Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Geology

Supervisor

Moser, Desmond E.

2nd Supervisor

Shieh, Sean R.

Joint Supervisor

Abstract

Established paradigms in materials science and experimentation were used to generate models of several well-known aspects of zircon constitutive behavior: (1) plastic deformation through twinning, (2) slip and (3) a phase transformation to reidite. (1) The shear stresses to homogeneously nucleate a (112)[1̄1̄1] twin were modeled for a variety of grain sizes, in order to re-evaluate this microstructure’s use as an indicator of shock deformation on Earth and other planetary bodies. The minimum stress is between 0.3 and 2.0 GPa for the range of grain sizes observed in a natural sample of twinned zircon from the Vredefort impact structure. The stress is largely pressure-independent and it is likely that twins can form over a wider range of shock pressures than currently estimated, as shown by the occurrence of twins in shocked zircon from distal regions of the impact basin. (2) The yield of zircon during plastic deformation by slip was measured by nanoindentation hardness testing at room temperature and 450 C, with the intention of characterizing the necessary stresses for significant plastic deformation in zircon. The hardness of zircon is 13 ± 0.5 GPa. Using the hardness as a measure of competence, comparison with other rock-forming minerals suggests that typical lower-crustal host rocks are most likely to promote deformation in zircon. (3) The martensitic transformation of zircon to reidite was modeled using the Eshelby formalism, with the intention of evaluating its use as a indicator of shock pressure. A typical martensitic mechanism predicts observed habit planes, especially (512), and internal deformation. A representiative elastic strain energy is between 3 and 5 GJ m−3 . Since a total conceivable activation energy has relatively comparable elastic strain energy and inelastic energies, transition pressures for shear-based mechanisms are expected to decrease significantly with temperature. Reconstructions of shock deformation pressures based on reidite and relying on temperature-insensitive boundaries should therefore be used with caution. This work can be extended to explain the material behavior of zircon in a more comprehensive way, and achieve direct strain chronometry and more precise impact pressure reconstruction.

Summary for Lay Audience

The amount and kind of deformation experienced by the mineral zircon in various settings in the crust are re-evaluated using established methods from materials science such as hardness testing and micromehcanical modeling. It is shown that many of the key observations of the consitutive behavior of zircon, relating to its other uses as a mineral used in geochronology and recording the effects of meteorite impact, can be explained and quantified with these methods. It is also shown that some of the previous methods tend to underestimate the proportion of zircon grains in a rock which may be deformed in a way of interest to geochronology or impact reconstruction. Although the range of modeled behaviors is relatively narrow, the methods are laid out in such a way that they can be developed further for use in evaluating zircon deformation more comprehensively. The ultimate goal of such research is to use zircon to precisely reconstruct the deformation conditions of meteorite impact or directly date strain events experienced by Earth’s crust.

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