Rheological Properties of Earth’s Upper Mantle at High Pressure: Roles of Melt, Water and Pressure

(proposal summary)

This proposal focuses on deformation of aggregates of olivine and olivine plus enstatite, key constituents of the upper mantle, at mantle pressures and temperatures. Our emphasis is not only on rheological behavior but also on development of crystallographic fabric, primarily under hydrous conditions. While mechanical properties of olivine-rich rocks have received significant attention over last few decades, only recently have hardware improvements allowed concentrated research of fabric, also known as lattice preferred orientation or LPO, resulting from deformation. Crystallographic fabric causes elastic properties and therefore seismic velocities to become anisotropic. Our knowledge of the process of fabric development will thus be an important tool for constraining the conditions and mechanisms of deformation both in laboratory experiments and in the mantle.

The proposed research builds on recent advances in studying LPO in deformed rocks and in interpreting LPO patterns based on crystal slip systems. Two state-of-the-art deformation apparatus will be used. The first is the D-DIA, with a working pressure up to 12 GPa, operated both on an x-ray beam line to provide well-resolved measurement of strain, stress and pressure as well as semi-quantitative monitoring of fabric development during deformation, and off line to produce numerous samples with deformed shear strains to ≈ 2 for microscopic fabric analysis. The second is the torsion gas-medium apparatus, which has high-resolution in stress, temperature and pressure. Importantly for mature fabric evolution, the latter can impart sample shear strains up to γ ≈ 20. In addition we will apply two well-known but essential analytical techniques: electron backscattered diffraction for rapid measurement of LPO on large numbers of grains in fine-grained and multi-phase rocks, and Fourier transform infrared spectroscopy, enabling quantitative determination of water (hydrogen) content of nominally anhydrous minerals.

Our experimental approach emphasizes the importance of separating the effects of water fugacity, pressure, temperature, and stress on LPO, and is unique in this regard in high-pressure (>2 GPa) research. To date, most studies in this area have varied at least two of these parameters at the same time but assigned associated changes in LPO to one of the parameters. As a result, it has not been possible to develop a clear understanding of fabric development or to apply laboratory results confidently to interpretation of seismic observations.

The results of the proposed study will be of significance for both geosciences and energy sciences. An understanding of the deformation fabrics and their dependence on pressure and water content will provide important constraints for understanding seismic anisotropy in the upper mantle, which in turn is a key link to the dynamics and structure of Earth’s interior. Our results will help us derive the deformation conditions (e.g., temperature, water content, stress) and the actual flow law (e.g., stress exponent) operating in the mantle at mantle conditions. Such information, for example, forms an important foundation for seismic exploration by energy industries.

The experiments described in this proposal will enable us to work productively through the synchrotron dark period associated with the transition from the original NSLS to NSLS II at Brookhaven National Lab. Shear experiments in the D-DIA will be continued on line at APS and off line at UMN. The UMN D-DIA is well calibrated for pressure and temperature; differential stress will be determined from dislocation density measurements. In addition, torsional deformation experiments in the gas-medium apparatus will complement the D-DIA experiments, permitting us to explore a wide range of pressure, temperature, and strain as the basis for understanding fabric development and it dependence deformation conditions.