Dense melt residues drive mid-ocean-ridge “hotspots”

ABSTRACT The geodynamic origin of melting anomalies found at the surface, often referred to as “hotspots,” is classically attributed to a mantle plume process. The distribution of hotspots along mid-ocean-ridge spreading systems around the globe, however, questions the universal validity of this concept. Here, the preferential association of hotspots with slow- to intermediate-spreading centers and not fast-spreading centers, an observation contrary to the expected effect of ridge suction forces on upwelling mantle plumes, is explained by a new mechanism for producing melting anomalies at shallow (<2.3 GPa) depths. By combining the effects of both chemical and thermal density changes during partial melting of the mantle (using appropriate latent heat and depth-dependent thermal expansivity parameters), we find that mantle residues experience an overall instantaneous increase in density when melting occurs at <2.3 GPa. This controversial finding is due to thermal contraction of material during melting, which outweighs the chemical buoyancy due to melting at shallow pressures (where thermal expansivities are highest). These dense mantle residues are likely to locally sink beneath spreading centers if ridge suction forces are modest, thus driving an increase in the flow of fertile mantle through the melting window and increasing magmatic production. This leads us to question our understanding of sub–spreading center dynamics, where we now suggest a portion of locally inverted mantle flow results in hotspots. Such inverted flow presents an alternative mechanism to upwelling hot mantle plumes for the generation of excess melt at near-ridge hotspots, i.e., dense downwelling of mantle residue locally increasing the flow of fertile mantle through the melting window. Near-ridge hotspots, therefore, may not require the elevated temperatures commonly invoked to account for excess melting. The proposed mechanism also satisfies counterintuitive observations of ridge-bound hotspots at slow- to intermediate-spreading centers, yet not at fast-spreading centers, where large dynamic ridge suction forces likely overwhelm density-driven downwelling. The lack of observations of such downwelling in numerical modeling studies to date reflects the generally high chemical depletion buoyancy and/or low thermal expansivity parameter values employed in simulations, which we find to be unrepresentative for melting at <2.3 GPa. We therefore invite future studies to review the values used for parameters affecting density changes during melting (e.g., depletion buoyancy, latent heat of melting, specific heat capacity, thermal expansivity), which quite literally have the potential to turn our understanding of mantle dynamics upside down.

Automating Finite Element Methods for Geodynamics via Firedrake

Firedrake is an automated system for solving partial differential equations using the finite element method. By applying sophisticated performance optimisations through automatic code-generation techniques, it provides a means to create accurate, efficient, flexible, easily extensible, scalable, transparent and reproducible research software, that is ideally suited to simulating a wide-range of problems in geophysical fluid dynamics. Here, we demonstrate the applicability of Firedrake for geodynamical simulation, with a focus on mantle dynamics. The accuracy and efficiency of the approach is confirmed via comparisons against a suite of analytical and benchmark cases of systematically increasing complexity, whilst parallel scalability is demonstrated up to 12288 compute cores, where the problem size and the number of processing cores are simultaneously increased. In addition, Firedrake's flexibility is highlighted via straightforward application to different physical (e.g. complex nonlinear rheologies, compressibility) and geometrical (2-D and 3-D Cartesian and spherical domains) scenarios. Finally, a representative simulation of global mantle convection is examined, which incorporates 230 Myr of plate motion history as a kinematic surface boundary condition, confirming its suitability for addressing research problems at the frontiers of global mantle dynamics research.

Simultaneous Inference of Plate Boundary Stresses and Mantle Rheology Using Adjoints: Large-Scale Two-Dimensional Models

Plate motions are a primary surface constraint on plate and mantle dynamics and rheology, plate boundary stresses, and the occurrence of great earthquakes. Within an optimization method, we use plate motion data to better constrain uncertain mantle parameters. For the optimization problem characterizing the maximum a posteriori rheological parameters we derive gradients using adjoints and expressions to approximate the posterior distributions for stresses within plate boundaries. We apply these methods to a 2-D cross section from the western to eastern Pacific, with temperature distributions and fault zone geometries developed primarily from seismic and plate motion data. We find that the best-fitting stress exponent, $n$, is about 2.8 and the yield stress about 100 MPa or less. The normal stress on the interplate fault zones is about 100 MPa and the shear stresses about 10 MPa or less.

Constraining the 410-km discontinuity and slab structure in the Kuril subduction zone with triplication waveforms

SUMMARY The detailed structure near the 410-km discontinuity provides key constraints of the dynamic interactions between the upper mantle and the lower mantle through the mantle transition zone (MTZ) via mass and heat exchange. Meanwhile, the temperature of the subducting slab, which can be derived from its fast wave speed perturbation, is critical for understanding the mantle dynamics in subduction zones where the slab enters the MTZ. Multipathing, i.e. triplicated, body waves that bottom near the MTZ carry rich information of the 410-km discontinuity structure and can be used to constrain the discontinuity depth and radial variations of wave speeds across it. In this study, we systematically analysed the trade-off between model parameters in triplication studies using synthetic examples. Specifically, we illustrated the necessity of using array-normalized amplitude. Two 1-D depth profiles of the wave speed below the Tatar Strait of Russia in the Kuril subduction zone are obtained. We have observed triplications due to both the 410-km discontinuity and the slab upper surface. And, seismic structures for these two interfaces are simultaneously inverted. Our derived 410-km discontinuity depths for the northern and southern regions are at 420$\pm $15 and 425$\pm $15 km, respectively, with no observable uplift. The slab upper surface is inverted to be located about 50–70 km below the 410-km discontinuity. This location is between the depths of the 1 and 2 per cent P-wave speed perturbation contours of a regional 3-D full-waveform inversion (FWI) model, but we found twice the wave speed perturbation amplitude. A wave speed increase of 3.9–4.6 per cent within the slab, compared to 2.0–2.4 per cent from the 3-D FWI model, is necessary to fit the waveforms with the shortest period of 2 s, indicating that high-frequency waves are required to accurately resolve the detailed structures near the MTZ.

Noise in the Cretaceous Quiet Zone uncovers plate tectonic chain reaction

Global plate reorganizations, intriguing but loosely defined periods of profoundly changing plate motions, may be caused by a single trigger such as a continental collision or a rising mantle plume. But whether and how such triggers propagate throughout a plate circuit remains unknown. Here, we show how a rising mantle plume set off a ‘plate tectonic chain reaction’. Plume rise has been shown to trigger formation of a subduction zone within the Neotethys Ocean between Africa and Eurasia at ~105 Ma. We provide new constraints on Africa-Eurasia convergence rates using variations in geomagnetic ‘noise’ within the Cretaceous Normal Superchron (the 126-83 Ma period without magnetic reversals) recorded in the Atlantic Quiet Zones crust. These new constraints are consistent with the timing of numerically predicted African Plate acceleration and deceleration associated with onset and arrest of the intra-Neotethyan subduction zone. The acceleration was associated with a change in Africa-Eurasia convergence direction, which in turn was accommodated by a next subduction initiation at ~85 Ma in the Alpine region that cascaded into regional tectonic events. Our concept of plate tectonic chain reactions shows how changes in plate motion, underpinned by mantle dynamics, may self-perpetuate through a plate circuit, making global plate reorganizations a key to unlock the driving mechanisms behind plate tectonics.

Self-Consistent Grain Size Evolution controls Strain Localization during Rifting

Geodynamic numerical models often employ solely grain-size-independent dislocation creep to describe upper mantle dynamics. However, observations from nature and rock deformation experiments suggest that shear zones can transition to a grain-size-dependent creep mechanism due to dynamic grain size evolution, with important implications for the overall strength of plate boundaries. We apply a two-dimensional thermo-mechanical numerical model with a composite diffusion-dislocation creep rheology coupled to a dynamic grain size evolution model based on the paleowattmeter. Results indicate average olivine grain sizes of 3–12 cm for the upper mantle below the LAB, while in the lithosphere grain size ranges from 0.3–3 mm at the Moho to 6–15 cm at the LAB. Such a grain size distribution results in dislocation creep being the dominant deformation mechanism in the upper mantle. However, deformation-related grain size reduction below 100 μm activates diffusion creep along lithospheric-scale shear zones during rifting, affecting the overall strength of tectonic plate boundaries.

Composition and Pressure Effects on Partitioning of Ferrous Iron in Iron-Rich Lower Mantle Heterogeneities

Both seismic observations of dense low shear velocity regions and models of magma ocean crystallization and mantle dynamics support enrichment of iron in Earth’s lowermost mantle. Physical properties of iron-rich lower mantle heterogeneities in the modern Earth depend on distribution of iron between coexisting lower mantle phases (Mg,Fe)O magnesiowüstite, (Mg,Fe)SiO3 bridgmanite, and (Mg,Fe)SiO3 post-perovskite. The partitioning of iron between these phases was investigated in synthetic ferrous-iron-rich olivine compositions (Mg0.55Fe0.45)2SiO4 and (Mg0.28Fe0.72)2SiO4 at lower mantle conditions ranging from 33–128 GPa and 1900–3000 K in the laser-heated diamond anvil cell. The resulting phase assemblages were characterized by a combination of in situ X-ray diffraction and ex situ transmission electron microscopy. The exchange coefficient between bridgmanite and magnesiowüstite decreases with pressure and bulk Fe# and increases with temperature. Thermodynamic modeling determines that incorporation and partitioning of iron in bridgmanite are explained well by excess volume associated with Mg-Fe exchange. Partitioning results are used to model compositions and densities of mantle phase assemblages as a function of pressure, FeO-content and SiO2-content. Unlike average mantle compositions, iron-rich compositions in the mantle exhibit negative dependence of density on SiO2-content at all mantle depths, an important finding for interpretation of deep lower mantle structures.

Burial and Heat Flux Modelling along a Southern Vøring Basin Transect: Implications for the Petroleum Systems and Thermal Regimes in the Deep Mid-Norwegian Sea

A key aspect on the evolution of rifted terranes and the prospectivity of the overlying sedimentary basins is heat. Temperature determines the deformation regime of crustal and mantle rocks and, thus, the style of rifting and geometry of rift basins. The generation of hydrocarbons from organic-rich rocks and reservoir conditions depend primarily on temperature. In this study, we model the thermal–burial history of the southern Vøring Basin (Mid-Norway Margin) along a regional transect (2-D), integrating basin- and lithospheric-scale processes. A model that accounts for the main extensional pulses that shaped the Mid-Norway Margin is in good general agreement with the present–past geothermal gradients inferred from borehole temperature and maturity data and the surface heat flux measurements in the proximal and intermediate margin. This supports a near steady-state, post-rift margin setting, following the break-up in the early Eocene. Significant discrepancies are, however, observed in the distal margin, where the borehole temperatures suggest (much) higher thermal gradients than model predicted and implied by the average surface heat flux. We speculate that the higher thermal gradients may result from deep-seated (mantle dynamics) thermal anomalies and/or recurrent hydrothermalism during periods of greater tectonic stress (regional compression and glacial loading rebound) and test the implications for the maturity of the Vøring Basin. The modelling results show, for example, that, depending on the thermal model assumptions, the depth and age of the optimal mid-Late Cretaceous source-rock horizons may vary by more than 2 km and 10 Ma, respectively.