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Laser Fusion 40Ar/39Ar Dating on Young Volcanic Rocks
详细信息   
摘要
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.

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