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.