Rhodri Davies | The Australian National University (original) (raw)

Papers by Rhodri Davies

We evaluate the spatial and temporal evolution of Earth's long-wavelength surface dynamic topogra... more We evaluate the spatial and temporal evolution of Earth's long-wavelength surface dynamic topography since the Jurassic using a series of high-resolution global mantle convection models. These models are Earth-like in terms of convective vigour, thermal structure, surface heat-flux and the geographic distribution of heterogeneity. The models generate a degree-2-dominated spectrum of dynamic topography with negative amplitudes above subducted slabs (i.e. circum-Pacific regions and southern Eurasia) and positive amplitudes elsewhere (i.e. Africa, northwestern Eurasia and the central Pacific). Model predictions are compared with published observations and subsidence patterns from well data, both globally and for the Australian and southern African regions. We find that our models reproduce the long-wavelength component of these observations, although observed smaller-scale variations are not reproduced. We subsequently define " geodynamic rules " for how different surface tectonic settings are affected by mantle processes: (i) locations in the vicinity of a subduction zone show large negative dynamic topography amplitudes; (ii) regions far away from convergent margins feature long-term positive dynamic topography; and (iii) rapid variations in dynamic support occur along the margins of overriding plates (e.g. the western US) and at points located on a plate that rapidly approaches a subduction zone (e.g. India and the Arabia Peninsula). Our models provide a predictive quantitative framework linking mantle convection with plate tectonics and sedimentary basin evolution, thus improving our understanding of how subduction and mantle convection affect the spatio-temporal evolution of basin architecture.

Numerical simulations of thermal convection in the Earth's mantle often employ a pseudoplas-tic r... more Numerical simulations of thermal convection in the Earth's mantle often employ a pseudoplas-tic rheology in order to mimic the plate-like behavior of the lithosphere. Yet the benchmark tests available in the literature are largely based on simple linear rheologies in which the viscosity is either assumed to be constant or weakly dependent on temperature. Here we present a suite of simple tests based on nonlinear rheologies featuring temperature, pressure, and strain rate-dependent viscosity. Eleven different codes based on the finite volume, finite element, or spectral methods have been used to run five benchmark cases leading to stagnant lid, mobile lid, and periodic convection in a 2-D square box. For two of these cases, we also show resolution tests from all contributing codes. In addition, we present a bifurcation analysis, describing the transition from a mobile lid regime to a periodic regime, and from a periodic regime to a stagnant lid regime, as a function of the yield stress. At a resolution of around 100 cells or elements in both vertical and horizontal directions, all codes reproduce the required diagnostic quantities with a discrepancy of at most $3% in the presence of both linear and nonlinear rheologies. Furthermore, they consistently predict the critical value of the yield stress at which the transition between different regimes occurs. As the most recent mantle convection codes can handle a number of different geometries within a single solution framework, this benchmark will also prove useful when validating viscoplastic thermal convection simulations in such geometries.

Komatiites are products of decompression melting of mantle so hot that they are almost exclusivel... more Komatiites are products of decompression melting of mantle so hot that they are almost exclusively restricted to the Archean. The high degree of partial melting (F) and pressure (P) required for their generation facilitates comparison between the magma composition and its mantle source. To investigate compositional variations in Archean komatiites, a global selection of 38 Archean komatiites spanning five cratons (Kaapvaal, Zimbabwe, Yilgarn, Pilbara, Superior) were analysed for their major and trace element contents. Included are the Aluminium-Depleted (ADK, Barberton-Type) and Aluminium-Undepleted (AUK, Munro-Type) petrogenetic types that have been equated with high P/moderate F and moderate P/high F, respectively, on the basis of their Al/Ti and Gd/Yb ratios. Following calculation of the primary magma composition of each suite, we show that the absolute Al content at a specified MgO proves a more sensitive indicator of P than either of the above two ratios and hence we introduce a new classification using Al. The Mg# is a reliable proxy for F, independent of the two endmember melting styles, fractional and batch. We demonstrate that most komatiites form by batch melting, ceding to fractional melting with decreasing pressure as the density contrast between the liquid and solid grows. The Munro AUKs are the only suite to show evidence of fractional melting, with melt extraction occurring at the lowest F and P, %25% melting at 5 GPa (mantle potential temperature, T P ¼ 1750 C) whereas the ADKs of Barberton segregated at the highest F and P (%40%, 9 GPa, T P ¼ 1950 C). The petrogenetic type is a combination of P and F, where, at a given pressure, higher F will produce AUKs over ADKs as majorite is consumed in the source. Through numerical simulations, it is shown that both types can occur within the same mantle plume, with ADKs forming in its cooler, distal fringes whereas AUKs occur along its axis. Furthermore, and contrary to previous views, there is no temporal distinction between the two komatiite types, with both AUKs and ADKs occurring throughout the Archean. By contrast, younger, 2Á7 Ga komatiites tend to have sources that are more depleted than those of older, 3Á5 Ga komatiites. Komatiites are invaluable records of the mantle's chemical and physical evolution during the Archean.

