Crystallographic preferred orientation produced by diffusional creep of harzburgite: Effects of chemical interactions among phases during plastic flow

Abstract

Experimental deformation experiments have been conducted on fine‐grained, two‐phase aggregates of olivine and orthopyroxene to investigate the role of grain and phase boundary sliding on rheology and fabric development. A suite of large‐strain (γ ≥ 1) general shear experiments conducted at T = 1200°C and P = 1.6 GPa on aggregates ranging from 65 to 0 vol % orthopyroxene, to characterize the evolution of fabric, were complemented by small‐strain axial compression experiments at T = 1200°C and P = 0.3 GPa, to better constrain the rheology. Microstructural and rheological data suggest that deformation of these two‐phase aggregates in the diffusion creep regime occurs via interface‐reaction‐controlled diffusion creep that is accompanied by extensive migration of olivine‐orthopyroxene phase boundaries. The resulting rheologies suggest that olivine + orthopyroxene composites are weaker than the olivine end‐member at the conditions tested. Physically, this behavior arises because long‐range, i.e., grain‐scale, diffusion of Si4+ is unnecessary in these pseudobinary two‐phase aggregates. We further demonstrate that interface‐controlled diffusion creep leads to strong crystallographic preferred orientations (CPO) of the component minerals, which develops in the near absence of dislocation activity. The CPO formed in these anhydrous, low‐stress experiments has the olivine a axis aligned perpendicular to the flow direction (“type B” fabric) argued by some to be the unique result of deformation under conditions of high differential stress and high water fugacity. Phase boundary dynamics, thus, are argued as a significant factor in the accumulation of strain in polyphase aggregates.