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

本文尝试用K-Ar法对年轻火山岩进行精确定年，为详细研究地磁场的古长期变化提供依据。在藏王火山群中采集了18个安山岩熔岩和岩脉样品，其中绝大多数给出了可靠的放射性年龄，只有岩脉样品"罗拔之见岩"的年龄为1Ma左右。显然，岩脉样品中捕获的过量氩是导致测年实验失败的原因。对日本东北地区5个火山群的未发表年龄数据进行了回顾和总结，并对藏王火山16个测点的安山岩熔岩和岩脉进行了古地磁研究。所有样品对AF和热退磁都是稳定的，并成功地确定了16个古地磁方向，年龄覆盖范围为0.07 - 1Ma。对8个测点中最稳定的50个样品，用Coe的Thelliers方法进行了古强度实验。然而，结果并不像定向数据那样成功。尽管有相当高的稳定性的剩磁AF和热退磁，一半的样品表现出不可靠的行为在paleointensity实验。对一个0.21Ma的浆川原熔岩流，在50和20mT的不同感应场下进行了两个系列的实验，每个实验中有5个样品。除一件标本外，两套资料之间的一致性较好。本文对Brunhes年龄的古方向资料集进行了统计长期变化研究，包括本研究新获得的资料。地磁场长期变化的程度大约是过去400年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 Review and summary were also given to unpublished age data for five volcano groups in Tohoku Japan obtained from the mass spectrometer of Yamagata University.Andesite lavas and dikes were collected for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.