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Paleomagnetic Secular Variation for the Last 2 million Years form Young Volcanic Rocks

年轻火山岩过去 200 万年的古地磁长期变化

基本信息

批准号：
01540329
负责人：
TANAKA Hidefumi
金额：
$ 1.34万
依托单位：
Tokyo Institute of Technology
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1989
资助国家：
日本
起止时间：
1989 至 1990
项目状态：
已结题

项目摘要

Precise dating of young volcanic rocks by K-Ar method was attempted to offer the basis of detailed paleosecular variation of the geomagnetic field. Samples for dating and paleomagnetic measurements were collected in the Zao Volcanic Group.Most of eighteen andesite lavas and dikes gave reliable radiometric ages, but exceptions are the dike "Roba no Mimi Rock" samples in which the ages of about 1 Ma were reported by the previous studies. Apparently some excess argon trapped in the dike samples is the cause of unsuccessful dating experiment. Review and summary were also given to unpublished age data for five volcano groups in Tohoku Japan obtained from the mass spectrometer of Yamagata University.Andesite lavas and dikes were collected for paleomagnetic study at sixteen sites in Zao Volcano. All samples are stable to AF and thermal demagnetizations, and 16 paleomagnetic directions were successfully determined in which the ages cover from 0.07-1 Ma. Paleointensity experiment by the Coe's version of the Thelliers' method was also made to the most stable 50 specimens selected from eight sites. However, the results were not as successful as the directional data. In spite of fairly high stability of remanence to both AF and thermal demagnetizations, half of the samples showed unreliable behavior in the paleointensity experiments. For one lava flow, Sainokawara lava of 0.21 Ma, two series of experiment were made in different inducing fields of 50 and 20 mT to five specimens in each experiment. Agreements were fairly good between the two sets except for one specimen.Statistical secular variation study was made for paleodirectional data set in Brunhes age including the newly obtained data in this study. The degree of the geomagnetic secular variation was about twice as large as the historical secular variation for the last 400 years.

尝试用K-Ar法对年轻火山岩进行精确测年，为详细研究地磁场的古分子变化提供依据。在赵氏火山群中采集了测年和古地磁测量样品，18个安山岩熔岩和岩墙中的大多数都给出了可靠的放射性年龄，但只有“Roba no Mimi Rock”岩样例外，其年龄约为1 Ma。显然，一些过剩的Ar被困在堤坝样品中是导致测年实验失败的原因。对山形大学质谱仪未发表的日本东北地区五个火山群的年龄数据进行了回顾和总结。采集了早火山16个地点的安山岩熔岩和岩脉进行古地磁研究。所有样品对AF和热退磁都是稳定的，并成功地确定了16个古地磁方向，年龄范围为0.07-1 Ma。用Coe版本的Thelliers方法对从8个地点选择的最稳定的50个标本进行了古强度实验。然而，结果并不像定向数据那样成功。尽管剩磁对AF和热退磁都有相当高的稳定性，但在古强度实验中，有一半的样品表现出不可靠的行为。对于0.21 Ma的Sainokawa熔岩流，在50和20Mt的不同诱导场进行了两系列实验，每次实验5个标本。除1个样本外，两套数据的一致性较好。对包括本研究新获得的数据在内的布伦斯时代的古方向数据集进行了统计长期变化研究。地磁长期变化的程度大约是过去400年历史长期变化的两倍。