Earth's Mantle Transition Zone Research Articles

Influence of water on the thermoelastic properties of Fe bearing ringwoodite: A first-principles study

Using first-principles density functional theory (DFT), we model the thermoelastic properties of hydrated ringwoodite (γ-(Fe, Mg)2SiO4), a potential water reservoir in the Earth's mantle transition zone (MTZ). Our calculations indicate that hydration, in general, leads to a reduction in the sound wave velocity of ringwoodite. However, increased pressure tends to suppress this reduction. For our 1.56 wt% H2O containing ringwoodite model, we find that the suppression is so large that at pressure and temperature (P-T) conditions relevant to the lower part of the MTZ the sound wave velocities for the hydrous (1.56 wt% H2O) ringwoodite become very similar to that of the anhydrous ringwoodite. However, when the water concentration is increased to 3.3 wt%, the pressure-induced suppression of the velocity reduction due to hydration is not so significant. We have given a plausible explanation for the same on the basis of the electronic structure of the hydrous ringwoodite models. We conclude that in the lower part of the MTZ, seismic wave data is sufficiently robust to detect regions of very high-water content (∼3.3 wt%). However, if the water concentration is less than 1.56 wt%, sound wave velocities may not be able to precisely detect the state of hydration of the MTZ.

3-D synthetic modelling and observations of anisotropy effects on SS precursors: implications for mantle deformation in the transition zone

SUMMARY The Earth's mantle transition zone (MTZ) plays a key role in the thermal and compositional interactions between the upper and lower mantle. Seismic anisotropy provides useful information about mantle deformation and dynamics across the MTZ. However, seismic anisotropy in the MTZ is difficult to constrain from surface wave or shear wave splitting measurements. Here, we investigate the sensitivity to anisotropy of a body wave method, SS precursors, through 3-D synthetic modelling and apply it to real data. Our study shows that the SS precursors can distinguish the anisotropy originating from three depths: shallow upper mantle (80–220 km), deep upper mantle above 410 km, and MTZ (410–660 km). Synthetic resolution tests indicate that SS precursors can resolve $\ge $3 per cent azimuthal anisotropy where data have an average signal-to-noise ratio (SNR = 7) and sufficient azimuthal coverage. To investigate regional sensitivity, we apply the stacking and inversion methods to two densely sampled areas: the Japan subduction zone and a central Pacific region around the Hawaiian hotspot. We find evidence for significant VS anisotropy (15.3 ± 9.2 per cent) with a trench-perpendicular fast direction (93° ± 5°) in the MTZ near the Japan subduction zone. We attribute the azimuthal anisotropy to the grain-scale shape-preferred orientation of basaltic materials induced by the shear deformation within the subducting slab beneath NE China. In the central Pacific study region, there is a non-detection of MTZ anisotropy, although modelling suggests the data coverage should allow us to resolve at least 3 per cent anisotropy. Therefore, the Hawaiian mantle plume has not produced detectable azimuthal anisotropy in the MTZ.

A reversed redox gradient in Earth's mantle transition zone

The Earth's mantle hosts a variety of reduced and oxidized phases, including iron-bearing alloys, diamond, and sulfide and carbonate melts. In the upper mantle, increasing pressure favors the stabilization of reduced iron-bearing phases via disproportionation of ferrous iron into ferric and metallic iron. Pressure-driven disproportionation is thought to continue into the transition zone, based on the extrapolation of experiments conducted at lower pressures. To test this hypothesis, we performed high-temperature and high-pressure experiments on basaltic and peridotitic compositions at pressures of 10 to 20 GPa, buffered at different oxygen fugacities. Under these conditions, majoritic garnet is the dominant ferric-iron bearing phase. We analyze our experimental run products for their ferric iron concentrations with EELS and Mössbauer spectroscopy. Contrary to expectations, results show that at iron saturation, ferric iron content of majorite peaks in the upper transition zone and then decreases between 500 and 650 km depth, destabilizing and resorbing reduced phases. This peak can be explained by decreases in the effective volume of ferrous minerals in transition zone assemblages. We also show that natural diamond-hosted majorite inclusions that equilibrated in the sublithospheric mantle grew from variably reduced fluids. These results are consistent with the idea that these diamonds formed during progressive reduction of an originally carbonatitic melt.

