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Lunar Paleomagnetism.

Lunar Paleomagnetism. Mike Fuller, HIGP-SOEST, University of Hawaii, and Ben Weiss, Department of Earth Atmospheric and Planetary Sciences, MIT. Apollo era “Paleointensities”. Calibration: 4700 . NRM (20mT)/ IRMs(20mT) -> B-field (µT). Cisowski et al., (1983). AF demagnetization (AFD).

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Lunar Paleomagnetism.

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  1. Lunar Paleomagnetism. Mike Fuller, HIGP-SOEST, University of Hawaii, and Ben Weiss, Department of Earth Atmospheric and Planetary Sciences, MIT.

  2. Apollo era “Paleointensities” Calibration: 4700 . NRM (20mT)/ IRMs(20mT) -> B-field (µT) Cisowski et al., (1983)

  3. AF demagnetization (AFD). Hoffman et al., (1979)

  4. Early Apollo Era Problems Multidomain nature of Fe easily contaminated, in contrast to terrestrial samples we use - Single domain tend to be more resistant to contamination. Thermal Demagnetization. (1) Failure to avoid irreversible changes on heating. AF demagnetization. (1) Fields used too low, (2) Analysis insufficient detail, (3) Lack of Principal Component Analysis Paleointensity. (1) Classical methods failed - irreversible changes on heating. (2) IRMs and ARM methods require calibration.

  5. Demagnetization of low field contamination - space vehicule. Weak field IRM contaminates spectra to the Curie point. Weak field IRM and SRM demagnetized by low AFD

  6. Mare Basalts

  7. 10049 High K Apollo 11 basalt (3.5 Ga) Directional analysis

  8. 10049 AF demagnetization characteristics

  9. Directional analysis and AF demagnetization characteristics Suavet et al., (2012)

  10. IRMs or ARM paleointensity. Compare AF demagnetization of NRM and IRMs or NRM and ARM. With calibration factor the ratio of NRM/IRMs or NRM/ARM gives paleofield estimate - NRM = k Paleofield ARM Lab. Field

  11. Paleointensities ARM method Mean Values: 62.5 + 7.3 µT 67.8 + 18.5 µT Grand Mean: 69.1 + 16.6 µT

  12. 12022.52. Ilmenite Basalt ~3.2 Ga Directional analysis

  13. AF demagnetization characteristics

  14. 12022 Direction analysis Paleointensity ARM method Tikoo et al., (2012)

  15. Melt Breccias

  16. 62235: Impact Melt Breccia: 3.9 Ga Collinson et al.,(1973) Hargraves and Dorety, (1975)

  17. Classical Koenigsberger Thellier Thellier method Basic idea replace NRM with TRM in known field NRM = Paleofield TRM Lab. Field Stepwise technique to monitor chemical changes

  18. 62235 Paleointensity ~100 µT. Sugiura and Strangway (1983): Collinson et al.,(1973) Lawrence et al.,(2008) Lawrence et al.,(2008)

  19. Melt breccias Magnetization: An analogue. Apollo 17 Boulder : Impact ejecta section. Pyroclastic flows: used to Get field and depositional Temperature.

  20. How does lunar paleomagnetism fit into Lunar History? Weber et al., (2011), possibly now stably stratified molten outer core Possibly from 4.2 Ga until ~ 3.2 Ga active lunar dynamo driven by convection, or other mechanism, such as precession.

  21. Energy sources for lunar dynamo. Convection, or inner core crystallization giving release of low density material in molten core, as on earth. Mechanical stirring of molten outer core by giant impacts, tidal forces, or precession. No time to be too dogmatic about energy source.

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