Arc volcanism, volatile cycling, mineralization, and continental crust formation are likely regul... more Arc volcanism, volatile cycling, mineralization, and continental crust formation are likely regulated by the mantle wedge's flow regime and thermal structure. Wedge flow is often assumed to follow a regular corner-flow pattern. However, studies that incorporate a hydrated rheology and thermal buoyancy predict internal small-scale-convection (SSC). Here, we systematically explore mantle-wedge dynamics in 3-D simulations. We find that longitudinal ''Richter-rolls'' of SSC (with trench-perpendicular axes) commonly occur if wedge hydration reduces viscosities to 1 Á 10 19 Pa s, although transient transverse rolls (with trench-parallel axes) can dominate at viscosities of 5Aˊ101821Aˊ1019Pas.Rollsbelowthearcandbackarcdiffer.Subarcrollshavesimilartrench−parallelandtrench−perpendiculardimensionsof100–150kmandevolveona1–5Myrtimescale.Subback−arcinstabilities,ontheotherhand,coalesceintoelongatedsheets,usuallywithapreferentialtrench−perpendicularalignment,displayawavelengthof150–400kmandvaryona5–10Myrtimescale.Themodulatinginfluenceofsubback−arcridgesonthesubarcsystemincreaseswithstrongerwedgehydration,highersubductionvelocity,andthickerupperplates.Wefindthattrench−parallelaveragesofwedgevelocitiesandtemperatureareconsistentwiththosepredictedin2−Dmodels.However,lithosphericthinningthroughSSCissomewhatenhancedin3−D,thusexpandinghydrousmeltingregionsandshiftingdehydrationboundaries.SubarcRichter−rollsgeneratetime−dependenttrench−paralleltemperaturevariationsofupto5 Á 10 18 21 Á 10 19 Pa s. Rolls below the arc and back arc differ. Subarc rolls have similar trench-parallel and trench-perpendicular dimensions of 100–150 km and evolve on a 1–5 Myr timescale. Subback-arc instabilities, on the other hand, coalesce into elongated sheets, usually with a preferential trench-perpendicular alignment, display a wavelength of 150–400 km and vary on a 5–10 Myr time scale. The modulating influence of subback-arc ridges on the subarc system increases with stronger wedge hydration, higher subduction velocity, and thicker upper plates. We find that trench-parallel averages of wedge velocities and temperature are consistent with those predicted in 2-D models. However, lithospheric thinning through SSC is somewhat enhanced in 3-D, thus expanding hydrous melting regions and shifting dehydration boundaries. Subarc Richter-rolls generate time-dependent trench-parallel temperature variations of up to 5Aˊ101821Aˊ1019Pas.Rollsbelowthearcandbackarcdiffer.Subarcrollshavesimilartrench−parallelandtrench−perpendiculardimensionsof100–150kmandevolveona1–5Myrtimescale.Subback−arcinstabilities,ontheotherhand,coalesceintoelongatedsheets,usuallywithapreferentialtrench−perpendicularalignment,displayawavelengthof150–400kmandvaryona5–10Myrtimescale.Themodulatinginfluenceofsubback−arcridgesonthesubarcsystemincreaseswithstrongerwedgehydration,highersubductionvelocity,andthickerupperplates.Wefindthattrench−parallelaveragesofwedgevelocitiesandtemperatureareconsistentwiththosepredictedin2−Dmodels.However,lithosphericthinningthroughSSCissomewhatenhancedin3−D,thusexpandinghydrousmeltingregionsandshiftingdehydrationboundaries.SubarcRichter−rollsgeneratetime−dependenttrench−paralleltemperaturevariationsofupto150 K, which exceed the transient 50–100 K variations predicted in 2-D and may contribute to arc-volcano spacing and the variable seismic velocity structures imaged beneath some arcs.

Keywords: lithosphere–asthenosphere boundary seismic anisotropy LPO development The depth of the ... more Keywords: lithosphere–asthenosphere boundary seismic anisotropy LPO development The depth of the oceanic lithosphere–asthenosphere boundary (LAB), as inferred from shear wave velocities, increases with lithospheric age, in agreement with models of cooling oceanic lithosphere. On the other hand, the distribution of radial anisotropy under oceanic plates is almost age-independent. In particular, radial anisotropy shows a maximum positive gradient at a depth of ∼70 km, which, if used as a proxy, indicates an age-independent LAB depth. These contrasting observations have fueled a controversy on the seismological signature of the LAB. To better understand the discrepancy between these observations, we model the development of lattice preferred orientation (LPO) in upper mantle crystal aggregates and predict the seismic anisotropy produced by plate-driven mid-ocean ridge flows. The model accounts for the progressive cooling of the lithosphere with age and can incorporate both diffusion and dislocation creep deformation mechanisms. We find that an age-independent distribution of radial anisotropy is the natural consequence of these simple flows. The depth and strength of anisotropy is further controlled by the deformation regime – dislocation or diffusion creep – experienced by crystals during their ascent towards, and subsequent motion away from, the ridge axis. Comparison to surface wave tomography models yield constraints on rheological parameters such as the activation volume. Although not excluded, additional mechanisms proposed to explain some geophysical signatures of the LAB, such as the presence of partial melt or changes in water content, are not required to explain the radial anisotropy proxy. Our prediction, that the age-independent radial anisotropy proxy marks the transition to flow-induced asthenospheric anisotropy, provides a way to reconcile thermal, mechanical and seismological views of the LAB.

Subduction zone mantle wedge temperatures impact plate interaction, melt generation, and chemical... more Subduction zone mantle wedge temperatures impact plate interaction, melt generation, and chemical recycling. However, it has been challenging to reconcile geophysical and geochemical constraints on wedge thermal structure. Here we chemically determine the equilibration pressures and temperatures of primitive arc lavas from worldwide intraoceanic subduction zones and compare them to kinematically driven thermal wedge models. We find that equilibration pressures are typically located in the lithosphere, starting just below the Moho, and spanning a wide depth range of 25 km. Equilibration temperatures are high for these depths, averaging 13008C. We test for correlations with subduction parameters and find that equilibration pressures correlate with upper plate age, indicating overriding lithosphere thickness plays a role in magma equilibration. We suggest that most, if not all, thermobarometric pressure and temperature conditions reflect magmatic reequilibration at a mechanical boundary, rather than reflecting the conditions of major melt generation. The magma reequilibration conditions are difficult to reconcile, to a first order, with any of the conditions predicted by our dynamic models, with the exception of subduction zones with very young, thin upper plates. For most zones, a mechanism for substantially thinning the overriding plate is required. Most likely thinning is localized below the arc, as kinematic thinning above the wedge corner would lead to a hot fore arc, incompatible with fore-arc surface heat flow and seismic properties. Localized subarc thermal erosion is consistent with seismic imaging and exhumed arc structures. Furthermore, such thermal erosion can serve as a weakness zone and affect subsequent plate evolution.

0 0 M o n t h 2 0 1 7 | V o L 0 0 0 | n A t U R E | 1 Mantle plumes are buoyant upwellings of hot... more 0 0 M o n t h 2 0 1 7 | V o L 0 0 0 | n A t U R E | 1 Mantle plumes are buoyant upwellings of hot rock that transport heat from Earth's core to its surface, generating anomalous regions of volcanism that are not directly associated with plate tectonic processes. The best-studied example is the Hawaiian–Emperor chain, but the emergence of two sub-parallel volcanic tracks along this chain 1 , Loa and Kea, and the systematic geochemical differences between them 2,3 have remained unexplained. Here we argue that the emergence of these tracks coincides with the appearance of other double volcanic tracks on the Pacific plate and a recent azimuthal change in the motion of the plate. We propose a three-part model that explains the evolution of Hawaiian double-track volcanism: first, mantle flow beneath the rapidly moving Pacific plate strongly tilts the Hawaiian plume and leads to lateral separation between high-and low-pressure melt source regions; second, the recent azimuthal change in Pacific plate motion exposes high-and low-pressure melt products as geographically distinct volcanoes, explaining the simultaneous emergence of double-track volcanism across the Pacific; and finally, secondary pyroxenite, which is formed as eclogite melt reacts with peridotite 4 , dominates the low-pressure melt region beneath Loa-track volcanism, yielding the systematic geochemical differences observed between Loa-and Kea-type lavas 3,5–9. Our results imply that the formation of double-track volcanism is transitory and can be used to identify and place temporal bounds on plate-motion changes.

