Continuous record of geomagnetic field variations during cooling of the Monchegorsk, Kivakka and Bushveld Early Proterozoic layered intrusions
Continuous record of geomagnetic field variations during cooling of the Monchegorsk, Kivakka and Bushveld Early Proterozoic layered intrusions
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Monchegorsk、Kivakka 和 Bushveld 早元古代层状侵入体冷却过程中地磁场变化的连续记录
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发表时间：
2005
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通讯作者：
V. S. Zakharov
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作者：
D. Pechersky;A. Lyubushin;V. S. Zakharov
首次对Monchegorsk（2.5Ga）、Kivakka（2.45Ga）和Bushveld（2.06Ga）层状侵入体冷却过程中的地磁场方向进行了连续记录。低Ti的钛磁铁矿（Tc = 530-580 ℃）是主要的自然还原磁化（NRM）载体。根据岩石磁学特征，高温NRM成分是热释电的，并且在侵入体的冷却阶段获得。540 ℃以上的磁性颗粒接近单畴状态。在求解Stefan问题和非稳态导热问题的基础上，计算了侵入体冷却过程中温度随时间的变化关系以及冷却速度随温度和时间的变化关系。地热梯度值取为20 km-1。在样品的热退磁过程中的非阻塞温度（Td）和从高温冷却过程中的阻塞温度（Tb）速率依赖性被用于Td到Tb的转换[Dodson和McClelland，1980]。从530 ° C到580 ° C，每隔2 ° C-3 ° C进行一次详细的热退磁，并获得地磁场方向的行为模式：（a）居里点在剖面上“运行”的时间，（B）从Tb = 580 ° C开始每个采样点侵入体的冷却时间。方向变化谱的主要韵律为3-4.5、5-7、8-10、12、15-17、19-20、30-40、50-60和90-100 kyr，它们随时间变化。节奏的长度是不同的，范围从1-2振荡（“飞溅”）到10- 1 - 2。Monchegorsk入侵。极点265.3东经，1.3北纬。记录的长度为170 kyr。记录到了持续不到2000年的地磁反极性漂移和约60 kyr的R亚时。Kivakka入侵。分离出侵入体冷却阶段的A1预折叠和A2共折叠分量。岩体在侵入就位后约85 kyr开始倾斜，并持续了20-25 kyr。A1的极点是17.8 ° S和247 ° E。记录的持续时间为35 kyr，在此期间只有一个R极性存在。灌木丛入侵。从粘滞磁化恢复了样品水平面的取向，并从NRM的高温N分量确定了古磁极（12 N/35.4 E/35.4 N）。获得了1500 kyr的古地磁记录;在此期间，磁场的极性只发生过一次变化。1俄罗斯科学院地球物理研究所，俄罗斯莫斯科2俄罗斯莫斯科莫斯科州立大学版权所有2004年俄罗斯地球科学杂志。论文编号TJE 04158。ISSN：1681-1208（online）本文的网络版于2005年1月12日出版。http://rjes.wdcb.ru/v06/tje04158/tje04158.htm简介地磁场的精细结构通常是从沉积和火山成因矿床的古地磁记录中研究的。绝大多数这样的部分，即使是最“连续”的部分也有空隙，因此地磁场的古地磁记录是不连续的。差距并不总是被发现的，它们涵盖了几年的时间间隔，
A continuous record of the geomagnetic field direction during of cooling of the Monchegorsk (2.5 Ga), Kivakka (2.45 Ga) and Bushveld (2.06 Ga) layered intrusions was conducted for the first time. The low-Ti titanomagnetites (Tc = 530–580◦C) are the main natural remanent magnetization (NRM) carriers. The high temperature NRM component according to petromagnetic characteristics is thermoremanent and was acquired at the cooling phase of the intrusion. The magnetic grains above 540◦C are close to a single-domain state. The temperature versus time and cooling velocity versus temperature and time dependences during cooling of intrusions were calculated on the basis of solution of the Stefan problem and the nonstationary heat conduction problem. The geothermal gradient value was taken as 20◦ km−1. The unblocking temperatures in the course of thermal demagnetization of samples (Td) and the blocking temperatures during cooling from high temperatures (Tb) rate dependences were used for conversion of the Td to Tb [Dodson and McClelland, 1980]. A detailed thermal demagnetization at 2◦–3◦ interval from 530◦C up to 580◦C was made and a pattern of behavior of the geomagnetic field direction was obtained (a) for the time of “running” the Curie points over the section and (b) for the cooling time of the intrusion at each sampling point from Tb = 580◦C. A wavelet analysis has been used to study the field variation etc. The main rhythms of the direction variation spectrum are 3–4.5, 5–7, 8–10, 12, 15–17, 19–20, 30–40, 50–60, and 90–100 kyr and they change in time. The length of rhythms is different and ranges from 1–2 oscillations (“splashes”) up to 10–12. The Monchegorsk intrusion. The pole 265.3◦E, 1.3◦N. The length of record is ∼70 kyr. The geomagnetic excursion of the reversal polarity lasting for less than 2000 years and the R-subchron of ∼60 kyr were recorded. Kivakka intrusion. The A1 prefolded and A2 synfolded components which occurred at the stage of cooling of the intrusion are isolated. The body started to tilt approximately 85 kyr after intrusion emplacement and it lasted for 20–25 kyr. The pole of the A1 is 17.8◦S and 247◦E. Duration of the record is 35 kyr and only one R-polarity existed during this period. Bushveld intrusion. Orientation of the horizontal plane of the samples has been restored from viscous magnetization and a paleomagnetic pole (12◦N, 35.4◦E) has been determined from a high-temperature N-component of the NRM. A paleomagnetic record for ∼500 kyr was obtained; polarity of the field has changed only once during this period. 1 Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia 2 Moscow State University, Moscow, Russia Copyright 2004 by the Russian Journal of Earth Sciences. Paper number TJE04158. ISSN: 1681–1208 (online) The online version of this paper was published 12 January 2005. URL: http://rjes.wdcb.ru/v06/tje04158/tje04158.htm Introduction A fine structure of the geomagnetic field is commonly studied from paleomagnetic records in sections of the sedimentary and volcanogenous deposits. The overwhelming majority of such sections, even the most “continuous” have gaps and, consequently, the paleomagnetic records of the geomagnetic field are discontinuous. The gaps are not always detected and they cover time intervals of several years to