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Download U and Th contents (heat production)! of the continental crust : a
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U and Th contents (heat production)! of the continental crust : a global perspective! ! Claude Jaupart! with more than a little help of Jean-Claude Mareschal! Main difficulties : ! (1) crust is highy heterogeneous at ALL depths ! Main difficulties : ! (2) bulk physical properties (Vp, Vs, ρ) are NOT sensitive! to trace element contents! Main difficulties : ! (3) Seismic models yield an “average” crustal structure! B03304 MUSACCHIO ET AL.: SUPERIOR PROVINCE LITHOSPHERIC STRUCTURE B03304 (From Musachio et al., J. Geophys. Res. 2004)! Figure 14. Interpretative sketch. See text for details. tectonic subcretion of oceanic crust. The observed directional anisotropy (i.e., E-W fast propagation) of this mantle layer is generally consistent with strain-induced lattice- [66] Extensive surface outcrop of Mesoproterozoic Keewenawan intrusives within the eastern Wabigoon subprovince (eastern line 1) suggests that the underlying crust Main difficulties : ! (3) Seismic models yield an “average” crustal structure! B03304 MUSACCHIO ET AL.: SUPERIOR PROVINCE LITHOSPHERIC STRUCTURE B03304 (From Musachio et al., J. Geophys. Res. 2004)! Figure 14. Interpretative sketch. See text for details. tectonic subcretion of oceanic crust. The observed directional anisotropy (i.e., E-W fast propagation) of this mantle layer is generally consistent with strain-induced lattice- [66] Extensive surface outcrop of Mesoproterozoic Keewenawan intrusives within the eastern Wabigoon subprovince (eastern line 1) suggests that the underlying crust Main difficulties : ! (3) Seismic models yield an “average” crustal structure! B03304 MUSACCHIO ET AL.: SUPERIOR PROVINCE LITHOSPHERIC STRUCTURE B03304 (From Musachio et al., J. Geophys. Res. 2004)! Figure 14. Interpretative sketch. See text for details. tectonic subcretion of oceanic crust. The observed directional anisotropy (i.e., E-W fast propagation) of this mantle layer is generally consistent with strain-induced lattice- [66] Extensive surface outcrop of Mesoproterozoic Keewenawan intrusives within the eastern Wabigoon subprovince (eastern line 1) suggests that the underlying crust Method no.1 : “model of continental crust formation”! Most popular : island arcs! Not appropriate:! (1) may not have been active! !at all times in Earth’s history! (2) crust gets modified by :! !- internal differentiation! ! !+ erosion ! !- tectonic processes! Method no.2 : complete cross-sections! Most popular (again) : island arcs! Example : Kohistan island arc, Pakistan! Method no.2 : complete cross-sections! Most popular (again) : island arcs! Problems:! (1) may not have been active! !at all times in Earth’s history! (2) outcrops on a scale that may not ! !be representative ! (3) variations of crustal thickness ?! ! Method no.3 : direct sampling! Problems : ! (1) large-scale sampling is required for a reliable average! ! Upper crust : brute force sampling! Eade & Fahrig (1973): >14,000 grid samples Space shuttle view of Thunder Bay, Ontario! Method no.3 : direct sampling! Problems : ! (1) large-scale sampling is required for a reliable average! (2) lower crustal levels are accessible in very few places! Method no.3 : direct sampling! Problems : ! (1) large-scale sampling is required to allow a reliable average! (2) lower crustal levels are accessible in very few places! (3) crustal thickness variations ?! Models use constraints from heat flow data …! Method no. 4 : Heat Flow Data! Qo = ΔQc + ΔQLM + Qb ≈ ΔQc + Qm! CRUST! Enriched in U, Th and K! ΔQc! Qm = heat flow at the Moho! (mantle heat flux)! Lithospheric mantle! ΔQLM ≈ 0! (rigid root)! Basal heat flux Qb! ! Main difficulties : ! (1) crust is highy heterogeneous at ALL depths ! Heat flow records the local average heat production! ! For z = 35 km, λ ≈ 220 km ! MAIN ADVANTAGES! ! ! (1) Heat flow provides us with an efficient natural averaging tool:! useful for studies of crustal composition.! ! (2) Heat flow records heat that is produced over ! !the whole crustal column.! !(No need to solve for crustal thickness).! ! (3) “Mantle” heat flow variations (due to changes of deep! !lithospheric structure > ≈ 150 km)! !are smoothed out over wavelengths ≈ 500 km.! ! ! ! Estimating the mantle heat flux : mantle xenoliths! Finsch kimberlite mine, South Africa" Estimating the Moho heat flux! Depth (km)! Temperature (°C)! Gradient ≈ 5 K km-1! Heat flux ≈ 15 mW m-2 ! Check ?! Check : seismic data! P-wave velocity as a function of! temperature! (and composition)! Pn velocity Heat Flow • Qm Tmoho • V(Pn) – T relationship dV(Pn)/dT=-0.60x10-3 ± 10% kms-1K-1 (consistent with mineral physics measurements) Does it make sense ?! Heat flux (mW m-2)! Heat Flux (mWm-2)! Temperature (K)! Temperature at the Moho at the time of crustal stabilization! 0 5! 10! 15! 20! Moho Heat Flow (mWm-2)! 25! Temperature (K)! Temperature at the Moho at the time of crustal stabilization! Solidus! 0 5! 10! 15! 20! Moho Heat Flow (mWm-2)! 25! Does it really matter ?! Cooling through formation of continental crust! Primitive mantle! Depleted mantle! Cooling through formation of continental crust! Enriched crust! Primitive mantle! Depleted mantle! What is more “thermally” efficient for a planet to cool ?! (1) To segregate its heat producing elements in a rigid crust at its upper surface! (2) Or to convect vigourously ?! Compare two cases: heat production in a thin crust or in the convecting mantle.! ! ! Thermal structure of convecting layer with internal heating! ΔTconv to evacuate mantle radiogenic heat! Compare internal temperatures for these two cases:! (1) conduction in a thin crust of thickness dc! (2) convection in a thick mantle of thickness h.! Most efficient cooling mechanism = segregate U and Th into the crust! Is the Earth working this way ?! Bulk Silicate Earth models! ! != 11 - 24 TW! ! ! Continental Crust (40% of total area)! ! != 6 - 8 TW! ! ! ! ! ! ! ! ! ! !! Is the Earth working this way ?! Bulk Silicate Earth models! ! != 11 - 24 TW! ! ! Continental Crust (40% of total area)! ! != 6 - 8 TW! ! Continental Crust over whole Earth’s surface! ! != 15 - 20 TW! ! ! ! ! ! ! ! ! Raglan heat flow site 62° North ! Venus and Earth! Two different dynamical regimes! yet similar present-day mantle temperatures! !
