精進碳酸鹽類稀土元素質譜分析技術及其在地球科學上之應用

Abstract

本論文研究工作為利用液相感應耦合電漿質譜術(solution nebulization-inductively coupled plasma mass spectrometry, SN-ICPMS) 及雷射剝蝕感應耦合電漿質譜術(laser ablation-inductively coupled plasma mass spectrometry, LA-ICPMS) 建立自然界中碳酸鹽內稀土元素之分析技術，並應用於古環境變遷研究。 首先，本論文工作建立一套可直接量測，不須繁複化學處理的稀土分析技術。此技術最大特點為可分析極低含量的稀土元素，其偵測極限可達飛克等級(femtogram quantity, 10-12 g)，同時可提供高精度 (high-precision, ±1.9-6.5%, 2σ)且高產率 (每小時可測量8-10個) 之碳酸鹽類樣本分析 (Analytical Chemistry, 2011)。應用此技術於南海地區所採集的現生珊瑚標本，我們發現在1991年夏季有明顯的稀土元素異常，同時結合微量元素鋁鈣比 (Al/Ca) 和微區影像 (micro-domain images) 的證據，表明造成該年夏季南海海域大規模珊瑚死亡事件極可能是由鄰近的菲律賓皮那土波 (Mt. Pinatubo) 火山大爆發所導致 (Wu et al., Geophysical Research Letters, 2018)。 為了提高樣本的空間解析度 (temporal resolution)，及縮短耗時的樣本處理步驟，本論文工作進一步利用雷射剝蝕質譜儀(LA-ICPMS)，建立世界最先進之高感度碳酸鹽雷射剝蝕質譜分析技術 (Wu et al., Analytica Chimica Acta, 2018)。此雷射剝蝕質譜術可成功量測到碳酸鹽內含量只有十億分之一 (parts-per-billion, ppb) 的稀土元素。應用此技術於自然界採自東帝汶的石筍標本，發現有兩次的稀土異常事件，發生在8萬8千年至7萬8千年前之間。比較其稀土元素蛛網圖 (REE diagram) 指示這可能是受控於當時鄰近區域火山噴發之火山灰保存於石筍樣本內所致。 本論文工作所建立的高感度 (high sensitivity) 與高空間解析 (high temporal resolution) 碳酸鹽類稀土族元素液相質譜術及雷射剝蝕質譜術，可廣泛應用於自然界的碳酸鹽類地質材料上，例如：珊瑚、石筍、石灰華、流石…等。對地球科學相關研究領域帶來顯著之貢獻。

Two state-of-the-art quantitative techniques to determine ultra-trace rare earth elements (REEs) in natural carbonates using solution nebulization-inductively coupled plasma mass spectrometry (SN-ICPMS) and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) with respective applications were presented in this dissertation. These techniques were applied to natural carbonates, including corals and stalagmites, to understand volcano eruptions and the impacts on modern biosystem and paleoclimate regimes. In the first SN-ICPMS protocol, direct measurements for femtogram quantity carbonate samples without chemical separation steps can offer accurate and high-precision analysis (±1.9-6.5%, 2σ) with a high sample throughput of 8-10 samples/hr routinely (Analytical Chemistry, 2011). Application to modern Porites corals collected from South China Sea region, the anomalies of REE contents and Al/Ca ratios associated with micro-domain images, register modern coral reefs could be exacerbated by volcanic eruptions (Wu et al., Geophysical Research Letters, 2018). In the second protocol, a high-sensitivity quantitative open-cell LA-ICPMS technique has been established to allow direct sampling on stalagmite surface in the atmospheric air (Wu et al., Analytica Chimica Acta, 2018). This technique improved limits of detection down to sub-ng/g range and promises analyses of carbonate REE profiles at the single digit parts-per-billion (ppb) levels. Application to a 15-cm stalagmite collected from East Timor, reveals two peaks of REE contents by at least one order of magnitude, possibly due to volcanic ash preserved in stalagmite. Both improved SN-ICPMS and LA-ICPMS techniques highlight the high-sensitivity and high-temporal-resolution carbonate REE analyses for corals and stalagmites, with great potential to other natural carbonates such as travertine, tufa, and flowstone, benefit our understanding of paleoclimatic and paleoenvironmental dynamics.