How fast is mantle convection

This view of a viscously two-layer mantle has been the focus of modelling and explanations for regional geochemical variations in oceanic crust, which is formed of mid ocean-ridge basalts MORB by partial melting in the upper mantle 5.

Many studies have focussed on the chemical origins of these anomalies e. Consensus on a unified model has remained elusive due to too many unquantifiable variables such as amount of material recycled, which part of the lithosphere recycled 9 , 10 , and the timing of incorporation into the convecting mantle 11 , As no consistent model can account for all lines of isotopic evidence, agreement also cannot be reached on whether the anomalies are perpetually produced 16 , are generated during supercontinent break-up 10 , 13 or have existed since the start of plate tectonics 8.

Focusing on the depleted mantle-MORB evidence for isotopic differences between Indian- and Pacific Ocean MORB, the Australian-Antarctic Discordance is clear evidence of a sharp transition between the two, leading to suggestions of a mantle convection boundary 17 , potentially separated by remnant slab material This evidence raises fundamental questions about how heterogeneities might be preserved within the upper mantle and counters any model of ubiquitous mixing throughout the upper mantle, suggesting that differences with height through the mantle radial layering are not the only form of long-duration mantle segregation.

This paper sets out to examine how long-term segregation can persist within the mantle, particularly regarding the maintenance of discrete large-scale reservoirs within the upper mantle. Hitherto, the best constraints derive from analysis of particle tracing in mantle circulation models that include present-day plate boundaries e.

TERRA solves equations for velocity, temperature and pressure, conserving mass, momentum, and energy 27 with an Earth-like convective vigour see Methods. Significantly, 4 we validate the models using long-lived radiogenic isotopes sampled from known MORB ophiolites across Europe and Asia. Our models using these two different plate history reconstructions, each explore how different regions of the mantle might have circulated as a result of plate motion history no density or chemical parameters are applied in the models for different mantle lithologies, therefore only spatial distribution assessed.

A 30x viscosity contrast between the upper and lower mantle was used but, otherwise, material was allowed to convect freely in response to plate motions. The results are used to examine material transfer from the upper mantle to the lower mantle and also its return. In particular we consider the behaviour of lateral convective flow within the mantle, as this affects how large-scale chemical heterogeneities might have persisted within the upper mantle during convective circulation.

Starting positions and end results for particles tracked for two different plate motion histories beginning at two starting depths. C and D Show the same model conditions, but for particles started in an intra-Pacific geographic locality.

Particles started in both upper and lower mantle beneath the Tethys-Indian Ocean circulate to the S. Atlantic but not the Pacific, in both plate motion histories. Those particles started beneath the Pacific spread towards Pacific subduction zones, but do not spread laterally to the Indian Ocean. Projections were selected to show all particles present on each layer.

Finally, to validate the significance and longevity of discrete convective regimes, that appear to suggest minimal interaction, we analysed Hf-Nd isotopes for depleted upper mantle compositions MORBs that span the duration of the model runs.

Some doubt on the significance of the original works comes from evidence that Pb is highly mobile in the marine and subduction environment Here, we have analysed Hf-Nd isotopes that are considered to better resist modification during low-temperature sea floor alteration and fluid mobility during subduction, and therefore preserve the composition of the basalt at the time of its formation.

We analysed low-grade metamorphosed MORBs from 1 six ophiolites across Europe and Asia 18 samples , 2 two ophiolites and three oceanic sites of old Pacific basalts to assess Pacific mantle compositions 12 samples , and 3 one example of old Atlantic MORB that formed at the margins of the Tethyan-Indian Ocean 2 samples; Fig. See Supplementary Information Part 2 for sample details. The final configuration of all the new mantle circulation models were similar to each other, regardless of the plate motion history used, or duration.

In each case, particles from Tethyan-Indian and intra-Pacific starting positions remained within the same hemisphere in which they started Fig. From each initial starting position, particles circulated to all depths of the mantle. Particles introduced beneath the intra-Pacific region in both the LBR and Seton models took longer to travel horizontally across the upper mantle before descending to mid- and lower mantle depths: this was a result of the larger size of the Pacific plate, and therefore farther distances to a subduction zone.

Supplementary Information Figs S6 and S7. In the case of the LBR plate history, with particles introduced to the Tethys-Indian Ocean region, particles initially descended to cluster close to the CMB, before travelling upwards in thin sheets to the upper mantle, still beneath the Tethys-Indian Ocean Supplementary Information Fig.

In the Seton model, a greater proportion of the particles introduced directly to lowermost mantle beneath the Tethys-Indian Ocean , moved up, away from the CMB in a shorter time-frame Supplementary Information Fig. The mantle circulation models appear to show that the mantle convects from top to bottom as a whole, but laterally within two discrete domains.

