RESULTS OF RADIOGENIC HEAT PRODUCTION MEASUREMENTS THROUGH THE ENTIRE THICKNESS OF THE SIERRA NEVADA BATHOLITH, CALIFORNIA: A NON-EXPONENTIAL DISTRIBUTION CONTROLLED BY HYDROTHERMAL PROCESSES?

A new paleodepth vs. radiogenic heat production database for the Sierra Nevada batholith, California suggests a pattern of radioelement distribution that is inconsistent with the downward-decreasing exponential function predicted from modeling of surface heat flow data. The database shows a wide range of heat production values at any given depth, but an overall pattern of moderate heat production of ~2 µW/m3 within the ~3-km-thick volcanic pile at the top of the batholith, below which heat production increases to an average value of ~3.5 µW/m3 at ~5.5 km, then decreases to ~0.5-1 µW/m3 at ~15 km depth and remains in this range through the entire crust below 15 km. Below the crust, from ~40-120 km depth, the mantle appears to have an average heat production rate of ~0.14 µW/m3, which is higher than the rates from most published xenolith studies, but consistent with the relatively young age of the Sierran lithosphere. The database suggests values of heat production in the lower crust that are 2-10 times higher than those predicted by an exponential distribution model. Calculations based on the observed distribution of radiogenic elements predict a reasonable range of geothermal gradients (with basal crustal temperatures highly dependent on the poorly constrained mantle heat flux). Interestingly, these data also predict that essentially all of the present day surface heat flow from the Sierra Nevada could be generated within the crust, requiring that there be very little heat flux from the mantle, and/or significant post-Cretaceous crustal modification. Variation of heat production with depth is principally due to variations in U and Th concentration; K concentration does not vary as greatly within the Sierran crust. Since silica content is also relatively constant through the upper ~30 km of the Sierran batholith, radiogenic heat production apparently does not vary with silica content, and magmatic differentiation processes are therefore unlikely to have been the principal control on the distribution of heat production within the Sierran crust. Instead, hydrothermal remobilization and deposition of U- and Th-rich minerals over a depth interval where crustal fluids might change from oxidizing to reducing in character is proposed as the more likely principal controlling mechanism.