Grain- to multiple-grain-scale deformation processes during diffusion creep of forsterite + diopside aggregate: 2. Grain boundary sliding-induced grain rotation and its role in crystallographic preferred orientation in rocks

2017 ◽  
Vol 122 (8) ◽  
pp. 5916-5934 ◽  
Author(s):  
G. Maruyama ◽  
T. Hiraga
Keyword(s):  
Grain Boundary ◽  
Grain Boundary Sliding ◽  
Preferred Orientation ◽  
Diffusion Creep ◽  
Crystallographic Preferred Orientation ◽  
Grain Rotation ◽  
Deformation Processes ◽  
Boundary Sliding

Reaction-enhanced ductile deformation during carbonation of serpentinized peridotite

2021 ◽  
Author(s):  
Manuel D. Menzel ◽  
Janos L. Urai ◽  
Peter B. Kelemen ◽  
Greg Hirth ◽  
Alexander Schwedt ◽  
...  
Keyword(s):  
Grain Boundary ◽  
Grain Boundary Sliding ◽  
Preferred Orientation ◽  
Ductile Deformation ◽  
Dislocation Creep ◽  
Crystallographic Preferred Orientation ◽  
Serpentinized Peridotite ◽  
Boundary Sliding ◽  
Penetrative Foliation ◽  
Drilling Project

<p>Carbonated serpentinites record carbon fluxes in subduction zones and are a possible natural analogue for carbon capture and storage via mineralization, but the processes by which the reaction of serpentinite to listvenite (magnesite-quartz rocks) goes to completion are not well understood. Large-scale hydration and carbonation of peridotite in the Oman Ophiolite produced massive listvenites, which have been drilled by the ICDP Oman Drilling Project (OmDP, site BT1) [1]. Here we report evidence for localized ductile deformation during serpentinite carbonation in core BT1B, based on observations from optical microscopy, cathodoluminescence microscopy, SEM, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) in segments of the core that lack a brittle overprint after listvenite formation [2].</p><p>Microstructural analysis of the serpentinized peridotite protolith shows a range of microstructures common in serpentinite with local ductile deformation manifested by a shape and crystallographic preferred orientation and kinking of lizardite. Listvenites with ductile deformation microstructures contain a penetrative foliation due to a shape preferred alignment of magnesite spheroids and/or dendritic magnesite, bending around Cr-spinel porphyroclasts. Locally the foliation can be due to aligned dendritic overgrowths on euhedral magnesite grains. Magnesite grains have a weak but consistent crystallographic preferred orientation with the c-axis perpendicular to the foliation, and show high internal misorientations. Locally, the microcrystalline quartz matrix also shows a crystallographic preferred orientation with the c-axes preferentially oriented parallel to the foliation. Folding and ductile transposition of early magnesite veins indicates that carbonation initiated before the ductile deformation stage recorded in listvenites with penetrative foliation. On the other hand, dendritic magnesite overgrowths on folded veins and truncated vein tips suggest that folding likely occurred before complete carbonation, when some serpentine was still present. TEM analysis of magnesite revealed that subgrain boundaries oriented at high angle to the foliation can consist of nano-cracks sealed by inclusion-free magnesite precipitates. High dislocation densities are not evident suggesting that dislocation creep was minor or negligible, in agreement with very low predicted strain rates for magnesite dislocation creep at the low temperatures (100 – 200 °C) of serpentinite carbonation. This points to dissolution-precipitation, possibly in addition to grain boundary sliding, as the main mechanism for the formation of the shape preferred orientation of magnesite. The weak magnesite crystallographic preferred orientation may be explained by a combination of initial growth competition in an anisotropic (sheared) serpentine medium with subsequent preferred dissolution of smaller, less favorably oriented grains. We infer that transient lithostatic pore pressures during listvenite formation promoted ductile deformation in the reacting medium through grain boundary sliding accommodated by dilatant granular flow and dissolution-precipitation. Because the reaction product listvenite is stronger than the reacting mass, deformation may be preferentially partitioned in the reacting mass, locally enhancing transient fluid flow and, thus, the carbonation reaction progress.</p><p>[1] Kelemen et al., 2020. Site BT1: fluid and mass exchange on a subduction zone plate boundary. In: Proceedings of the Oman Drilling Project: College Station, TX</p><p>[2] Menzel et al., 2020, JGR Solid Earth 125(10)</p>

Strain localisation during diffusion creep: influence of grain coarsening and grain boundary sliding

2020 ◽  
Author(s):  
John Wheeler ◽  
Lynn Evans ◽  
Robyn Gardner ◽  
Sandra Piazolo
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Earth Pressure ◽  
Grain Boundary Sliding ◽  
Mechanical Anisotropy ◽  
Limited Attention ◽  
Diffusion Creep ◽  
Pressure Solution ◽  
Grain Coarsening ◽  
Boundary Sliding

<p>Diffusion creep and the wet low temperature version, pressure solution, are major deformation mechanisms in the Earth. Pressure solution operates in many metamorphosing systems in the crust and may contribute to slow creep on fault surfaces. Diffusion creep prevails in areas of the upper mantle deforming slowly, and possibly in most of the lower mantle. Both mechanisms contribute to localisation since small grain sizes can deform faster.</p><p>However, there has been limited attention paid to the evolution of microstructure during diffusion creep. In some experiments grains coarsen; in some but not all experiments grains remain rather equant. We have developed a grain-scale numerical model for diffusion creep, which indicates that those processes are very important in influencing evolving strength. Our models illustrate three behaviours.</p><ol><li>Strain localises along slip surfaces formed by aligned grain boundaries on all scales. This affects overall strength.</li> <li>Diffusion creep is predicted to produce elongate grains and then the overall aggregate has intense mechanical anisotropy. Thus strength during diffusion creep, and localisation on weak zones, is influenced not just by grain size but by other aspects of microstructure.</li> <li>Grain coarsening increases grain size and strength. Our most recent work shows how it interacts with ongoing deformation. In particular grain growth can lead to particular grain shapes which are directly related to strain rate, and influence strength. Consequently, understanding localisation during diffusion creep must encompass the effects of diffusion itself, grain boundary sliding and grain coarsening.</li> </ol>

Comments on grain-boundary sliding in diffusion creep

1976 ◽  
Vol 10 (2) ◽  
pp. 161-162
Author(s):  
E.H. Aigeltinger ◽  
R.C. Gifkins
Keyword(s):  
Grain Boundary ◽  
Grain Boundary Sliding ◽  
Diffusion Creep ◽  
Boundary Sliding
Sign in / Sign up

Export Citation Format

Share Document