岩石鋰同位素的分析方法:應用多接收感應耦合電漿質譜術

Abstract

鋰具有兩個穩定同位素6Li與7Li，兩者質量差異大，在近地表環境的流體與固體交互作用中會產生明顯分化，使得鋰同位素深具潛力作為近地表地質作用中的示蹤劑 (如：隱沒帶脫水作用與地表化學風化等)。本研究工作亟欲建立岩石的鋰同位素分析方法，在純化過程中避免造成鋰同位素分化，並使用多接收感應耦合電漿質譜儀 (multiple-collector inductively coupled plasma mass spectrometry, MC-ICP-MS)，挾其優勢以精準、快速地測量經純化過後之低鋰濃度樣品 ([Li]：20 ppb) 的鋰同位素比值 (7Li/6Li)。本研究以矽酸鹽火成岩參考樣建立鋰同位素分析方法；25 mg樣品經酸溶並完全溶解後，以1.6 ml AG 50W-X8氫型陽離子交換樹脂與1 M硝酸 + 80% (v/v)甲醇的提洗液，使用層析法先建立不同岩性樣品的提洗曲線，除了確認鋰的提洗區間，也可得知會出現在鋰樣品溶液中的元素，以鋰同位素標準品L-SVEC混合不同濃度的其它元素。將純化後達> 99%回收率的鋰樣品溶液，續由MC-ICP-MS進行鋰同位素分析，儀器分析時，直接在質量峰測量零值，並以標準品—樣品—標準品包圍法計算樣品的鋰同位素數值δ7Li。結果顯示，不同岩性樣品中，鋰的提洗行為略有些差異，當鋰回收率> 99%時，基性岩至酸性岩的鋰萃取溶液中會有較高的鈉鋰比值與鈦鋰比值，其中基性岩含有高鈦，超基性岩含有高鈉，但對於鋰同位素數值的量測未造成明顯影響。儀器長期的分析精度為0.5‰ (2 S.D.)，USGS參考樣BHVO-2、AGV-2、G-2與RGM-1的δ7Li測值分別是+4.5‰、+7.3‰、-0.4‰與+2.8‰，皆與文獻值一致。

Lithium (Li) has two stable isotopes, Li6and Li7, with large mass difference. Their isotopic fractionation significantly occurs in the interaction between soild/rock and aqueous phase/water in low termperature, and thus lithium isotopes have high potential as a geochemical tracer for aqueous-related geological processes, e.g. weathering on the Earth’s surface and dehydration of altered oceanic crust in subduction zones. The goal of this study is to establish the method of Li isotopes in igneous rocks in NTU as a base of future researches. At beginning, approximately 25 mg powder of international reference rocks, from peridotite to rhyolite or granite, were digested by inorganic acids completely. Chemical separation was followed and sample solution was eluted through 1.6 ml AG50W-X8 cation exchange resin by 1 M HNO3-80% (v/v) methanol to set up elution curves. According to the elution curves, the Li fraction of each sample with >99% recovery yield was collected and then quantitatively analysed by ICP-QMS and MC-ICP-MS for its matrix elements and lithium isotopes, δ7Li, respectively. In MC-ICP-MS analysis, zero was determined at each mass peak center of a blank solution, and the “standard-sample-standard bracketing” method was adopted in order to correct for the mass bias. Our preliminary results indicate: The elution curve of Li slightly varies with lithology. Li fraction of ultramafic and mafic rock samples may have matrices that concentrations are higher than Li, i.e. sodium and titanium, respectively. However, such high-abundance impurities do not affect our lithium isotopic measurements demonstrated by analysing doped with L-SVEC. Long-term precision of this study is 0.5‰ (2 S.D.). δ7Li values measured of USGS RMs (BHVO-2: +4.5‰; AGV-2: +7.3‰; G-2: -0.4‰; RGM-1: +2.8‰) are consistent with literature values.