Greg Hirth

Water in the oceanic upper mantle: implications for rheology, melt extraction and the evolution of the lithosphere

Abstract The influence of water on the dynamics of the oceanic upper mantle is re-evaluated based on recent experimental constraints on the solubility of water in mantle minerals and earlier experimental studies of olivine rheology. Experimental results indicate that the viscosity of olivine aggregates is reduced by a factor of ∼ 140 in the presence of water at a confining pressure of 300 MPa and that the influence of water on viscosity depends on the concentration of water in olivine. The water content of olivine in the MORB source is estimated to be810±490H10/6 Si, a value greater than the solubility of water in olivine at a confining pressure of 300 MPa (∼ 250H10/6 Si). We therefore conclude that the viscosity of the mantle in the MORB source region is 500±300 times less than that of dry olivine aggregates. The dependence of the solubility of water in olivine on pressure and water fugacity is used in conjunction with other petrological constraints to estimate the depth at which melting initiates beneath mid-ocean ridges. These calculations indicate that melting begins at a depth of ∼ 115 km, consistent with other geochemical observations. Owing to the relatively small amount of water present in the MORB source, only ∼ 1–2% melt is produced in the depth interval between the water-influenced solidus and the dry solidus. A discontinuity in mantle viscosity can develop at a depth of ∼ 60–70 km as a result of the extraction of water from olivine during the MORB melting process. In the mid-ocean ridge environment, the mantle viscosity at depths above this discontinuity may be large enough to produce lateral pressure gradients capable of focusing melt migration to the ridge axis. These observations indicate that the base of an oceanic plate is defined by a compositional rather than thermal boundary layer, or at least that the location of the thermal boundary layer is strongly influenced by a compositional boundary, and that the evolution of the oceanic upper mantle is strongly influenced by a viscosity structure that is controlled by the extraction of water from olivine at mid-ocean ridges.

Rheology of the Upper Mantle and the Mantle Wedge: A View from the Experimentalists

In this manuscript we review experimental constraints for the viscosity of the upper mantle. We first analyze experimental data to provide a critical review of flow law parameters for olivine aggregates and single crystals deformed in the diffusion creep and dislocation creep regimes under both wet and dry conditions. Using reasonable values for the physical state of the upper mantle, the viscosities predicted by extrapolation of the experimental flow laws compare well with independent estimates for the viscosity of the oceanic mantle, which is approximately 10 19 Pa s at a depth of ∼100 km. The viscosity of the mantle wedge of subduction zones could be even lower if the flux of water through it can result in olivine water contents greater than those estimated for the oceanic asthenosphere and promote the onset of melting. Calculations of the partitioning of water between hydrous melt and mantle peridotite suggest that the water content of the residue of arc melting is similar to that estimated for the asthenosphere. Thus, transport of water from the slab into the mantle wedge can continually replenish the water content of the upper mantle and facilitate the existence of a low viscosity asthenosphere.

Dislocation creep regimes in quartz aggregates

Abstract Using optical and TEM microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates, depending on the relative rates of grain boundary migration, dislocation climb and dislocation production. Within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization. At lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion-controlled dislocation climb to be an effective recovery mechanism. In this regime recovery is accommodated by strain-induced grain boundary migration recrystallization. With an increase in temperature or decrease in strain rate, the rate of dislocation climb becomes sufficiently rapid to accommodate recovery. In this regime dynamic recrystallization occurs by progressive subgrain rotation. With a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery. However, in this regime grain boundary migration is rapid, thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation. The identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers. In addition, the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred.

Experimental constraints on the dynamics of the partially molten upper mantle: deformation in the diffusion creep regime

Experiments have been conducted to investigate the effect of melt on the creep behavior of water-free olivine aggregates deformed in the dislocation creep regime. The influence of the melt phase is modest at melt fractions less than ∼0.04. However, at melt fractions > 0.04, the creep rate of melt-added samples is enhanced by more than an order of magnitude relative to melt-free aggregates. This unexpectedly large influence of melt on strain rate arises because deformation occurs by grain boundary sliding (GBS) accommodated by a dislocation creep process. Four observations support this hypothesis. (1) The strain rate enhancement observed in the dislocation creep regime can be related to the stress concentration caused by the reduction in the solid-solid grain boundary area. (2) Both melt-free and melt-added samples exhibit strain rates indicating that deformation is limited by slip on (010)[100], the easiest slip system in olivine. (3) The GBS mechanism occurs near the transition between diffusion and dislocation creep. (4) Grains in specimens deformed in the GBS regime are not significantly flattened, even after ∼50% shortening. In melt-free aggregates, a transition from the GBS mechanism to dislocation creep limited by slip on (010)[001], the hardest slip system, is observed with an increase in grain size. A transition to (010)[001] limited creep was not observed for partially molten aggregates because grain growth was inhibited by the presence of melt. The results of this study indicate that the viscosity of the upper mantle may decrease by at least an order of magnitude if the retained melt fraction exceeds 0.04 or if the onset of melting results in a reduction in grain size and a concomitant transition from (010)[001] to (010)[100] limited creep.

Strength of slightly serpentinized peridotites: Implications for the tectonics of oceanic lithosphere

We deformed cores of peridotite with ∼10%–15% lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation. The strength, the pressure dependence of strength, and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites. These results indicate that deformation is accommodated primarily by serpentine, while olivine, despite being the more abundant component, remains nominally undeformed. On the basis of these data and previous work, we determine that the transition from a “strong,” dilatant dunite rheology to a “weak,” nondilatant serpentinite rheology is not a linear function of the degree of serpentinization. Instead, an abrupt transition in strength is observed at low degrees of serpentinization. The pressure of the transition from localized to distributed deformation also decreases abruptly, from >1000 MPa to 150–350 MPa. The change in rheological behavior occurs at a serpentine content of 10%–15% or less, which corresponds to published compressional seismic velocity of >7.8–7.5 km/s at a pressure of 200 MPa. The seismic velocity of the oceanic lithosphere, particularly of that formed at slow spreading ridges, can thus provide constraints on its mechanical properties at depth. Because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite, significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis.