There appears to be minimal mixing or transfer of material laterally between the hemispherical divisions, and convection within the upper mantle is as much constrained by the division as the lower mantle. But is this really what we see from robust geochemical isotope data in the natural system? See Fig. The Ligurian ophiolite is thought to represent crust that formed in a subsidiary basin at the western-most limit of Tethys, at a time when the Tethys Ocean to the east was closing.

Discussion of this will not be within the remit of this paper. Rare, anomalous Indian-type compositions have been noted previously in the Pacific 36 and similarly the other way around with anomalous occurrences of Pacific compositions within the Indian Ocean crust, e.

Masirah 13 , and may represent minor heterogeneities within the upper mantle due to incomplete mixing. This will be discussed further below. As in the models, there has been an overwhelming dominance of two large hemispheric convection cells, and only small, localised domains e.

Liguria where there has been some partial mixing or heterogeneity. We have, in addition for the first time used robust Hf-Nd isotope pairs, and this suggests that previous distinctions based on Pb isotopes 13 are valid. Determining the origin of the various MORB signatures is beyond the scope of the present study. A stand-out result of the mantle circulation models is that, despite a level of variation between models, all the model runs showed that material in the upper mantle did not spread and mix ubiquitously, neither through descent to the lowermost depths of the mantle nor on its subsequent return flow to more shallow depths.

Furthermore, this overall pattern was not diminished for the two alternative current plate motion history constructions, and their different duration times. The Hf isotope data, which is not affected by low-moderate temperature subduction-alteration, is entirely consistent with the mantle circulation models and demonstrates that the mantle below the present Indian Ocean could have maintained a distinct difference from that beneath the Pacific during plate re-organisations.

We designed the study to extend the significance of the results, by incorporating tracer particles initially positioned at mid- and lowermost mantle levels, as well as upper mantle see Supplementary Information Part 1.

The general results of our models show a dominantly up-down convective pattern with upper mantle ending up in the lower mantle — geographically below where it started — and lower mantle ending up in the upper mantle — geographically above where it started. It seems that the overall pattern of convection adopted by these models is approximately degree-2, in response to the nature of the imposed plate motions.

Prior to super-continent break-up subduction around the edge of Pangaea would have provided an effective curtain to full mantle mixing, only enhancing the potential for separation of two regions within the mantle. However, it is hoped that mantle circulation models, utilising longer plate motion histories being developed, will in the future be able to test this.

In all the Tethys-Indian Ocean models, regardless of the depth of introduction of particles or the plate motion history, particles spread towards the central or southern parts of the Atlantic Fig.

Despite the modelled spread of tracer particles into the southern Atlantic, no particles travelled laterally to the Pacific region, nor, interestingly, into the northern Atlantic.

We noted that the particles initially placed in the intra-Pacific region generally did not leave the Pacific area using either plate motion history reconstruction; Supplementary Information Figs S6 , S7 , S10 , S11 , S14 , S However, on a more detailed scale the models reveal localised, complex interplay between circulating mantle and subducting slab. The effect is consistent with Indian Ocean MORB compositions that are recorded in parts of the present western Pacific basins 29 , which could result from alteration of slab vergence beneath the North Fiji and Lau basins.

At the scale of the models, it is difficult to assess whether a small number of particles take subsidiary, capricious excursions see Supplementary Information Fig. S15 for example departing from the dominant convection patterns. Such behaviour was noted, for example, in other 3D numerical models Such convective behaviour is supported by geochemical data, in the form of isolated occurrences of Indian Ocean-type MORB in areas away from the Indian Ocean, such as the Lomonosov ridge in the Arctic 36 and in the Northwest Pacific Overall, this study reveals that lateral, geographic heterogeneities are as important as radial depth layering in preserving and developing chemical heterogeneities within the mantle.

The models and chemical evidence combined demonstrate that lateral segregations within the mantle, and a dominant large-scale convection pattern, can persist and match the long-term position of sinking plates; this is consistent with early speculations of an interplay between isotope anomalies and subduction patterns 7.

We propose that this process could have been a dominant feature of mantle convection ever since competent plates were able to descend deep into the mantle. The conclusion that the mantle preserves heterogeneities through deep time and that it achieves this by quasi-steady planetary-scale convection, is robust in that it is consistent with evidence from elsewhere that use different assumptions, approaches, and geometric set-ups that differ from Earth.

Long-term physical isolation of upper mantle regions by curtains of descending slab material provides, for the first time, a mechanism to explain large-scale chemical heterogeneities e. The results confirm a persistent global disparity in upper mantle composition that maps onto large hemispheric cells. The modelled pattern of convection provides a mechanism by which large regions of the mantle could evolve different isotope compositions, which would otherwise homogenise during convective mixing.