Metallic iron limits silicate hydration in Earth’s transition zone

The Earth's mantle transition zone (MTZ) is often considered an internal reservoir for water because its major minerals wadsleyite and ringwoodite can store several oceans of structural water. Whether it is a hydrous layer or an empty reservoir is still under debate. Previous studies suggested the MTZ may be saturated with iron metal. Here we show that metallic iron reacts with hydrous wadsleyite under the pressure and temperature conditions of the MTZ to form iron hydride or molecular hydrogen and silicate with less than tens of parts per million (ppm) water, implying that water enrichment is incompatible with iron saturation in the MTZ. With the current estimate of water flux to the MTZ, the iron metal preserved from early Earth could transform a significant fraction of subducted water into reduced hydrogen species, thus limiting the hydration of silicates in the bulk MTZ. Meanwhile, the MTZ would become gradually oxidized and metal depleted. As a result, water-rich region can still exist near modern active slabs where iron metal was consumed by reaction with subducted water. Heterogeneous water distribution resolves the apparent contradiction between the extreme water enrichment indicated by the occurrence of hydrous ringwoodite and ice VII in superdeep diamonds and the relatively low water content in bulk MTZ silicates inferred from electrical conductivity studies.

Evolution of Diamond-Forming Systems of the Mantle Transition Zone: Ringwoodite Peritectic Reaction (Mg,Fe)2SiO4 (Experiment at 20 GPa)

Abstract—The peritectic reaction of ringwoodite (Mg,Fe)2SiO4 and silicate–carbonate melt with formation of magnesiowustite (Fe,Mg)O, stishovite SiO2, and Mg, Na, Ca, K-carbonates is revealed by experimental study at 20 GPa of phase relations in the multicomponent diamond-forming MgO–FeO–SiO2–Na2CO3–CaCO3–K2CO3 system of the Earth mantle transition zone. An interaction of CaCO3 and SiO2 with a formation of Ca-perovskite CaSiO3 is also detected. It is shown that the peritectic reaction of ringwoodite and melt with the formation of stishovite controls physicochemically the fractional ultrabasic-basic evolution of both magmatic and diamond-forming systems of deep horizons of the transition zone up to its boundary with the Earth lower mantle.

High-pressure phase of brucite stable at Earth’s mantle transition zone and lower mantle conditions

We investigate the high-pressure phase diagram of the hydrous mineral brucite, Mg(OH)2, using structure search algorithms and ab initio simulations. We predict a high-pressure phase stable at pressure and temperature conditions found in cold subducting slabs in Earth's mantle transition zone and lower mantle. This prediction implies that brucite can play a much more important role in water transport and storage in Earth's interior than hitherto thought. The predicted high-pressure phase, stable in calculations between 20 and 35 GPa and up to 800 K, features MgO6 octahedral units arranged in the anatase-TiO2 structure. Our findings suggest that brucite will transform from a layered to a compact 3D network structure before eventual decomposition into periclase and ice. We show that the high-pressure phase has unique spectroscopic fingerprints that should allow for straightforward detection in experiments. The phase also has distinct elastic properties that might make its direct detection in the deep Earth possible with geophysical methods.

Comparative compressibility of hydrous wadsleyite and ringwoodite: Effect of H2O and implications for detecting water in the transition zone

AbstractReview of recent mineral physics literature shows consistent trends for the influence of Fe and H2O on the bulk modulus (K0) of wadsleyite and ringwoodite, the major phases of Earth's mantle transition zone (410–660 km). However, there is little consensus on the first pressure derivative, K0′ = (dK/dP)P=0, which ranges from about 4 to >5 across experimental studies and compositions. Here we demonstrate the importance of K0′ in evaluating the bulk sound velocity of the transition zone in terms of water content and provide new constraints on the effect of H2O on K0′ for wadsleyite and ringwoodite by conducting a comparative compressibility study. In the experiment, multiple crystals of hydrous Fo90 wadsleyite containing 2.0 and 0.25 wt % H2O were loaded into the same diamond anvil cell, along with hydrous ringwoodite containing 1.4 wt % H2O. By measuring their pressure‐volume evolution simultaneously up to 32 GPa, we constrain the difference in K0′ independent of the pressure scale, finding that H2O has no effect on K0′, whereas the effect of H2O on K0 is significant. The fitted K0′ values of hydrous wadsleyite (0.25 and 2.0 wt % H2O) and hydrous ringwoodite (1.4 wt % H2O) examined in this study were found to be identical within uncertainty, with K0′ ~3.7(2). New secondary‐ion mass spectrometry measurements of the H2O content of these and previously investigated wadsleyite samples shows the bulk modulus of wadsleyite is reduced by 7.0(5) GPa/wt % H2O, independent of Fe content for upper mantle compositions. Because K0′ is unaffected by H2O, the reduction of bulk sound velocity in very hydrous regions of transition zone is expected to be on the order of 1.6%, which is potentially detectible in high‐resolution, regional seismology studies.

