An Ultralow Geomagnetic Field Intensity in the Mesoproterozoic Based on Studies of 1380 Ma Old Intrusive Bodies from the Udzha Aulacogen of the Siberian Platform

IF 0.9 4区地球科学 Q4 GEOCHEMISTRY & GEOPHYSICS Izvestiya, Physics of the Solid Earth Pub Date : 2023-09-26 DOI:10.1134/S1069351323050105

V. V. Shcherbakova, A. M. Pasenko, G. V. Zhidkov, V. P. Shcherbakov, N. A. Aphinogenova

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Abstract—Estimating the time of formation of the Earth’s solid inner core is a first-order problem in the thermal evolution of the Earth as a planet, which can be solved in particular by paleointensity (B_anc) determinations. For this purpose, we have studied a collection of ~1380 Ma rocks sampled in the Udzha River valley within the Udzha aulacogen from the dolerite sill near the Khapchanyr River. The sill is an intrusion 5–7 m thick that cuts through the siltstones and carbonates of the Udzha Formation. To obtain reliable paleointensity (B_anc) determinations, magnetic and thermomagnetic properties of the studied rocks have been investigated, and their X-ray diffraction patterns and electron-microscopic images have been collected. It is shown that the carriers of the characteristic component of natural remanent magnetization are single-domain (SD) and small pseudo-single-domain (PSD) magnetite grains. Two methods have been used to determine the B_anc: the Thellier–Coe procedure including reheating to lower temperatures (the pTRM checkpoints procedure) and the Wilson express method. Paleointensity determinations have been obtained for 9 samples (30 duplicates) that met the selection criteria. The mean value of B_anc is extremely low and amounts to 4.54 ± 0.49 μT; the corresponding calculated value of the virtual dipole moment (VDM) is (11.1 ± 1.2) × 10²¹ Am², which is almost an order of magnitude lower than the mean VDM in modern epoch (≈80 × 10²¹ Am²) and more than six times lower than the mean VDM in the Cenozoic (6.44 × 10²² Am²). The VDM data over the 350–3500 Ma interval presented in the world paleointensity database (WPD), Borok, have been analyzed. An alternation of periods of low and high paleointensity is observed in the Precambrian and Paleozoic, which indicates a large variability in the operation mode of the geomagnetic dynamo regardless of the existence or absence of an inner solid core of the Earth. It is important to note that the number of reliable VDM values over such a long interval, 350–3500 Ma, is too small for a complete statistical analysis to single out any time interval as the most probable for the formation of the inner core.

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中元古代的超低地磁场强度——基于对西伯利亚地台乌扎-奥拉金1380 Ma古侵入体的研究

摘要——估算地球固体内核的形成时间是地球作为一颗行星热演化中的一个一阶问题，尤其可以通过古强度（Banc）测定来解决。为此，我们研究了从Khapchanyr河附近的粗玄岩岩床在Udzha aulacogen内的Udzha河流域取样的约1380 Ma岩石。岩床是一个5–7 m厚的侵入体，穿过Udzha组的粉砂岩和碳酸盐岩。为了获得可靠的古强度（Banc）测定结果，对所研究岩石的磁性和热磁性进行了研究，并收集了它们的X射线衍射图和电子显微镜图像。结果表明，自然剩余磁化特征分量的载流子是单畴（SD）和小的伪单畴（PSD）磁铁矿晶粒。已经使用了两种方法来确定Banc：Thellier–Coe程序，包括重新加热到较低温度（pTRM检查点程序）和Wilson express方法。已对符合选择标准的9个样本（30个重复样本）进行了古强度测定。Banc的平均值极低，为4.54±0.49μT；相应的虚拟偶极矩计算值为（11.1±1.2）×。对Borok世界古强度数据库（WPD）中提供的350–3500 Ma区间的VDM数据进行了分析。在前寒武纪和古生代观察到低古强度和高古强度周期的交替，这表明无论是否存在地球内部固体核心，地磁发电机的运行模式都有很大的变化。值得注意的是，在如此长的时间间隔（350–3500 Ma）内，可靠的VDM值的数量太少，无法进行完整的统计分析，无法将任何时间间隔单独列为最有可能形成内核的时间间隔。

本文章由计算机程序翻译，如有差异，请以英文原文为准。

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来源期刊

Izvestiya, Physics of the Solid Earth 地学-地球化学与地球物理

CiteScore

1.60

自引率

30.00%

发文量

审稿时长

6-12 weeks

期刊介绍： Izvestiya, Physics of the Solid Earth is an international peer reviewed journal that publishes results of original theoretical and experimental research in relevant areas of the physics of the Earth''s interior and applied geophysics. The journal welcomes manuscripts from all countries in the English or Russian language.