Astronomy and Astrophysics – Astronomy
Scientific paper
May 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009geoji.177..383b&link_type=abstract
Geophysical Journal International, Volume 177, Issue 2, pp. 383-394.
Astronomy and Astrophysics
Astronomy
6
Paleointensity
Scientific paper
The San Quintín volcanic field (SQVF) in Baja California (Mexico) consists of at least 13 volcanic complexes with about 30 lava flows. 40Ar/39Ar step-heating and 3He surface exposure dating experiments suggest eruption ages between 22 and 165 ka. Paleodirections of 14 flows reported here are consistent with normal secular variation. Because of low Curie temperatures and thermal alteration starting at ~350 °C, classical Thellier-Thellier paleointensity (PI) experiments were deemed unsuccessful. Microwave PI experiments were carried out on 139 samples with 97 being interpreted to be successful, yielding values smaller than 20 μT for all sites and averaging 16.5 μT. Another sample set was studied with the multispecimen parallel differential pTRM method; temperatures for acquisition of pTRM were chosen between 200-and 225 °C. In two cases of higher unblocking temperatures, it was set at 480 °C. The experiments were successful for 104 out of 120 specimens with PI values ranging between 14 and 21 μT and clustering around 18 μT. Three noticeably higher values between 28 and 35.7 μT were obtained. Both methods yield in general well comparable PI data with the microwave PI showing a tendency to be slightly smaller than those from the multispecimen PI. The reason for it is presently not known with certainty. Possible causes include 1) incipient thermal alteration, anticipated to be slightly more prominent in the multispecimen method. 2) Cooling time effects: the microwave method involves heating during only 10-30 s compared to 2400 or more seconds in the multispecimen method. This may account for a relative underestimate of the field intensity by about 4 per cent. 3) A general PI underestimate (by ~4 per cent) inherent to the microwave perpendicular field method applied to multidomain particles in comparison to parallel field methods. 4) A PI overestimate for small multidomain particles in the multispecimen method. However, both PI methods produced very similar results even for difficult materials as the SQVF lavas giving credit to the veracity of the obtained PI values. The multispecimen parallel differential pTRM paleointensity method thus presents a valuable alternative for traditional thermal PI determinations, as it can be applied using standard laboratory equipment. The microwave paleointensity method may yield better results but requires specialized equipment not yet available commercially.
Böhnel Harald N.
Dekkers Mark J.
Delgado-Argote Luis A.
Gratton Martin N.
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