Furthermore, for the first time, this work demonstrates a process that can account for the observed e. Regardless of the specific plate motion histories used in the modelling, our results suggest that a dominantly up-down motion of convection, restricted laterally by downwelling slabs, could have been a long-lived process resulting in large-scale lateral isolation of the mantle.

This process is potentially as important as radial layering, within the mantle, at influencing long-term chemical evolution and could have led to different histories of isotope development, even in the upper mantle. Convection restricted by physical barriers of downwelling slabs could have existed throughout the Phanerozoic and potentially since the onset of modern plate tectonics. A prescribed initial temperature condition was used to calculate initial pressure and velocity fields. The upper-lower mantle viscosity ratio is broadly consistent with estimates from geophysical studies e.

Passive tracer particles were implemented using a Lagrangian particle tracking method where particles were advected using a second order Runge-Kutta method; this is the order warranted by the linear shape functions of TERRA 49 , Model input parameters, which were not varied, are given in Supplementary Information Table S1.

Traditionally, mantle models are permitted to run for sufficient time before analysis, to erase the influence of the initial conditions. In a model with constant heat inputs, this is beyond the point at which heat fluxes show no long-term trend. We output the 3D temperature and particle distribution fields at the end of each plate stage.

Particles were counted for each of the radial model layers through the mantle profile and determined as percentage of volume adjusted for decreasing cell volume with depth Supplementary Information Figs S4 to S15 , parts D. Samples were selected from the least altered available material. Following strict criteria Supplementary Information Part 2 , some lava or dyke samples from each of the selected ophiolites were found to preserve MORB sensu stricto chemistry, and therefore reflects shallow asthenospheric mantle melting without the involvement of arc fluids, e.

Samples were carefully checked using a binocular microscope for any residual phases following dissolution, prior to elemental separation. Minimum uncertainties are derived from external precision of standard measurements for JMC Samples were run in several sessions over three years. To allow for inter-laboratory bias, results are quoted relative to a preferred value for JMC of 0. Repeat analyses of BCR2 run with the samples gave a mean value of 0. Neodymium was run in multidynamic mode, as the metal species on double Re filaments using a Triton multi-collector mass spectrometer.

Solution standards run with the samples over the three-year duration of the project gave results as follows: La Jolla, 0. Mantle mixing: the generation, preservation, and destruction of chemical heterogeneity. Annual Reviews of Earth and Planetary Science 30 , —, doi: Andersen, M. The terrestrial uranium isotope cycle. Nature , —, doi: Mitrovica, J. A new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data.

Earth and Planetary Science Letters , —, doi: But the cause of the unexpected mismatch between the new observations and previous models is difficult to unravel. Up-and-down movement of the surface can have massive global impacts. In the future, Hoggard says he plans to expand this study of present conditions back into the past.

All rights reserved. Temperature-dependent viscosity, a semi-rigid lithosphere held together by lateral compressive stresses and buoyant continents and thick crust regions completely change this. Gravitational forces on cooling plates cause them to move. Dissipation takes place in and between the plates, causing them to self-organize and to organize the underlying weaker mantle.

Pressure decreases the expansivity and Rayleigh number so it is difficult to generate buoyancy or vigorous small-scale convection at the base of the mantle. In addition, heat flow across the CMB is about an order of magnitude less than at the surface so it takes a long time to build up buoyancy. In contrast to the upper TBL, which involves frequent ejections of narrow dense slabs into the interior, the lower TBL is sluggish and does not play an active role in mantle convection.

CMB upwellings are expected to be thousands of kilometers in extent and embedded in high-viscosity mantle. Lithospheric architecture and slabs set up lateral temperature gradients that drive small-scale convection. This lateral temperature difference sets up convection. Shallow upwellings resulting from this mechanism are intrinsically three-dimensional and plume-like. Materials usually expand when heated. This causes them to rise when embedded in compositionally similar material.

Pressure drives atoms closer together and suppresses the ability of high temperatures to create buoyancy. This is unimportant in the laboratory but it also means that laboratory simulations of mantle convection, including theinjection experiments used to generate plumes, are not relevant to the mantle. Unfortunately, computer simulations are generally used to confirm the laboratory results and, when applied to the mantle, also ignore the effects of pressure on material properties.

In fact, the effects of temperature are also generally ignored except the effect of temperature on density. This is called the Boussinesq approximation. This works fine in the laboratory, but does not apply to the mantle.

Lateral variations in temperature are what drives thermal convection. Lateral variations in pressure are generally unimportant since the pressure in the material outside of the rising element is about the same as inside.

