MagNetZ with Yuhji Yamamoto (2021/10/06) - The Tsunakawa-Shaw method applied "aged" TRMs
Channel: MagNetZ
Date: 2021-10-07

Application of the Tsunakawa-Shaw method to lava samples with  "aged" thermoremanent magnetizations in laboratory

The video is also available for download at: https://earthref.org/ERDA/2484/
and can be referenced with the doi: 10.7288/V4/ERDA/2484

Title:Application of the Tsunakawa-Shaw method to lava samples with  "aged" thermoremanent magnetizations in laboratory
Author: Yuhji Yamamoto from Kochi University, Japan
Author Website: https://www.researchgate.net/profile/Yuhji-Yamamoto

Absolute paleointensity (API) of the geomagnetic field can be estimated from a volcanic rock based on a comparison between a natural remanent magnetization (NRM) and a laboratory-induced thermoremanent magnetization (Lab-TRM). It is known that a plot of NRM vs Lab-TRM (Arai plot) based on their blocking temperature spectra, which can be obtained using a Thellier-type method, often exhibits a non-ideal curvature. Recent investigations have found that such a curvature generally increase through time except an ideal sample, and the increase mainly originates from a shift to high unblocking temperatures across the entire temperature range after aging.

API can also be estimated in a coercivity space, using a Shaw-type method. One of the latest version of the method is formerly called the LTD-DHT Shaw method and now called the Tsunakawa-Shaw (TS) method. In the present study, we applied the TS method to 20 specimens aged in the laboratory field of 70.0 µT for two years. Total 17 API estimates were successfully obtained and they resulted in an overall average (median) of 69.3 (69.6) µT with a standard deviation of 2.7 µT, if we excluded one estimate of 54.2 µT as an outlier because it was significantly deviated from the other estimates.

API estimates by the IZZI Thellier method were previously obtained from sister specimens from the same samples used in the present study, and their Arai plots resulted in variable curvatures. Causes of such IZZI curvatures seem to have certain relations to remanences originating from the magnetocrystalline anisotropy, as there is a tendency that amounts of anhysteretic remanent magnetization demagnetized by low-temperature demagnetization (LTD) were larger for the samples with the larger IZZI curvatures. The LTD treatment in the TS method could not necessarily result in straight NRM-TRM1 and ARM0-ARM1 plots, though degree of the curvatures in the two plots are similar for the same specimens resulting in more or less straight NRM-TRM1* plots with reasonable API estimates.