Have you done the experiments to determine how much iron goes into the solid part and how much into the melted part? If so, have you considered that the formerly melted part is now represented by the lava rock at the surface and the solid part is the stuff inside the Moon (e.g. Moon mantle)? You can learn the concentration of iron in each of the solid and liquid parts and calculate the ratio of these. At what temperature will you get the observed ratio of iron in the solid part (the mantle) to iron in the liquid part (the basalt)? Rethink your answer.