Viscoelastic relaxation in olivine: an experimental study of alternative mechanisms

Abstract

Observation and analysis of seismic waves is the predominant mechanism in which geophysicists infer the structure and state of the Earth’s interior, including aspects such as major element composition, grain size, temperature, melt fraction and water content. However, imperative to this inversion, is obtaining a robust understanding of how changes in any of these specific physical characteristics manifest as seismic anomalies.

In this thesis we conduct carefully controlled laboratory experiments using the methods of forced oscillation and microcreep to further elucidate the micromechanical details of anelastic relaxation for commonly invoked, yet still poorly constrained, mechanisms of seismic wave attenuation. Polycrystalline olivine specimens were prepared from either synthetic (solution-gelation derived) or natural (San Carlos) origins were by hot-pressing at 1200°C and 300 MPa. The use of synthetic precursors allowed the chemical composition to be tailored to address specific questions concerning the anelastic behavior. Additionally, large olivine crystals were grown for future work on intracrystalline relaxation, along with polycrystalline (Mg,Ni)2GeO4 materials for potential studies of transformational anelasticity involving coexisting olivine and spinel phases.

Fabrication and testing of a melt-bearing olivine specimen with complimentary modes of torsional, and newly developed flexural oscillation, assessed the possibility of melt-induced relaxation of bulk modulus. In shear, a conspicuous melt-related attenuation peak is observed, similar to previously conducted studies. Flexural data demonstrate a large decrease in Young’s modulus with increasing temperature, inclusive of trans-solidus conditions. This modulus reduction accords well with the reduction in shear modulus through the approximate relation E ~ 3G. These results indicate that when shear modulus is relatively low (i.e. at high homologous temperatures), resolution of bulk modulus through combined shear and flexural/extensional modes is significantly curtailed, thus making experimental detection of bulk modulus dispersion and attenuation difficult or impossible.

Assessment of the role of water in the attenuation of seismic waves was achieved through the fabrication of eight specimens containing different hydrous defects at varying concentrations, as well as a range of molecular water contents. Specimens tested under conditions that are both oxidizing and water-undersaturated display markedly stronger strain-energy dissipation and modulus dispersion than those tested under conditions that are both reducing and dry. However, it is unambiguously demonstrated that neither Ti-hydroxyl defects nor molecular water significantly affect the measured seismic properties. Rather, a relationship between the prevailing redox conditions and strength of anelastic relaxation was observed, with the approximate scaling of Q-1 ~ (fO2)1/3. These results suggest that low velocities and high attenuation above subducting slabs, typically attributed to the presence of elevated water contents, are instead a consequence of the prevailing redox conditions. Show more