HR: 14:20h
AN: P43C-03    [Abstracts]
TI: New Lunar Paleointensity Measurements, Ancient Lunar Dynamo or Lunar Dud?
AU: * Lawrence, K P
EM: klawrence@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0220, United States
AU: Johnson, C L
EM: cjohnson@eos.ubc.ca
AF: University of British Columbia, Department of Earth and Ocean Sciences, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada
AU: Tauxe, L
EM: ltauxe@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0220, United States
AU: Gee, J S
EM: jsgee@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0220, United States
AB: We analyze published and new paleointensity data from Apollo samples to reexamine the hypothesis of an early (3.9 to 3.6 Ga) lunar dynamo. Our new paleointensity experiments on four Apollo samples use modern absolute and relative measurement techniques. Our samples (60015, 76535, 72215, 62235) have ages ranging from 3.3 to 4.2 Ga, bracketing the putative period of a lunar dynamo. Samples 60015 (anorthosite) and 76535 (troctolite) failed during absolute paleointensity experiments, using the IZZI-modified Thellier-Thellier method. Samples 72215 and 62235 recorded a complicated, multi-component magnetic history that includes a low temperature (< 500°C) component with a high intensity (~90 μT), and a high temperature (> 500°C) component with a low intensity (~2 μT). These two samples were also subjected to a relative paleointensity experiment (sIRM), from which neither provided unambiguous evidence for a thermal origin of the recorded remanent magnetization. We found similar multi-component behavior in several published experiments on lunar samples. We test and present several magnetization scenarios in an attempt to explain the complex magnetization recorded in lunar samples. Specifically, an overprint from exposure to a small magnetic field (i.e. IRM) results in multi-component behavior (similar to lunar sample results), from which we could not recover the correct magnitude of the original TRM. The non-unique interpretation of these multi-component results combined with IRM (isothermal remanent magnetization) contamination during Apollo sample return ( Strangway et al., 1973), indicates that techniques incapable of distinguishing between single- and multi-component records (e.g., sIRM), cannot be reliably used to infer magnetic conditions of the early Moon. In light of these new experiments and a thorough reevaluation of existing paleointensity measurements, we conclude that there is a paucity of lunar samples that demonstrate a primary thermal remanent magnetization. As relative paleointensity measurements for lunar samples are calibrated using absolute paleointensities, the lack of acceptable absolute paleointensity measurements renders the interpretation of relative paleointensity measurements unreliable. Consequently, current lunar paleointensity measurements are inadequate to determine the existence and strength of an early lunar magnetic field. Surface magnetometry measurements and the return of magnetically uncontaminated samples from future missions are much needed for further progress in understanding the characteristics and origin of lunar crustal remanent magnetization.
DE: 1521 Paleointensity
DE: 1595 Planetary magnetism: all frequencies and wavelengths
DE: 5440 Magnetic fields and magnetism
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting