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Electrical Images of the Mantle: A review of Magnetotellurics

Rob. L. Evans Dept of Geology and Geophysics WHOI. Electrical Images of the Mantle: A review of Magnetotellurics. Outline. What is Magnetotellurics (MT)? How is it done ? What is Electrical Conductivity sensitive to? What does MT show?

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Electrical Images of the Mantle: A review of Magnetotellurics

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  1. Rob. L. Evans Dept of Geology and Geophysics WHOI. Electrical Images of the Mantle:A review of Magnetotellurics

  2. Outline • What is Magnetotellurics (MT)? • How is it done ? • What is Electrical Conductivity sensitive to? • What does MT show? Examples from Oceanic Mantle, Cratonic Mantle and Subduction Zones

  3. MT Source: Ionospheric Currents.Not event driven, but does depend on sunspot activity.

  4. Hz Hx Hy Ex Ey 90° X Y MT – Basic Acquisition Layout GPS Acq Unit Batteries

  5. Land MT Acquisition

  6. Seafloor MT Instruments

  7. Raw Data: Electric and Magnetic Field Time Series Ex Ey Hx Hy

  8. From Maxwell’s Equations(over a uniform Earth) Taking the magnitude gives:

  9. MT Strategy Depth of Penetration Depends On Resistivity Structure

  10. Transportable Array Earthscope:

  11. MT Measures Electrical Conductivity • Temperature • Composition • Fluid Content (e.g. melt) • Hydration • Other Conducting Phases (e.g. Carbon) Sensitive to: All these important in the mantle, especially in convergent Margin settings.

  12. Temperature Constable et al. (1992)

  13. Compositional Dependence Based on Lab Results of Xu et al. (2000)

  14. Compositional Control: Xenolith Constraints Xenolith Compositions from Kaapvaal Craton: Saltzer et al. (2001)

  15. Impact of Fluids (e.g. Melt) Depends on Connectivity

  16. More Formally….

  17. Effect of Water (Dissolved Hydrogen): Anisotropy Hirth et al. (2000)

  18. Composition and non-perfect alignment of olivine reduces maximum electrical anisotropy Simpson (2002)

  19. Grain Size Dependence (?) ten Grotenhuis et al. (2004)

  20. Thoughts on Conduction Mechanisms: Effect of Hydration Still to be Confirmed in Laboratory Grain Size Data Intriguing but Conflicting Results (Earlier data sets don’t show effect). Our Interpretations are Only as Good as the Best Laboratory Data

  21. Examples: Oceanic Mantle: MELT Area Cratonic Mantle: Superior, Slave, Kaapvaal Convergent Margins: Mariana, Argentina,Cascadia

  22. MELT Area

  23. Young Oceanic Plates: MELT Area off-Axis Evans et al. (2005)

  24. - - - - - - - - Log Conductivity What About MELT? (Baba et al., JGR 2006)

  25. Archean Cratons Superior: Single Station 1-D Profiles Seismic Anisotropy No Electrical Anisotropy = Dry Resistive Archean lithosphere stabilized against convective instability by high viscosity due to dry composition Hirth et al (2000)

  26. Kaapvaal Craton: Extensive MT Coverage

  27. Kaapvaal: Preliminary Models Limpopo Mobile Belt Bushveld Karoo Kimberley Thick Resistive Lithosphere

  28. Some Cratons Are More Complicated: e.g. Slave Jones et al. (2000)

  29. Also, Some Cratons Do Show Electrical Anisotropy Australia Simpson (2002) Fennoscandia Lahti et al. (2005)

  30. …..Some are more complicated Southern Africa Is there a link between Seismic and Electrical Anisotropy?

  31. Passive Marginsand ConvergenceInduced Anisotropy Figures Courtesy Phil Wannamaker

  32. Subduction Factory Evans et al. (2002)

  33. Mariana Subduction Factory: MT Transect

  34. Mariana: Preliminary Model Baba et al. In prep

  35. Argentina Booker et al., (2004)

  36. Cascadia Soyer and Unsworth (2006)

  37. There’s More !..... New Zealand Chile And Soon to Come…… Costa Rica Taiwan (no offshore) Marianas (Full profile)

  38. Summary MT Surveys are complementary to seismic studies MT provides constraints on thermal structure, compositional variations as well as hydration and fluid contents of the mantle On land, MT is relatively cheap and easy to carry out. Seafloor MT capability continues to grow, particularly in light of recent boom in Oil industry use of EM. Interpretations improve with increased computing power and laboratory measurements

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