Keywords: mantle plumes hotspot volcanism LLSVPs Hawaii mantle structure and dynamics thermochemi... more Keywords: mantle plumes hotspot volcanism LLSVPs Hawaii mantle structure and dynamics thermochemical convection It has been proposed that the spatial variations recorded in the geochemistry of hotspot lavas, such as the bilateral asymmetry recorded at Hawaii, can be directly mapped as the heterogeneous structure and composition of their deep-mantle source. This would imply that source-region heterogeneities are transported into, and preserved within, a plume conduit, as the plume rises from the deep-mantle to Earth's surface. Previous laboratory and numerical studies, which neglect density and rheological variations between different chemical components, support this view. However, in this paper, we demonstrate that this interpretation cannot be extended to distinct chemical domains that differ from surrounding mantle in their density and viscosity. By numerically simulating thermo-chemical mantle plumes across a broad parameter space, in 2-D and 3-D, we identify two conduit structures: (i) bilaterally asymmetric conduits, which occur exclusively for cases where the chemical effect on buoyancy is negligible, in which the spatial distribution of deep-mantle heterogeneities is preserved during plume ascent; and (ii) concentric conduits, which occur for all other cases, with dense material preferentially sampled within the conduit's centre. In the latter regime, the spatial distribution of geochemical domains in the lowermost mantle is not preserved during plume ascent. Our results imply that the heterogeneous structure and composition of Earth's lowermost mantle can only be mapped from geochemical observations at Earth's surface if chemical heterogeneity is a passive component of lowermost mantle dynamics (i.e. its effect on density is outweighed by, or is secondary to, the effect of temperature). The implications of our results for: (i) why oceanic crust should be the prevalent component of ocean island basalts; and (ii) how we interpret the geochemical evolution of Earth's deep-mantle are also discussed.

Earth and Planetary Science Letters, 2012

Two large low-shear-velocity provinces (LLSVPs) in the deep mantle beneath Africa and the Pacific... more Two large low-shear-velocity provinces (LLSVPs) in the deep mantle beneath Africa and the Pacific are generally interpreted as hot but chemically dense 'piles', which have remained isolated from mantle circulation for several hundred million years. This interpretation largely hinges on four seismic observations: (i) their shear wave velocity anomalies are considered too large for purely thermal structures; (ii) shear wave velocity gradients at their edges are sharp; (iii) their shear to compressional wave-speed anomaly ratios are high; and (iv) their shear and bulk-sound velocity anomalies appear to be anti-correlated. However, using compressible global mantle circulation models driven by 300 Myr of plate motion history and thermodynamic methods for converting from physical to seismic structure, we show that observed lower mantle shear wave velocity anomalies do not require, and are most likely incompatible with, large-scale chemical 'piles'. A prescribed core-mantle-boundary temperature of 4000 K, which is consistent with current estimates, combined with anelastic seismic sensitivity to temperature, ensures that purely thermal LLSVPs, strongly focussed beneath Africa and the Pacific by subduction history, can reconcile observed shear wave velocity anomalies and gradients. By contrast, shear wave velocity anomalies from models that include dense chemical 'piles' at the base of Earth's mantle, where 'piles' correspond to only 3% of the mantle's volume, are substantially stronger than the tomographic model S40RTS, even after accounting for limited tomographic resolution. Our results also suggest that in the presence of post-perovskite, elevated ratios between shear and compressional wavespeed anomalies and the correlation between shear and bulk-sound velocity anomalies cannot be used to discriminate between thermal and compositional heterogeneity at depth: in all calculations, an anticorrelation only occurs within the post-perovskite stability field. Taken together, this implies that although there must be considerable chemical heterogeneity within Earth's mantle, large, coherent 'piles' are not required to reconcile the seismic observations examined here. Indeed, our results suggest that if chemical heterogeneity is present in these regions, its dynamical and seismic significance is far less than has previously been inferred.

The two large low shear-wave velocity provinces (LLSVPs) that dominate lower-mantle structure may... more The two large low shear-wave velocity provinces (LLSVPs) that dominate lower-mantle structure may hold key information on Earth’s thermal and chemical evolution. It is generally accepted that these provinces are hotter than background mantle and are likely the main source of mantle plumes. Increasingly, it is also proposed that they hold a dense (primitive and/or recycled) compositional component. The principle evidence that LLSVPs may represent thermo-chemical ‘piles’ comes from seismic constraints, including the following: (i) their long-wavelength nature; (ii) sharp gradients in shear-wave velocity at their margins; (iii) non-Gaussian distributions of deep mantle shear-wave velocity anomalies; (iv) anti-correlated shear-wave and bulk-sound velocity anomalies (and elevated ratios between shear- and compressional-wave velocity anomalies); (v) anti-correlated shear-wave and density anomalies ; and (vi) 1-D/radial profiles of seismic velocity that deviate from those expected for an i...

It has been proposed that volcanic hotspots and the reconstructed eruption sites of large igneous... more It has been proposed that volcanic hotspots and the reconstructed eruption sites of large igneous provinces (LIPs) are preferentially located above the margins of two deep mantle large low shear-wave velocity provinces (LLSVPs), beneath the African continent and the Pacific Ocean. This spatial correlation has been interpreted to imply that LLSVPs represent long-lived, dense, stable thermo-chemical piles, which preferentially trigger mantle plumes at their edges and exert a strong influence on lower-mantle dynamics. Here, we re-analyse this spatial correlation, demonstrating that it is not global: it is strong for the African LLSVP, but weak for the Pacific. Moreover, Monte Carlo based statistical analyses indicate that the observed distribution of African and Pacific hotspots/reconstructed LIPs is consistent with the hypothesis that they are drawn from a sample that is uniformly distributed across the entire areal extent of each LLSVP: the stronger spatial correlation with the margin of the African LLSVP is expected as a simple consequence of its elongated geometry, where more than 75% of the LLSVP interior lies within 10 • of its margin. Our results imply that the geographical distribution of hotspots and reconstructed LIPs does not indicate the extent to which chemical heterogeneity influences lower-mantle dynamics.

Physics of the Earth and Planetary Interiors, 2012

Free surface Topography Dynamics of lithosphere and mantle Tectonics and landscape evolution Nume... more Free surface Topography Dynamics of lithosphere and mantle Tectonics and landscape evolution Numerical methods and analysis a b s t r a c t Identifying the dominant controls on Earth's surface topography is of critical importance to understanding both the short-and long-term evolution of geological processes and past-and present-day dynamics of Earth's coupled mantle-lithosphere system. The ability to simulate a stress free -or a so-called 'free surface' -boundary condition is required to examine such processes via numerical models. However, at present, geodynamical models incorporating a free surface are limited, as most underlying free surface algorithms place severe restrictions on the computational timestep. Consequently, the simulations are often intractable. In this study, we introduce a new approach for incorporating a free surface within geodynamical models: an algorithm, in which free surface elevation is treated as an independent variable and is solved for in conjunction with the momentum and continuity equation, using implicit time integration. We demonstrate that the method is straightforward to implement in existing models and, using a series of analytical and benchmark comparisons, we show that it does not suffer from the timestep constraints of previous schemes. Furthermore, the scheme can be made second order accurate in time, at no additional cost. The method therefore dramatically improves the computational efficiency of geodynamical simulations including a free surface, whilst maintaining solution accuracy.

