Influences of CO2 on the Microstructure in Sheared Olivine Aggregates

Huihui Zhang,Ningli Zhao,Greg Hirth,Xiaoge Huang,Chao Qi

doi:10.3390/min11050493

Abstract

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.

Highlights

In the mantle of the Earth, deformation often occurs in regions where melt is produced and transported, such as plate boundaries, plumes, intra-plate rifts, and boundary layers
Shear deformation of partially molten, olivine aggregates produces a crystallographic preferred orientation (CPO), which is characterized by a cluster of [010] axes sub-normal to the shear plane and girdles of [100] and [001] axes sub-parallel to the shear plane [2,10]
Piercing through the Ni jacket with the sample submerged in ethanol, gas bubbles were released from the blisters for several minutes—indicating CO2 was successfully encapsuled in the assembly by the jacket at our experimental conditions

Summary

Introduction

In the mantle of the Earth, deformation often occurs in regions where melt is produced and transported, such as plate boundaries, plumes, intra-plate rifts, and boundary layers. Shear deformation of partially molten, olivine aggregates produces a crystallographic preferred orientation (CPO), which is characterized by a cluster of [010] axes sub-normal to the shear plane and girdles of [100] and [001] axes sub-parallel to the shear plane [2,10]. This CPO, often referred to as AG-type fabric [11], can form due to the development of a shape preferred orientation (SPO), where the grain shape is crystallographically controlled in partially molten olivine aggregates [10]

Methods

Results

Conclusion