Magma Buoyancy Couples Lunar Mantle Convection and Magmatism: Long-Lasting Mare Volcanism and Radius Change in a 3-D Regional Model

To understand the mantle evolution that has caused the observed histories of mare volcanism and radius change of the Moon, we developed a 3-D regional spherical shell mantle convection model with magmatism. Magma generated by decompression melting segregates from the convecting matrix and percolates upward through it. In our model, internal heating forms a basal partially molten layer during the first several hundred million years. Finger-like magma ascents then grow from the top of this layer and evolve into buoyant, partially molten plumes. The plumes ascend by the buoyancy of magma, while decompression melting in the ascending plumes generates more magma that further drives the ascent. Magma supply to the top of the mantle peaks between 4 and 3.5 Gyr ago, coincident with a maximum in radial expansion driven by volume change of the mantle associated with melting, and both decline as the mantle cools and solidifies. Magma supply, however, persists for billions of years because earlier plumes leave thermally softened conduits, through which later plumes repeatedly rise at fixed locations. These stable conduits are a consequence of the 3-D geometry. Recycling of magmatic products formed in the uppermost mantle also triggers renewed magma ascent through the conduits. Our results provide a mechanism for the observed long-lasting localized volcanism that peaked at 4-3.5 Gyr ago and quantitatively account for early expansion followed by contraction. The conditions required for consistency with the observed history imply an initially hot but mostly solid mantle with a dense basal layer.