Absolute Paleointensity Experiments on Aged Thermoremanent Magnetization: Assessment of Reliability of the Tsunakawa‐Shaw and Other Methods With Implications for "Fragile" Curvature. (28th March 2022)

• Record Type: Journal Article
• Authors: Yamamoto, Y.; Tauxe, L.; Ahn, H.; Santos, C.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 23:Number 4(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-03-28
• Subjects: Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2022GC010391
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: Absolute paleointensity (API) of the geomagnetic field can be estimated from volcanic rocks by comparing the natural remanent magnetization (NRM) to a laboratory‐induced thermoremanent magnetization (Lab‐TRM). Plots of NRM unblocking versus Lab‐TRM blocking from API experiments often exhibit nonideal curvature, which can result in biased estimates. Previous work showed that curvature can increase with age; however, selection criteria designed to eliminate such behavior yielded accurate estimates for two‐year‐aged specimens (70.3 ± 3.8 μT; N = 96 specimens out of 120 experiments). API can also be estimated in coercivity space. Here, we use the Tsunakawa‐Shaw (TS) method applied to 20 specimens aged in the laboratory field of 70.0 μT for 4 years, after acquisition of zero‐age (fresh) Lab‐TRM in the same field. Selection criteria for the TS experiment also yielded accurate results (68.5 ± 4.5 μT; N = 17 specimens). In thermal API experiments, curvature is related to internal structure with more single domain‐like behavior having the least curvature. Here we show that the fraction of anhysteretic remanent magnetization demagnetized by low‐temperature treatment was larger for samples with larger thermal curvatures suggesting a magnetocrystalline anisotropy source. We also tested experimental remedies that have been proposed to improve the accuracy of paleointensity estimates. In particular, we test the efficacy of the multi‐specimen approach and a strategy pretreating specimens with low field alternating field demagnetization prior to the paleointensity experiment. Neither yielded accurate results. Plain Language Summary: The strength of the geomagnetic field in the past is a fundamental property of the Earth and volcanic rocks are magnetized as they cool, potentially retaining a record of the ancient field. However, because natural materials were not "designed" to retain records of the field, there are many difficulties in obtaining reliable estimates. Consequently, there are many experimental approaches and methods of analyzing the data. Here, we explore several very different experimental approaches on replicate specimens from a set of samples chosen to represent the range of behaviors observed in lava flows. Specimens were given a fresh magnetization and some were allowed to "age" in a laboratory field for up to 5 years. We find that two methods work quite well while other approaches fail to provide accurate estimates of the field. Key Points: Both the Tsunakawa‐Shaw and the IZZI paleointensity methods give accurate results when applied to aged laboratory thermal remanences Introducing alternating‐field demagnetization steps into the IZZI method resulted in inaccurate results, as did the multi‐specimen protocol Low temperature demagnetization (targeting grains with magnetocrystalline anisotropy) is strongly correlated with "fragile" curvature