冲绳海槽中段热液活动的沉积记录及其对热液物质输运过程的指示

The Okinawa Trough distributed widely vents is a natural laboratory to investigate the hydrothermal activities in back-arc basins. Iheya and Izena hydrothermal areas located at the middle Okinawa Trough are the the main study areas in this program. Sediment samples (mainly surface sediments, supplemented with some core sediments) were collected from both Iheya and Izena hydrothermal areas, and each sediment samples were divided into four sub-samples,such as >63μm, 63-30μm, 30-10μm and <10μm, according to sensitive grain- size of hydrodynamic condition. We analyzed the element components of all bulk sediments, and measured the mineral assemblages,element compositions and S- Sr-Nd isotopes of four sub-samples ( >63μm, 63-30μm, 30-10μm and <10μm) for all samples. We also leached the <10μm sub-samples to separate both a Fe-Mn oxide faction and a detrital faction, and then determined the compositions of elements, Sr-Nd isotope of these two factions. We compared the element components of bulk sediments with previous reported data, and deeply analyzed and discussed all the data and comparied the differenc between these two hydrothermal areas. The objectives of this program are: to determine the spatial distribution pattern of metalliferous sediments; to establish the geochemical background parameters and hydrothermal activity index system; to identify the mass sources of the two hydrothermal systems; to reveal the transport processes of hydrothermal products, such as particulate matter and solute states, in marine systems; and to clarify the relationships between geological features and hydrothermal activities.The results of this research will provide basical data and guidance for enriching element cycles and locating unknown hydrothermal vents.

冲绳海槽发育广泛的热液活动，是了解弧后盆地热液活动的天然实验室。课题以热液活动最为集中的海槽中段为靶区，以伊平屋和伊是名区沉积物为主要研究对象，分析其元素组成，结合前人资料初步建立区内环境背景参数和热液活动指标体系，确定含金属沉积物分布范围。根据水动力敏感粒级，将沉积物分为>63、63-30、30-10和<10μm四个子样，测定矿物、元素和S、Sr-Nd同位素组成；淋洗<10μm子样颗粒外包铁锰氧化物/氢氧化物薄膜，测定淋洗液和残留物元素、Sr-Nd同位素组成。对比正常沉积物-含金属沉积物矿物、地球化学数据，完善区内背景参数和热液指标；归纳数据空间变化规律，以厘清海槽热液系统物质来源、颗粒态和溶质态产物在海洋系统中输运过程；比较海槽中段不同热液靶区热液系统物质来源、产物输运过程的差异，讨论地质条件对中段热液活动的影响机制；为充实海洋系统物质循环、未知喷口定位提供基础资料。

本次研究以冲绳海槽热液区沉积物和火山岩为研究对象，运用矿物学和元素-同位素地球化学的方法，探索冲绳海槽热液活动特征、产物分布规律及其对环境的影响。区内热液硫化物矿物组合主要包括黄铜矿、黄铁矿、方铅矿、闪锌矿以及少量次生矿物。松散沉积物明显富集Cu、Pb、Zn、Ba、Cd、U。在小于63μm粒径的沉积物组分（包括63-30μm、30-10μm和<10μm组分）中，目标元素含量相对稳定；大于63μm的粒级组分中，元素含量变化明显，热液沉积物目标元素含量可高于的背景值2-3个数量级；但区内单个热液喷口影响范围较小，约百米左右。研究区出露的酸性火山岩富集Pb、Ba等目标元素；岩石蚀变后元素组成变化与海底热液活动富集的元素相吻合，表明岩浆体系可能为热液活动提供了物质组成。海槽中段岩浆-热液事件层位中的沉积物，其自生铁锰组分含有来自岩浆体系的物质组分，显示岩浆-热液活动对底层水体的影响。正常海洋沉积物自生铁锰相元素与同位素组成显示，末次冰消期-早全新世，北太平洋中层水与南海中层水（南极中层水）之间交换活跃，导致海槽中段底层水呈氧化性; 早全新世-现在，底层水逐渐转换为亚氧化性，反映海槽现代深层水体侧向和垂向交换减弱。本次工作获得的成果将有利于提升弧后盆地热液成矿以及北半球海洋-气候系统演化特征方面的研究。