| abstract: |
The size of a planet exerts control over its tectonic and volcanic activities. Our linear stability analysis of thermal convection of a variable viscosity fluid in a basally heated spherical shell suggests that the present mantle of the Moon that appears to be tectonically dead is indeed convectively stable, while that the mantles of Mars and larger planets that are more tectonically active are unstable. We further carried out two-dimensional numerical experiments of a coupled magmatism-mantle convection system, and found that magmatism also depends on the size of planet. In the Moon, mantle upwelling flow causes mild magmatism that continues for several hundred Myr. In larger planets, however, magmatism that is caused by a mantle upwelling flow boosts the flow itself. This positive feedback (the magmatism-mantle upwelling, or MMU, feedback) makes the magmatism vigorous and episodic. In Venus and the Earth, the solid-solid phase transitions at the top of the lower mantle also plays an important role in magmatism in the early mantle that contains a large amount of heat producing elements (HPEs); the phase transitions cause episodic upwelling flow, or burst, of the hot lower mantle materials into the upper mantle that triggers vigorous magmatism. In spite of compositional differentiation of the mantle by this magmatism, the mantle remains rather homogeneous because of a strong convective stirring due to the MMU feedback. We then numerically reproduced tectonic plates by a stress-history dependent rheology of the lithosphere, and obtained a two-stage evolution model of the Earth. On the early stage, bursts frequently occur to make plate motion chaotic and keep the mantle rather compositionally homogeneous. As HPEs decay, however, the mantle evolves into the second stage where bursts subside, tectonic plates move more steadily, and subducted basaltic crusts accumulate on the core-mantle boundary to form a large thermo-chemical pile. For more comprehensive understanding of the evolution of terrestrial planets, it is crucial to develop three-dimensional spherical models of the coupled magmatism-mantle convection system, and also to clarify the relationship between planetary formation processes and mantle evolution.
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