Geophysical Journal International, 2011

Keywords: mantle plume seismic tomography plume dynamics shear velocity Seismic detection of a ma... more Keywords: mantle plume seismic tomography plume dynamics shear velocity Seismic detection of a mantle plume may resolve the debate about the origin of hotspots and the role of plumes in mantle convection. In this paper, we test the hypothesis that whole-mantle plumes exist below major hotspots, by quantitatively comparing physically plausible plume models with seismic images following three steps. We (1) simulate a set of representative thermal plumes by solving the governing equations for Earth-like parameters in an axisymmetric spherical shell, (2) convert the thermal structure into shear-velocity anomalies using self-consistent thermo-dynamic relationships, and (3) project the theoretical plumes as seismic images using the S40RTS tomographic filter to account for finite seismic resolution. Simulated plumes with excess potential temperatures of 375 K map into negative shear-wave anomalies of up to 4-8% between 300 and 660 km depth, and 2.0-3.5% in the mid-lower mantle. Given the heterogeneous resolution of S40RTS, plumes of this strength are not easily detectable if tails are narrower than 150-250 km in the upper-mantle or 400-700 km in the lower mantle. In S40RTS, more than half of the forty hotspots we studied overlie low-velocity anomalies that extend through most of the lower mantle. These anomalies exceed 0.6% in the lower mantle, compatible with thermal plume strengths. They have widths mostly with the range 800-1200 km, which is at the high end of plausible thermal plume structures, and at the low end to be resolved in S40RTS. In the upper mantle, the shear velocity is low beneath more than ninety percent of the hotspots. For about ten, including Iceland, the East African hotspots, Hawaii, and the Samoa/Tahiti and Cobb/Bowie pairs, S40RTS low-velocity anomalies extending through the transition zone imply 200-300 K excess temperatures over a~1000 km wide region. This is substantially broader than expected for thermal plume tails. Such anomalies may be compatible with deep-seated plumes, but only if plume flux is strongly variable due to, for example, interaction with phase transitions and/or chemical entrainment.

Geochemistry, Geophysics, Geosystems, 2011

1] We present a new computational modeling framework, Fluidity, for application to a range of two... more 1] We present a new computational modeling framework, Fluidity, for application to a range of two-and three-dimensional geodynamic problems, with the focus here on mantle convection. The approach centers upon a finite element discretization on unstructured simplex meshes, which represent complex geometries in a straightforward manner. Throughout a simulation, the mesh is dynamically adapted to optimize the representation of evolving solution structures. The adaptive algorithm makes use of anisotropic measures of solution complexity, to vary resolution and allow long, thin elements to align with features such as boundary layers. The modeling framework presented differs from the majority of current mantle convection codes, which are typically based upon fixed structured grids. This necessitates a thorough and detailed validation, which is a focus of this paper. Benchmark comparisons are undertaken with a range of two-and three-dimensional, isoviscous and variable viscosity cases. In addition, model predictions are compared to experimental results. Such comparisons highlight not only the robustness and accuracy of Fluidity but also the advantages of anisotropic adaptive unstructured meshes, significantly reducing computational requirements when compared to a fixed mesh simulation. (2011), Fluidity: A fully unstructured anisotropic adaptive mesh computational modeling framework for geodynamics, Geochem. Geophys. Geosyst., 12, Q06001,

Earth and Planetary Science Letters, 2012

Two large low-shear-velocity provinces (LLSVPs) in the deep mantle beneath Africa and the Pacific... more Two large low-shear-velocity provinces (LLSVPs) in the deep mantle beneath Africa and the Pacific are generally interpreted as hot but chemically dense 'piles', which have remained isolated from mantle circulation for several hundred million years. This interpretation largely hinges on four seismic observations: (i) their shear wave velocity anomalies are considered too large for purely thermal structures; (ii) shear wave velocity gradients at their edges are sharp; (iii) their shear to compressional wave-speed anomaly ratios are high; and (iv) their shear and bulk-sound velocity anomalies appear to be anti-correlated. However, using compressible global mantle circulation models driven by 300 Myr of plate motion history and thermodynamic methods for converting from physical to seismic structure, we show that observed lower mantle shear wave velocity anomalies do not require, and are most likely incompatible with, large-scale chemical 'piles'. A prescribed core-mantle-boundary temperature of 4000 K, which is consistent with current estimates, combined with anelastic seismic sensitivity to temperature, ensures that purely thermal LLSVPs, strongly focussed beneath Africa and the Pacific by subduction history, can reconcile observed shear wave velocity anomalies and gradients. By contrast, shear wave velocity anomalies from models that include dense chemical 'piles' at the base of Earth's mantle, where 'piles' correspond to only 3% of the mantle's volume, are substantially stronger than the tomographic model S40RTS, even after accounting for limited tomographic resolution. Our results also suggest that in the presence of post-perovskite, elevated ratios between shear and compressional wavespeed anomalies and the correlation between shear and bulk-sound velocity anomalies cannot be used to discriminate between thermal and compositional heterogeneity at depth: in all calculations, an anticorrelation only occurs within the post-perovskite stability field. Taken together, this implies that although there must be considerable chemical heterogeneity within Earth's mantle, large, coherent 'piles' are not required to reconcile the seismic observations examined here. Indeed, our results suggest that if chemical heterogeneity is present in these regions, its dynamical and seismic significance is far less than has previously been inferred.

Earth and Planetary Science Letters, 2012

The thermal diffusivity (κ) of perovskite and post-perovskite CaIrO 3 has been measured, at eleva... more The thermal diffusivity (κ) of perovskite and post-perovskite CaIrO 3 has been measured, at elevated pressure and temperatures up to 600°C, using the X-radiographic Ångström method. At high temperatures we find that the thermal diffusivity of post-perovskite is slightly below twice that of isochemical perovskite over the temperature range investigated. Assuming a similar effect occurs in MgSiO 3 post-perovskite, the effect of the contrasting thermal transport properties between perovskite and post-perovskite on mantle dynamics has been investigated using simple two-dimensional convection models. These show a reduction in extent and increase in depth of post-perovskite lenses, as well as increased core-mantle-boundary heat-flux, broader upwellings and more vigorous downwellings when compared to the reference, constant a, case.

