Mantle Wedge Hydration
Mantle wedge hydration and its effects on mineralogy and rheology
Sam Roy, 11/21/2011, University of Maine School of Earth and Climate Sciences
Though considered to be a secondary influence behind temperature and strain rate (Billen, 2008), water does play an important role in mantle wedge rheology. Water released from a subducting oceanic lithosphere quickly saturates the mantle wedge, allows the creation of stable hydrous phases, and reduces viscosity by as much as 2 orders of magnitude (Arcay et al., 2006). The excess water that is not dissolved in the wedge continues to migrate upward into the overlying plate, where it can cause thermal erosion. I explore the effects of water on the mantle wedge through a thermal-mechanical model of Yakutat block subduction under the North American plate and use pseudosections to determine the amount of water to be released by the subducting slab. The influence on viscosity is then explored using a flow law that considers water abundance within the pre-exponential coefficient.
2. Thermal-Mechanical Model
A thermal-mechanical model is used as a 2D spatial framework for temperature and pressure that can be used to determine the expected phase assemblages within pseudosections. The model used is from Abers et al. (2006) and is meant to replicate the subduction front of the Yakutat block located beneath southern Alaska (Figure 1a). Plate geometry was attained by seismic tomography. The mantle wedge viscosity is based on a non-Newtonian dry olivine dislocation creep flow law. The mantle is coupled to the subducting and overriding slabs except within the wedge nose, where a mechanically brittle mantle is host to localized frictional sliding between plate and wedge. The geothermal gradient and advective heat transfer from wedge flow control thermal gradients within the subduction zone, and 17 mW m-2 of heat generation comes from shear heating along the fault (Figure 1b). Plate velocity is 55 mm a-1 (Nuvel-1A). The oceanic crust is 16 km thick.
3. H2O saturated MORB Phase Assemblage Analysis
I use an average bulk rock composition for north Pacific MORB from Melson et al. (1976) as a surrogate for Yakutat plate composition (Table 1). MORB compositions typically do not vary except for REE abundances which would not affect this study. The subducting MORB can experience large pressure and temperature ranges. The oceanic plate is host to excessive hydrothermal advection at the rift zone in addition to water percolation into normal faults that form by crustal bending at the trench and is therefore considered to be saturated in H2O when it subducts (Schmidt and Poli, 1998; Arcay et al., 2005). Bulk rock composition data is used to determine phase stability in Perple_X, a thermodynamic calculation program, to determine which phases have the lowest Gibbs free energy within a specified temperature and pressure range (Connolly, 2009). I used the hp04ver.dat database file (Holland and Powell, 1998) and the solut09.dat solution model file to create pressure versus temperature pseudosections for both MORB and mantle compositions. Considered solution models for the MORB composition are listed in Table 2 and the resulting pseudosections are shown in Figure 2a-c. Water is held in the rock’s crystal structure until lawsonite is reacted out at higher T and P, but the release of water is gradual, as seen by the red contours (Figure 2a). Lawsonite (Figure 2b) holds about 11.5 wt% water at ultrahigh pressures and can make up to 33 wt% of the entire rock at relatively high pressure and low temperature, therefore it is an important factor in mantle wedge hydration and its abundance controls H2O abundance at high pressure (Pawley and Holloway, 1993). Lawsonite abundance decreases with increasing temperature until it is completely reacted out to form eclogite facies rock. Dehydration and compression also increases the plate’s negative buoyancy (Figure 2c). Figure 2d displays the same pseudosection but with PT paths for three positions (top, middle, and base) in the MORB crust. The starting position of the arrows is dependent on pressure at depth and temperature in the steady-state oceanic crust geothermal gradient. Plate subduction leads to a reversal of the geothermal gradient by rapid thermal advection from wedge flow into the top of the subducting slab. The plate passes through greenschist, amphibolite, blueschist, and finally eclogite facies when all water is dehydrated, and the stable phase assemblages are typical for the basaltic rendition of these facies (Schmidt and Poli, 1998). The top of the MORB crust quickly dehydrates at ~30 kbar and releases 4.55 wt% H2O, as does the center of the plate at ~50 kbar (Figure 2e). The base of the MORB crust dehydrates at ~68 kbar, but releases only 2 wt% H2O due to its initially hotter temperature. Water is released from the subducting slab continuously until the depth of 200 km (~68 kbar).