Kinetics of the olivine–ringwoodite transformation and seismic attenuation in the Earth's mantle transition zone

In regions of the mantle where multi-phases coexist like at the olivine–wadsleyite–ringwoodite transitions, the stress induced by the seismic waves may drive a mineralogical reaction between the low to high pressure phases, a possible source of dissipation. In such a situation, the amount of attenuation critically depends on the timescale for the phase transformations to reach equilibrium relative to the period of the seismic wave. Here we report synchrotron-based measurements of the kinetics of the olivine to ringwoodite transformation at pressure-temperature conditions of the co-stability loop, for iron-rich olivine compositions. Both microstructural and kinetic data suggest that the transformation rates are controlled by growth processes after the early saturation of nucleation sites along olivine grain boundaries. Transformation-time data show an increase of reaction rates with temperature and iron content, and have been fitted to a rate equation for interface-controlled transformation: G=k0⋅T⋅exp⁡[n⋅XFa]⋅exp⁡[−(ΔHa+PV⁎)/RT]×[1−exp⁡(ΔGr/RT)], where XFa is the fayalite fraction, the exponential factor n=9.7, ln⁡k0=−9.1 ms−1. XFa−1 and ΔHa=199 kJ/mol, assuming V⁎=0 cm3/mol. Including these new kinetic results in a micro-mechanical model of a two-phase loop (Ricard et al., 2009), we predict QK−1 and Qμ−1 significantly higher than the PREM values for both body waves and normal modes. This attests that the olivine–wadsleyite transition can significantly contribute to the attenuation of the Earth's mantle transition zone.

Continental flood basalts derived from the hydrous mantle transition zone.

It has previously been postulated that the Earth's hydrous mantle transition zone may play a key role in intraplate magmatism, but no confirmatory evidence has been reported. Here we demonstrate that hydrothermally altered subducted oceanic crust was involved in generating the late Cenozoic Chifeng continental flood basalts of East Asia. This study combines oxygen isotopes with conventional geochemistry to provide evidence for an origin in the hydrous mantle transition zone. These observations lead us to propose an alternative thermochemical model, whereby slab-triggered wet upwelling produces large volumes of melt that may rise from the hydrous mantle transition zone. This model explains the lack of pre-magmatic lithospheric extension or a hotspot track and also the arc-like signatures observed in some large-scale intracontinental magmas. Deep-Earth water cycling, linked to cold subduction, slab stagnation, wet mantle upwelling and assembly/breakup of supercontinents, can potentially account for the chemical diversity of many continental flood basalts.

Rheology of Mg2GeO4 olivine and spinel harzburgite: Implications for Earth's mantle transition zone

AbstractGeophysical modeling requires an upper mantle low‐viscosity zone (LVZ); recent results suggest that the LVZ could be within the mantle transition zone (MTZ) or above it, depending on boundary conditions. The rheology of the olivine‐dominated uppermost mantle is well constrained experimentally but not the MTZ. Here we report first‐time measurement of the rheology of Mg2GeO4 spinel (analogue of ringwoodite, the dominant mineral of the lower MTZ) in the dislocation creep regime and compare it with that of Mg2GeO4 olivine. In both cases, we incorporate 15–20% pyroxene, the second most abundant mineral in the upper mantle to provide a realistic comparison of rock rheologies. Under similar conditions, spinel‐dominated material is about twice as strong as olivine‐dominated material. Our results suggest that unless H2O weakening of ringwoodite or phase‐transformation‐induced grain size reduction occurs in the lower MTZ, it will be more viscous than above the 410 km discontinuity.

