印尼蘇拉瓦阿甘火山特殊稀土元素富集現象探討

Abstract

稀土元素（rare earth elements; REEs）為關鍵金屬（critical metals）之一，是現代高科技與能源轉型發展中不可或缺的原料，然而對產業至關重要的中、重稀土元素在自然界中少有發生富集現象。自印尼蘇門答臘島西北端、第四紀島弧火山蘇拉瓦阿甘採集而來的十一件火山岩中，有四件中酸性樣品出現顯著的輕、中，甚至重稀土元素富集（LaN: 114-761; DyN: 29-141; YbN: 32-126），同時伴隨著強烈的鈰負異常（Ce/Ce*: 0.28-0.67）。類似的特殊稀土元素富集現象在文獻中僅有少數案例，且對其成因尚無定論，本研究透過比較「具有」與「不具」特殊稀土元素富集的樣本，根據岩象學、地球化學資料探討其形成機制。 與同岩性但無異常稀土元素富集的樣品相比對，具富集的樣品有相似的岩象（礦物組成、岩石組織）及地球化學（主量元素含量、稀土以外微量元素濃度及鍶、釹、鉿放射性同位素比值）資料；唯獨在四件富集樣本中出現罕見的次生含水稀土磷酸鹽礦物，其中富輕稀土的水磷鈰礦（rhabdophane）通常呈斑塊狀出現在基質或篩狀斜長石斑晶中，偶見置換磷灰石；富重稀土的水磷釔礦（churchite）則僅零星發現於其中兩件特殊稀土富集岩樣的篩狀斜長石中。水磷鈰礦及水磷釔礦含有大量稀土元素（ΣREE2O3 ~ 55 wt.%），因此僅須微量（0.12-0.16 wt.%）的水磷鈰礦及水磷釔礦加入，即可將全岩稀土元素濃度提升10倍，造成此特殊稀土元素富集。 兩含水稀土磷酸鹽礦物具有強烈鈰負異常，說明稀土源區曾經歷過氧化作用，且稀土元素富集樣本的釹同位素比值與無富集樣本相似，因此外來稀土元素應源自位在近地表氧化環境中的蘇拉瓦阿甘火山岩。前人文獻指出REE3+能夠被含Cl-或SO42-的流體所溶解、搬運，則富集稀土的蘇拉瓦阿甘溫泉水在與新鮮火山岩接觸後，離子交換使pH值上升，導致水磷鈰礦及水磷釔礦於基質、篩狀斜長石中析出，或者成為磷灰石假象，造成此特殊稀土富集現象 彙整前人案例，可知此類稀土富集的形成不影響其他微量元素之濃度，不受地質架構（島弧、洋島或板內火山）、岩性（基性至中酸性）或氣候（熱帶、溫帶，乾燥或海洋性）侷限，且可在地質尺度上極短的時間內（<0.3 Ma）形成。

Rare earth elements (REEs), as part of the critical metal, are indispensable to modern high-tech products and global energy transition. However, natural enrichment of the most useful middle and heavy REEs is relatively rare. Of the eleven volcanic samples collected from Seulawah Agam, a Quaternary arc volcano situated at the northwestern tip of Sumatra, Indonesia, four intermediate to acidic rocks show significant enrichment across all REEs (LaN: 114-761; DyN: 29-141; YbN: 32-126), with an extremely negative Ce anomaly (Ce/Ce*: 0.28-0.67). There are only a few cases of such unique REE enrichment reported in literature, and no consensus on its formation mechanism has been reached. By comparing the petrographic and geochemical data of the REE-enriched and normal samples, this study aims to understand the formation mechanism behind such REE enrichment. Compared to non-enriched samples, the REE-enriched samples have similar petrographic (mineral assemblages and textures) and geochemical (major elements and non-REE trace element concentrations, as well as Sr-Nd-Hf radiogenic isotopic ratios) features. The difference lies in the presence of secondary hydrous REE phosphate minerals. LREE-enriched “Rhabdophane” occurs in groundmass or as inclusions within sieved plagioclase phenocrysts, occasionally as apatite pseudomorph; HREE-enriched churchite was only found in sieved plagioclase in two of the four REE-enriched samples. Rhabdophane and churchite both contain large amounts of REE oxides (~55 wt.%), therefore, the addition of 0.12-0.16 wt.% of both minerals is enough to raise the whole-rock REE content by tenfold, causing the observed REE enrichment. Both REE phosphate minerals contain extremely negative Ce anomaly, likely formed under oxidizing condition. While their Nd isotopic ratio is similar to the non-enriched samples, indicating the additional REE came from volcanic rocks originating from near surface oxidizing environment of Seulawah Agam. Previous studies have shown that REE3+ can be mobilized by Cl- or SO42--bearing acidic fluid. Therefore, when REE-bearing hot spring water from Seulawah Agam came into contact with fresh volcanic rock, ion exchange between the two can cause the pH to rise, inducing the precipitation of rhabdophane and churchite in groundmass or sieved plagioclase, as well as forming apatite pseudomorph, resulting in the REE enrichment. The integration of literature suggests that this REE-enrichment process does not alter the concentrations of other trace elements, and is not limited by geological setting (e.g., island arcs, ocean islands or intraplate volcanism), rock type (from mafic, intermediate to felsic) or climate (tropical, temperate, arid or oceanic). Moreover, it can occur over a geologically short period of time (<0.3 Ma).