We evaluate the spatial and temporal evolution of Earth's long-wavelength surface dynamic topogra... more We evaluate the spatial and temporal evolution of Earth's long-wavelength surface dynamic topography since the Jurassic using a series of high-resolution global mantle convection models. These models are Earth-like in terms of convective vigour, thermal structure, surface heat-flux and the geographic distribution of heterogeneity. The models generate a degree-2-dominated spectrum of dynamic topography with negative amplitudes above subducted slabs (i.e. circum-Pacific regions and southern Eurasia) and positive amplitudes elsewhere (i.e. Africa, northwestern Eurasia and the central Pacific). Model predictions are compared with published observations and subsidence patterns from well data, both globally and for the Australian and southern African regions. We find that our models reproduce the long-wavelength component of these observations, although observed smaller-scale variations are not reproduced. We subsequently define " geodynamic rules " for how different surface tectonic settings are affected by mantle processes: (i) locations in the vicinity of a subduction zone show large negative dynamic topography amplitudes; (ii) regions far away from convergent margins feature long-term positive dynamic topography; and (iii) rapid variations in dynamic support occur along the margins of overriding plates (e.g. the western US) and at points located on a plate that rapidly approaches a subduction zone (e.g. India and the Arabia Peninsula). Our models provide a predictive quantitative framework linking mantle convection with plate tectonics and sedimentary basin evolution, thus improving our understanding of how subduction and mantle convection affect the spatio-temporal evolution of basin architecture.

Numerical simulations of thermal convection in the Earth's mantle often employ a pseudoplas-tic r... more Numerical simulations of thermal convection in the Earth's mantle often employ a pseudoplas-tic rheology in order to mimic the plate-like behavior of the lithosphere. Yet the benchmark tests available in the literature are largely based on simple linear rheologies in which the viscosity is either assumed to be constant or weakly dependent on temperature. Here we present a suite of simple tests based on nonlinear rheologies featuring temperature, pressure, and strain rate-dependent viscosity. Eleven different codes based on the finite volume, finite element, or spectral methods have been used to run five benchmark cases leading to stagnant lid, mobile lid, and periodic convection in a 2-D square box. For two of these cases, we also show resolution tests from all contributing codes. In addition, we present a bifurcation analysis, describing the transition from a mobile lid regime to a periodic regime, and from a periodic regime to a stagnant lid regime, as a function of the yield stress. At a resolution of around 100 cells or elements in both vertical and horizontal directions, all codes reproduce the required diagnostic quantities with a discrepancy of at most $3% in the presence of both linear and nonlinear rheologies. Furthermore, they consistently predict the critical value of the yield stress at which the transition between different regimes occurs. As the most recent mantle convection codes can handle a number of different geometries within a single solution framework, this benchmark will also prove useful when validating viscoplastic thermal convection simulations in such geometries.

Komatiites are products of decompression melting of mantle so hot that they are almost exclusivel... more Komatiites are products of decompression melting of mantle so hot that they are almost exclusively restricted to the Archean. The high degree of partial melting (F) and pressure (P) required for their generation facilitates comparison between the magma composition and its mantle source. To investigate compositional variations in Archean komatiites, a global selection of 38 Archean komatiites spanning five cratons (Kaapvaal, Zimbabwe, Yilgarn, Pilbara, Superior) were analysed for their major and trace element contents. Included are the Aluminium-Depleted (ADK, Barberton-Type) and Aluminium-Undepleted (AUK, Munro-Type) petrogenetic types that have been equated with high P/moderate F and moderate P/high F, respectively, on the basis of their Al/Ti and Gd/Yb ratios. Following calculation of the primary magma composition of each suite, we show that the absolute Al content at a specified MgO proves a more sensitive indicator of P than either of the above two ratios and hence we introduce a new classification using Al. The Mg# is a reliable proxy for F, independent of the two endmember melting styles, fractional and batch. We demonstrate that most komatiites form by batch melting, ceding to fractional melting with decreasing pressure as the density contrast between the liquid and solid grows. The Munro AUKs are the only suite to show evidence of fractional melting, with melt extraction occurring at the lowest F and P, %25% melting at 5 GPa (mantle potential temperature, T P ¼ 1750 C) whereas the ADKs of Barberton segregated at the highest F and P (%40%, 9 GPa, T P ¼ 1950 C). The petrogenetic type is a combination of P and F, where, at a given pressure, higher F will produce AUKs over ADKs as majorite is consumed in the source. Through numerical simulations, it is shown that both types can occur within the same mantle plume, with ADKs forming in its cooler, distal fringes whereas AUKs occur along its axis. Furthermore, and contrary to previous views, there is no temporal distinction between the two komatiite types, with both AUKs and ADKs occurring throughout the Archean. By contrast, younger, 2Á7 Ga komatiites tend to have sources that are more depleted than those of older, 3Á5 Ga komatiites. Komatiites are invaluable records of the mantle's chemical and physical evolution during the Archean.

