- 學位論文
固體樣本的稀有氣體同位素分析系統之建立與其應用
The Analyzing System for Noble Gas Isotopes on Solid Samples and its Applications
摘要
稀有氣體同位素在地球科學及定年學皆有廣泛的應用，包含岩石成因、地體構造、礦物定年等。可應用的樣本分為氣體、液體、及固體三種，各有著不同的純化與分析步驟。目前台灣大學氣體地球化學實驗室已完成氣體及液體的稀有氣體分析純化系統，唯尚未設立固體樣本的分析系統，本研究目的為建立一套固體樣本的分析系統，以進一步討論其分析結果之意義。 為了使稀有氣體從固體樣本中釋出並進行分析，本研究使用電阻加熱式真空高溫爐，可應用的固體樣本有岩石樣本及礦物顆粒等，在高真空環境下階段式加熱樣本，使其逐段釋出稀有氣體，並經由純化線純化，及超低溫幫浦富集，再導入稀有氣體質譜儀分析其同位素比值。本研究針對稀有氣體中的氦(helium)、氬(argon)同位素進行分析，氦氣為分析3He及4He兩種同位素，而氬氣部分針對較常用的36Ar及40Ar進行實驗；分析得到之同位素比值則利用標準空氣與人工合成標準氣體，進行分析結果之校正。 本研究初步分析了台灣大學標準岩石樣本及6種礦物樣本。台灣大學標準岩石樣本的4He含量介於16.7~9087 nccSTP/g，3He/4He比值為0.00002~0.028Ra (Ra為大氣中的3He/4He比值，1.39×10-6)。六種礦物樣本分別是：中國漢諾壩橄欖石、澎湖橄欖石、觀音山斜輝石、嶺頂斜輝石，綠島斜輝石及綠島堇青石。礦物樣本的4He含量介於2.34∼10.64 nccSTP/g，3He/4He比值為1.12~7.04Ra。 氦氣含量及同位素分析結果顯示台灣大學標準岩石樣本樣本皆受到風化作用或是研磨過程影響，已經失去了90%左右的氦氣。嶺頂橄欖石為受到脫氣作用影響，而使4He含量及3He/4He比值皆降低；澎湖橄欖石之3He/4He比值與世界中洋脊玄武岩（MORB）之平均值相近；中國漢諾壩橄欖石之3He/4He比值和其他文獻比較，表現出相當大的異質性。觀音山斜輝石、綠島斜輝石及綠島堇青石等樣品之氦同位素比值分析結果均指出有地殼混染的訊號。 綠島岩漿成因長久以來存在有地殼混染或是源區混染的爭論，本研究利用綠島斜輝石及堇青石巨晶的3He/4He比值及其母岩漿的87Sr/86Sr比值，得到了綠島岩漿經歷過5%左右地殼混染的結果。配合上岩石學的觀察，本研究推論綠島火成岩曾受到地殼混染作用影響，而產生低3He/4He比值、高鍶同位素及低釹同位素比值的地化特性。
並列摘要
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.