Paradigms versus paradoxes: Developing a new paradigm for the mantle

1. Paradigms versus paradoxes: Developing a new paradigm for the mantle Attreyee Ghosh, Ricardo Arevalo Jr., Ved Lekic, and Victor Tsai with Adam Dziewonski, Barbara Romanowicz, Louise Kellogg, and Wendy Panero (names arranged in alphabetical order by first name)

2. Geochemistry likes a layered mantle • DMM cannot account for the planet’s budget of: • Incompatible elements • UDMM + UCC≠ UBSE • Radiogenic heat production • HDMM + HCC≠ HBSE • Noble gas abundances • 40ArDMM + 40ArCC + 40Aratm ≠ 40ArBSE

3. Seismology finds a variety of behaviors for slabs in the transition zone • Tomographic images illustrate mass flux across the 660 km discontinuity Li et al. (2008) Van der Hilst et al. (1998) Li et al. (2008)

4. Geodynamics like well-mixed mantle reservoirs • Mantle layering difficult to maintain for multiple Ga without significant mixing Naliboff and Kellogg (2007)

5. 1 layer  whole- mantle convection 2 layers  traditional limited exchange Hybrid  limited exchange 2 layers  isolated upper and lower mantle reservoirs Hofmann (1997)

6. “Marble-cake mantle” Sobolev et al. (2005)

7. Morgan & Morgan (1999) “Plum-pudding mantle”

8. Becker et al. (1999) “Blob mantle”

9. Several geochemical studies have called upon an early, differentiated reservoir that has remained “hidden” at the core-mantle boundary What about this ‘D”’? Boyet and Carlson (2006)

10. What about this ‘D”’? • Seismology and mineral physics observations indicate a heterogeneous layer at the core-mantle boundary Power Spectra

11. Temperature oC 1000 1500 2500 3000 2000 5 150 km 10 Zone of neutrally or negatively buoyant melt Pressure (GPa) 410’ 15 20 660’ 25 Lee and Luffi Lee et al., (2007) Transition Zone

12. Tolstikhin and Hofmann (2005)

13. What about the role of thermochemical piles/superplumes? • Increasing the volume of a deep mantle reservoir (e.g., including superplumes) dilutes the required incompatible/ radioactive element budget of this reservoir

14. 2800 km depth seismic profiles Kustowski (2006)

15. Romanowicz and Gung (2002) Upper mantle:Q - lower mantle: Vsh Degree 2 only

16. Romanowicz and Gung (2002)

17. Defining the volume of a superplume superplumes in S362ANI (1% slow anomaly)

18. Area (sq km) Area (sq km) Depth (km) Depth (km) S362ANI (0.6% contours) SAW24B16 (1% contours) Defining the volume of a superplume

19. Defining the volume of a superplume Conservative estimates  only consider depths >1000 km

20. Geochemical implications • If we know the composition of the Continental Crust (CC; e.g., Rudnick and Gao, 2003), the Depleted MORB Mantle (DMM; e.g., Su, 2002) and the Bulk Silicate Earth (BSE; McDonough and Sun, 1995)… • The size of the DMM dictates the required composition of a deep, Enriched Mantle Reservoir (EMR)

21. Geochemical implications *Concentration range calculated from uncertainties in compositional models of CC and DMM

22. Thermal implications

23. Thermal implications

24. Solve: Maintaining neutral buoyancy • Assume: • Fe is most important chemical variant • Fe has no effect on modulus or thermal expansion • Thermal and chemical effects are linear wrt velocity • Fe partitioning between mw and pv • Fe has a linear effect on density:

25. Stixrude & Lithgow- Bertelloni, 2005

27. What does this mean? • Uncertainty in partitioning behavior has a first order effect • Velocity drop at base of the mantle is >2.5% • Additional 1.5% Fe (reasonable) • Excess temperature of 450-700 K • Velocity drop in mid-mantle is ~1% • Additional 0.5% Fe • Excess temperature of 180-275 K • Super piles are neither on constant adiabat or isochemical if they are neutrally buoyant

28. Future questions to address • How stationary are these superplumes? • Do surface tectonics dictate the large scale flow in the mantle, or vice-versa? • Slab reconstructions (over the last 200 Ma) and degree-2 signals are well correlated (Upper mantle comparison) Vs model S362ANI Slab model of L-B & R (1998)

29. (Mid-mantle depths) Slab model of Lithgow- Bertelloni & Richards (1998) Vs model S362ANI

30. Slab model of Lithgow-Bertelloni & Richards (1998) Vs model S362ANI (Comparison at CMB) The degree-2 velocity anomalies at the CMB are extremely well correlated with the integrated slab signal: the sum of all the slabs deposited during the last 200 Ma.

31. Future questions to address • How long could such a thermochemical reservoir be dynamically stable for? • “Bottom-up” dynamical test • Starting conditions: 2 rigid conical masses attached to the CMB - representative size of superplumes • The transition zone must be able to arrest, at least temporarily, sinking subducted materials • The convection experiments, spanning a sufficiently large parameter space would give us insight into lower mantle mixing and return flow

32. H=150 km B=2 Some typical snapshots at t ~ 4.55Ga H=500 km B=0.7 H=1000 km B=1 H=1600 km B=0.7 Kellogg and Ferrachat Dynamic criteria: stability over several Ga, topography of the interface, net density, and magnetic field

33. Future work/questions • What is the mass flux of material into the lower mantle? Reaching D”? • How much becomes incorporated in our deep reservoir? Fukao et al. (2001)

34. A new paradigm • We propose that the lowermost mantle pattern of the two chemically and thermally distinct super-plumes dictates the planform of mantle dynamics for at least the last 200 Ma. • The superplumes may have stable locations for at long periods of time, anchoring mantle plumes and influencing the paths of Wilson cycles. • The transition zone plays an important role in the interaction between subducted slabs and the superplumes

35. A new paradigm • Transition zone may be a “leaky” boundary layer • Subducted slabs pond in the transition zone, with sufficient residence time for some oceanic crust to be re-circulated in the upper mantle • Ponded material breaks through the 660 km discontinuity in avalanche-like events and is deposited around the upwellings giving rise to the ring of fast velocities girdling the Pacific • Low-pass filter removes high wavenumber features from slab signal • Temperature contrast sufficient to produce plumes at the 660 km discontinuity

36. Hawaii Generic Questions?

37. 72 km 362 km 652 km 942 km 1377 km 2102 km 2827 km Slab integration model of Lithgow-Bertelloni and Richards (1998)

38. Further Required Assumptions • Fe partitioning between MgO and MgSiO3 Andrault, 2001

39. Assume D=5

40. Andrault D

41. Badro D (HS->LS)

42. EMR = Enriched Mantle Reservoir CC = Continental Crust • 40Ar produced by decay of 40K (t1/2 = 11.93 Gyr) • Too heavy to be lost from atm • >99.9% Ar is 40Ar • We know: • 280 ppm K in equals >150 Eg (1018 g) of 40Ar produced over 4.5 Ga • 66 Eg in atm, 10-20 Eg may be in crust, the rest must reside in the mantle • 40ArBSE = 40Aratm + 40ArCC + 40ArDMM +40ArEMR – 40Ardegassed