Arc volcanism, volatile cycling, mineralization, and continental crust formation are likely regul... more Arc volcanism, volatile cycling, mineralization, and continental crust formation are likely regulated by the mantle wedge's flow regime and thermal structure. Wedge flow is often assumed to follow a regular corner-flow pattern. However, studies that incorporate a hydrated rheology and thermal buoyancy predict internal small-scale-convection (SSC). Here, we systematically explore mantle-wedge dynamics in 3-D simulations. We find that longitudinal ''Richter-rolls'' of SSC (with trench-perpendicular axes) commonly occur if wedge hydration reduces viscosities to 1 Á 10 19 Pa s, although transient transverse rolls (with trench-parallel axes) can dominate at viscosities of 5Aˊ101821Aˊ1019Pas.Rollsbelowthearcandbackarcdiffer.Subarcrollshavesimilartrench−parallelandtrench−perpendiculardimensionsof100–150kmandevolveona1–5Myrtimescale.Subback−arcinstabilities,ontheotherhand,coalesceintoelongatedsheets,usuallywithapreferentialtrench−perpendicularalignment,displayawavelengthof150–400kmandvaryona5–10Myrtimescale.Themodulatinginfluenceofsubback−arcridgesonthesubarcsystemincreaseswithstrongerwedgehydration,highersubductionvelocity,andthickerupperplates.Wefindthattrench−parallelaveragesofwedgevelocitiesandtemperatureareconsistentwiththosepredictedin2−Dmodels.However,lithosphericthinningthroughSSCissomewhatenhancedin3−D,thusexpandinghydrousmeltingregionsandshiftingdehydrationboundaries.SubarcRichter−rollsgeneratetime−dependenttrench−paralleltemperaturevariationsofupto5 Á 10 18 21 Á 10 19 Pa s. Rolls below the arc and back arc differ. Subarc rolls have similar trench-parallel and trench-perpendicular dimensions of 100–150 km and evolve on a 1–5 Myr timescale. Subback-arc instabilities, on the other hand, coalesce into elongated sheets, usually with a preferential trench-perpendicular alignment, display a wavelength of 150–400 km and vary on a 5–10 Myr time scale. The modulating influence of subback-arc ridges on the subarc system increases with stronger wedge hydration, higher subduction velocity, and thicker upper plates. We find that trench-parallel averages of wedge velocities and temperature are consistent with those predicted in 2-D models. However, lithospheric thinning through SSC is somewhat enhanced in 3-D, thus expanding hydrous melting regions and shifting dehydration boundaries. Subarc Richter-rolls generate time-dependent trench-parallel temperature variations of up to 5Aˊ101821Aˊ1019Pas.Rollsbelowthearcandbackarcdiffer.Subarcrollshavesimilartrench−parallelandtrench−perpendiculardimensionsof100–150kmandevolveona1–5Myrtimescale.Subback−arcinstabilities,ontheotherhand,coalesceintoelongatedsheets,usuallywithapreferentialtrench−perpendicularalignment,displayawavelengthof150–400kmandvaryona5–10Myrtimescale.Themodulatinginfluenceofsubback−arcridgesonthesubarcsystemincreaseswithstrongerwedgehydration,highersubductionvelocity,andthickerupperplates.Wefindthattrench−parallelaveragesofwedgevelocitiesandtemperatureareconsistentwiththosepredictedin2−Dmodels.However,lithosphericthinningthroughSSCissomewhatenhancedin3−D,thusexpandinghydrousmeltingregionsandshiftingdehydrationboundaries.SubarcRichter−rollsgeneratetime−dependenttrench−paralleltemperaturevariationsofupto150 K, which exceed the transient 50–100 K variations predicted in 2-D and may contribute to arc-volcano spacing and the variable seismic velocity structures imaged beneath some arcs.

Keywords: lithosphere–asthenosphere boundary seismic anisotropy LPO development The depth of the ... more Keywords: lithosphere–asthenosphere boundary seismic anisotropy LPO development The depth of the oceanic lithosphere–asthenosphere boundary (LAB), as inferred from shear wave velocities, increases with lithospheric age, in agreement with models of cooling oceanic lithosphere. On the other hand, the distribution of radial anisotropy under oceanic plates is almost age-independent. In particular, radial anisotropy shows a maximum positive gradient at a depth of ∼70 km, which, if used as a proxy, indicates an age-independent LAB depth. These contrasting observations have fueled a controversy on the seismological signature of the LAB. To better understand the discrepancy between these observations, we model the development of lattice preferred orientation (LPO) in upper mantle crystal aggregates and predict the seismic anisotropy produced by plate-driven mid-ocean ridge flows. The model accounts for the progressive cooling of the lithosphere with age and can incorporate both diffusion and dislocation creep deformation mechanisms. We find that an age-independent distribution of radial anisotropy is the natural consequence of these simple flows. The depth and strength of anisotropy is further controlled by the deformation regime – dislocation or diffusion creep – experienced by crystals during their ascent towards, and subsequent motion away from, the ridge axis. Comparison to surface wave tomography models yield constraints on rheological parameters such as the activation volume. Although not excluded, additional mechanisms proposed to explain some geophysical signatures of the LAB, such as the presence of partial melt or changes in water content, are not required to explain the radial anisotropy proxy. Our prediction, that the age-independent radial anisotropy proxy marks the transition to flow-induced asthenospheric anisotropy, provides a way to reconcile thermal, mechanical and seismological views of the LAB.

Subduction zone mantle wedge temperatures impact plate interaction, melt generation, and chemical... more Subduction zone mantle wedge temperatures impact plate interaction, melt generation, and chemical recycling. However, it has been challenging to reconcile geophysical and geochemical constraints on wedge thermal structure. Here we chemically determine the equilibration pressures and temperatures of primitive arc lavas from worldwide intraoceanic subduction zones and compare them to kinematically driven thermal wedge models. We find that equilibration pressures are typically located in the lithosphere, starting just below the Moho, and spanning a wide depth range of 25 km. Equilibration temperatures are high for these depths, averaging 13008C. We test for correlations with subduction parameters and find that equilibration pressures correlate with upper plate age, indicating overriding lithosphere thickness plays a role in magma equilibration. We suggest that most, if not all, thermobarometric pressure and temperature conditions reflect magmatic reequilibration at a mechanical boundary, rather than reflecting the conditions of major melt generation. The magma reequilibration conditions are difficult to reconcile, to a first order, with any of the conditions predicted by our dynamic models, with the exception of subduction zones with very young, thin upper plates. For most zones, a mechanism for substantially thinning the overriding plate is required. Most likely thinning is localized below the arc, as kinematic thinning above the wedge corner would lead to a hot fore arc, incompatible with fore-arc surface heat flow and seismic properties. Localized subarc thermal erosion is consistent with seismic imaging and exhumed arc structures. Furthermore, such thermal erosion can serve as a weakness zone and affect subsequent plate evolution.

0 0 M o n t h 2 0 1 7 | V o L 0 0 0 | n A t U R E | 1 Mantle plumes are buoyant upwellings of hot... more 0 0 M o n t h 2 0 1 7 | V o L 0 0 0 | n A t U R E | 1 Mantle plumes are buoyant upwellings of hot rock that transport heat from Earth's core to its surface, generating anomalous regions of volcanism that are not directly associated with plate tectonic processes. The best-studied example is the Hawaiian–Emperor chain, but the emergence of two sub-parallel volcanic tracks along this chain 1 , Loa and Kea, and the systematic geochemical differences between them 2,3 have remained unexplained. Here we argue that the emergence of these tracks coincides with the appearance of other double volcanic tracks on the Pacific plate and a recent azimuthal change in the motion of the plate. We propose a three-part model that explains the evolution of Hawaiian double-track volcanism: first, mantle flow beneath the rapidly moving Pacific plate strongly tilts the Hawaiian plume and leads to lateral separation between high-and low-pressure melt source regions; second, the recent azimuthal change in Pacific plate motion exposes high-and low-pressure melt products as geographically distinct volcanoes, explaining the simultaneous emergence of double-track volcanism across the Pacific; and finally, secondary pyroxenite, which is formed as eclogite melt reacts with peridotite 4 , dominates the low-pressure melt region beneath Loa-track volcanism, yielding the systematic geochemical differences observed between Loa-and Kea-type lavas 3,5–9. Our results imply that the formation of double-track volcanism is transitory and can be used to identify and place temporal bounds on plate-motion changes.

