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Geodynamics Heat conduction and production Lecture 7.3 - Heat production Lecturer: David Whipp [email protected] Geodynamics www.helsinki.fi/yliopisto 1 Goals of this lecture • Discuss radiogenic heat production in the Earth, how it varies and the implications for geodynamic processes 2 Radiogenic heat production • Radiogenic heat production, 𝐴 or 𝐻, results from the decay of radioactive elements in the Earth, mainly 238U, 235U, 232Th and 40K. 𝐴 is generally used for volumetric heat production and 𝐻 for heat production by mass. • These elements occur in the mantle, but are concentrated in 248 Heat Transfer the crust, where radiogenic heating can be significant • Table Typicalheat Concentrations of the Heat-Producing Elements The4.3 surface flow in continental regions is ~65inmW m-2 Several Rock Types and Average Concentrations in Chondritic and ~37 mW m-2 isthefrom radiogenic heat production (57%) Meteorites Type Rock Reference undepleted (fertile) mantle “Depleted” peridotites Tholeiitic basalt Granite Shale Average continental crust Chondritic meteorites U (ppm) Concentration Th (ppm) K (%) 0.031 0.001 0.07 4.7 3.7 1.42 0.008 0.124 0.004 0.19 20 12 5.6 0.029 0.031 0.003 0.088 4.2 2.7 1.43 0.056 Turcotte and Schubert, 2014 3 Radiogenic heat production • Radiogenic heat production, 𝐴 or 𝐻, results from the decay of radioactive elements in the Earth, mainly 238U, 235U, 232Th and 40K. 𝐴 is generally used for volumetric heat production and 𝐻 for heat production by mass. • These elements occur in the mantle, but are concentrated in 248 Heat Transfer the crust, where radiogenic heating can be significant • Table Typicalheat Concentrations of the Heat-Producing Elements The4.3 surface flow in continental regions is ~65inmW m-2 Several Rock Types and Average Concentrations in Chondritic and ~37 mW m-2 isthefrom radiogenic heat production (57%) Meteorites Type Rock Reference undepleted (fertile) mantle “Depleted” peridotites Tholeiitic basalt Granite Shale Average continental crust Chondritic meteorites U (ppm) Concentration Th (ppm) K (%) 0.031 0.001 0.07 4.7 3.7 1.42 0.008 0.124 0.004 0.19 20 12 5.6 0.029 0.031 0.003 0.088 4.2 2.7 1.43 0.056 Turcotte and Schubert, 2014 4 Radiogenic heat production Heat production Rock type By mass, 𝐻 [W kg-1] Reference undepleated (fer/le) mantle 7.39E-12 2.44E-08 0.024 "Depleated" perido/tes 3.08E-13 1.02E-09 0.001 Tholeii/c basalt 1.49E-11 4.41E-08 0.044 Granite 1.14E-09 3.01E-06 3.008 Shale 7.74E-10 1.86E-06 1.857 Average con/nental crust 3.37E-10 9.26E-07 0.927 Chondri/c meteorites 3.50E-12 1.15E-08 0.012 By volume, 𝐴 By volume, 𝐴 [W m-3] [µW m-3] Calculated from Turcotte and Schubert, 2014 5 Radiogenic heat production Heat production Rock type By mass, 𝐻 [W kg-1] Reference undepleated (fer/le) mantle 7.39E-12 2.44E-08 0.024 "Depleated" perido/tes 3.08E-13 1.02E-09 0.001 Tholeii/c basalt 1.49E-11 4.41E-08 0.044 Granite 1.14E-09 3.01E-06 3.008 Shale 7.74E-10 1.86E-06 1.857 Average con/nental crust 3.37E-10 9.26E-07 0.927 Chondri/c meteorites 3.50E-12 1.15E-08 0.012 By volume, 𝐴 By volume, 𝐴 [W m-3] [µW m-3] Calculated from Turcotte and Schubert, 2014 • Typical heat production values for upper crustal rocks are 2-3 µW m-3, but the crustal average is <1 µW m-3 • What does this suggest, and why might this occur? 6 Radiogenic heat production Stüwe, 2007 • 𝐻 Because radiogenic heat production is the result of radioactive decay, the parent isotope concentrations have continually decreased throughout Earth’s history • Today, the total amount of radiogenic heat is about half of what was produced in the ArcheanS at ~3 Ga Crustal heat production over time 7 Radiogenic heat production Stüwe, 2007 • 𝐻 Because radiogenic heat production is the result of radioactive decay, the parent isotope concentrations have continually decreased throughout Earth’s history • Today, the total amount of radiogenic heat is about half of what was produced in the ArcheanS at ~3 Ga Crustal heat production over time 8 LETTER RESEARCH LETTER doi:10.1038/nature13728 Spreading continents kick-started plate tectonics