Earth's deep water reservoir

A tiny sample of a mineral included in a diamond confirms predictions from high-pressure laboratory experiments that a water reservoir comparable in size to all the oceans combined is hidden deep in Earth's mantle. See Letter p.221 It is not clear just how much water resides within the solid Earth, and where it is to be found, with many indirect measurements yielding conflicting results. Here Graham Pearson and co-authors present evidence from a diamond inclusion from Juina, Brazil, for the first known terrestrial occurrence of ringwoodite — a high-pressure polymorph of olivine first identified in meteorites and thought to be a major constituent of the Earth's mantle transition zone. The water-rich nature of this inclusion provides direct evidence that, at least locally, the transition zone is hydrous, to about 1 weight per cent.

Phase transformations of Ca 3Al 2Si 3O 12 grossular garnet to the depths of the Earth's mantle transition zone

High-pressure and high-temperature phase transformations of Ca 3Al 2Si 3O 12 grossular garnet were examined at 19–26 GPa and 700–2000 K using Kawai-type multi-anvil apparatus coupled with in situ X-ray diffraction (XRD). Recovered samples were analyzed by a combination of micro-focused X-ray diffraction (μ-XRD) and transmission electron microscopy (TEM). The results show that grossular garnet gradually transforms to an Al-rich CaSiO 3 perovskite at 22–26 GPa and 1000–1400 K. The transition boundary can be expressed as P (GPa) = −0.0082 × T (K) + 33.05. When the garnet completely disappears, we observed orthorhombic CaSiO 3 perovskite with a grossular composition. At 20–24 GPa and temperatures above 1500 K the CAS phase with the composition CaAl 4Si 2O 11 appears to accommodate excess Al from the perovskite along with two distinct populations of Al-bearing CaSiO 3 perovskites, with Al content of 3.7 and 10.0 wt% Al 2O 3, respectively. The pressure and temperature of these transitions correspond to the lowermost part of the transition zone and therefore it suggests that Ca-rich aluminosilicates could provide alternative candidates to explain multiple seismic reflections near the 660 km depth discontinuity.

Forsterite to wadsleyite phase transformation under shear stress and consequences for the Earth's mantle transition zone

We have studied the phase transformation of forsterite to wadsleyite under shear stress at the Earth's transition zone pressure and temperature conditions. Two-step experiments were performed using a multi-anvil press. First, we hot pressed iron-free forsterite at 6 or 11 GPa and 1100 °C. Then we deformed a slab of this starting material using a direct simple shear assembly at 16 GPa and 1400 °C for 1, 15, 35, 40, or 60 min. Both the starting material and the deformed samples were characterized using optical and scanning electron microscopy including measurements of crystal preferred orientations (CPO) by electron back scattered diffraction (EBSD), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). The phase transformation occurs very rapidly, in less than 1 min, and metastable forsterite relics are not observed after deformation. The grain size of wadsleyite is slightly smaller than the forsterite starting material. The water contents obtained from FTIR analyses in forsterite and wadsleyite are 65–124 wt ppm H 2O and 114–736 wt ppm H 2O, respectively, which are well below water solubility at similar conditions in the presence of free water. Wadsleyite aggregates display weak CPO patterns with [1 0 0] axes concentrated at low angle to the shear direction, [0 1 0] axes perpendicular to the shear plane and nearly random [0 0 1] axes. Only a few dislocations were observed in wadsleyite with TEM. This observation is consistent with the assumption that most dislocations formed during the initial high-stress stages of these stress-relaxation experiments, were consumed in the phase transformation, probably enhancing the transformation rate. CPO patterns vary as a function of the water content: with increasing water content the density of [1 0 0] axes parallel to the shear direction decreases, and the density of [0 0 1] axes increases. Viscoplastic self-consistent modeling of CPO evolution using previously reported glide systems for wadsleyite, i.e., [1 0 0]{0 k l} and 1/2 〈1 1 1〉{1 0 1}, cannot reproduce the measured CPO, unless the [0 0 1](0 1 0) system, for which dislocations have not been observed by TEM, is also activated. In addition, wadsleyite grain growth suggests the participation of diffusion-assisted processes in deformation. Calculated anisotropies for P and S-waves using measured CPO are always below 1%. This very low anisotropy is due to both the low finite strain achieved in the experiments, which leads to weak wadsleyite CPO, and to the diluting effect of added majorite. The present experiments emphasize the importance of stress, grain size evolution and water content in the forsterite to wadsleyite phase transformation and subsequent deformation in the transition zone.

