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

试图用K-Ar法对年轻火山岩进行精确测年，以提供详细的古地球磁场变化的基础。在枣火山群采集了样品进行年代测定和古地磁测量。18个安山岩熔岩和岩脉中的大多数给出了可靠的辐射年龄，但例外的是岩脉“Roba no Mimi Rock”样本，其年龄在1 Ma左右，这是先前研究报告的。显然，岩脉样品中含有过量的氩气是测年实验失败的原因。对山形大学质谱计获得的日本东北地区5个火山群未发表的年龄数据进行了回顾和总结。收集了枣火山16个地点的安山岩熔岩和岩脉进行了古地磁研究。所有样品对AF和热退磁稳定，并成功确定了16个古磁方向，年龄范围为0.07 ~ 1 Ma。采用Coe版本的Thelliers方法对从8个地点选出的最稳定的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。尽管对AF和热退磁都有较高的剩余物稳定性，但一半的样品在古强度实验中表现出不可靠的行为。以0.21 Ma的Sainokawara熔岩流为例，分别在50和20 mT的不同诱导场下进行两组实验，每次实验5个试件。除了一个标本外，两组之间的一致性相当好。对包括本研究新获得的数据在内的Brunhes时代古定向数据集进行了统计长期变化研究。近400年来地磁长期变化的程度约为历史长期变化的两倍。

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 Review and summary were also given to unpublished age data for five volcano groups in Tohoku Japan obtained from the mass spectrometer of Yamagata University.Andesite lavas and dikes were collected for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.