| 东太平洋海隆热液活动区沉积岩心的地球化学研究 |
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| 引用本文：薛发玉,翟世奎.2005.东太平洋海隆热液活动区沉积岩心的地球化学研究[J].地球学报,26(z1):200-201. |
| DOI：10.3975/cagsb.2005.s1.66 |
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| 中文摘要:东太平洋隆是现代海底热液活动及热液沉积分布最为广泛的构造环境之一。本文对采自 EPR13°N 现代海底热液活动区附近的两个沉积物岩心分别进行了元素地球化学分析，并结合沉积物中有孔虫壳体对岩心进行关键层位 14C 年代学研究。目的在于通过对比距现代海底热液喷口不同距离的两岩心的地球化学特征，研究热液活动在岩心沉积物中所何存的地球化学记录，探讨海底热液活动对洋中脊附近沉积作用的贡献，以及热液活动在一定地质历史时期内的变化规律。显示，距热液喷口区相对较近的 B13－26 岩心沉积物含有丰富的热液特征组分，绝大多数微量元素和 REE 都与 Fe和Mn 成较强的正相关关系，而与 Al、Na、Ca、Mg、Ti 等元素相关性较弱或呈负相关，说明这些元素的富集机制主要是热液成因非晶质 Fe－Mn 化合物对热液及海水中元素的吸附作用。距热液喷口区相对较远的 B13－62岩心上部沉积物中元素的富集特征与 B13－26 岩心相似，但中下部沉积物中 Fe 与 Mn 无明显的相关关系，说明这些元素的富集主要受深海粘土沉积的影响，而受热液活动的影响减弱或消失。由于与海隆扩张轴距离的不同，两岩心受热液活动影响的强度不同。由海隆扩张轴向外，沉积作用受热活动的影响逐渐减弱，正常深海沉积作用逐渐加强。两个岩心沉积物地球化学上的共性是 Ca 和 Sr 都与其他元素呈现负相关关系或极微弱的正相关关系，表明生物碳酸盐沉积作用的独特性。有孔虫 14C 年龄测定结果表明，B13－26 岩心 35～250cm 段年龄十分接近，前后相差仅在几百年内。根据年龄测量中的误差范围，可以断定这段沉积物岩心是短时间内快速沉积的产物。结合热液活动特征性元素在岩心中的分布特征，判断该岩心段记录一次强烈的热液活动事件，从而形成了典型的事件沉积。在 B13－62 岩心的45～50cm 处测得有孔虫的 14C 年龄为距今 32900±530 yr。由于 14C 测年方法上的局限性，在岩心 135～140cm 处 14C 测年只能给出大于40000 yr 的结果。显然，B13－62 岩心处早期并没有受到热液活动明显的影响，沉积速率非常缓慢，只是在近期受到了热液活动的影响，形成了上部富含 Fe、Mn、Zn、Cr、Co、Ni、V 等热液活动特征元素的沉积。 |
| 中文关键词:热液活动 沉积岩心 地球化学 14C年龄 东太平洋海隆 |
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| Geochemical Studies of Sediment Cores from the Hydrothermal Field on the East Pacific Rise |
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| Abstract:The East Pacific Rise is one of the tectonic environments characterized by very abundant modern hydrothermal activities and hydrothermal sediments. Two sediment cores from the hydrothermal fields near the EPR 13°N were chosen for systematic elemental geochemical analysis. The analytical techniques used included Inductively Coupled Plasmas-Atomic Emission Spectroscopy (ICP-AES) and Inductively Coupled Plasmas-Mass Spectrometry (ICP-MS). In addition, the Carbon-14 ages of some foraminifer samples in the selected layers were measured by AMS. The sediments in Core B13-26 are enriched in typical hydrothermal components. Most trace elements and REE are well positively correlated with Fe and Mn,and are weakly positively correlated or negatively correlated with Al,Na,Ca,Mg and Ti.All this indicates that most trace elements and REE are probably scavenged by Fe-Mn oxyhydroxide materials from hydrothermal fluids and seawater. In the upper Core B13-62,the concentrations and distribution of the elements are similar to those in Core B13-26. In the lower parts,the contents of Fe and Mn decrease,while the content of Al increases distinctly, with little or no hydrothermal signature. As the distances between the two cores and the ridge axis or the vents are different,the hydrothermal signatures preserved in the two cores behave differently. The common characteristic of the two cores is that Ca and Sr in both cores are negatively or slightly positively correlated with other elements. Carbon-14 ages of sediments from different layers between 35～250cm below the seafloor in Core B13-26 shows a limited range from 10500 to 12000 years B P,with the difference of only several hundred years,even lower than the errors.Combined with the distribution pattens of typical hydrothermal components along the core,it is held that the sediments in Core B13-26 have recorded an intense hydrothermal event with a high sedimentation rate. This event should have started from 12000 yr B P and weakened a little 10000 yr B P. Carbon-14 age of the foraminifer in the sediments at 45～50cm below the seafloor from Core B13-62 is 32900±530 years B P. Owing to the limitation of this dating method,the age of sediments at 135～140cm below the seafloor was not acquired. It should be older than 40000 years B P. Obviously, Core B13-26 was not affected by the hydrothermal activities at the early period,and with a low sedimentation rate. Only recently,in the upper 0～35cm,the sediments were affected by hydrothermal activities,resulting in the enrichment of Fe,Mn,Zn,Cr,Co,Ni,V,etc. |
| keywords:hydrothermal activity sediment cores geochemistry Carbon-14 ages East Pacific Rise |
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