Figure 3 displays the pseudosection for a saturated mantle composition (Hoffman, 1988). I also consider the subducting mantle to be saturated due to hydrothermal advection at the rift zone. There is a thin (~1km) layer of mantle lying right below the MORB crust that contains ~60 wt% Serpentinite, holding ~8.5 wt% H2O. Although thin, this layer holds a very large proportion of the subducting slab water budget, and the chemical discontinuituy between MORB and serpentinized mantle can lead to decoupling in some situations (Billen, 2008). Amphibole is stable within deeper parts of the mantle but it holds only miniscule amounts of water. Both the 1km thick serpentinized mantle layer and the 16km thick MORB crust contain a total average of 4.96 wt% H2O that can be released into the upper mantle.
5. H2O Limited Mantle Wedge Phase Assemblage Analysis
The water content (Table 1) used to create the mantle wedge pseudosection (Figure 4a) is limited to the amount of water that is released by lawsonite and serpentinite dehydration in the subducting slab (4.96 wt%). The blue line of Figure 4a represents the transition from all water utilized in hydrous phase assemblages to a water saturated system that requires less water. Serpentinite is restricted to a maximum of 29.46 wt% (rather than the maximum 60 wt% for a saturated mantle) within the blue range due to the limited amount of water within the wedge. The cold wedge nose also contains chlorite and amphibole (Figure 4b), and is the only location within the wedge where water is held in the crystal structure (Figure 4c). All water that is released under the wedge is taken into hydrous phases, therefore no excess water remains in the wedge nose. The red line represents the last dehydration reactions before a completely anhydrous mineral assemblage becomes stable. The only water that can exist within the rock beyond this point is dissolved water locked within olivine, cpx, opx, or garnet, and is limited to 0.146 wt% (Arcay et al., 2005; 2006).
Mantle viscosity is reduced by the introduction of water according to the viscosity equation of Figure 5. The equation takes into account the weakening factor of hydration through the pre-exponential coefficient. The weakening factor controls the drop in viscosity through a given weight percent of water. Although temperature and strain rate are the major components that control viscosity, water can alter viscosity values by 1 order of magnitude if dissolved water is present, or 2 orders of magnitude if water is able to enter the crystal structure of a hydrous mineral (e.g., Arcay et al., 2005; 2006). All hydrous minerals exist at low temperatures, therefore their reduction in viscosity is negligible because their viscosity value is already several orders of magnitude larger. Water enhances transportability of ions and reduces cohesion between grains, allowing for more efficient diffusion and dislocation creep mechanisms under constant strain (Arcay et al., 2006; Manea and Gurnis, 2007). This contrasts with the non-Newtonian dry olivine dislocation creep flow law used in the Arcay et al. (2005) model, therefore the thermal gradients produced therein would be altered in a wet wedge system. Additionally, hydration leads to a reduced solidus for mantle compositions and can lead to melt which further reduces wedge viscosity. Melt can occur within the wedge where temperatures are highest, but reduced wedge viscosity will enhance thermal advection into the now hydrous overlying plate and initiate low viscosity mechanics and melting there as well (Billen, 2008).
7. Concluding Remarks
Water should be expected in abundance within all subduction zones, and it has large effects on mantle wedge mineralogy and rheology. Water is released predominantly by dehydration of lawsonite in the MORB crust and serpentinite in the upper layer of the subducting mantle. Hydration allows growth of serpentinite and other hydrous minerals in the wedge nose, which leads to flow decoupling between the wedge and the subducting plate. Dehydration of the subducting slab leads to seismisity at greater depths (Abers et al., 2005). H2O dissolution in the anhydrous mantle wedge mineral assemblage can reduce viscosity by 1-2 orders of magnitude on top of the enhanced ability to advect heat more rapidly. Finally, excess water not taken into the wedge, which is substantial, is driven upward into the overlying plate, leading to thermal erosion and melting at its base, which will lead to volcanism and extension of the backarc.
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