Diffusion Creep Characteristics of Anorthite Revealed by Uniaxial and Pure Shear Deformation Experiments

Abstract

Plagioclase, a major rock-forming mineral in the Earth's crust, often shows microstructural evidence of ductile deformation in crustal rocks, suggesting that crustal flow is largely controlled by the high-temperature deformation behavior of this mineral. Here, we present uniaxial and pure shear deformation experiments that reveal various diffusion creep characteristics of anorthite (CaAl2Si2O8), a calcium-rich plagioclase endmember. We synthesized fine-grained (∼1 μm) anorthite aggregates with different Al/Si ratios (Al/Si = 1 and 0.97) that were either undoped or doped with 1 wt% MgO. We determined that the synthesized samples deformed by interface (reaction)-controlled and grain-boundary diffusion creep mechanisms. At similar conditions of stress, temperature, and grain size, strain rates varied by ∼4 orders of magnitude among the samples. A reduction in Al/Si ratio weakens the aggregate by ∼2 orders of magnitude, while doping with MgO, which probably becomes concentrated at grain boundaries, further weakens samples with different Al/Si ratios to the same low strength level. The resulting low viscosity due to MgO is comparable to that of grain-boundary diffusion creep in anorthite in previous studies. Grain boundary sliding (GBS)-induced rigid-body-like grain rotation was identified by analysis of a marker-etched sample surface after deformation. Crystallographic preferred orientation and shape preferred orientation developed after the samples were deformed to strains of ≥0.7, which is well explained by preferential GBS along grain boundaries parallel to (010) and also in the [100] direction within the grain-boundary plane. These important, but previously unknown, characteristics of anorthite diffusion creep originate from the nature of the grain boundaries.