Influences of CO₂ on the Microstructure in Sheared Olivine Aggregates

• Main Authors: Huihui Zhang, Ningli Zhao, Chao Qi, Xiaoge Huang, Greg Hirth
• Format: Article
• Language: English
• Published: MDPI AG 2021-05-01
• Series: Minerals
• Subjects: olivine aggregates; CO₂; crystallographic preferred orientation; AG-type fabric
• Online Access: https://www.mdpi.com/2075-163X/11/5/493

Shear deformation of a solid-fluid, two-phase material induces a fluid segregation process that produces fluid-enriched bands and fluid-depleted regions, and a crystallographic preferred orientation (CPO) characterized by girdles of [100] and [001] axes sub-parallel to the shear plane and a cluster of [010] axes sub-normal to the shear plane, namely the AG-type fabric. Based on experiments of two-phase aggregates of olivine + basalt, a two-phase flow theory and a CPO formation model were established to explain these microstructures. Here, we investigate the microstructure in a two-phase aggregate with supercritical CO₂ as the fluid phase and examine the theory and model, to evaluate differences in rheological properties due to the presence of CO₂ or basaltic melt. We conducted high-temperature and high-pressure shear deformed experiments at 1 GPa and 1100 °C in a Griggs-type apparatus on samples made of olivine + dolomite, which decomposed into carbonate melt and CO₂ at experimental conditions. After deformation, CO₂ segregation and an AG-type fabric were observed in these CO₂-bearing samples, similar to basaltic melt-bearing samples. An SPO-induce CPO model was used to explain to the formation of the fabric. Our results suggest that the influences of CO₂ as a fluid phase on the microstructure of a two-phase olivine aggregate is similar to that of basaltic melt and can be explained by the CPO formation model for the solid-fluid system.