1. Dynamically viable early plate tectonics? Geoff Davies Research School of Earth Sciences, Australian National University

2. Convection with plates and tracers (present mantle)

3. Tracers : Trace basaltic component of mantle composition Upper melting zone: - defined by melting depth - move tracers into “crust” Store times of “sampling”

5. Crust thickness and tracer concentration profile

6. Tracer settling vs Temp

7. Thickness of oceanic crust

8. Present cool mantle Early hot mantle? ?

9. Effect of buoyant oceanic crust

10. Standard thermal evolution

11. Effect of thick oceanic crust:

12. Effect of buoyant crust (reduced by 1.8)

13. Conclusions Strong early depletion of the upper mantle Thin oceanic crust from hot, refractory upper mantle Viable plate tectonics? Bimodal thermal evolution?

14. Spacer

15. Thinning crust at high T:

16. Reduced effect of crust (like eclogite effect)

17. To explain: apparent ages ~1.8Ga (Pb-Pb), MORB-OIB differences

18. Results with heavy tracers: mean ages > 2 Ga dense layer at bottom gradient through upper mantle

19. Run at present mantle temperature: same number of overturns

20. Hot early mantle, 1550°C

21. Hot mantle, 1550°C

22. Thickness of oceanic crust