Patrice F. Rey1, Nicolas Coltice2,3 & Nicolas Flament1 Temperature (K) Continent c d e Depth (km) b Depth (km) Depth (km) Aa that of present-day tectonic forces driving orogenesis1. To explore the Stresses acting on cold, thick and negatively buoyant oceanic litho- 2,000 0 1,000 0 sphere are thought to be crucial to the initiation of subduction and tectonic impact of a thick and buoyant continent surrounded by a stag1,2 the operation of plate tectonics , which characterizes the present- nant lithospheric lid, we produced a series of two-dimensional thermo200 was hotter mechanical 1,820 K numerical models of the top 700 km of the Earth, using day geodynamics of the Earth. Because the Earth’s interior 293 K in the Archaean eon, the oceanic crust may have been thicker, thereby temperature-dependent densities and visco-plastic rheologies that depend 400 making the oceanic lithosphere more buoyant than at present3, and on temperature, melt fraction and depletion, stress and strain rate (see 400 km whether subduction and plate tectonics occurred during this time is Methods). The initial temperature field is the horizontally averaged tem600 ambiguous, both in the geological record and in geodynamic models4. perature profile of a stagnant-lid convection calculation for a mantle Here we show that because the oceanic crust was thick and buoyant5, ,200 K hotter than at present (Fig. 1A, a and Extended Data Fig. 2). The lateral temperature gradients ensures that no convective stresses(g cm–3) early continents may have produced intra-lithospheric gravitational absence ofB a Deviatoric Reference density stress (MPa) stresses large enough to drive their gravitational spreading, to initi- act on the lid, allowing100 us to isolate dynamic effects2.8 of the3.0 continent. 300 the 500 3.2 3.4 0 depleted mantle 0 continent 225 km thick (strongly ate subduction at their margins and to trigger episodes of subduc- A buoyant and stiff Basaltic crust tion. Our model predicts the co-occurrence of deep to progressively root 170 km thick overlain by felsic crust 40 km thick; see Fig. 1B, a) is shallower mafic volcanics and arc magmatism within continents in inserted within the lid, on the left side of the domain to exploit the symCrust 40 40 a self-consistent geodynamic framework, explaining the enigmatic metry of the problem (Fig. 1A, a). A mafic crust 15 km thick covers the multimodal volcanism and tectonic record of Archaean cratons6. More- whole system (Fig. 1A, a), consistent with the common occurrence of Strongly over, our model predicts a petrological stratification and tectonic struc- thick greenstone80 covers on continents, as well as80 thick basaltic crust on 3 depleted ture of the sub-continental lithospheric mantle, two predictions that the oceanic lid . are consistent with xenolith5 and seismic studies, respectively, and conOur numerical solutions show that the presence oflithospheric a buoyant consistent with the existence of a mid-lithospheric seismic discontinuity7. tinent imparts 120 Depletion a horizontal force large enough120 to induce mantle a long period The slow gravitational collapse of early continents could have kick- (,50–150 Myr) of slow collapse of the whole continental lithosphere Strain rate started transient episodes of plate tectonics until, as the Earth’s inte- (Fig. 1 and Extended Fig. 3), in agreement with the dynamics of 160 Data –15 s–1 a continent of larger volume leads 10 19. Hence, rior cooled and oceanic lithosphere became heavier, plate tectonics spreading for gravity currents 225 became self-sustaining. to larger gravitational power and faster collapse. BecauseAsthenosphere of lateral spreadPresent-day plate tectonics is primarily driven by the negative buoy- ing of the continent, the100 adjacent 3.2 3.4 2.8pushed 3.0 under 300lithospheric 500 lid is slowly b 0 b and Extended Data Fig. 3A,0a). For gravitational ancy of cold subducting plates. Petrological and geochemical proxies of its margin (Fig. 1A, Basaltic crust subduction preserved in early continents point to subduction-like pro- stress lower than the yield stress of the oceanic lid, thickening