No Plume Beneath Iceland

1. No Plume Beneath Iceland talk given at the Colorado School of Mines, 2nd March 2006 Gillian R. Foulger Durham University, U.K.

2. Evidence in support of a plume beneath Iceland • History of magmatism • Uplift • High temperatures • Crustal structure • Mantle structure

3. DISKO FAROES & E GREENLAND ODP 158 BRITISH PROVINCE 1. History of magmatism Jones (2005) 61-59 Ma 54 Ma

4. Formed over the last 54 Million years Thick crust 1. History ofmagmatism:Iceland

5. 2. Uplift 400-900 m 420-620 m 380-590 m 180-425 m 0-100 m 500-800 m 0 - 200 m 0-200 m Jones (2005)

6. 2. Uplift • Uplift rapid • Approached 1 km in some places 400-900 m 420-620 m 380-590 m 180-425 m 0-100 m 500-800 m 0 - 200 m 0-200 m Jones (2005)

7. 3. High-temperatures ~ 100 K temperature anomaly for Iceland relative to MORB Arndt (2005)

8. 4. Crustal structure Foulger et al. (2003) Crustal structure from receiver functions

9. 5. Mantle structure Whole-mantle tomography: A plume from the core-mantle boundary. Bijwaard & Spakman (1999)

10. The Iceland plume? A slam dunk!

11. Let us look in detail, to find out more about what the Iceland plume is like.

12. Seismological studies of Iceland Foulger et al. (2003)

13. Crustal structure • Variations in crustal thickness should be parallel to spreading direction • Crust should be thickest in the west, behind the plume Foulger et al. (2003)

14. Crustal structure The melting anomaly has always been centred on the mid-Atlantic ridge

15. Iceland: Mantle tomography • Over 2,000,000 data • S-wave arrival times (S, SS, SSS, ScS & SKS) • fundamental- & higher-mode Rayleigh-wave phase velocities • normal-mode frequencies • Probably best spherical harmonic model for the transition zone & mid-mantle Ritsema et al. (1999)

16. Hudson Bay plume? Whole-mantle tomography Bijwaard & Spakman (1999)

17. Transition zone discontinuities Predicted topography on the 410-km and 650-km discontinuities Du et al. (2006)

18. Transition zone discontinuities • 410 warps down by 15 km • 650 flat • No evidence for anomalous structure or physical conditions at 650 km beneath Iceland Du et al. (2006)

19. Temperature Can be investigated using: • Petrology • Seismology • Modeling bathymetry • Modeling vertical motion • Heat flow

20. Petrological temperature ~ 100 K temperature anomaly for Iceland relative to MORB Arndt (2005)

21. ? Iceland? Petrological temperature Hawaii 1570˚ MORs 1280-1400˚ Gudfinnsson et al. (2003)

22. Temperature: Seismology Vs Vertical scale x 10 DT ~ 200˚C DT ~ 100˚C Ritsema & Montagner (2003) Vertical scale x 1 Iceland

23. Temperature: Iceland Foulger et al. (2005)

24. Uplift: Magnitude & Duration • 61 Ma uplift associated with British igneous activity variable, low amplitude (few 100 m) & localised. • 54 Ma uplift associated with igneous activity distant from proposed plume, high amplitude (up to 1 km) & widespread. • Time between onset and peak uplift for both igneous phases probably << 1 Myr. • Uplift history complex & not satisfactorily explained by any single published model.

25. DISKO FAROES & E GREENLAND ODP 158 BRITISH PROVINCE 1. History of magmatism Jones (2005) 61-59 Ma 54 Ma

26. Summary • Variations in crustal thickness inconsistent with plume predictions • Mantle anomaly confined to upper mantle • No reliable evidence for plume-like temperatures • Uplift history complex and not well explained • Distribution of magmatism inconsistent with plume predictions

27. An alternative model Plate tectonic processes (“PLATE”) • Two elements: • Variable source fertility • Extensional stress A cool, shallow, top-driven model

28. Mid-ocean ridges (1/3 of all “hot spots”) Many others intraplate extensional areas PLATE: Lithospheric extension

29. PLATE: Variable mantle fertility • Possible sources: • recycling of subducted slabs in upper mantle Peacock (2000)

30. PLATE: Variable mantle fertility • Possible sources: • delamination of continental lithosphere Schott et al. (2000)

31. Pyrolite Eclogite The liquidus & solidus of subducted crust are lower than peridotite • Subducted crust transforms to eclogite at depth • Eclogite is extensively molten at the peridotite solidus Cordery et al. (1997)

32. Geochemistry of “hot spot” lavas • Can be modeled as fractional melting of MORB • Ocean Island Basalt (OIB) comes from recycled near-surface materials e.g., subducted oceanic crust Hofmann & White (1982)

33. Iceland

34. Iceland: Extension Jones (2005) Iceland has been persistently centred on the mid-Atlantic ridge

35. Relationship to the Caledonian suture Recycled Iapetus crust in source? Can remelting of Iapetus slabs account for the excess melt, geochemistry & petrology? Iceland: Mantle fertility Closure of Iapetus

36. Melt fraction : Temperature Yaxley (2000) A 30/70 eclogite-peridotite mixture can generate several times as much melt as peridotite

37. Geochemical evidence for crustal recycling • Recent papers: Korenaga & Keleman (2000); Breddam (2002); Chauvel & Hemond (2000) • Estimated primary mantle melt from Iceland, E & SE Greenland shows source mantle enriched in Fe; Mg# is as low as 0.87 • Heterogeneity suggests MORB mantle also involved • Sr-Nd-Hf-Pb isotopes & dO18 suggest recycling of subducted, aged oceanic crust, ± sub-arc magmatism, ± sediments

38. Iceland: REE patterns Foulger et al. (2005) Iceland REE can be modeled by extensive melting of subducted crust + small amount of alkali olivine basalt

39. The alternative hypothesis is... • Iceland is a “normal” part of the MAR where excess melt is produced from remelting Iapetus slabs • However, the amount of melt produced by isentropic upwelling of eclogite cannot at present be calculated

40. Foulger et al. (2003) Tectonics & crustal structure Iceland is also a region of local, persistent tectonic instability

41. Iceland: Tectonic evolution Foulger (in press)

42. Iceland: Tectonic evolution Foulger (2002)

43. Crustal structure The thickspot beneath Iceland may be a submerged oceanic microplate

44. Iceland: The mantle anomaly • Can be explained by 0.1% partial melt • a more fusible mantle composition • CO2 fluxing • Could simply be a place where the low-velocity zone is thicker Iceland

45. Summary • Superficially, several observations are consistent with plume theory • Closer examination virtually never fulfills the predictions of plume theory

46. Summary • 2 approaches: • adapt plume theory to fit • accept that plume theory fails and boldly go where no man has gone before

47. Resources: http://www.mantleplumes.org/

48. That’s all folks