Relative Contributions of Uranium, Thorium, and Potassium to Heat Production in the Earth

Data from a wide variety of igneous rock types show that the ratio of potassium to uranium is approximately 1 $\times $ 10$^{4}$. This suggests that the value of K/U $\approx $ 1 $\times $ 10$^{4}$ is characteristic of terrestrial materials and is distinct from the value of 8 $\times $ 10$^{4}$ found in chondrites. In a model earth with K/U $\approx $ 10$^{4}$, uranium and thorium are the dominant sources of radioactive heat at the present time. This will permit the average terrestrial concentrations of uranium and thorium to be 2 to 4.7 times higher than that observed in chondrites. The resulting models of the terrestrial heat production will be considerably different from those for chondritic heat production because of the longer half-life of U$^{238}$ and Th$^{232}$ compared with K$^{40}$.