Fluid migration in the mantle wedge: Influence of mineral grain size and mantle compaction

Abstract

Mineral grain size in the mantle affects fluid migration by controlling mantle permeability; the smaller the grain size, the less permeable the mantle is. Mantle shear viscosity also affects fluid migration by controlling compaction pressure; high mantle shear viscosity can act as a barrier to fluid flow. Here we investigate for the first time their combined effects on fluid migration in the mantle wedge of subduction zones over ranges of subduction parameters and patterns of fluid influx using a 2‐D numerical fluid migration model. Our results show that fluids introduced into the mantle wedge beneath the forearc are first dragged downdip by the mantle flow due to small grain size (<1 mm) and high mantle shear viscosity that develop along the base of the mantle wedge. Increasing grain size with depth allows upward fluid migration out of the high shear viscosity layer at subarc depths. Fluids introduced into the mantle wedge at postarc depths migrate upward due to relatively large grain size in the deep mantle wedge, forming secondary fluid pathways behind the arc. Fluids that reach the shallow part of the mantle wedge spread trench‐ward due to the combined effect of high mantle shear viscosity and advection by the inflowing mantle and eventually pond at 55–65 km depths. These results show that grain size and mantle shear viscosity together play an important role in focusing fluids beneath the arc.

Water Migration in the Subduction Mantle Wedge: A Two‐Phase Flow Approach

“…The effect of fluid pressure on frictional yield, which may weaken for instance the subduction interface and lead to dehydration embrittlement (Jung et al, ; Podladchikov & Miller, ; Skarbek & Rempel, ), is also missing from our models. Lastly, future models could include grain‐size‐dependent and anisotropic permeability, which may play a role in the development of the fluid pathways in subduction zones (Cerpa et al, ; Kawano et al, ).…”

Fluid Migration in a Subducting Viscoelastic Slab

“…Permeability is generally proportional to / n where n ranges from 2 to 3. We use n 5 2 in equation (6), although we investigate the effects of different value for n in Appendix C. Note that although the spatial and temporal variations in grain size also affect permeability (e.g., Cerpa et al, 2017;Wada & Behn, 2015), we do not consider the effect here. Fluid viscosity is given as a function of temperature and also the concentration of dissolved silicate, which increases significantly with pressure (from about 30 wt % at 3 GPa to about 70 wt % at 8 GPa; Mibe et al, 2002).…”

“…These include the effects of dynamic pressure caused by shear deformation of matrix (e.g., Faccenda et al, 2009Faccenda et al, , 2012Spiegelman & McKenzie, 1987), hydration-dehydration reactions and melting (Iwamori, 1998), compaction (Cerpa et al, 2017;Wilson et al, 2014), grain size distribution (Cerpa et al, 2017;Wada & Behn, 2015), and 3-D slab geometry combined with permeability anisotropy (Morishige & van Keken, 2017). These include the effects of dynamic pressure caused by shear deformation of matrix (e.g., Faccenda et al, 2009Faccenda et al, , 2012Spiegelman & McKenzie, 1987), hydration-dehydration reactions and melting (Iwamori, 1998), compaction (Cerpa et al, 2017;Wilson et al, 2014), grain size distribution (Cerpa et al, 2017;Wada & Behn, 2015), and 3-D slab geometry combined with permeability anisotropy (Morishige & van Keken, 2017).…”

“…There are several numerical studies dealing with the fluid migration in subduction zones with focus on specific controlling mechanisms. These include the effects of dynamic pressure caused by shear deformation of matrix (e.g., Faccenda et al, 2009Faccenda et al, , 2012Spiegelman & McKenzie, 1987), hydration-dehydration reactions and melting (Iwamori, 1998), compaction (Cerpa et al, 2017;Wilson et al, 2014), grain size distribution (Cerpa et al, 2017;Wada & Behn, 2015), and 3-D slab geometry combined with permeability anisotropy (Morishige & van Keken, 2017). Wilson et al (2014) demonstrated that some amount of fluid is trapped in and migrates along the fluid source when the effects of viscous resistance to volume change is included.…”

Effects of fluid influx, fluid viscosity, and fluid density on fluid migration in the mantle wedge and their implications for hydrous melting

“…In our previous study, we developed numerical models of fluid migration in the mantle wedge accounting for both grain-size distribution and mantle compaction (Cerpa et al, 2017). The modeling results showed that spatial variations of grain size and solid shear viscosity both serve to focus fluids beneath the arc, and that their contributions as fluid-focusing mechanisms vary spatially in the mantle wedge.…”