Global variations of temperature and water content in the mantle transition zone from higher mode surface waves

We performed a fully nonlinear joint inversion of surface wave overtone phase velocities for velocity structure down to a depth of 1500 km and topography of transition zone discontinuities using neural networks. The resulting models are compared with existing mantle models from linearized inversions techniques and the agreement between vertically averaged S-wave velocity models and various 660-km topography models is very good, whereas the agreement between all the 400-km topography models is very poor. The latter might be due to a broader than previously reported 400-km discontinuity where the minerals co-exist with water induced melt. From our models of transition zone thickness and S-wave velocity we estimated lateral variations of temperature and water content in Earth's mantle transition zone, linearly relating variations in S-wave velocity and transition zone thickness to variations in temperature and water content. Within the transition zone we observe relatively cold subduction zones as well as relatively warm regions beneath continents and hot spots. We find that the transition zone is wettest away from subduction zones.

Water‐induced convection in the Earth's mantle transition zone

Water enters the Earth's mantle by subduction of oceanic lithosphere. Most of this water immediately returns to the atmosphere through arc volcanism, but a part of it is expected as deep as the mantle transition zone (410–660 km depth). There, slabs can be deflected and linger before sinking into the lower mantle. Because it lowers the density and viscosity of the transition zone minerals (i.e., wadsleyite and ringwoodite), water is likely to affect the dynamics of the transition zone mantle overlying stagnant slabs. The consequences of water exchange between a floating slab and the transition zone are investigated. In particular, we focus on the possible onset of small‐scale convection despite the adverse thermal gradient (i.e., mantle is cooled from below by the slab). The competition between thermal and hydrous effects on the density and thus on the convective stability of the top layer of the slab is examined numerically, including water‐dependent density and viscosity and temperature‐dependent water solubility. For plausible initial water content in a slab (≥0.5 wt %), an episode of convection is likely to occur after a relatively short time delay (5–20 Ma) after the slab enters the transition zone. However, water induced rheological weakening is seen to be a controlling parameter for the onset time of convection. Moreover, small‐scale convection above a stagnant slab greatly enhances the rate of slab dehydration. Small‐scale convection also facilitates heating of the slab, which in itself may prolong the residence time of the slab in the transition zone.

Crystallographic preferred orientation of akimotoite and seismic anisotropy of Tonga slab

The mineral akimotoite, ilmenite-structured MgSiO(3), exists at the bottom of the Earth's mantle transition zone and within the uppermost lower mantle, especially under low-temperature conditions. Akimotoite is thought to be a major constituent of the harzburgite layer of subducting slabs, and the most anisotropic mineral in the mantle transition zone. It has been predicted that if akimotoite crystals are preferentially oriented by plastic deformation, a cold subducted slab would be extremely anisotropic. However, there have been no studies of crystallographic preferred orientations and very few reports of plastic deformation experiments for MgSiO(3) ilmenite. Here we present plastic deformation experiments on polycrystalline akimotoite, which were conducted at confining pressures of 20-22 GPa and temperatures of 1,000-1,300 degrees C. We found a change in crystallographic preferred orientation pattern of akimotoite with temperature, where the c-axis maximum parallel to the compression direction develops at high temperature, whereas the c axes are preferentially oriented parallel to the shear direction or perpendicular to the compression direction at lower temperature. The previously reported difference in compressional-wave seismic anisotropy between the northern and southern segments of the Tonga slab at depths of the mantle transition zone can conceivably be attributed to the difference in the crystallographic preferred orientation pattern of akimotoite at varying temperature within the slab.

Plastic deformation of wadsleyite and olivine at high-pressure and high-temperature using a rotational Drickamer apparatus (RDA)

Large-strain plastic deformation experiments of wadsleyite and olivine were conducted using a rotational Drickamer apparatus (RDA) up to pressure and temperature conditions corresponding to the Earth's mantle transition zone. Sintered ring-shaped (Mg,Fe) 2SiO 4 wadsleyite and olivine samples were deformed at P ∼ 16 GPa and T = 1600 and 1800 K, and P ∼ 11 GPa and T = 1800 K, respectively, with equivalent strain rate of ε ˙ E ∼ 6 × 10 − 5 s − 1 . In situ observations of deforming samples were carried out using the synchrotron radiation facility at Brookhaven National Laboratory, NSLS, X17B2. Stress was measured by X-ray diffraction at six different angles with respect to the compression axis. The stress estimated by X-ray diffraction was in good agreement with the stress estimated from dislocation density (for olivine). Strain was determined using X-ray radiographs of a strain marker (Re or Mo foil). Deformation of samples with a RDA involves both uniaxial compression and simple shear. A new formulation is developed to analyze both components to determine the rheological properties of a sample. Stress–strain curves show strain-hardening up to the equivalent strain of ɛ E ∼ 0.2 followed by the quasi-steady state deformation. Wadsleyite is found to be stronger than olivine compared at similar conditions and the creep strength of olivine at P ∼ 11 GPa is much higher than those at lower pressures.

