As outlined in Turcotte & Schubert (Geodynamics, Applications of continuum mechanics to geological problems, John Wiley & Sons, p140), the heat production for any rock at past time, t, is given by

0.9928 * CUo * HU238 * exp( t ln(2)/ t0.5U238) +
0.0072 * CUo * HU235 * exp( t ln(2)/ t0.5U235) +

CTho * HTh * exp( t ln(2)/ t0.5Th ) +

1.28 *10-4 * CKo * HK * exp( t ln(2)/ t0.5K) +