But the increase in pressure with depth means that viscosity, thermal conductivity and expansivity change, making it harder for material to convect. The system responds by increasing the dimensions of the thermal instabilities in order to maintain buoyancy and to overcome viscous resistance.

A closed or isolated system at equilibrium returns to equilibrium if perturbed. In a tank of fluid, or the mantle, with a cold surface and a hot bottom, heat will flow by conduction alone unless the temperature gradient gets too large.

The stable conduction situation is called equilibrium. A far-from-equilibrium dissipative system, provided with a steady source of energy or matter from the outside world, can organize itself via its own dissipation.

It is sensitive to small internal fluctuations and prone to massive reorganization. The fluid in a pot heated on a stove evolves rapidly through a series of transitions with complex pattern formation even if the heating is spatially uniform and slowly varying in time. The stove is the outside source of energy and the fluid provides the buoyancy and the dissipation via viscosity. Far-from-equilibrium self-organization and reorganization require an open system, a large, steady, outside source of matter or energy, non-linear interconnectedness of system components, dissipation, and a mechanism for exporting entropy products.

Under these conditions the system responds as a whole, and in such a way as to minimize entropy production dissipation. Certain fluctuations are amplified and stabilized by exchange of energy with the outside world.

Structures appear which have different time and spatial scales than the energy input. Similar considerations apply to a fluid cooled from above. The cold surface layer organizes the flow and drives the convection. If the fluid has a strongly temperature-dependent viscosity, or if buoyant things are floating on top, only part of the surface layer is able to circulate into the interior. If the upper mantle is near or above the melting point there are other sources of buoyancy and dissipation and the possibility of lubrication.

Volcanic chains can form as a result of buoyant dikes breaking through the surface layer at regions of relative extension. Melts are predicted to pond beneath regions of lateral compression. The idea that a deep TBL may be responsible for narrow structures such as volcanic chains is based on heating-from-below experiments and calculations, or injection experiments.

The effects of pressure on thermal properties are not considered the Boussinesq approximation. In the Earth the effects of temperature and pressure on convection parameters cannot be ignored and these must be determined as part of the solution in a self-consistent way. Furthermore, it is the cooling of the mantle that controls the rate of heat loss from the core. The core does not play an active role in mantle convection. The magnitude of the bottom TBL depends on the cooling rate of the mantle, the pressure and temperature dependence of the physical properties and the radioactivity of the deep mantle.

The local Rayleigh number of the deep mantle is very low. Chemical boundaries are hard to detect by seismic techniques but the evidence favors one such boundary near 1, km. The seismic boundary at an average depth of km is primarily due to a solid-solid phase change, in mantle minerals, with a negative Clapeyron slope.

Slabs can be halted at such a boundary but if they accumulate they can punch through. Observations also suggest the presence of one or more chemically distinct layers at the base of the mantle that may extend, in places, as high as 1, km from the CMB. This region exhibits large-scale sluggish behavior as appropriate for high Prandt number, low Rayleigh number convection.

This kind of chemical and gravitational stratification resolves various geodynamic and geochemical paradoxes and is more consistent with petrology and mineral physics than the one- and two-layer models that are most discussed in the literature. It is generally believed that geochemical observations support models of layered mantle convection and the popular geochemical box models. However, it is chemical heterogeneity that is demonstrated by the data, which cannot determine the depth, size or configurations of the inhomogeneities.

Changes in iron content can give larger variations. Such density differences have been thought too small to stabilize stratification. However, when pressure is taken into account such slightly denser layers become trapped, although their effect on seismic velocities can be slight. Multilayered convection simulations are avoided for the reason that modelers think there is no evidence for it, and the calculations are difficult and time consuming. This is a major hindrance for advances in mantle dynamics and geochemistry and an opportunity for future research.

A chemically stratified mantle with variable-depth chemical boundaries near 1, and 2, km and a lower mantle depleted in radioactive elements appears to satisfy available geochemical and geophysical constraints. Spherical harmonic power spectrum of velocity throughout the mantle from Gu et al. The Earth is clearly characterised by strong heterogeneity in the longer-wavelength components in the lithosphere and upper mantle and the lowermost mantle, with little heterogeneity in between.

The three major regions of the mantle are evident. There may also be chemically distinct layers at the very base of the mantle D" and a buoyant, refractory layer at the top of the mantle the perisphere. The creation of new plates at ridges, the subsequent cooling of these plates, and their ultimate subduction at trenches introduce forces which drive and break up the plates.

They also introduce chemical and thermal inhomogeneities into the mantle. Plate forces such as ridge push, slab pull, and trench suction are basically gravitational forces generated by cooling plates. They are resisted by transform fault, bending and tearing resistance, mantle viscosity and bottom drag.

If convection currents dragged plates around, the bottom drag force would be the most important.