Microwave palaeointensities from a recent Mexican lava flow, baked sediments and reheated pottery

Details Microwave palaeointensities from a recent Mexican lava flow, baked sediments and reheated pottery Microwave palaeointensities from a recent Mexican lava flow, baked sediments and reheated pottery

Sep 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003e%26psl.214..221b&link_type=abstract

Earth and Planetary Science Letters, Volume 214, Issue 1-2, p. 221-236.

Physics

11

Microwave Method, Palaeointensities, Holocene, Lava Flow, Mexico

Scientific paper

Microwave palaeointensity (PI) estimates have been produced from samples from the 1670-yr old Xitle lava flow, Mexico. Seventeen out of 19 experiments were successful, producing high-quality estimates and a mean PI of 58.3+/-9.5 μT, within errors of that expected from global data for the time period. The dispersion is high but could be reduced to 5.5 μT by using stricter selection criteria. Previous data, obtained by the Thellier-Coe method using samples from the same lava flow, were less successful and of much lower quality, producing a higher mean of 67.9+/-9.8 μT (25 out of 65 samples). This difference is most probably caused by the presence of multi-domain particles producing concave-up Arai plots combined with alteration processes affecting the Thellier-Coe experiments at higher temperatures. PI calculations restricted to the initial part of an Arai plot therefore tend to overestimate the correct values. In addition to lava samples, microwave PIs were also determined from pottery fragments recovered from the contact zone between the lava flow and the underlying baked sediments, which were also studied. These materials also provided good-quality results, with a higher mean PI of 66.8+/-7.1 μT, which is still statistically indistinct from that produced by the microwave analysis of the lava samples. Nevertheless, these samples seem to be less suitable for PI determinations, because they are characterised by larger magnetic grains and apparently more prone to thermally induced alteration than the lava samples. We conclude that in the case of the Xitle lava the microwave PI method is superior to the conventional Thellier-Coe method in many respects and that the results produced by the latter method, even when satisfying strict acceptance criteria, may be unreliable. However, we also draw attention to the fact that microwave-produced PI determinations, though of high technical quality, may still be prone to inaccuracy when rock magnetic parameters indicate that the material is likely to be a poor PI recorder, as for the pottery, sediment, and some of the lava samples described here.

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