Lunar Paleomagnetism. Lunar Paleomagnetism.

Mike Fuller, HIGP-SOEST, Mike Fuller, HIGP-SOEST, University of Hawaii, University of Hawaii,

andand

Ben Weiss, Department of Earth Ben Weiss, Department of Earth Atmospheric and Planetary Atmospheric and Planetary

Sciences, MIT.Sciences, MIT.

Apollo era “Paleointensities”Apollo era “Paleointensities”

Cisowski et al., (1983)

Calibration: 4700 . NRM (20mT)/ IRMs(20mT) -> B-field (µT)

AF demagnetization (AFD).AF demagnetization (AFD).

Hoffman et al., (1979)

Early Apollo Era ProblemsEarly 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 Analysis

Paleointensity.(1) Classical methods failed - irreversible changes

on heating.(2) IRMs and ARM methods require calibration.

Demagnetization of low field Demagnetization of low field contamination - space vehicule.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 characteristics

Suavet 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 µT 67.8 + 18.5 µT Grand Mean: 69.1 + 16.6 µT

12022.52. Ilmenite Basalt ~3.2 Ga Directional analysis

AF demagnetization characteristics

12022 Direction analysis 12022 Direction analysis Paleointensity ARM methodPaleointensity ARM method

Tikoo et al., (2012)

Melt Breccias

62235: Impact Melt Breccia: 3.9 Ga62235: Impact Melt Breccia: 3.9 Ga

Hargraves and Dorety, (1975)Collinson et al.,(1973)

Classical Koenigsberger Thellier Thellier method

Basic idea replace NRM with TRM in known field

NRM = PaleofieldTRM Lab. Field

Stepwise technique to monitor chemical changes

62235 Paleointensity ~100 62235 Paleointensity ~100 µT..

Collinsonet al.,(1973)

Sugiura andStrangway (1983):

Lawrence et al.,(2008)

Lawrence et al.,(2008)

Melt breccias Magnetization:Melt breccias Magnetization: An analogue. An analogue.

Apollo 17 Boulder :Impact ejecta section.

Pyroclastic flows: used toGet field and depositionalTemperature.

How does lunar paleomagnetism How does lunar paleomagnetism fit into Lunar History?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 Energy sources for lunar dynamo.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.