西南印度洋中脊热液区海底玄武岩元素地球化学原位分析

摘要

In this study,we analyze element geochemistry of submarine basalt in situ,which is sampled in hydrothermal areas from ultraslow spreading Southwest Indian Ridge,including the fresh basalt rocks (B19-9,B15-13)and altered basalt (B5-2). And we can confirm that altered mineral in B5-2 is celadonite by microscope and Raman Spectrum.Furthermore,amygdaloidal celadonites are analyzed by electron microprobe (EPMA)and EDS-line scanning.The results show that K-contents decrease and Na-contents increase from the core to the edge in these altered minerals,indicating the transition from celadonite to saponite. Celadonite is an altered minerals,forming in low temperature(<50 ℃)and oxidizing condition,while saponite form in low wa-ter/rock and more reducing condition.As a result,the transition from celadonite to saponite suggests environment change from oxidizing to reducing condition.Using the result of EPMA as internal standard,we can analyze rare earth elements (REE)in al-tered mineral in situ.Most of result show positive Eu anomaly (δEu ),indicating hydrothermal fluid transform from oxidizing to reducing,and reducing fluid rework on the early altered minerals.Comparison with REE in matrix feldspar both in altered and unaltered zoning,we find that reducing fluid can leach REE from the matrix feldspar,leading to lower total REE concentrations and positive Eu anomaly.So leaching process play an important role in hydrothermal system.%对西南印度洋中脊壬液区海底新鲜和蚀变玄武岩样品进行了原位元素地球化学分析，发现其中的蚀变矿物主要为绿鳞石。电子探针及能谱线扫分析显示杏仁状绿鳞石从核部到边部具有 K 含量降低而 Na含量有所升高的特点，指示边部绿鳞石转化为皂石。绿鳞石为低温（＜50℃）的氧化条件下的蚀变矿物，而皂石则在较低水／岩值、还原性更强的条件下形成，转化过程暗示了蚀变环境由氧化环境转化为还原环境。以电子探针为内标，利用 LA-ICP-MS 对蚀变矿物做原位的稀土元素分析，大部分结果均有 Eu 正异常的特点，指示了蚀变流体从氧化性向还原性转变，并且还原性流体对早期蚀变矿物进行了叠加改造。而蚀变区和未蚀变区的基质长石原位稀土元素对比研究则表明后期还原性热液蚀变流体对稀土元素进行了淋滤，从而导致了蚀变区稀土总量下降且出现 Eu 正异常的特点。由此可见，热液系统中淋滤作用具有重要意义。