Grain‐ to multiple‐grain‐scale deformation processes during diffusion creep of forsterite + diopside aggregate: 1. Direct observations

Abstract

We uniaxially deformed fine‐grained (~ 1 μm) forsterite + diopside (5 and 20 vol %) aggregates in the diffusion creep regime. Prior to deformation, line markers were milled on a lateral surface of a cylindrical sample to detect single‐ to multiple‐grain‐scale deformation. We performed deformation experiments and observations of the marker‐etched surface after sample cooling multiple times on the same specimens. The strain measured at the scale of several tens of grains from macroscopic shortening of the markers parallel to the compression axis is consistent with the total strain of the sample. However, microscopically, the markers are intensely segmented and rotated at the grain scale increasing with the sample strain. Meanwhile, essentially, no deformation is observed within the grains in most of the samples. The surface microstructures, including the deformation of the markers, reveal the serial operations of grain boundary migration, grain boundary sliding, rigid body grain rotation, and grain neighbor switching, which correspond well to processes expected in diffusion‐controlled superplasticity. This sequence is commonly observed in both samples consisting of forsterite grains of tabular and equiaxed grain shapes, which have been shown to develop notable crystallographic preferred orientation (CPO) and random (or weak) CPO, respectively, during diffusion creep. Intragranular regions of relatively larger forsterite grains in the specimens deformed at stresses near the transition between deformation mechanisms from diffusion creep to dislocation creep reveal marker deformation and formation of surface creases and subgrain boundaries, which indicate intragranular dislocation processes. Overall, the surface microstructures reflect the deformation state of the materials well.

Numerical Modeling of Mantle Flow Beneath Madagascar to Constrain Upper Mantle Rheology Beneath Continental Regions

“…LPO beneath this channel might not exist as they could potentially be erased by diffusion creep. There are a few studies that suggest that anisotropy may develop in the diffusion creep regime (Maruyama & Hiraga, ; Miyazaki et al, ; Wheeler, ), but our results indicate shallow sources of anisotropy favoring its formation in the dislocation regime.…”

“…LPO beneath this channel might not exist as they could potentially be erased by diffusion creep. There are a few studies that suggest that anisotropy may develop in the diffusion creep regime (Maruyama & Hiraga, 2017;Miyazaki et al, 2013;Wheeler, 2009), but our results indicate shallow sources of anisotropy favoring its formation in the dislocation regime. Figure 13a shows dominance of dislocation creep in the shallow asthenosphere beneath Madagascar, where strain rate magnitude is relatively large (Figure 13b).…”

Microstructural Evidence for Grain Boundary Migration and Dynamic Recrystallization in Experimentally Deformed Forsterite Aggregates

“…Grain boundary sliding is often invoked as a source of high strain gradients at the GBs. Microstructural evidence for sliding at GBs have indeed been reported in olivine, either deformed in the diffusion creep regime [45,46] or in the dislocation creep regime [18]. However, in the present post mortem study, we did not have a marker to confirm or dismiss the possible contribution of GBs in the nucleation of new grains.…”

Weak B‐Type Olivine Fabric Induced by Fast Compaction of Crystal Mush in a Crustal Magma Reservoir

“…Coaxial compression experiments on fine‐grained forsterite plus 30 vol.% melt at room pressure and 1,385 °C also verified that the crystallography‐controlled orientation of olivine grain boundaries makes GBS frequently occur on (010) plane and even in [001] axis, which in turn contributes to grain rotation and the B‐type fabric (Miyazaki et al, ). This mechanism has been confirmed by deformation experiments on forsterite + diopside aggregates in diffusion creep (Maruyama & Hiraga, , ) and was considered as the formation mechanism of the B‐type olivine fabric in the Songshugou dunite cumulates under low stress (~10 MPa; Cao et al, ). For dunite sample 1247, the [001] and [010] axes of most tabular olivine grains are parallel to the interpreted lineation (Figure a) and the uniaxial compression direction (Figure b), respectively.…”