南沖繩海槽熱泉系統之鐳同位素分布與地化特徵

Abstract

海底熱泉循環過程中，因水岩換質作用，鐳同位素會釋出進入熱液；相較於一般大洋水體，臨近熱泉系統區域之海水呈現較高鐳同位素活度值；因此，鐳同位素為海底熱泉的良好示蹤劑。然而過去熱泉系統之鐳同位素研究，多集中於中洋脊熱泉系統之熱液－火成岩反應研究，鮮少探討具厚層沈積物特徵區域熱泉系統之海水中鐳同位素分布及熱液－沈積物反應。位於台灣東北部海域的南沖繩海槽屬於具有巨厚沉積物之弧後擴張構造區域，其火成活動相當活躍，並已發現數處海底熱泉活動位址，為研究具厚沉積物特徵區域熱液系統之鐳同位素分布特徵的良好場址。本研究利用鐳同位素(224Raex、226Ra、228Ra)，輔以海水層柱/間隙水之氦同位素比值、溶解甲烷氣體濃度、及沉積物之主量及微量元素，探討南沖繩海槽海底熱泉系統中天然放射性鐳同位素核種的分布及其地化特徵。 海水層柱/間隙水中氦同位素比值、溶解甲烷氣體及地化分析結果，顯示蓬萊斷層帶(P1、P1T、P1B)及石林隆堆(G1、G2)呈現熱液訊號。鐳同位素空間分布結果顯示，蓬萊斷層帶P1、P1B站位底水中鐳同位素活度為其他站位的數倍，同步顯示出熱液輸出的訊號。石林隆堆G1站位底水中鐳同位素活度並無呈現極高的活度，推測為熱液中的鐳同位素於沉積物上的沉澱所造成的虧損。此外，石林隆堆G1站位的複管岩心沉積物發現金、砷、鋅、鉍、銅、鉻等熱液富集之微量金屬沉澱層位，且鐳同位素(226Ra、228Ra)於該特定層位處亦出現高值。透過228Ra及226Ra的儲量比值及210Pbex推算，熱液噴發產生沉澱層位時間間隔約為15~ 20年，推測石林隆堆沉積物中的熱液富集元素沉澱發生於熱液-海床介面，且噴發活動屬偶發性事件(episodic event)，熱液來源之210Pbex儲量占整體的64%。

Compare to seawater, radium isotopes are enriched in hydrothermal fluids due to rock/basalt-hydrothermal alteration. The geochemical distributing and characteristics of radium isotopes within hydrothermal systems have been intensively investigated at sediment-starved mid-oceanic ridge. However, in sediment-hosted hydrothermal systems, such as the southern Okinawa Trough, only few studies have been conducted. This study aims to investigate the geochemical distributions of radium isotopes within sediment-hosted hydrothermal systems in the southern Okinawa Trough. Helium isotope ratios, methane concentrations and geochemical characteristics in seawater column and porewater samples indicated the Penglai Fault Zone and Geolin Mounds are relating to active hydrothermal venting sites. Radium isotopes (224Ra and 226Ra) activities in seawater increase with supplied from the bottom. In the near bottom seawater at Penglai Fault Zone (P1 and P1B stations), the 224Ra and 226Ra activities are much higher than the background sites, implying the influence of hydrothermal fluids. However, the near bottom seawater at Geolin Mounds (G1 site) doesn’t show the same feature. The radium isotopes in sediment shows high activities in specific layers, the precipitation from radium isotopes in the bottom seawater onto the sediment might be responsible for relatively low Ra isotopes activity in the near bottom seawater. In addition, radium isotopes (226Ra and 228Ra) in the sediment shows high activities at few specific layers with high concentrations of Au, Ag, As, Bi, Cu, Cr. Combined with 228Ra/226Ra and 210Pbex chronology results, the enrichment of metal elements and radium isotopes in the sediment, is related to episodic venting events and the latest hydrothermal venting interval being approximately 15-20 years.