Dry mantle transition zone inferred from the conductivity of wadsleyite and ringwoodite

The Earth's mantle transition zone could potentially store a large amount of water, as the minerals wadsleyite and ringwoodite incorporate a significant amount of water in their crystal structure. The water content in the transition zone can be estimated from the electrical conductivities of hydrous wadsleyite and ringwoodite, although such estimates depend on accurate knowledge of the two conduction mechanisms in these minerals (small polaron and proton conductions), which early studies have failed to distinguish between. Here we report the electrical conductivity of these two minerals obtained by high-pressure multi-anvil experiments. We found that the small polaron conductions of these minerals are substantially lower than previously estimated. The contributions of proton conduction are small at temperatures corresponding to the mantle transition zone and the conductivity of wadsleyite is considerably lower than that of ringwoodite for both mechanisms. The dry model mantle shows considerable conductivity jumps associated with the olivine-wadsleyite, wadsleyite-ringwoodite and post-spinel transitions. Such a dry model explains well the currently available conductivity-depth profiles obtained from geoelectromagnetic studies. We therefore conclude that there is no need to introduce a significant amount of water in the mantle transition to satisfy electrical conductivity constraints.

Experimental studies of mineralogical assemblages of metasedimentary rocks at Earth's mantle transition zone conditions

AbstractMetasedimentary rocks, a major component of the continental crust, are abundant within ultra‐high pressure (UHP) metamorphic terranes related to continental collisions. The presence of diamond, coesite, and relics of decompressed minerals in these rocks suggests that they were subducted to a depth of more than 150–250 km. Reconnaissance experiments at 9–12 GPa and 1000–1300 °C on compositions corresponding to felsic rocks from diamond‐bearing UHP terranes of Germany and Kazakhstan show that at higher pressures they consist of majoritic garnet, Al‐Na‐rich clinopyroxene, stishovite, solid solution of KAlSi3O8‐NaAlSi3O8 hollandite, topaz‐OH, and TiO2 with α‐PbO2 structure. Comparison of our data with experiments conducted by others at similar P–T conditions shows differences, which are due to variations in bulk chemistry and the type of starting material (gel, oxides, minerals). These differences may affect correct establishment of the ‘point of no return’ of subducted continental lithologies. This paper discusses the implication of the experimental data with regard to naturally existing UHP metamorphic rocks and their significance for our understanding of the deep subduction of continental material.

Slab dehydration in the Earth's mantle transition zone

Because of the high water solubility in wadsleyite and ringwoodite, the mantle transition zone is possibly a large water reservoir. The potentially high water content of the Earth's mantle transition zone is a key element of several global mantle dynamic models. Nevertheless, to keep the transition zone relatively wet, the tendency for convection to distribute water over the entire mantle has to be offset by other mechanisms. One such mechanism could be linked to the dehydration of the slab. Studies of slab dehydration mainly focus on hydrous minerals phase diagrams that provide the depth of water exsolution. In this study, we investigate the water diffusion process that occurs prior to such exsolution. The diffusion process is controlled by both chemical gradients and thermal gradients via the temperature dependence of water solubility. We have addressed how this diffusion phenomenon influences slab water transport into the transition zone by developing a theoretical model of coupled thermal and chemical diffusion in a flat lying slab stalled in the transition zone. Model solutions demonstrate that even if intrinsic water solubility is decreasing with increasing temperature (which normally would act to impede a chemical diffusion from the slab), the water concentration profile adjusts to reduce the adverse effects of temperature-dependent solubility. The self-adjustment entails the concentration of water reaching a local maximum below the top edge of the stagnant slab; under specific conditions, this concentration maximum can reach the solubility limit and thus exsolve fluid (i.e. create of hydrous melt or aqueous silicate fluid). Consequently, model water flux computations are relatively independent of solubility variation and indicate that much of the water flux introduced by relatively well hydrated slabs is taken up into the transition zone by diffusion.