Palaeointensity study of the Hawaiian 1960 lava: implications for possible causes of erroneously high intensities

Abstract

S U M M A R YIt is known that the Hawaiian 1960 lava sometimes yields higher palaeointensities than the IGRF (36.2 µT). In order to clarify the causes, we have performed a comprehensive investigation of palaeointensity measurements on 19 cores from the lava. According to various rock magnetic analyses, they are classified into three groups with degrees of the deuteric oxidation. In Coe's version of the Thellier method, a significantly larger mean palaeointensity of 49.0 ± 9.6 µT (N = 17) was observed similar to previous studies. These palaeointensities showed a dependence on the oxidation indices, which can be explained by thermochemical remanent magnetization (TCRM) acquisition during the natural cooling stage of the lava. Although it is generally difficult to distinguish the TCRM from thermoremanent magnetization (TRM), a lowtemperature demagnetization (LTD) treatment on NRM and TRM in the Thellier experiment seems to allow us to detect the influence of the TCRM on NRM. Another possible tool for TCRM detection is the double-heating technique (DHT) of the Shaw method combined with LTD (LTD-DHT Shaw method). It gave a better average of 39.4 ± 7.9 µT (N = 9), though the success rate was much lower. Most of the inappropriate results were screened by a combination of the double-heating test, the ARM correction and the LTD treatment. If two outliers are further excluded, the average is improved to be 35.7 ± 3.3 µT (N = 7), which agrees well with the expected intensity. The results of both methods emphasize the importance of taking and measuring samples from the parts of various oxidation degrees in a lava flow.

Archeointensity study on baked clay samples taken from the reconstructed ancient kiln: implication for validity of the Tsunakawa-Shaw paleointensity method

“…Thus, AF-based paleointensity techniques are thought to be suitable for these samples. The Tsunakawa-Shaw method, which has been previously called the LTD-DHT Shaw method (Tsunakawa and Shaw 1994;Yamamoto et al 2003), is one of such techniques. Its applicability and validity have been elucidated for various types of volcanic rocks from historical lava flows (e.g., Yamamoto et al 2003;Mochizuki et al 2004;Oishi et al 2005;Yamamoto and Hoshi 2008) but remain unassessed for archeological artifacts including baked clay.…”

Magnetic force microscopy reveals meta-stable magnetic domain states that prevent reliable absolute palaeointensity experiments

“…Etna. For Hawaii, however, palaeointensity experiments that fail to deliver a proper estimate of the palaeofield are usually overestimates 6,61 .…”

“…Directional information is generally straightforward to obtain from various recorders, including well-dated lavas and (lake) sediments for shorter timescales and sedimentary sequences for reconstructions going further back into geological history. Despite recent methodological progress [3][4][5][6][7][8][9][10][11][12] , however, acquiring reliable absolute palaeointensity data remains a formidable task. Only materials that acquired their remanent magnetization by cooling in the Earth's ambient magnetic field are suited for absolute palaeointensity determinations.…”

“…Over the past decades, various tests and protocols have been proposed to identify and (partially) overcome the MD effects during palaeointensity experiments [3][4][5][6][7][8][9][10][11][12] , but success rates of such experiments are still notoriously low [17][18][19][20] . It is even more disturbing that-in spite of extensive testing-palaeointensity experiments sometimes yield technically successful estimates of the palaeofield that actually over-or underestimate the palaeofield by tens of percent 6,21 .…”

“…It is even more disturbing that-in spite of extensive testing-palaeointensity experiments sometimes yield technically successful estimates of the palaeofield that actually over-or underestimate the palaeofield by tens of percent 6,21 .…”