The longevity of lunar volcanism: Implications of thermal evolution calculations with 2D and 3D mantle convection models

Lunar volcanism, as is generally accepted, started well before the emplacem ent of the mare hh at 3.1-39 Ga b.p. and likely extended well into the Erat osthenian (3.1-1.1 Ga b.p.), The early volcanism is relatively easily under stood. The extension of the activity to, possibly, 1.5 Ga b.p., albeit at a decreasing rate, poses a problem since a relatively small body such as the Moon may be expected to cool rapidly and freeze a partially molten mantle layer quickly. We present thermal history models in which a region of parti al melt forms in the mantle underneath the lithosphere almost immediately a fter the start of the model at depths between 300 and 700 km depending on t he chosen initial depth of the magma ocean. To calculate the thermal histor ies we use axisymmetric 2D and fully 3D spherical shell convection codes wi th viscosity depending on the azimuthally averaged temperature. The thermal evolution of the Moon is found to be characterized by the growth of a mass ive 700-to 800-km-thick lithosphere while the lower mantle and core cool on ly by 100-200 K. The partial melt zone decreases in thickness with time fro m top to bottom until it vanishes at times between 3.4 and 2.2 Ga b.p. The maximum degree of partial melting is between roughy 10 and 20%. The partial melt layer is initially global and is disrupted by cold downwellings as co oling proceeds. The melt zone freezes from above due to the thickening of t he lithosphere, which implies that the source region of volcanic rock proce eds to increasing depth with time. This corresponds nicely with the variati on of titanium with age of lunar mare basalts. Mixing of the melt zone with the underlying mantle chemically rejuvenates the melt zone to some degree and, thus, the source region of the mare basalts. (C) 2001 Academic Press.