3+) to Fe(2+) and Fe(0). However, the formation of ε-FeOOH in the α-Fe2O3 starting material suggests that H may participate in the chemical reaction of iron oxides.
ContributorsKulka, Britany Lynn (Author) / Shim, Sang-Heon (Thesis advisor) / Sharp, Thomas (Committee member) / Leinenweber, Kurt (Committee member) / Hervig, Richard (Committee member) / Arizona State University (Publisher)
Created2021