Pressure Effect on Diffusion Creep of Olivine: Implications for the Upper Mantle Rheology
Abstract
Diffusion and dislocation creep are two dominant mechanisms of deformation in Earth's mantle. The relative importance of these two mechanisms may change with depth and location (temperature and water content). Although the temperature (T) and water content dependence of both deformation mechanisms has been studied, pressure (P) dependence of high temperature creep is still poorly constrained. As a consequence, the dominant mechanism of deformation in Earth's upper mantle is only poorly constrained. The existing uncertainties pose serious issues in understanding the nature of seismic anisotropy and flow geometry in the upper mantle. The obvious reason for this is the fact that in the lab experiments, much higher stress is applied to a sample than a typical stress in Earth, and consequently, dislocation creep is observed in most cases (e.g. Karato & Jung 2003).
In order to determine the pressure dependence of diffusion creep, we need to synthesize a sample with fine grain-size and identify the operation of diffusion creep, and determine the pressure dependence of strength in diffusion creep regime. We use Deformation-DIA (D-DIA) coupled with synchrotron X-ray facility (6-BM,B beamline at APS ), to perform in-situ high pressure deformation of fine-grained samples. Samples are synthetic fine-grained olivine either from crushed San Carlos olivine or samples made from sol-gel synthesis. Typical starting grain-size is ~200 nm. Well controlled grain growth during the sample hot-pressing and subsequent annealing as well as during the deformation is achieved through doping the olivine samples with up to 5wt% of nano-alumina and orthopyroxene. The deformation behavior of synthetic and natural samples with different grain size is compared. The strain rates are generally maintained at ~10-5 s-1 - 10-6 s-1. We use the pyrope stress sensors to monitor the stress evolution in the cell during the experiments. P-T is estimated using EOS of MgO and Pt. Modified furnace and internal cell designs are employed to minimize the thermal gradient throughout the experimental cell. The preliminary results demonstrate that we were able to deform our samples in the diffusion creep regime. References: Karato, S.I., & Jung, H. (2003) Philosophical Magazine, 83(3), 401-414.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMMR51B0055G
- Keywords:
-
- 5112 Microstructure;
- PHYSICAL PROPERTIES OF ROCKS;
- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 8034 Rheology and friction of fault zones;
- STRUCTURAL GEOLOGY;
- 8159 Rheology: crust and lithosphere;
- TECTONOPHYSICS