The Analyzing System for Noble Gas Isotopes onSolid Samples and its Applications

• Main Authors: Chen-Chneg Wang, 王振丞
• Other Authors: 楊燦堯
• Format: Others
• Language: zh-TW
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/84982343491501751826

碩士 === 國立臺灣大學 === 地質科學研究所 === 93 === Noble gases isotopes are generally applied to chronology and geosciences, including petrology, tectonics, mineral dating and so on. Three types of samples can be analyzed, including gas, liquid and solid samples. There are different purification and analytic procedures for those samples. The procedures for gas and liquid samples are well established in NTU geochemistry laboratory. However, the procedures for solid samples are still not available. The aims of this research are to establish the analytic procedures for solid samples and to discuss the implications of those results. To extract noble gases from solid samples, high-vacuum electric resistance heating furnace has been applied. Available samples of this study are rock powders and mineral crystals. Samples are transferred to the high-vacuum furnace, and heated with three steps of temperature. The extracted gas is purified and cryogenically concentrated. Helium concentration and isotopic composition are determined by mess spectrometer. 3He and 4He are analyzed for helium isotopes, while 36Ar and 40Ar are determined for argon isotopes. The isotopic results are calibrated by standard-air and artificial standard-gas. NTUG (National Taiwan University, Department of Geosciences) rock standards and six mineral samples are analyzed. Helium abundances of NTUG rock standards range from 16.7 to 9087 nccSTP/g, and helium ratios (3He/4He) range from 0.00002 to 0.028Ra (Ra is the helium ratio of air, 1.39×10-6). Six mineral samples are Hannuoba (漢諾壩) olivine of China, Penghu (澎湖) olivine, Kuanyinshan (觀音山) clino-pyroxene, Lingding (嶺頂) clino-pyroxene, Lutao (綠島) clino-pyroxene and cordierite. Helium abundances of mineral samples range from 2.34 to 10.64 nccSTP/g, and helium ratios (3He/4He) range from 1.12 to 7.04Ra. Helium abundances and ratios of NTUG rock standards show that those samples have lost approximately 90% of helium because of alteration or grinding. Helium ratio of Lingding olivine indicates helium degassing which decreases both helium ratio and concentration. Helium isotopic ratio of Penghu olivine shows MORB (Mid Ocean Ridge Basalt) value. Compared to previous study, Hannuoba samples show a wide range of 3He/4He ratios. Helium results of Kuanyinshan clino-pyroxene, Lutao clino-pyroxene and cordierite all indicate the signature of crust contamination. There is a long standing debate about the magma origin of Lutao. Two different models of crust contamination and source contamination are proposed. Combining helium results with strontium ratios of Lutao lavas, it shows that Lutao arc magma has experienced approximately 5% of crust contamination. In addition to available data, we can conclude that crust contamination of Lutao arc magma has resulted in low helium ratios, high strontium ratios and low neodymium ratios of Lutao lavas.