The Lf-Hf evolution of KREEP

The most widely accepted theory for lunar evolution is the cumulate remelting model. According to the model, the outer several hundred kilometers of the Moon were molten early in its history at about 4.4 to 4.5 billion years ago. As this magma ocean cooled minerals began to crystallize and accumulate either at the base or the top of the magma ocean depending upon the density of the mineral. The accumulated minerals at the base were partially remelted at about 3.9 to 3.2 billion years to produce the mare basalts.