Keywords: mantle plumes hotspot volcanism LLSVPs Hawaii mantle structure and dynamics thermochemi... more Keywords: mantle plumes hotspot volcanism LLSVPs Hawaii mantle structure and dynamics thermochemical convection It has been proposed that the spatial variations recorded in the geochemistry of hotspot lavas, such as the bilateral asymmetry recorded at Hawaii, can be directly mapped as the heterogeneous structure and composition of their deep-mantle source. This would imply that source-region heterogeneities are transported into, and preserved within, a plume conduit, as the plume rises from the deep-mantle to Earth's surface. Previous laboratory and numerical studies, which neglect density and rheological variations between different chemical components, support this view. However, in this paper, we demonstrate that this interpretation cannot be extended to distinct chemical domains that differ from surrounding mantle in their density and viscosity. By numerically simulating thermo-chemical mantle plumes across a broad parameter space, in 2-D and 3-D, we identify two conduit structures: (i) bilaterally asymmetric conduits, which occur exclusively for cases where the chemical effect on buoyancy is negligible, in which the spatial distribution of deep-mantle heterogeneities is preserved during plume ascent; and (ii) concentric conduits, which occur for all other cases, with dense material preferentially sampled within the conduit's centre. In the latter regime, the spatial distribution of geochemical domains in the lowermost mantle is not preserved during plume ascent. Our results imply that the heterogeneous structure and composition of Earth's lowermost mantle can only be mapped from geochemical observations at Earth's surface if chemical heterogeneity is a passive component of lowermost mantle dynamics (i.e. its effect on density is outweighed by, or is secondary to, the effect of temperature). The implications of our results for: (i) why oceanic crust should be the prevalent component of ocean island basalts; and (ii) how we interpret the geochemical evolution of Earth's deep-mantle are also discussed.

Earth and Planetary Science Letters, 2012

Two large low-shear-velocity provinces (LLSVPs) in the deep mantle beneath Africa and the Pacific... more Two large low-shear-velocity provinces (LLSVPs) in the deep mantle beneath Africa and the Pacific are generally interpreted as hot but chemically dense 'piles', which have remained isolated from mantle circulation for several hundred million years. This interpretation largely hinges on four seismic observations: (i) their shear wave velocity anomalies are considered too large for purely thermal structures; (ii) shear wave velocity gradients at their edges are sharp; (iii) their shear to compressional wave-speed anomaly ratios are high; and (iv) their shear and bulk-sound velocity anomalies appear to be anti-correlated. However, using compressible global mantle circulation models driven by 300 Myr of plate motion history and thermodynamic methods for converting from physical to seismic structure, we show that observed lower mantle shear wave velocity anomalies do not require, and are most likely incompatible with, large-scale chemical 'piles'. A prescribed core-mantle-boundary temperature of 4000 K, which is consistent with current estimates, combined with anelastic seismic sensitivity to temperature, ensures that purely thermal LLSVPs, strongly focussed beneath Africa and the Pacific by subduction history, can reconcile observed shear wave velocity anomalies and gradients. By contrast, shear wave velocity anomalies from models that include dense chemical 'piles' at the base of Earth's mantle, where 'piles' correspond to only 3% of the mantle's volume, are substantially stronger than the tomographic model S40RTS, even after accounting for limited tomographic resolution. Our results also suggest that in the presence of post-perovskite, elevated ratios between shear and compressional wavespeed anomalies and the correlation between shear and bulk-sound velocity anomalies cannot be used to discriminate between thermal and compositional heterogeneity at depth: in all calculations, an anticorrelation only occurs within the post-perovskite stability field. Taken together, this implies that although there must be considerable chemical heterogeneity within Earth's mantle, large, coherent 'piles' are not required to reconcile the seismic observations examined here. Indeed, our results suggest that if chemical heterogeneity is present in these regions, its dynamical and seismic significance is far less than has previously been inferred.

The two large low shear-wave velocity provinces (LLSVPs) that dominate lower-mantle structure may... more The two large low shear-wave velocity provinces (LLSVPs) that dominate lower-mantle structure may hold key information on Earth’s thermal and chemical evolution. It is generally accepted that these provinces are hotter than background mantle and are likely the main source of mantle plumes. Increasingly, it is also proposed that they hold a dense (primitive and/or recycled) compositional component. The principle evidence that LLSVPs may represent thermo-chemical ‘piles’ comes from seismic constraints, including the following: (i) their long-wavelength nature; (ii) sharp gradients in shear-wave velocity at their margins; (iii) non-Gaussian distributions of deep mantle shear-wave velocity anomalies; (iv) anti-correlated shear-wave and bulk-sound velocity anomalies (and elevated ratios between shear- and compressional-wave velocity anomalies); (v) anti-correlated shear-wave and density anomalies ; and (vi) 1-D/radial profiles of seismic velocity that deviate from those expected for an i...

It has been proposed that volcanic hotspots and the reconstructed eruption sites of large igneous... more It has been proposed that volcanic hotspots and the reconstructed eruption sites of large igneous provinces (LIPs) are preferentially located above the margins of two deep mantle large low shear-wave velocity provinces (LLSVPs), beneath the African continent and the Pacific Ocean. This spatial correlation has been interpreted to imply that LLSVPs represent long-lived, dense, stable thermo-chemical piles, which preferentially trigger mantle plumes at their edges and exert a strong influence on lower-mantle dynamics. Here, we re-analyse this spatial correlation, demonstrating that it is not global: it is strong for the African LLSVP, but weak for the Pacific. Moreover, Monte Carlo based statistical analyses indicate that the observed distribution of African and Pacific hotspots/reconstructed LIPs is consistent with the hypothesis that they are drawn from a sample that is uniformly distributed across the entire areal extent of each LLSVP: the stronger spatial correlation with the margin of the African LLSVP is expected as a simple consequence of its elongated geometry, where more than 75% of the LLSVP interior lies within 10 • of its margin. Our results imply that the geographical distribution of hotspots and reconstructed LIPs does not indicate the extent to which chemical heterogeneity influences lower-mantle dynamics.

Physics of the Earth and Planetary Interiors, 2012

Free surface Topography Dynamics of lithosphere and mantle Tectonics and landscape evolution Nume... more Free surface Topography Dynamics of lithosphere and mantle Tectonics and landscape evolution Numerical methods and analysis a b s t r a c t Identifying the dominant controls on Earth's surface topography is of critical importance to understanding both the short-and long-term evolution of geological processes and past-and present-day dynamics of Earth's coupled mantle-lithosphere system. The ability to simulate a stress free -or a so-called 'free surface' -boundary condition is required to examine such processes via numerical models. However, at present, geodynamical models incorporating a free surface are limited, as most underlying free surface algorithms place severe restrictions on the computational timestep. Consequently, the simulations are often intractable. In this study, we introduce a new approach for incorporating a free surface within geodynamical models: an algorithm, in which free surface elevation is treated as an independent variable and is solved for in conjunction with the momentum and continuity equation, using implicit time integration. We demonstrate that the method is straightforward to implement in existing models and, using a series of analytical and benchmark comparisons, we show that it does not suffer from the timestep constraints of previous schemes. Furthermore, the scheme can be made second order accurate in time, at no additional cost. The method therefore dramatically improves the computational efficiency of geodynamical simulations including a free surface, whilst maintaining solution accuracy.