of the margin cesses already operating before 3 billion years (Gyr) ago8,9 and perhaps of the lid is slow, and viscous drips (that is, Rayleigh–Taylor instabilities) 40 (Extended Data Fig. 3A, a and40 Lithospheric as early as 4.1 Gyr ago10. However, they are not unequivocal, and geo- detach from its base b). These instabilities, 20 mantle of dynamic modelling suggests that the thicker basaltic crust produced by typical of stagnant-lid convection , mitigate the thermal thickening partial melting of a hotter Archaean or Hadean mantle would have had the lid. 80 80 increased lithospheric buoyancy and inhibited subduction3,4. Mantle conWhen gravitational stresses overcome the yield stress of the lithospheric vection under a stagnant lid with extensive volcanism could therefore lid, subduction is initiated (Fig. 1A, b and c). Depending on the half-width have preceded the onset of subduction11. In this scenario, it is classically of the continent120 and its density contrast with the120 adjacent oceanic lid (that Strain rate assumed that the transition from stagnant-lid regime to mobile-lid regime is, its gravitational power) three situations can arise: first, subduction Asthenosphere –15 s–1 10 and the onset of plate tectonics require that convective stresses overcame initiates and stalls (Extended Data Fig. 3b); second, 160the slab detaches and 160 the strength of the stagnant lid12 at some stage in the Archaean. the lid stabilizes (Fig. 1A, d and e and Extended Data Fig. 3c); or third, On the modern Earth, gravitational stresses due to continental buoyancy recurrent detachment of the slab continues until recycling of the oceanic can contribute to the initiation of subduction2,13. The role of continental lid is completed, followed by stabilization (Extended Data Fig. 3d). When 9 LETTER doi:10.1038/nature13728 Spreading continents kick-started plate tectonics Patrice F. Rey1, Nicolas Coltice2,3 & Nicolas Flament1 Temperature (K) Continent Depth (km) Aa that of present-day tectonic forces driving orogenesis1. To explore the Stresses acting on cold, thick and negatively buoyant oceanic litho- 2,000 0 1,000 0 sphere are thought to be crucial to the initiation of subduction and tectonic impact of a thick and buoyant continent surrounded by a stag1,2 the operation of plate tectonics , which characterizes the present- nant lithospheric lid, we produced a series of two-dimensional thermo200 was hotter mechanical 1,820 K numerical models of the top 700 km of the Earth, using day geodynamics of the Earth. Because the Earth’s interior 293 K in the Archaean eon, the oceanic crust may have been thicker, thereby temperature-dependent densities and visco-plastic rheologies that depend 400 making the oceanic lithosphere more buoyant than at present3, and on temperature, melt fraction and depletion, stress and strain rate (see 400 km whether subduction and plate tectonics occurred during this time is Methods). The initial temperature field is the horizontally averaged tem600 ambiguous, both in the geological record and in geodynamic models4. perature profile of a stagnant-lid convection calculation for a mantle Here we show that because the oceanic crust was thick and buoyant5, ,200 K hotter than at present (Fig. 1A, a and Extended Data Fig. 2). The lateral temperature gradients ensures that no convective stresses(g cm–3) early continents may have produced intra-lithospheric gravitational absence ofB a Deviatoric Reference density stress (MPa) stresses large enough to drive their gravitational spreading, to initi- act on the lid, allowing100 us to isolate dynamic effects2.8 of the3.0 continent. 