HR: 1340h
AN: T23A-1213    [Abstracts]
TI: Secular Evolution of Mantle Heat Flow in Precambrian Terrains
AU: * Thakur, M
EM: mthakur@smu.edu
AF: Southern Methodist University, Dept. of Geological Sciences, Dallas, TX 75275-0395, United States
AU: Blackwell, D D
EM: blackwel@smu.edu
AF: Southern Methodist University, Dept. of Geological Sciences, Dallas, TX 75275-0395, United States
AB: We complied a global continental average heat flow - heat production data (Q-A plot) for 32 Precambrian terrains. A linear fit to the data gives Q = 10 X + 32 . There is no correlation between reduced heat flow and age. On the global average Q-A plot, high average surface heat flow in Precambrian terrains correlates well with the high average heat production in the crust. Mantle heat flow predicated by the average Q-A relationship is 25 ± 5 mWm^-2 for Archean, Proterozoic and Paleozoic terrains. Using the characteristic depth D =10 km as empirically determined by the global average Q-A plot and applying a decay correction, we generate the past surface heat flow versus age best fit line (32 terrains) as Q = 64 +1.9t (t time in Ga), in contrast to uncorrected relationship which is best fit line to the present surface heat flow versus age as of Q = 64 - 5.3t . This shows that the present best fit line to the surface heat flow versus age is mainly controlled by the decay of radioactive elements. Consequently, the lower surface heat flow predicated for terrains in the age range of 3.9 - 2.5 Ga is a result of decay of crustal radioactivity to ~ ≥ half its initial value, as the half life of crustal radioactivity is comparable to the ages of Archean cratons ~ (2.5 - 3.9 Ga) . Thus it is clear that variations in surface heat flow are mainly controlled by shallow crustal radioactivity rather than deep lithosphere thermal processes. Modeling of the crustal radioactivity in the Archean crust through time compared with the present best fit line to the surface heat flow versus age, also favors Archean crustal radioactivity in the range of ~ 0.4-0.5 μ Wm^-3, assuming constant mantle heat flow. What we found is in contrast to the concept that Q_o = 0.6Q, (where Q_o is the reduced heat flow) i.e., that the radioactivity is same in terrains of different ages and variations in surface heat flow are mainly controlled by variations in the mantle heat flow. Using linear fit to the surface heat flow Q = 65 - 9t and reduced heat flow Q_o = 48 - 9t versus crustal age of Precambrian terrains, it has been proposed that there is secular increase in the mantle heat flow with decreasing age. In the Archean crust mantle heat flow is 10 - 30 mWm^-2 and in the late Proterozoic crust it is 30 - 45 mWm^-2, with a total variation of 10 - 45 mWm^-2 . A surface heat flow difference of ~ 15 mWm^-2 has been implied by the difference in the mantle heat flow between Archean and Late Proterozoic terrain which in turn is inferred to be due to lithospheric thickness contrast. Seismic tomography implies a difference of ~ 100 - 200 km in the thickness of lithosphere between Archean cratons (3.9 - 2.5 Ga) and post -Archean terrains (< 2.5 Ga). However this analysis shows that the low surface heat flow in the Archean cratons is due to the decay of radioactivity in comparison to the model of low mantle heat flow due to thick lithosphere. Therefore, heat flow data complements xenolith data which do not support proposed lithospheric thickness differences between PreMesozoic terrains of different ages.
DE: 1020 Composition of the continental crust
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 5400 PLANETARY SCIENCES: SOLID SURFACE PLANETS
DE: 5418 Heat flow
DE: 8125 Evolution of the Earth (0325)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting