Guizhi Zhu

Three-dimensional dynamics of hydrous thermal-chemical plumes in oceanic subduction zones

Hydration and partial melting along subducting slabs can trigger Rayleigh-Taylor-like instabilities. We use 3-D petrological-thermomechanical numerical simulations to investigate small-scale convection and hydrous, partially molten, cold plumes formed in the mantle wedge in response to slab dehydration. The simulations were carried out with the I3ELVIS code, which is based on a multigrid approach combined with marker-in-cell methods and conservative finite difference schemes. Our numerical simulations show that three types of plumes occur above the slab-mantle interface: (1) finger-like plumes that form sheet-like structure parallel to the trench, (2) ridge-like structures perpendicular to the trench, and (3) flattened wave-like instabilities propagating upward along the upper surface of the slab and forming zigzag patterns parallel to the trench. The viscosity of the plume material is the main factor controlling the geometry of the plumes. Our results show that lower viscosity of the partially molten rocks facilitates the Rayleigh-Taylor-like instabilities with small wavelengths. In particular, in low-viscosity models (1018–1019 Pa s) the typical spacing of finger-like plumes is about 30–45 km, while in high-viscosity models (1020–1021 Pa s) plumes become rather sheet-like, and the spacing between them increases to 70–100 km. Water released from the slab forms a low-viscosity wedge with complex 3-D geometries. The computed spatial and temporal pattern of melt generation intensity above the slab is compared to the distribution and ages of volcanoes in the northeast Japan. Based on the similarity of the patterns we suggest that specific clustering of volcanic activity in this region could be potentially related to the activity of thermal-chemical plumes.

Subduction of fracture zones controls mantle melting and geochemical signature above slabs.

For some volcanic arcs, the geochemistry of volcanic rocks erupting above subducted oceanic fracture zones is consistent with higher than normal fluid inputs to arc magma sources. Here we use enrichment of boron (B/Zr) in volcanic arc lavas as a proxy to evaluate relative along-strike inputs of slab-derived fluids in the Aleutian, Andean, Cascades and Trans-Mexican arcs. Significant B/Zr spikes coincide with subduction of prominent fracture zones in the relatively cool Aleutian and Andean subduction zones where fracture zone subduction locally enhances fluid introduction beneath volcanic arcs. Geodynamic models of subduction have not previously considered how fracture zones may influence the melt and fluid distribution above slabs. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations of subduction, we show that enhanced production of slab-derived fluids and mantle wedge melts concentrate in areas where fracture zones are subducted, resulting in significant along-arc variability in magma source compositions and processes.

Subduction initiates at straight passive margins

Subduction initiation at straight passive margins can be investigated with two-dimensional (2-D) numerical models, because the geometry is purely cylindrical. However, on Earth, straight margins rarely occur. The construction of 3-D models is therefore critical in the modeling of spontaneous subduction initiation at realistic, curved passive margins. Here we report on the results obtained from gravitationally driven, 3-D thermomechanical numerical models using a visco-plastic rheology and a passive margin with a single curved section in the middle. The models show that the curvature angle β can control subduction initiation: the greater β is, the more difficult subduction initiation becomes. The 3-D thermomechanical models provide an in-depth physical understanding of the processes. Specifically, we find that pressure gradients, arising from density differences between oceanic and continental rocks, drive subduction initiation, and strongly influence the timing. The main difference between straight (cylindrical) and curved margins is that the orientation of the pressure gradient in 3-D is no longer constant, thus producing a horizontal, along-margin component of flow. We thus conclude that the reason for the impedance of subduction initiation is the result of partitioning of the vertical velocity component into a horizontal component, which therefore decreases the effective slab pull. We infer that, although favorable for subduction initiation in a 2-D model, because the estimated force balance is adequate, the pronounced curvature in the southeast Brazilian margin is a likely explanation why subduction initiation is hampered there.