300 the 500 3.2 3.4 0 depleted mantle 0 continent 225 km thick (strongly ate subduction at their margins and to trigger episodes of subduc- A buoyant and stiff Basaltic crust tion. Our model predicts the co-occurrence of deep to progressively root 170 km thick overlain by felsic crust 40 km thick; see Fig. 1B, a) is shallower mafic volcanics and arc magmatism within continents in inserted within the lid, on the left side of the domain to exploit the symCrust 40 40 a self-consistent geodynamic framework, explaining the enigmatic metry of the problem (Fig. 1A, a). A mafic crust 15 km thick covers the multimodal volcanism and tectonic record of Archaean cratons6. More- whole system (Fig. 1A, a), consistent with the common occurrence of Strongly over, our model predicts a petrological stratification and tectonic struc- thick greenstone80 covers on continents, as well as80 thick basaltic crust on 3 depleted ture of the sub-continental lithospheric mantle, two predictions that the oceanic lid . are consistent with xenolith5 and seismic studies, respectively, and conOur numerical solutions show that the presence oflithospheric a buoyant consistent with the existence of a mid-lithospheric seismic discontinuity7. tinent imparts 120 Depletion a horizontal force large enough120 to induce mantle a long period The slow gravitational collapse of early continents could have kick- (,50–150 Myr) of slow collapse of the whole continental lithosphere Strain rate started transient episodes of plate tectonics until, as the Earth’s inte- (Fig. 1 and Extended Fig. 3), in agreement with the dynamics of 160 Data –15 s–1 a continent of larger volume leads 10 19. Hence, rior cooled and oceanic lithosphere became heavier, plate tectonics spreading for gravity currents 225 became self-sustaining. to larger gravitational power and faster collapse. BecauseAsthenosphere of lateral spreadPresent-day plate tectonics is primarily driven by the negative buoy- ing of the continent, the100 adjacent 3.2 3.4 2.8pushed 3.0 under 300lithospheric 500 lid is slowly b 0 b and Extended Data Fig. 3A,0a). For gravitational ancy of cold subducting plates. Petrological and geochemical proxies of its margin (Fig. 1A, Basaltic crust subduction preserved in early continents point to subduction-like pro- stress lower than the yield stress of the oceanic lid, thickening of the margin cesses already operating before 3 billion years (Gyr) ago8,9 and perhaps of the lid is slow, and viscous drips (that is, Rayleigh–Taylor instabilities) 40 (Extended Data Fig. 3A, a and40 Lithospheric as early as 4.1 Gyr ago10. However, they are not unequivocal, and geo- detach from its base b). These instabilities, 20 mantle of dynamic modelling suggests that the thicker basaltic crust produced by typical of stagnant-lid convection , mitigate the thermal thickening partial melting of a hotter Archaean or Hadean mantle would have had the lid. 80 80 increased lithospheric buoyancy and inhibited subduction3,4. Mantle conWhen gravitational stresses overcome the yield stress of the lithospheric vection under a stagnant lid with extensive volcanism could therefore lid, subduction is initiated (Fig. 1A, b and c). Depending on the half-width have preceded the onset of subduction11. In this scenario, it is classically of the continent120 and its density contrast with the120 adjacent oceanic lid (that Strain rate assumed that the transition from stagnant-lid regime to mobile-lid regime is, its gravitational power) three situations can arise: first, subduction Asthenosphere –15 s–1 10 and the onset of plate tectonics require that convective stresses overcame initiates and stalls (Extended Data Fig. 3b); second, 160the slab detaches and 160 10 the strength of the stagnant lid12 at some stage in the Archaean. the lid stabilizes (Fig. 1A, d and e and Extended Data Fig. 3c); or third, On the modern Earth, gravitational stresses due to continental buoyancy recurrent detachment of the slab continues until recycling of the oceanic can contribute to the initiation of subduction2,13. The role of continental lid is completed, followed by stabilization (Extended Data Fig. 3d). When b d Larger horizontal stresses force subduction of buoyant oceanic lithosphere Slab detaches, margin stabilizes e Depth (km) c Depth (km) Continent weakens, spreads laterally Let’s see what you’ve learned… • If you’re watching this lecture in Moodle, you will now be automatically directed to the quiz! • References: Rey, P. F., Coltice, N., & Flament, N. (2015). Spreading continents kick-started plate tectonics. Nature, 513(7518), 405–408. doi:10.1038/nature13728 11