arXiv subjects

Masanori Kameyama

Publications and source records attributed to Masanori Kameyama.

5 recordsLinked to original sources

arXiv2025

Partially Molten Plumes and Melt-Fingers: Two Modes of Magma-Transport through the Mantle in Terrestrial Bodies

To understand the dynamics of partially molten mantle in terrestrial bodies, we carried out a linear perturbation analysis and 2-D numerical simulations of magma-matrix flow in a horizontal layer, where decompression melting generates magma that percolates through the convecting matrix. Our study shows that there are two regimes for the upward migration of magma, depending on the melt-buoyancy parameter B_m, which is the ratio of the Stokes velocity of matrix to the percolation velocity of melt, both driven by the melt-buoyancy. At large B_m, the magmatism-mantle upwelling (MMUb) feedback dominates the convective flow in the layer: decompression melting during upwelling enhances magma buoyancy, which further strengthens the upwelling. When a solid layer is overlaid on the partially molten layer, the MMUb feedback induces partially molten plumes that ascend through the solid layer by their melt-buoyancy. At lower B_m, in contrast, a perturbation in the melt-content in the partially molten layer propagates upward as a porosity wave: the perturbation induces a spatial variation in the rate of expansion or contraction of matrix caused by magma migration, leading to an upward shift of the perturbation. When a solid layer is overlaid, the porosity wave develops also along the layer boundary to induce a finger-like magma structure, or melt-finger, that extends upward into the solid layer. The threshold value of B_m for MMUb feedback suggests that it can explain volcanism forming Large Igneous Provinces, but not hotspot volcanism on Earth. Since B_m increases with decreasing matrix viscosity, volcanism caused by the MMUb feedback is likely to have been more important in earlier terrestrial planets where the mantle was hotter and softer. Melt-fingers are, in contrast, expected to have developed in the lunar mantle if a partially molten layer has developed at its base in the history of the Moon.

physics.geo-ph↗

arXiv2024

Resurgence of Lunar Volcanism: Role of Localized Radioactive Enrichment in a Numerical Model of Magmatism and Mantle Convection

We develop a 2-D numerical model of magmatism and mantle convection to understand the volcanism on the Procellarum KREEP terrane (PKT) of the Moon, which continued for billions of years with two peaks of activities at 3.5-4 Gyr ago and around 2 Gyr ago. In our model, the effects of the PKT on lunar evolution are considered by initially imposing a region of localized radioactive enrichment. The calculated volcanism has two peaks induced by different mechanisms. The first peak occurs at 3.5-4 Gyr ago when magma generated in the deep mantle by internal heating ascends to the surface as partially molten plumes. The basaltic blocks in the uppermost mantle formed by this magmatism, then, sink to the deep mantle, triggering further plumes that cause the resurgence of volcanism at $\sim$2 Gyr ago. Our model shows that localized radioactive enrichment is important for the long-lasting volcanism with a couple of peaks.

physics.geo-ph↗

arXiv2023

The volcanic and radial expansion/contraction history of the Moon simulated by numerical models of magmatism in the convective mantle

To understand the evolution of the Moon, we numerically modeled mantle convection and magmatism in a two-dimensional polar rectangular mantle. Magmatism occurs as an upward permeable flow of magma generated by decompression melting through the convecting matrix. The mantle is assumed to be initially enriched in heat-producing elements (HPEs) and compositionally dense ilmenite-bearing cumulates (IBC) at its base. Here, we newly show that magma generation and migration play a crucial role in the calculated volcanic and radial expansion/contraction history. Magma is generated in the deep mantle by internal heating for the first several hundred million years. A large volume of the generated magma ascends to the surface as partially molten fingers and plumes driven by melt-buoyancy to cause a volcanic activity and radial expansion of the planet with the peak at 3.5-4 Gyr ago. Eventually, however, the planet begins to radially contract when the mantle solidifies by cooling from the surface boundary. As the mantle is cooled, the activity of partially molten plumes declines but continues for billions of years after the peak because some basal materials enriched in the dense IBC components hold HPEs. The calculated volcanic and radial expansion/contraction history is consistent with the observed history of the Moon. Our simulations suggest a substantial fraction of the mantle was solid, and there was a basal layer enriched in HPEs and the IBC components at the beginning of the history of the Moon.

astro-ph.EP↗

arXiv2004

Multigrid iterative algorithm using pseudo-compressibility for three-dimensional mantle convection with strongly variable viscosity

A numerical algorithm for solving mantle convection problems with strongly variable viscosity is presented. Equations for conservation of mass and momentum for highly viscous and incompressible fluids are solved iteratively by a multigrid method in combination with pseudo-compressibility and local time stepping techniques. This algorithm is suitable for large-scale three-dimensional numerical simulations, because (i) memory storage for any additional matrix is not required and (ii) vectorization and parallelization are straightforward. The present algorithm has been incorporated into a mantle convection simulation program based on the finite-volume discretization in a three-dimensional rectangular domain. Benchmark comparisons with previous two- and three-dimensional calculations including the temperature- and/or depth-dependent viscosity revealed that accurate results are successfully reproduced even for the cases with viscosity variations of several orders of magnitude. The robustness of the numerical method against viscosity variation can be significantly improved by increasing the pre- and post-smoothing calculations during the multigrid operations, and the convergence can be achieved for the global viscosity variations up to 1e10.

physics.geo-ph↗

arXiv2004

A 15.2 TFlops Simulation of Geodynamo on the Earth Simulator

For realistic geodynamo simulations, one must solve the magnetohydrodynamic equations to follow time development of thermal convection motion of electrically conducting fluid in a rotating spherical shell. We have developed a new geodynamo simulation code by combining the finite difference method with the recently proposed spherical overset grid called Yin-Yang grid. We achieved performance of 15.2 Tflops (46% of theoretical peak performance) on 4096 processors of the Earth Simulator.

physics.geo-ph↗