Geophysical Journal International, 2011

Keywords: mantle plume seismic tomography plume dynamics shear velocity Seismic detection of a ma... more Keywords: mantle plume seismic tomography plume dynamics shear velocity Seismic detection of a mantle plume may resolve the debate about the origin of hotspots and the role of plumes in mantle convection. In this paper, we test the hypothesis that whole-mantle plumes exist below major hotspots, by quantitatively comparing physically plausible plume models with seismic images following three steps. We (1) simulate a set of representative thermal plumes by solving the governing equations for Earth-like parameters in an axisymmetric spherical shell, (2) convert the thermal structure into shear-velocity anomalies using self-consistent thermo-dynamic relationships, and (3) project the theoretical plumes as seismic images using the S40RTS tomographic filter to account for finite seismic resolution. Simulated plumes with excess potential temperatures of 375 K map into negative shear-wave anomalies of up to 4-8% between 300 and 660 km depth, and 2.0-3.5% in the mid-lower mantle. Given the heterogeneous resolution of S40RTS, plumes of this strength are not easily detectable if tails are narrower than 150-250 km in the upper-mantle or 400-700 km in the lower mantle. In S40RTS, more than half of the forty hotspots we studied overlie low-velocity anomalies that extend through most of the lower mantle. These anomalies exceed 0.6% in the lower mantle, compatible with thermal plume strengths. They have widths mostly with the range 800-1200 km, which is at the high end of plausible thermal plume structures, and at the low end to be resolved in S40RTS. In the upper mantle, the shear velocity is low beneath more than ninety percent of the hotspots. For about ten, including Iceland, the East African hotspots, Hawaii, and the Samoa/Tahiti and Cobb/Bowie pairs, S40RTS low-velocity anomalies extending through the transition zone imply 200-300 K excess temperatures over a~1000 km wide region. This is substantially broader than expected for thermal plume tails. Such anomalies may be compatible with deep-seated plumes, but only if plume flux is strongly variable due to, for example, interaction with phase transitions and/or chemical entrainment.

Geochemistry, Geophysics, Geosystems, 2011

1] We present a new computational modeling framework, Fluidity, for application to a range of two... more 1] We present a new computational modeling framework, Fluidity, for application to a range of two-and three-dimensional geodynamic problems, with the focus here on mantle convection. The approach centers upon a finite element discretization on unstructured simplex meshes, which represent complex geometries in a straightforward manner. Throughout a simulation, the mesh is dynamically adapted to optimize the representation of evolving solution structures. The adaptive algorithm makes use of anisotropic measures of solution complexity, to vary resolution and allow long, thin elements to align with features such as boundary layers. The modeling framework presented differs from the majority of current mantle convection codes, which are typically based upon fixed structured grids. This necessitates a thorough and detailed validation, which is a focus of this paper. Benchmark comparisons are undertaken with a range of two-and three-dimensional, isoviscous and variable viscosity cases. In addition, model predictions are compared to experimental results. Such comparisons highlight not only the robustness and accuracy of Fluidity but also the advantages of anisotropic adaptive unstructured meshes, significantly reducing computational requirements when compared to a fixed mesh simulation. (2011), Fluidity: A fully unstructured anisotropic adaptive mesh computational modeling framework for geodynamics, Geochem. Geophys. Geosyst., 12, Q06001,

Earth and Planetary Science Letters, 2012

Two large low-shear-velocity provinces (LLSVPs) in the deep mantle beneath Africa and the Pacific... more Two large low-shear-velocity provinces (LLSVPs) in the deep mantle beneath Africa and the Pacific are generally interpreted as hot but chemically dense 'piles', which have remained isolated from mantle circulation for several hundred million years. This interpretation largely hinges on four seismic observations: (i) their shear wave velocity anomalies are considered too large for purely thermal structures; (ii) shear wave velocity gradients at their edges are sharp; (iii) their shear to compressional wave-speed anomaly ratios are high; and (iv) their shear and bulk-sound velocity anomalies appear to be anti-correlated. However, using compressible global mantle circulation models driven by 300 Myr of plate motion history and thermodynamic methods for converting from physical to seismic structure, we show that observed lower mantle shear wave velocity anomalies do not require, and are most likely incompatible with, large-scale chemical 'piles'. A prescribed core-mantle-boundary temperature of 4000 K, which is consistent with current estimates, combined with anelastic seismic sensitivity to temperature, ensures that purely thermal LLSVPs, strongly focussed beneath Africa and the Pacific by subduction history, can reconcile observed shear wave velocity anomalies and gradients. By contrast, shear wave velocity anomalies from models that include dense chemical 'piles' at the base of Earth's mantle, where 'piles' correspond to only 3% of the mantle's volume, are substantially stronger than the tomographic model S40RTS, even after accounting for limited tomographic resolution. Our results also suggest that in the presence of post-perovskite, elevated ratios between shear and compressional wavespeed anomalies and the correlation between shear and bulk-sound velocity anomalies cannot be used to discriminate between thermal and compositional heterogeneity at depth: in all calculations, an anticorrelation only occurs within the post-perovskite stability field. Taken together, this implies that although there must be considerable chemical heterogeneity within Earth's mantle, large, coherent 'piles' are not required to reconcile the seismic observations examined here. Indeed, our results suggest that if chemical heterogeneity is present in these regions, its dynamical and seismic significance is far less than has previously been inferred.

Earth and Planetary Science Letters, 2012

The thermal diffusivity (κ) of perovskite and post-perovskite CaIrO 3 has been measured, at eleva... more The thermal diffusivity (κ) of perovskite and post-perovskite CaIrO 3 has been measured, at elevated pressure and temperatures up to 600°C, using the X-radiographic Ångström method. At high temperatures we find that the thermal diffusivity of post-perovskite is slightly below twice that of isochemical perovskite over the temperature range investigated. Assuming a similar effect occurs in MgSiO 3 post-perovskite, the effect of the contrasting thermal transport properties between perovskite and post-perovskite on mantle dynamics has been investigated using simple two-dimensional convection models. These show a reduction in extent and increase in depth of post-perovskite lenses, as well as increased core-mantle-boundary heat-flux, broader upwellings and more vigorous downwellings when compared to the reference, constant a, case.