Three‐dimensional simulations of the southern polar giant impact hypothesis for the origin of the Martian dichotomy

We demonstrate via numerical simulations that the impact of a ~lunar-sized body with Mars is capable of creating a hemispherical magma ocean that upon cooling and solidification resulted in the formation of the southern highlands and thus the Martian dichotomy. The giant impact may have contributed a significant amount of iron to the Martian core and generated a deep thermal anomaly that led to the onset and development of the volcanism in the southern highlands. Our model predicts several mantle plumes converging to the South Pole from the equatorial regions as well as new plumes forming in the equatorial region and also an absence of significant large-scale volcanism in the northern lowlands. The core heat flux evolution obtained from our numerical models is consistent with the decline of the magnetic field. We argue that such a scenario is more consistent with a range of observations than a northern giant impact (excavating the Borealis basin) for the formation of the Martian dichotomy.

Electro-pulse-boring (EPB): Novel super-deep drilling technology for low cost electricity

The inexhaustible heat deposit in great depths (5–10 km) is a scientific fact. Such deposit occurs around the globe. Thereby, everybody is enabled to generate autonomously clean and renewable energy, ample electricity and heat. The economical exploration and exploitation of this superdeep geothermal heat deposit requires a novel drilling technique, because the currently only deep drilling method (Rotary) is limited to about 5 km, due to the rising costs, depending exponentially on depth. Electro-pulse-boring (EPB) is a valuable option to Rotary drilling. EPB, originally investigated in Russia, is ready to be developed for industrialization. The feasibility of EPB is proven by many boreholes drilled up to 200 m in granite (crystalline). Estimates show outstanding low costs for drilling by EPB: 100 zs/m for a borehole with a large diameter (Ø) such as 20″ (50 cm), independent on depth and applicable likewise for sediments and crystalline rocks, such as granite. The current rate of penetration (ROP) of 3 m per hour is planned to be augmented up to 35 m per hour, and again, irrespective whether in sedimentary or crystalline formations. Consequently, a 10 km deep borehole with Ø 50 cm will ultimately be drilled within 12 days. EPB will create new markets, such as: (i) EPB shallow drilling for geotechnics, energy piles, measures in order to mitigate natural hazards, etc., (ii) EPB deep drilling (3–5 km) for hydro-geothermics, exploration campaigns etc. and (iii) EPB super-deep drilling (5–10 km) for petro-geothermics, enabling the economic generation of electricity. The autonomous and unlimited supply with cost efficient electricity, besides ample heat, ensures reliably clean and renewable energy, thus, high supply security. Such development will provide a substantial relief to cope with the global challenge to limit the climate change below 2 °C. The diminution of fossil fuels, due to the energy transition in order to mitigate the climate change, implies likewise the decrease of air pollution.

On the dynamics of 3-D single thermal plumes at various Prandtl numbers and Rayleigh numbers

Three-dimensional (3-D) numerical simulations of single turbulent thermal plumes in the Boussinesq approximation are used to understand more deeply the interaction of a plume with itself and its environment. In order to do so, we varied the Rayleigh and Prandtl numbers from Ra ∼ 105 to Ra ∼ 108 and from Pr ∼ 0.025 to Pr ∼ 70. We found that thermal dissipation takes place mostly on the border of the plume. Moreover, the rate of energy dissipation per unit mass ε T has a critical point around Pr ∼ 0.7. The reason is that at Pr greater than ∼0.7, buoyancy dominates inertia and thermal advection dominates wave formation whereas this trend is reversed at Pr less than ∼0.7. We also found that for large enough Prandtl number (Pr ∼ 70), the velocity field is mostly poloidal although this result was known for Rayleigh–Bénard convection (see Schmalzl et al. [On the validity of two-dimensional numerical approaches to time-dependent thermal convection. Europhys. Lett. 2004, 67, 390--396]). On the other hand, at small Prandtl numbers, the plume has a large helicity at large scale and a non-negligible toroidal part. Finally, as observed recently in details in weakly compressible turbulent thermal plume at Pr = 0.7 (see Plourde et al. [Direct numerical simulations of a rapidly expanding thermal plume: structure and entrainment interaction. J. Fluid Mech. 2008, 604, 99--123]), we also noticed a two-time cycle in which there is entrainment of some of the external fluid to the plume, this process being most pronounced at the base of the plume. We explain this as a consequence of calculated Richardson number being unity at Pr = 0.7 when buoyancy balance inertia.