Thermal structure of continental lithosphere from heat flow and seismic constraints: Implications for upper mantle compo

1. Thermal structure of continental lithosphere from heat flow and seismic constraints: Implications for upper mantle composition and geodynamic models Claire Perry GEOTOP-UQAM-McGill, Montreal, Canada

2. Stability of continental lithosphere • equilibrium between chemical and • thermal buoyancy (e.g.,Jordan 1979) ? δFe# δT 150 km Perry et al. GJI (2003); Forte & Perry Science (2000) Accurate lithospheric thermal models required(heat flow, crustal heat production)

3. Introduction : Global Terrestrial Heat Loss Pollack et al. (1993)

4. Continental Heat Flow : example from Canadian Shield

5. Heteogenity of Continents … • geological • compositional • link between • surface geology • and lateral • variations in Qs Canadian Shield

6. generic thermal model for all cratons ? • influence of temperature + composition on • seismic velocity precise thermal model

7. Thermal Structure of the Continental Lithosphere Gung et al. (2003) • variable seismic thickness • d3 detected by tomography

8. Presentation Outline • Lithospheric thermal structure, upper mantle temperatures, and Pn velocity-temperature conversions from heat flow and seismic refraction studies • The thermal boundary layer of continental lithosphere and average mantle temperatures from a geodynamic flow model How does continental heat production affect lithospheric and mantle temperatures ?

9. Variables of Continental Thermal Structure Problem

10. Variables of Continental Thermal Structure Problem (Aavg~0.7 µWm-3) : distribution of radiogenic elements ? small (~0.02µWm-3)

12. Archean Superior Province, Canada

13. Heat Flow Data … • Qs Tmoho • correlation VP – T • mechanical resistance of lithosphere

14. Distribution of Radiogenic elements _____________ Differentiation Index: DI= <Asurf> Ac Province DI Perry et al. JGR 2006a

15. Distribution of Radiogenic elements _____________ Differentiation Index: DI= <Asurf> Ac Province DI Perry et al. JGR 2006a

16. Crustal Model • distribution of ACR in crustal columns • Moho temperature estimated using using k(T) LITH5.0 (Perry et al. GJI, 2002) + more recent data • Hc, Pn Fixed Parameters: Qs, A0, k(T), Hc Free Parameter : Qm (constrained by xenolith + heat flow, A(z) constrained by Qm, Qs, Hc Principal unknown Qm

17. Pn velocity

18. Crustal Thickness

19. Moho depth

21. dV(Pn)/dT=-0.60x10-3 ± 10% kms-1K-1 (close to mineral physics estimates)

22. Average Cratonic Mantle Composition Perry et al. JGR 2006b • on-craton VP-T ≠ off-craton VP-T • predicted/measured VP Qm≥ 12 mWm-2

23. Preferred Mineralogical Composition :Superior upper-mantle joint Qs + Pn lithospheric mantle composition + Qm Perry et al. JGR 2006b

24. Conclusions – Part I • Comparison of large-scale empirical geophysical data and in-situ experiments of mantle composition provide further confidence in mantle temperatures from seismic studies and heat flow • Joint inversions of heat flow and seismic Pn velocity constrain : • mantle mineralogical composition • effects of water ? • Average composition of cratonic mantle in southern Superior Province : ‘Proton’ or ‘Archon’ ? • Superior crust was rejuvenated by Keweenawan rifting at 1.1 Ga – metasomatism ?

25. Using V-T conversions + upper mantle temperature from heat flow ++ crustal models (test tomographic model) • subcontinental mantle dynamics : • Thermo-chemical structure of cratonic roots Refine thermo-chem structure

26. Thermal Boundary Layer at the base of Continents ‘rheological’ thickness of continent

27. Example from Kaapvaal xenoliths

28. Model Geometry

29. Oceanic vs. Continental Geotherms • δc»δo • δc depends on A • (dT/dz)cond = • O(dT/dz)a

30. Effect of Heat Production

31. Distribution of Heat Production

32. Δt = 0.25 Ga

34. Continental thickness from seismic tomography d d from Nettles (2004)

35. Continental thickness from seismic tomography d d d from Nettles (2004)

36. Continental Thermal Boundary Layer

37. Lateral Temperature Anomalies

38. Scaling Law for Average Mantle Temperature Θ C = 1.02 Sotin & Labrosse (1999) Total oceanic area, F

39. Continental geometry and average mantle temperature Perry, Jaupart & Tackley, in prep.

40. Continent thermal structure and average mantle temperature Perry, Jaupart & Tackley, in prep.

41. Hm + Vo + A × Vc = Ct = Htotal × Vtotal Effect of crustal accretion on the mantle’s thermal history ? Model Setup : To w d A D H To+ΔT Example Present-day Model :Example Archean Model : Htotal = 5 pW/kg Htotal = 10 pW/kg A = 300 pW/kg (~0.9μWm-3) A = 300 pW/kg RaH = 5 × 106 RaH = 5 × 107

42. 1.0 0.5 0.0 Today Potential temperature Archean Same mean mantle temperature from two models after 1Ga

43. 1.0 0.5 0.0 Today Potential temperature Archean Vrms continent/Vrms max RaH

44. 1.0 0.5 0.0 Today Potential temperature Archean Tmanto~Tmant(t) A/H Tmanto>>Tmant(t) RaH

45. Conclusions - II • Lateral temperature anomalies between ocean/continent diminished as A increases • Thickness of the thermal b.l. below continents depends strongly on A (A+ δ-) • Average mantle temperature may be scaled as a function of the total oceanic area • Implications for time evolution of mantle temperature • Average mantle temperature (and heat flow) may not be have been significantly higher than today : Feedback between mantle & continents : Ra, Acont