K-Ar dating of volcanic rocks younger than 0.1 m.y.

在该研究项目中，准确测定岩石样品的Ar同位素比值是非常重要的。为了建立质谱仪，我们首先编写了一个计算机程序来模拟离子轨迹。该程序绘制了任意形状磁铁和离子源位置下的离子轨迹，对质谱计的设计非常有用。我们在第一年收集了质谱仪的所有部件，并安装了机器。收集了离子源、收集器、钽炉、气体净化系统的基本数据。我们注意到，同位素比值的鉴别因子很大程度上受电子倍增器的高电压的影响。本文还研制了一套由计算机和A-D转换器组成的自动化系统，可直接获得年龄数据，质谱仪的灵敏度稳定，鉴别因子很大，但很恒定，这使我们对实际岩石样品进行了年龄测定，并对两个已知年龄小于0.1百万年的样品进行了年龄测定。如果仅从鉴别因子的变化分配误差，则获得的年龄为0.056 <-！+> 5百万年朝鲜粗面岩和0.028 <-！+> 0.017百万年安山岩，分别。粗面岩和安山岩的相对误差分别为50%和100%。我们还测量了三个岩石样品，年龄约为300万年。如果空气污染程度较低，则精确测定这些样品的年龄。

In this research project it was very important to measure Ar isotopic ratios of rock samples precisely. To set up the mass spectrometer we at first made a computer program to simulate ion trajectory. This computer program draws ion trajectory for any magnet shape and ion source position, which is very useful to design a mass spectrometer. We gathered all parts of the mass spectrometer during the first year and set up the machine. Basic and necessary data were collected for ion source, collecter, Ta furnace, and gas purification system. We noticed that the discrimination factor of isotopic ratios were largely influenced by the high voltage of the electron multiplier. Complete automotion system, using a computer and A-D converter, was also developed to get the age data directly.Sensivity of the mass spectrometer have been stable and the discrimination factor was very large but was very much constant, which lead us to do age determination of actual rock samples.The age determinations were performed for two samples which ages were already known and younger than 0.1 m.y. If errors were assigned only from the variation of the discrimination factor, the obtained ages were 0.056 <-!+> 0.005 m.y. for Korean trachyte and 0.028 <-!+> 0.017 m.y. for andesite, respectively. The errors from the hot blank correction gave the relative error of 50% for trachyte and 100% for andesite. We also measured three rock smaples which ages were about 3 m.y. The ages were precisely determined for these samples if air contaminations were low.