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The evidence actually says all the earth's heating takes place in the mantle and crust, not the core.

All models of the inner Earth depend on indirect evidence. Leading models of the kind known as bulk silicate Earth (BSE) assume that the mantle and crust contain only lithophiles ("rockloving" elements) and the core contains only siderophiles (elements that "like to be with iron"). Thus all the heat from radioactive decay comes from the crust and mantle - about eight terawatts from uranium $(238\text{U)}$, another eight terawatts from thorium $232$($232$Th), and four terawatts from potassium $40$($40\text{K}$).So the heating from $\text{2.89脳10152.89脳1015办驳}$of uranium, thorium, and potassium isn't in the core but it surrounds the core. The earth generates $20-29$(varies by source) terawatts from radioactive decay of heavy metals in the core and will be generating heat for over $90$billion years.

There are a lot of different elements in the core brought on by a period of material differentiation within the crust and the mantle. The heavy metals migrated to the core when the earth was re-liquefied shortly after its formation and during the creation of the moon from a planet-sized collision.

Hence Iron is abundant in the core because as the earth was re-liquified after its formation, the heavy metals migrated to the core of the earth.