Systematics of Crustal Radioactivity Distribution and Implication for Mantle Thermal Evolution Through Time
Mahesh Thakur and David D. Blackwell
Huffington Department of Earth Sciences, Southern Methodist University, Dallas, Texas
The implications of the global terrain average heat flow - heat production relation are profound in understanding the stabilization of the lithosphere and sublithospheric heat flow through time. The Q-A plot is able to distinguish the changes in (Qo) mantle heat flow component from radioactivity in the crust. For example in the Sierra Nevada an outer arc of subduction zone, the Qo is ~ 20 mWm-2 , in the active extension zone of the Basin and Range Qo is ~ 60 mWm-2 ; and in stable area of Eastern United States (craton) Qo is ~ 30mWm-2. The fact that the linear fits to the surface heat flow and radioactivity in the eastern US and the terrain average relationship for all continents (36 data points) have same intercept of ~ 32 mWm-2, is a strong constrain on the mantle heat flow in PreMesozoic terrains and on lithosphere thickness. The relationship also provides the proof of the extreme upward concentration of radioactivity in the continental crust necessary for Archean lithosphere stabilization. The 32±5 mWm-2 intercept value provides reasonable mantle heat flow and sublithospheric estimates for the present (27±5 mWm-2) and the past. Estimates of Bulk Silicate Earth radioactivity imply a mantle heat production of 0.013 μWm-3, which considering only the lower mantle thickness of 2230 km, gives a rough estimate of the basal heat flow ~ 28 mWm-2. On the other hand, considering the models (in Slave craton and Superior), which predict mantle heat flow in the range of 10 – 15 mWm-2, if radioactivity in the cratonic mantle is applied (0.02μWm- 3) these geotherms predict the basal heat flow in the range of ~ 1.4 – 5.7 mWm-3 with large lithospheric thicknesses of 328-548 km. Excess radioactivity of 0.4μWm-3 in the lower crust causes the inferred low mantle heat flow, low basal heat flow and large lithospheric thickness.
Thus, the global Q-A relationship is able to explain that the variations of surface heat flow are due to radioactivity which in turn is related of age differences in the terrains which stabilized at different time in the history of Earth. The vertical distribution of radioactivity only makes geological sense when it is concentrated in the upper crust (constant 10 ± 4 km), or decreases exponentially with depth.
The linear fit Q = 65.64-5.6 t to the surface heat flow with age implies (where Q is the surface heat flow and t is time in billion years) that decrease in surface heat flow is 5.6 mWm-2 per billion years. Assuming a mantle heat flow of 20 mWm-2 and modeling the decay of various bulk radioactivities with age (0.4-0.9 μWm-3) implies that bulk radioactivity of the crust is 0.5 ± 0.1 μWm-3 . This means that total heat generation by radioactivity in the crust is 3.3- 4.9 Tera Watt, which imples that 16-24 % of the total heat producing elements are in the crust.
AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas