Mathematics – Logic
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmgp41b0830f&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #GP41B-0830
Mathematics
Logic
[1519] Geomagnetism And Paleomagnetism / Magnetic Mineralogy And Petrology, [1595] Geomagnetism And Paleomagnetism / Planetary Magnetism: All Frequencies And Wavelengths, [5440] Planetary Sciences: Solid Surface Planets / Magnetic Fields And Magnetism, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
The introduction of principal component analysis by Kirschvink (1980) and the routine use of orthogonal demagnetization plotting techniques brought about a fundamental improvement in the analysis of demagnetization data. Unfortunately, principal component analysis was not available when much of the early paleomagnetic work was done on the Apollo samples. We have therefore carried out a reanalysis of past lunar sample demagnetization data using the principal component analysis technique combined with orthogonal demagnetization plots. Our results show that AF demagnetization of some samples reveals a characteristic remanent magnetization. Other samples were clearly not demagnetized to sufficiently high fields to identify a characteristic remanent magnetization. In retrospect, it is clear that the failure to carry out AF demagnetization to much more than ~50 mT was a critical problem with the early work. High fields are particularly important for separating the effects of shock from any possible primary thermoremanent magnetization (TRM) acquired on the lunar surface. Results from Gattacceca et al. (2007, 2008) document shock remanent magnetization (SRM) acquired in only a few GPa, which is below the level of shock currently identifiable using petrologic techniques. Moreover these SRMs were stable against AF demagnetization to ~20-30 mT. However, they were softer than TRM in the same samples. Few mare basalts show evidence of extreme shock sufficient to generate maskelynite, but some show mild or intense fracturing, suggesting shock of up to ~10 - 15 GPa. Hence recognition of SRM is essential for satisfactory interpretation of the natural remanent magnetization. With the lunar samples, this is best done at the present time by AF demagnetization because it avoids the difficulties sample alteration during heating and because SRM can have a broad range of blocking temperatures and yet be restricted to mainly low coercivity grains. Examples of principal component analyses on AF demagnetization data will be shown. While many samples fail to reveal an interpretable characteristic remanent magnetization, there are other samples which appear to have retained a primary NRM from when they cooled on the lunar surface in a lunar field.
Fuller Michael M.
Weiss Benjamin P.
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