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[PPS09-14] Evolution of the lunar interior caused by magmatism and mantle convection inferred from a 3-D regionally spherical model
Keywords:Moon, Numerical Analysis, Mantle Convection, Magmatism
To understand the mantle evolution of the Moon, we newly developed a numerical model of magmatism and mantle convection in a 3-D regionally spherical model. Magma is generated by decompression melting and/or internal heating. The generated magma migrates upward as a permeable flow through the coexisting matrix driven by the density difference between the solid and liquid phases. The migrating magma transports the basaltic component and incompatible heat-producing elements (HPEs) that decay with time.
Magma generation and mantle convection driven by melt-buoyancy control the calculated history of volcanism and radial expansion/contraction. Magma is generated in the deep mantle and rises as partially molten plumes to the uppermost mantle for the first 800 million years of the calculated history. The Moon expands by 1 km owing to volume change caused by melting of the mantle. The magma transports HPEs and the basaltic component to the uppermost mantle to form regional basaltic blocks in the uppermost mantle. The basaltic blocks then sink into the deep mantle owing to their compositionally induced negative buoyancy, and the upward counterflows caused by the descending basaltic blocks induce partially molten plumes that ascend to cause further volcanism for around 2 billion years. After that, the Moon gradually contracts as the mantle solidifies at the rate of around -1 km Gyr-1 until the end of the calculated history. By comparing the 3-D spherical model with the 2-D polar rectangular model we developed in a previous study, we found that the dimension of the lunar model does not affect the overall features of mantle evolution but that quantitative values, such as the amplitude of the radial expansion and the rate of magma flux, become smaller; the values are more consistent with observations. Besides, the locations of upwelling partially molten plumes tend to stay steady throughout the calculated history despite the perturbations caused by descending basaltic blocks. The result could impose a constraint on the origin of the volcanic heterogeneity of the Moon; the lunar volcanism was active in some regions and continued there for more than a few billion years.
The figure shows a snapshot of the distribution of melt fraction at around 700 million years after the start of the calculation. The contour lines show the magma distribution at 1%.
Magma generation and mantle convection driven by melt-buoyancy control the calculated history of volcanism and radial expansion/contraction. Magma is generated in the deep mantle and rises as partially molten plumes to the uppermost mantle for the first 800 million years of the calculated history. The Moon expands by 1 km owing to volume change caused by melting of the mantle. The magma transports HPEs and the basaltic component to the uppermost mantle to form regional basaltic blocks in the uppermost mantle. The basaltic blocks then sink into the deep mantle owing to their compositionally induced negative buoyancy, and the upward counterflows caused by the descending basaltic blocks induce partially molten plumes that ascend to cause further volcanism for around 2 billion years. After that, the Moon gradually contracts as the mantle solidifies at the rate of around -1 km Gyr-1 until the end of the calculated history. By comparing the 3-D spherical model with the 2-D polar rectangular model we developed in a previous study, we found that the dimension of the lunar model does not affect the overall features of mantle evolution but that quantitative values, such as the amplitude of the radial expansion and the rate of magma flux, become smaller; the values are more consistent with observations. Besides, the locations of upwelling partially molten plumes tend to stay steady throughout the calculated history despite the perturbations caused by descending basaltic blocks. The result could impose a constraint on the origin of the volcanic heterogeneity of the Moon; the lunar volcanism was active in some regions and continued there for more than a few billion years.
The figure shows a snapshot of the distribution of melt fraction at around 700 million years after the start of the calculation. The contour lines show the magma distribution at 1%.