石英的^（40）Ar／^（39）Ar定年方法及Ar同位素质谱分析

￣(40)Ar/￣(39)Ar Dating Method and Ar Isotopic Mass Spectrometry Analysis of Quartz

石英中Ｋ含量极低，而且一般含有包体成分（气相、液相或固相）的干扰，用常规Ｋ—Ａｒ稀释法很难测定其同位素地质年龄。采用４０Ａｒ／３９Ａｒ快中子活化定年法，通过石英中３９Ｋ（ｎ·Ｐ）３９Ａｒ核反应，把微量Ｋ的化学分析转化为３９Ａｒ静态质谱法测定，从样品挑选、样品用量、照射时间、实验流程、质谱分析及数据处理诸环节严格把关，使Ｃｌ和过剩Ａｒ的干扰降低到最小限度，就能得到比较可靠的年龄数据，进而获得含金石英脉的成矿时代。

The potassium content in the quartz is very low and contains inclusion composition which usually occurs interference of the excess Ar and Cl for 40Ar/39Ar dating. There are two chlorine isotopes (35Cl and 37Cl) in the quartz minerals. When they are irradiated in a reactor, producing the reaction 35Cl(n. γ)36Cl and 37Cl(n.γ)38Cl, followed by β-decay to 36Cl Ar and 38Ar, respectively. The half life of 36Cl is very long (3. 1 × 105 years) and 38Cl is very short (37. 3min). Therefore, by the time of Ar analysis in a typical irradiation experiment a small amout of 36Ar is produced, while all the 38Cl has decayed to 38Ar.In the 40Ar/39Ar isotopic dating technique for quartz or other minerals, the Cl and excess Ar can be corrected by samples separation, 36Cl is adsorpted at low temperature (-80～-120℃) in the experiment process. Some formulas are used to calculate 'eAr and Fvalue, and thus to obtain precise 40Ar/39Ar age of the quartz which represents the forming time of gold deposits. All the isotopes of argon are analysed using a new RGA-10 gas-source mass spectrometer (VSS Company, U. K. ) operating in the static mode.Its sensitivity is 1. 12 × 10-4A/torr(1. 42 × 10-14 mol/mV). The data are calculated with a Stone-386 computer.