| 摘要: |
| 对1988年取自大西洋中脊TAG热液活动区的海底表层热液沉积物中36件硫化物样品进行了硫同位素组成分析。结合ODPLeg158的近期成果,对TAG区热液沉积物的硫同位素组成及其时空演变、硫源、硫同位素偏重的原因和硫的演化进行了探讨,得出如下结论:(1)TAG区表层热液沉积物的δ34S值从3.9‰—7.6‰变化,均值为5.98‰,与其它洋中脊热液活动区相比明显偏重。(2)从海底表层区到蚀变玄武岩区,热液沉积物的硫同位素组成有增大的趋势,而时间对热液沉积物的硫同位素组成则没有明显的控制。(3)热液沉积物中硫主要来自玄武岩,部分来自海水硫酸盐,是海水硫酸盐和玄武岩中硫不同程度混合的结果,且各硫源在不同阶段提供硫的比例分配和方式明显不同。(4)与其它无沉积物覆盖洋中脊中热液活动区相比,相对较大比例海水来源硫的加入是导致TAG区硫同位素组成偏重的一个重要原因,而TAG区具备有利海水硫酸盐还原作用进行的温度和水/岩比值条件则是促使该区热液沉积物中海水来源硫相对较多的主要原因。(5)海水的直接混入、流体-玄武岩相互作用、先期形成硫化物的重溶作用和硫酸盐矿物的还原作用是海水和玄武岩直接或间接提供硫的主要方式，也是导致本区硫同位素演化复杂化的主要原因，且硫的演化与海底岩浆作用和构造运动紧密相关。 |
| 关键词: 硫同位素组成 海底表层热液沉积物 TAG热液区 |
| DOI：10.11693/hyhz200005010010 |
| 分类号: |
| 基金项目:山东省自然科学基金资助项目，Q98E02139号和国家自然科学基金资助项目，49625609号 |
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| SULFUR ISOTOPIC COMPOSITION OF SEAFLOOR SURFACE HYDROTHERMAL SEDIMENTS IN THE TAG HYDROTHE-RMAL FIELD OF MID-ATLANTIC RIDGE AND ITS GEOLOGICAL IMPLICATIONS |
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ZENG Zhi-gang1, QIN Yun-shan1, ZHAO Yi-yang1, ZHAI Shi-kui2
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1.Institute of Oceanology,The Chinese Academy of Sciences,Qingdao266071;2.College of Marine Geosciences, Ocean University of Qingdao, Qingdao, 266003
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| Abstract: |
| The Trans-Atlantic Geotraverse (TAG) hydrothermal field is located on the slow-spreading Mid-Atlantic Ridge at 26°08'N, 44°50'W, near the east wall of rift valley of the mid-ocean ridge, and within a 25km2 area of the floor. Previous investigations have described the sulfur-isotope characteristics of various sulfides and sulfates from the TAG hydrothermal field and proposed some models for the sulfur-isotope systematics of modern seafloor volcanic-hosted massive deposit. However, the cause of heavy δ34S signature (compared to other sediment-free hydrothermal system), and the model for the time-space variation of sulfur-isotopic composition from hydrothermal sulfide and sulfate in the TAG hydrothermal field are still questions with uncertainty.
Samples for this study were collected by dredges in the TAG hydrothermal field of the Mid-Atlantic Ridge during the R. R. S. Discovery cruise 176 in 1998. A total of 36 sulfide samples from seafloor surface the TAG hydrothermal sediments were analyzed using sulfur-isotopic method. Combining the recent scientific results of ODP Lege158, the authors have undertaken analyses of the sulfur isotopic characteristics of sulfides from the seafloor surface hydrothermal sediments, the spatial distribution of the sulfur-isotopic values, the sources of sulfur in surface hydrothermal sediments, the cause of the heavy δ34S values in the hydrothermal sediments, and the evolution of sulfur for the TAG hydrothermal system. The conclusions obtained in this study are listed below.
1. Sulfides from the TAG surface hydrothermal sediments have δ34S ranging from 3.9‰—7.6‰, with a mean of 5.98‰. It is remarkable for the heavy δ34S signature compared to other hydrothermal sulfides from sediment-starved mid- ocean ridge axial and off-axial settings in the Atlantic and Pacific Oceans.
2. From the surface hydrothermal sediment zone to the altered basalt zone, the sulfur-isotopic values of hydrothermal sulfides have an increasing trend, and the variation of sulfur-isotopic composition of hydrothermal sulfide and sulfate essentially has no relationship with the time for forming the hydrothermal sulfide and sulfate.
3. The sulfur to surface hydrothermal sediments is derived mainly from Mid-Atlantic Ridge basalt, and partial from reduced seawater sulfate; it is the result of totally to partially reduced seawater sulfate mixes with basaltic sulfur. At different stages of seafloor hydrothermal activity, the portion and the way of supplying sulfur from seawater and basalt are different.
4. Comparing to other sediment-free hydrothermal fields, a considerably reduced seawater sulfate mixes with a deep hydrothermal fluid is a principal reason for causing the heavy δ34S signature of TAG hydrothermal fields, and the temperature and water/rock ratio for the reduction of seawater sulfate in the TAG hydrothermal fields is a major factor controlling the portion of sulfur from seawater in the hydrothermal sulfides.
5. The principal way of supplying sulfur from seawater and/or basalt is by the direct mixture of seawater, the fluid-basalt interaction, the dissolution of previously formed sulfide, and the reduction of sulfate minerals. The evolution of sulfur in the seafloor hydrothermal system is associated with magmatism and tectonism of the Mid-Atlantic Ridge. |
| Key words: Sulfur-isotopic composition, seafloor surface hydrothermal sediment, TAG hydrothermal field |
