An Experimental Study of Grain-Size Evolution and Its Rheological Consequences During the Phase Transitions in Olivine to Its High-Pressure Polymorphs [electronic resource].

9781088315101

Ann Arbor : ProQuest Dissertations & Theses, 2019.

1 online resource (209 p.)

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Deformation of oceanic lithosphere at a trench dissipates a large amount of energy and hence plays an important role in the thermal evolution and dynamics of the Earth. Rheological properties of constituent minerals control the deformation of the lithosphere and therefore the knowledge of rheological properties of minerals is fundamental to our understanding of the processes in the Earth. Olivine is the weakest and volumetrically dominant mineral in the upper mantle and therefore, the strength of subducted slabs in the mantle is governed by that of olivine. Olivine in the cold interiors of subducted slabs most likely deforms by low-temperature plasticity but the effect of pressure on the strength of olivine in this regime is not well constrained. We conduct in-situ deformation experiments at higher-pressures and combine these data with relatively lower-pressure data, to determine the effect of pressure on low-temperature plasticity in olivine. Our results suggest that, in the absence of any mechanism to weaken subducted slabs, it is very difficult to explain the observed slab deformation by low-temperature plasticity in olivine. It has been hypothesized that grain-size reduction due to phase transformation might weaken subducted slabs. To understand how phase transitions change grain-size, we experimentally investigated the grain-scale microstructural evolution during phase transformation of olivine to wadsleyite. We found that the grain-size of daughter phase depends on the temperature at which the phase transformation takes place. The resultant grain-size of daughter phase is less than 0.1 µm at relatively lower temperatures, whereas it is larger at relatively higher temperatures. Using scaling laws, we conclude that in Earth, at relatively lower temperatures (<1100 K), very fine-grained daughter phase is formed and sustained for a long period of time. We also studied how the change in grain-size during phase transformation of olivine affects its rheological properties. To do so, we conducted high-pressure in-situ deformation experiments during the phase transformation of olivine to ringwoodite. We found that transformed ringwoodite was much weaker but the pre-synthesized ringwoodite stronger than the parent olivine. Moreover, the strength of transformed ringwoodite increased with time whereas that of pre-synthesized ringwoodite and parent olivine was constant with time. We constructed a model to explain the strength evolution in transformed ringwoodite and used it to extrapolate our results to Earth. Our results suggest that grain-size reduction during phase transformation and the consequent weakening of slabs leads to slab deformation in the mantle transition zone.Subducted slabs deform the mantle surrounding them. This deformation results in the development of crystallographic preferred orientation of mantle minerals. We used source-side shear-wave splitting technique to investigate the seismic anisotropy in the mid-mantle around subducted slabs. We found ample evidence for the presence of anisotropy around subducted slabs in the mantle transition zone and the uppermost lower mantle. Our results can be used to interpret mid-mantle flow associated with subducted slabs when experimental data on the deformation of transition zone minerals and lower mantle minerals become available.

Dissertations & Theses @ Yale University.

Books / Online / Dissertations & Theses

English

January 17, 2020

Thesis (Ph.D.)--Yale University, 2019.

Yale University. Geology and Geophysics.