lunar paleomagnetism

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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 (T). Cisowski et al., (1983). AF demagnetization (AFD). - PowerPoint PPT Presentation

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

  • Apollo era PaleointensitiesCisowski et al., (1983)Calibration: 4700 . NRM (20mT)/ IRMs(20mT) -> B-field (T)

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

  • Early Apollo Era ProblemsMultidomain 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 AnalysisPaleointensity.(1) Classical methods failed - irreversible changes on heating.(2) IRMs and ARM methods require calibration.

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

  • Mare Basalts

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

  • 10049 AF demagnetization characteristics

  • Directional analysis and AF demagnetization characteristicsSuavet et al., (2012)

  • IRMs or ARM paleointensity.Compare AF demagnetization of NRM and IRMs orNRM and ARM.With calibration factor the ratio of NRM/IRMs or NRM/ARM gives paleofield estimate -

    NRM = k PaleofieldARM Lab. Field

  • Paleointensities ARM method Mean Values: 62.5 + 7.3 T 67.8 + 18.5 T Grand Mean: 69.1 + 16.6 T

  • 12022.52. Ilmenite Basalt ~3.2 Ga Directional analysis

  • AF demagnetization characteristics

  • 12022 Direction analysis Paleointensity ARM methodTikoo et al., (2012)

  • Melt Breccias

  • 62235: Impact Melt Breccia: 3.9 GaHargraves and Dorety, (1975)Collinson et al.,(1973)

  • Classical Koenigsberger Thellier Thellier methodBasic idea replace NRM with TRM in known field

    NRM = PaleofieldTRM Lab. Field

    Stepwise technique to monitor chemical changes

  • 62235 Paleointensity ~100 T.Collinsonet al.,(1973)Sugiura andStrangway (1983): Lawrence et al.,(2008)Lawrence et al.,(2008)

  • Melt breccias Magnetization: An analogue.Apollo 17 Boulder :Impact ejecta section.Pyroclastic flows: used toGet field and depositionalTemperature.

  • How does lunar paleomagnetism fit into Lunar History?Weber et al., (2011), possibly nowstably 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.

  • 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 bygiant impacts, tidal forces, or precession.

    No time to be too dogmatic about energy source.

    *Many of us who worked with the samples Apollo in the early days are getting a bit long in the tooth, so I thought it would be useful to provide a summary of the earlier results in a standard modern form, to help the younger workers, who have taken over so well.