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No Plume Beneath Iceland talk given at the Colorado School of Mines, 2nd March 2006 Gillian R. Foulger Durham University, U.K. 1 Evidence in support of a plume beneath Iceland 1. 2. 3. 4. 5. 2 History of magmatism Uplift High temperatures Crustal structure Mantle structure 1. History of magmatism DISKO FAROES & E GREENLAND ODP 158 BRITISH PROVINCE 3 61-59 Ma 54 Ma Jones (2005) 1. History of magmatism: Iceland • Formed over the last 54 Million years • Thick crust 4 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 5 Jones (2005) 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 6 Jones (2005) 3. High-temperatures Arndt (2005) 7 ~ 100 K temperature anomaly for Iceland relative to MORB 4. Crustal structure Foulger et al. (2003) Crustal structure from receiver functions 8 5. Mantle structure Whole-mantle tomography: A plume from the core-mantle boundary. 9 Bijwaard & Spakman (1999) The Iceland plume? A slam dunk! 10 Let us look in detail, to find out more about what the Iceland plume is like. 11 Seismological studies of Iceland 12 Foulger et al. (2003) Crustal structure Foulger et al. (2003) • Variations in crustal thickness should be parallel to spreading direction • Crust should be thickest in the west, behind the plume 13 Crustal structure The melting anomaly has always been centred on the mid-Atlantic ridge 14 Iceland: Mantle tomography • Over 2,000,000 data – S-wave arrival times (S, SS, SSS, ScS & SKS) – fundamental- & highermode Rayleigh-wave phase velocities – normal-mode frequencies • Probably best spherical harmonic model for the transition zone & midmantle 15 Ritsema et al. (1999) Whole-mantle tomography Hudson Bay plume? Bijwaard & Spakman (1999) 16 Transition zone discontinuities Predicted topography on the 410-km and 650-km discontinuities 17 Du et al. (2006) Transition zone discontinuities • 410 warps down by 15 km • 650 flat • No evidence for anomalous structure or physical conditions at 650 km beneath Iceland 18 Du et al. (2006) Temperature Can be investigated using: • • • • • 19 Petrology Seismology Modeling bathymetry Modeling vertical motion Heat flow Petrological temperature Arndt (2005) 20 ~ 100 K temperature anomaly for Iceland relative to MORB Petrological temperature Hawaii 1570˚ ? Iceland? MORs 1280-1400˚ Gudfinnsson et al. (2003) 21 Temperature: Seismology Vs Vertical scale x 10 DT ~ 200˚C DT ~ 100˚C Ritsema & Montagner (2003) 22 Iceland Vertical scale x 1 Temperature: Iceland 23 Foulger et al. (2005) 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. 24 1. History of magmatism DISKO FAROES & E GREENLAND ODP 158 BRITISH PROVINCE 25 61-59 Ma 54 Ma Jones (2005) 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 26 An alternative model Plate tectonic processes (“PLATE”) • Two elements: – Variable source fertility – Extensional stress A cool, shallow, top-driven model 27 PLATE: Lithospheric extension • Mid-ocean ridges (1/3 of all “hot spots”) • Many others intraplate extensional areas 28 PLATE: Variable mantle fertility • Possible sources: – recycling of subducted slabs in upper mantle 29 Peacock (2000) PLATE: Variable mantle fertility • Possible sources: – delamination of continental lithosphere QuickTime™ and a GIF decompressor are needed to see this picture. 30 Schott et al. (2000) 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 31 Pyrolite Eclogite Cordery et al. (1997) 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) 32 Iceland 33 Iceland: Extension Jones (2005) 34 Iceland has been persistently centred on the mid-Atlantic ridge Iceland: Mantle fertility • Relationship to the Caledonian suture • Recycled Iapetus crust in source? • Can remelting of Iapetus slabs account for the excess melt, geochemistry & petrology? 35 Closure of Iapetus Melt fraction : Temperature Yaxley (2000) 36 A 30/70 eclogite-peridotite mixture can generate several times as much melt as peridotite 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 37 Iceland: REE patterns Foulger et al. (2005) Iceland REE can be modeled by extensive melting of subducted crust + small amount of alkali olivine basalt 38 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 39 Tectonics & crustal structure Iceland is also a region of local, persistent tectonic instability Foulger et al. (2003) 40 Iceland: Tectonic evolution 41 Foulger (in press) Iceland: Tectonic evolution Foulger (2002) 42 Crustal structure The thickspot beneath Iceland may be a submerged oceanic microplate 43 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 lowvelocity zone is thicker Iceland 44 Summary 1. Superficially, several observations are consistent with plume theory 2. Closer examination virtually never fulfills the predictions of plume theory 45 Summary 3. 2 approaches: 1. adapt plume theory to fit 2. accept that plume theory fails and boldly go where no man has gone before 46 Resources: http://www.mantleplumes.org/ 47 That’s all folks 48