Absolute Paleointensity Experiments on Aged Thermoremanent Magnetization: Assessment of Reliability of the Tsunakawa‐Shaw and Other Methods With Implications for “Fragile” Curvature
Absolute Paleointensity Experiments on Aged Thermoremanent Magnetization: Assessment of Reliability of the Tsunakawa‐Shaw and Other Methods With Implications for “Fragile” Curvature
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DOI：
10.1029/2022gc010391
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发表时间：
2022-03
期刊：
影响因子：
3.7
通讯作者：
Y. Yamamoto;L. Tauxe;H. Ahn;C. Santos
中科院分区：
文献类型：
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作者：
Y. Yamamoto;L. Tauxe;H. Ahn;C. Santos
通过将自然剩磁 (NRM) 与实验室诱发的热剩磁 (Lab-TRM) 进行比较，可以从火山岩中估计地磁场的绝对古强度 (API)。 API 实验中的 NRM 解锁与 Lab-TRM 阻塞图通常表现出非理想曲率，这可能会导致估计有偏差。之前的研究表明，曲率会随着年龄的增长而增加；然而，旨在消除这种行为的选择标准对两年龄样本产生了准确的估计（70.3 ± 3.8 μT；N = 120 个实验中的 96 个样本）。 API 也可以在矫顽力空间中估计。在这里，我们使用 Tsunakawa‐Shaw (TS) 方法对 20 个在 70.0 μT 的实验室场中老化 4 年的样本进行处理，并在同一场获得零老化（新鲜）Lab-TRM 后。 TS 实验的选择标准也产生了准确的结果（68.5 ± 4.5 μT；N = 17 个样本）。在热 API 实验中，曲率与具有最小曲率的单域行为的内部结构有关。在这里，我们表明，对于热曲率较大的样品，通过低温处理退磁的非磁滞剩磁的比例较大，这表明磁晶各向异性源。我们还测试了为提高古强度估计准确性而提出的实验补救措施。特别是，我们在古强度实验之前测试了多样本方法和低场交变场退磁预处理样本策略的有效性。两者都没有产生准确的结果。
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.