Mantle flow, melting, and dehydration of the Iceland mantle plume

Abstract Recent studies have shown that the extraction of water from the mantle due to partial melting beneath mid-ocean ridges may increase the viscosity of the residuum by 2–3 orders of magnitude. We examine this rheological effect on mantle flow and melting of a ridge-centered mantle plume using three-dimensional numerical models. Results indicate that the viscosity increase associated with dehydration prevents buoyancy forces from contributing significantly to plume upwelling above the dry solidus. Consequently, upwelling in the primary melting zone is driven passively by plate spreading and melt production rates are substantially lower than predicted by models that do not include the rheological effect of dehydration. Predictions of along-axis crustal thickness, bathymetric, and gravity variations are shown to be consistent with observations at Iceland and along the Mid-Atlantic Ridge. Furthermore, these predictions result from a model of a plume with relatively high excess temperature (180°C) and narrow radius (100 km) — properties that are consistent with estimates previously inferred from geochemical and seismological observations. Calculations of incompatible trace-element concentrations suggest that observed along-axis geochemical anomalies primarily reflect incompatible element heterogeneity of the plume source.

Effects of serpentinization on the lithospheric strength and the style of normal faulting at slow-spreading ridges

Abstract Rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere. Strength versus depth profiles, calculated for conditions appropriate for slow-spreading ridges, indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30%. Results from flexural fault models indicate that if serpentinization is isolated to fault zones, strain localization should be enhanced, providing an explanation for the variations in the style of normal faulting along slow-spreading ridge segments. At segment centers, where serpentinites are scarce, deformation is accommodated on closely spaced faults with small throws. At the segment ends, where serpentinites are most abundant, faults are widely spaced and have large throws.

The brittle‐plastic transition in experimentally deformed quartz aggregates

Deformation experiments have been conducted to provide constraints on the processes responsible for the brittle-plastic transition in quartz aggregates. A correlation between mechanical behavior and distinctive microstructural characteristics indicates that the brittle-plastic transition in nonporous quartzite involves at least three transitions in deformation mechanism that occur with increasing temperature and/or pressure. First there is a transition from cataclastic faulting to semibrittle faulting; microstructural observations indicate that this transition occurs due to the activation of dislocations. In addition, faulting is more stable in the semibrittle faulting regime due to the blunting of the stresses at the advancing fault tip by dislocation glide. Second, there is a transition from semibrittle faulting to semibrittle flow; this transition corresponds to a change from localized to distributed deformation. Microstructural observations indicate that microcracks nucleate in response to stress concentrations at dislocation pileups in the semibrittle flow regime. We conclude that the transition to semibrittle flow occurs when the stress intensity at crack tips is insufficient to allow propagation across grain boundaries. Third, there is a transition from semibrittle flow to dislocation creep. Microstructural observations suggest that this transition occurs as a result of an increase in grain boundary mobility with increasing temperature. In addition, microstructural observations indicate that a transition from dominantly mode I (axial) to mode II (shear) microcracking occurs with an increase in confining pressure from 0.4 to 0.8 GPa, regardless of temperature. The differential stresses supported by the experimentally deformed samples are higher than those expected under geologic conditions. However, a comparison of the experimentally produced microstructures to those reported from natural fault zones suggests that similar processes are operative in the laboratory and in the Earth. The results of this study provide further evidence to indicate that the brittle-plastic transition in the continental crust occurs over a relatively wide range of conditions.

Geophysical evidence from the MELT area for compositional controls on oceanic plates

Magnetotelluric and seismic data, collected during the MELT experiment at the southern East Pacific Rise, constrain the distribution of melt beneath this mid-ocean-ridge spreading centre and also the evolution of the oceanic lithosphere during its early cooling history. Here we focus on structures imaged at distances ∼100 to 350 km east of the ridge crest, corresponding to seafloor ages of ∼1.3 to 4.5 million years (Myr), where the seismic and electrical conductivity structure is nearly constant and independent of age. Beginning at a depth of about 60 km, we image a large increase in electrical conductivity and a change from isotropic to transversely anisotropic electrical structure, with higher conductivity in the direction of fast propagation for seismic waves. Conductive cooling models predict structure that increases in depth with age, extending to about 30 km at 4.5 Myr ago. We infer, however, that the structure of young oceanic plates is instead controlled by a decrease in water content above a depth of 60 km induced by the melting process beneath the spreading centre.

Nondilatant brittle deformation of serpentinites: Implications for Mohr-Coulomb theory and the strength of faults

We conducted deformation experiments to investigate the strength, deformation processes, and nature of the brittle-ductile transition of lizardite and antigorite serpentinites. A transition from localized to distributed deformation occurs as confining pressure increases from ∼200 to ∼400 MPa at room temperature. Deformation in both brittle (localized) and ductile (distributed) regimes is accommodated by shear microcracks, which form preferentially parallel to the (001) cleavage. Axial microcracks (mode I) are infrequently observed. Volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant, consistent with the shear-dominated microcracking. Three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow: (1) a lack of evidence for crystal plastic deformation, (2) a positive pressure dependence of the maximum differential stress, and (3) abundant evidence for brittle microcracking. The weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the (001) cleavage, combined with the low pressure dependence of strength. The transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite, rather than the more general Byerlees law. If brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity, serpentinites define an end-member style of nondilatant brittle deformation. This deformation style may result in extremely weak faults in nature, and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present.