(极)年轻火山岩激光熔蚀^(40)Ar/^(39)Ar定年

Laser fusion ^(40)Ar/^(39)Ar dating on young volcanic rocks

对中国大量年轻或/和极年轻火山岩的定年实践研究表明,(极)年轻火山岩的激光熔蚀40Ar/39Ar定年具有不同于第四纪以前喷发火山岩定年的显著特点。激光熔蚀40Ar/39Ar定年技术因为本底低、样品用量小以及与现代惰性气体同位素质谱设备在灵敏度、高精度方面的相一致,在年轻火山岩的定年中得到深入运用。借助激光在年轻或/和极年轻火山岩的40Ar/39Ar定年中,实践证明,样品形成时限越年轻(特别是相当于第四纪时期的样品),Nier值与样品中初始氩比值的偏离会引起K-Ar和40Ar/39Ar表观年龄的偏差越大。对于小于0.2Ma的样品,Nier值与样品中初始氩比值的偏离对K-Ar和40Ar/39Ar表观年龄的偏差影响呈指数增长;当样品年龄相对较老(老于第四纪)时,Nier值和初始氩比值的偏离对K-Ar和40Ar/39Ar表观年龄的影响较小。以40Ar/39Ar定年为出发点,定量给出界定年轻与极年轻火山岩的年龄:2～0.2Ma的火山岩界定为年轻火山岩,0.2Ma以来的火山岩称为极年轻火山岩。实验结果还证实,测定(极)年轻火山岩基质年龄时要尽量剔除非同源分馏的斑晶,以便去除斑晶可能带来的过剩氩影响;年轻火山岩样品的测年,应根据岩石结构和粒度特征选取合适的粒度,通常情况下,推荐0.2mm颗粒直径(60～80目)为理想粒径;年轻火山岩样品在快中子辐照后冷却放置时间不宜过长,否则造成37Ar测不准,影响数据结果,带来较大偏差;激光40Ar/39Ar精细定年对标准样品的均一性有很高的要求,通过标定常用的国内外监测标样发现,标样SB-778-Bi,Bern4M,BT-1均一性很好,适合用作激光熔蚀40Ar/39Ar定年监测;测试数据的处理中,火山岩喷发后冷却结晶中同时形成的斑晶和基质的等时线处理能够帮助获得客观真实和精细的年龄结果。在此基础上,北京大学惰性气体同位素实验室建成了专用于(极)年轻火山岩精细定年的激光熔蚀40Ar/39Ar定年实验流程。

It is extremely difficult to date young volcanic rocks. Accurate 40Ar/39Ar dating research and practice of young volcanic rocks in China put forward several notable and important points during the process of 40Ar/39Ar dating for young volcanic rocks. Because of the low background level,small sample quantity,and high sensitivity of modern noble gas isotope mass spectrometry device,laser ablation 40Ar/39Ar dating technique has been used in depth application in volcanic rocks dating. With the laser in 40Ar/39Ar dating of young and modern volcanic rocks,the discrepancy of Nier value and initial argon isotope ratio in Quaternary samples may cause a remarkable deviation between K-Ar apparent age and 40Ar/39Ar apparent age. The deviation show exponential increase for samples younger than 0. 2Ma,but it is not so notable for samples older than 2Ma. There out,the boundary of young and modern volcanic rocks,which is applied to 40Ar/39Ar system,is assigned at 0. 2Ma： rocks aged 2 ～ 0. 2Ma is defined as young volcanic rock, while rocks aged younger than 0. 2Ma is defined as modern volcanic rock. The phenocrysts,which are from different argon isotope fractionation system,should be rejected from the sample during 40Ar/39Ar dating. According to rock texture and grain size of mineral, the matrix should be picked with ideal grain size. In normal cases,about 0. 2mm （or 60～80 mesh） is recommended; Cooling time after irradiation of fast neutron reactor should not be no more than three months,or else it may cause an error on 37Ar measurement, and consequently on final results. As neutron-flux monitor,standards should be characteristic of high homogeneity during laser 40Ar/39Ar dating. In calibrating the international and domestic standards we have,we find that SB-778-Bi,Bern4M and BT-1 can be ideal standard samples with good homogeneity. We also find that the isochron age calculated from the laser 40Ar/39Ar data of the phenocryst and matrix,which were cooled and crystallized at the same time during volcanic eruption,is an objective and precise age. For volcanic rocks with almost the same crystallization time,the same magma source,and perfect argon-argon closed system,phenocryst-matrix isochron method can be adopted to gain more precise and reliable results.