Estimating the “thickness” of the Boulder Batholith, Montana, from heat-flow and heat-productivity data

Estimates of minimum thickness of the Boulder batholith, computed using the linear relation between heat flow and heat productivity and assuming constant heat productivity with depth, are highly nonspecific. They can vary between about 3 and 20 km, depending on values of surface-rock heat productivity and values of assumed contribution of nonbatholith heat sources (such as lower crustal and upper mantle) to the measured surface heat flow used in the calculations. Models involving radiogenic heat sources decreasing with depth in the batholith lead to significantly greater estimates of thickness by as much as a factor of two or more. A reappraisal of data and arguments related to earlier conflicting estimates, based only on one heat-flow determination and within the context of several newly published additional heat-flow measurements, indicates that the previous differences of opinion are negligible and acceptable, in view of the enormous uncertainties inherent in the method of estimation.