Searcharxiv⌕ Search

arXiv subjects

Masaki Ogawa

Publications and source records attributed to Masaki Ogawa.

12 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

Nielsen equivalence and multisections of 4-manifolds

Islambouli showed that there exist infinitely many 4-manifolds admitting non-isotopic trisections using a Nielsen equivalence, which can be used to construct non-isotopic Heegaard splittings. In this paper, we show that there exist infinitely many 4-manifolds admitting non-isotopic bisections in the same way. Moreover, we show that there exist infinitely many 4-manifolds admitting non-isotopic 4-sections by considering the doubles of the bisections.

math.GT↗
arXiv2024

Shadow-complexity and trisection genus

The shadow-complexity is an invariant of closed $4$-manifolds defined by using $2$-dimensional polyhedra called Turaev's shadows, which, roughly speaking, measures how complicated a $2$-skeleton of the $4$-manifold is. In this paper, we define a new version $\mathrm{sc}_{r}$ of shadow-complexity depending on an extra parameter $r\geq0$, and we investigate the relationship between this complexity and the trisection genus $g$. More explicitly, we prove an inequality $g(W) \leq 2+2\mathrm{sc}_{r}(W)$ for any closed $4$-manifold $W$ and any $r\geq1/2$. Moreover, we determine the exact values of $\mathrm{sc}_{1/2}$ for infinitely many $4$-manifolds, and also we classify all the closed $4$-manifolds with $\mathrm{sc}_{1/2}\leq1/2$.

math.GT↗
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

Trisections induced by the Gluck surgery along certain spun knots

Gay and Meier asked whether or not a trisection diagram obtained by the Gluck twist on a spun or a twist spun 2-knot obtained from some method is standard. In this paper, we depict the trisection diagrams explicitly when the 2- knot is the spun $(2n + 1, -2)$-torus knot, where $n\geq1$, and show that the trisection diagram is standard when $n = 1$. Moreover, we introduce a notion of homologically standard for trisection diagrams and show that the trisection diagram is homologically standard for all $n$.

math.GT↗
arXiv2023

Weinstein trisections of trivial surface bundles

Weinstein trisection is a trisection of a symplectic 4-manifold whose 1-handlebodies are the Weinstein domain for the symplectic structure induced from an ambient manifold. Lambert-Cole, Meier, and Starkston showed that every closed symplectic 4-manifold admits a Weinstein trisection. In this paper, we construct a Weinstein trisection of $Σ_g\times Σ_h$. As a consequence of this construction, we construct a little explicit Weinstein trisection of $S^2\times S^2$.

math.GT↗
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↗
arXiv2023

Some lower bounds for the Kirby-Thompson invariant

Kirby and Thompson introduced a non-negative integer-valued invariant, called the Kirby-Thompson invariant, of a $4$-manifold using trisections. In this paper, we give some lower bounds for the Kirby-Thompson invariant of certain $4$-manifolds. As an application, we determine the Kirby-Thompson invariant of the spin of $L(2,1)$, which is the first example of a $4$-manifold with non-trivial Kirby-Thompson invariant. We also show that there exist $4$-manifolds with arbitrarily large Kirby-Thompson invariant.

math.GT↗
arXiv2022

Trisections with Kirby-Thompson length 2

Kirby and Thompson introduced a length of a trisection. They also defined the length of a 4-manifold as the minimum of length among all lengths of trisection of a 4-manifold. In this paper, we consider trisections whose Kirby-Thompson length is 2. Kirby and Thompson conjectured that length 2 trisection is a trisection of 4-manifold with length 0. We shall prove this conjecture in this paper.

math.GT↗
arXiv2022

Handlebody decompositions of 3-manifolds and polycontinuous patterns

We introduce the concept of a handlebody decomposition of a 3-manifold, a generalization of a Heegaard splitting, or a trisection. We show that two handlebody decompositions of a closed orientable 3-manifold are stably equivalent. As an application to materials science, we consider a mathematical model of polycontinuous patterns and discuss a topological study of microphase separation of a block copolymer melt.

math.GT↗
arXiv2021

Decompositions of the 3-sphere and lens spaces with three handlebodies

In this paper, we consider decompositions of 3-manifolds with three handlebodies. We classify such decompositions of the 3-sphere and lens spaces with small genera. These decompositions admit operations called stabilizations. We also determine whether these decompositions are stabilized.

math.GT↗