Vertical distribution of heat generation in the Idaho batholith

Abstract

The technique of γ-ray spectrometry has been used to measure the radioactive heat generation (from K, U, Th) for a wide variety of rocks that comprise the Idaho batholith of central Idaho. These heat generation values are then plotted against depth of emplacement, a parameter that can be estimated from various geologic and petrographic information. Two groups of data are presented. In each case, heat generation decreases exponentially with increasing depth. In the first case, only quartz monzonites whose potassium content varies between 2.7% and 3.4% are considered to show that heat generation decreases with increasing depth in a manner essentially independent of magmatic evolution. In the second case, only the apparently genetically related gabbro to granite suite of the northwest corner of the batholith is considered. In both cases, heat generation is related to depth by the equation A(z) = A0 exp (−z/b), where A0 is surface heat generation, z is depth, and b is a constant calculated to be in the range 8–10 km. The exponential model suggested by both groups of data is consistent with the linear relation between heat flow and heat generation for plutonic bodies.