綠島火山岩中晶體與溫泉噴氣氦同位素比值之研究

Abstract

在探討島弧岩漿成因時，氦同位素是一個實用的工具。因為在板塊隱沒時所攜帶之沈積物所含的氦氣多在淺部即會釋放殆盡，不會被帶至地函楔深處，所以不會顯著地改變地函楔的氦同位素比值。利用此一特性，我們可以很容易看出島弧岩漿在從地函上升的過程中是否有地殼氦氣的加入，如果有地殼混染（Crustal contamination）的情形，那麼同位素比值將會因為地殼值而被拉低，反之，則將會是接近典型的上部地函值。 綠島地區的岩漿成因存在著地殼混染與地函源區混染的爭議，前人曾經從核飛跡定年、氧、釹、鍶同位素組成、礦物反應邊及其包裹體之成分等研究來尋求解釋，但至今仍未獲定論。本研究則嘗試利用礦物及溫泉噴氣中之氦氣同位素比值（3He/4He）來探討此一議題。 王振丞（2005）的研究中發現了綠島的礦物與氣體樣本具有不同的氦同位素比值，他推論氣體樣本的高值可能源自未受地殼混染冷凝中的岩漿庫脫氣，而礦物的低值則顯示其母岩漿曾受到地殼的混染。然而，其研究中所分析礦物之氦同位素值皆由高溫爐燒融法所得，所分析得到的結果是流體包裹體及晶格中氣體成分的總和，可能無法代表岩漿的原始成分。 本研究利用真空擊碎法、結合高溫爐的方法，分析礦物中的氦同位素，結果發現在礦物的氦氣中並沒有顯著的地殼訊號，同時在溫泉氣泡中也得到相同的結果，故推論綠島岩漿在演化的過程中並未受到明顯的地殼混染作用。

Noble gas isotopes have several important applications in geochemistry, such as petrogenesis, volcanism, tectonics and mineral dating. In this study, I focus on the helium isotopic ratios (3He/4He) in volcanic mineral crystals and hot spring bubbles. Helium is not able to be recycled by plate tectonics into the Earth’s interior, making the 3He/4He ratio unique among isotopic tracers of mantle sources involved in volcanism. As magma ascends it may suffer He exchange with crust material which with lower He isotopic values. Even minor amounts of crustal interaction with the ascending magma through the process of crust contamination can be easily recognized through the helium isotopic ratios. Therefore, He isotopes have been proven as a powerful tool for assessing crustal contributions to the magmatic systems in subduction-related settings. Gases exist in two different positions inside a mineral grain: fluid inclusion and crystal lattice. There are two major ways to extract gases from minerals or rocks. One is vacuum crushing, the other is total fusion. Using vacuum crusher, the gases in fluid inclusions may be separated from those in mineral matrix (lattice). Together with the total fusion method, we can measure the gas composition of fluid inclusions and mineral matrix respectively. There is a long-standing debate in the petrogenetic processes of Lutao volcanic rocks, surrounding a main issue that is: if it is source contamination or crust contamination for the magma genesis? Previous studies have reported 87Sr/86Sr, εNd and δ18O results on this issue. In this study, helium isotopes have unique properties that enable them to provide crucial information regarding this debate. In our results, we did not find significant crustal signal in helium isotopes ratios of crystals. It is consistent with the results of hot spring bubbles. Therefore, we can conclude the crustal contamination is not the major process for the genesis of Lutao magmatism.