Improvements to the Shaw-Type Absolute Palaeointensity Method

Abstract

Palaeointensity information enables us to define the strength of Earth’s magnetic field over geological time, providing a window into Earth’s deep interior. The difficulties in acquiring reliable measurements are substantial, particularly from older rocks. Two of the most significant causes of experimental failure are laboratory induced alteration of the magnetic remanence carriers and effects relating to multidomain magnetic carriers. One method that has been claimed to overcome both of these problems is the Shaw method. Here we detail and evaluate the method, comparing various selection criteria in a controlled experiment performed on a large, non-ideal dataset of mainly Precambrian rocks. Monte Carlo analyses are used to determine an optimal set of selection criteria; the end result is a new, improved experimental protocol that lends itself very well to the automated Rapid 2G magnetometer system enabling experiments to be carried out expeditiously and with greater accuracy.

Absolute Paleointensity Experiments on Aged Thermoremanent Magnetization: Assessment of Reliability of the Tsunakawa‐Shaw and Other Methods With Implications for “Fragile” Curvature

“…All the 17 successful results were accompanied by $$\vert \vec{{k}^{\prime }}\vert $$ values not more than 0.2 for the NRM‐TRMl* diagrams (Table 2). A criterion of $$\vert \vec{{k}^{\prime }}\vert \le 0.2$$ has been suggested as an alternative selection criterion in the Shaw‐type method (Lloyd, Paterson, et al., 2021). They argued that the ARM correction might occasionally fail resulting in inaccurate API estimate, if $$\vert \vec{{k}^{\prime }}\vert > 0.1$$ for an NRM‐TRMl* diagram.…”

“…With this as background, the TS method has been applied to old igneous rocks in Africa (Ahn et al., 2016; Yoshimura et al., 2020), China (Yamamoto, Tsunakawa, et al., 2007), French Polynesia (Mochizuki et al., 2011; Yamamoto, Ishizuka, et al., 2007; Yamamoto & Tsunakawa, 2005), Hawaii (Singer et al., 2019; Yamamoto & Yamaoka, 2018), Japan (Kato et al., 2018; Mochizuki et al., 2013; Okayama et al., 2019; Tsunakawa et al., 2009; Yamamoto et al., 2010), Korea (Ahn & Yamamoto, 2019), Louisville seamount (Yamazaki & Yamamoto, 2014), New Zealand (Mochizuki et al., 2006), and archeological materials in Japan (Kitahara et al., 2018, 2021). Although LTD treatments are not necessarily included, the TS method has also been applied to Precambrian rocks (Lloyd, Biggin, et al., 2021; Thallner, Biggin, & Halls, 2021, Thallner, Biggin, McCausland, & Fu, 2021), partly with consideration for different sets of selection criteria (Lloyd, Paterson, et al., 2021). It has also been shown that the APIs determined by the TS method are linearly related to the sedimentary paleointensities of the last 1.5–2 million years (Mochizuki et al., 2021).…”

New Paleointensity Data Suggest Possible Phanerozoic‐Type Paleomagnetic Variations in the Precambrian

“…|k'| (Paterson, 2011;Paterson et al, 2016) Defined in Table 1 caption and used in addition to the R 2 correlation, which is not as strict a measure of linearity; this is particularly important in Shaw-DHT pseudo-Arai plots because the paleointensity slope is insensitive to alteration (Tanaka & Komuro, 2009). We apply a strict minimum of |k'| ≤ 0.2 to all Shaw results (e.g., Lloyd, Paterson, et al, 2021).…”