| 蛇纹石化和俯冲带蛇纹岩变质脱水过程中流体活动性元素的行为 |
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| 引用本文： | 吴凯,袁洪林,吕楠,张丽鹏.蛇纹石化和俯冲带蛇纹岩变质脱水过程中流体活动性元素的行为[J].岩石学报,2020,36(1):141-153. |
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| 作者姓名： | 吴凯 袁洪林 吕楠 张丽鹏 |
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| 作者单位： | 西北大学地质学系, 大陆动力学国家重点实验室, 西安 710069,西北大学地质学系, 大陆动力学国家重点实验室, 西安 710069,西北大学地质学系, 大陆动力学国家重点实验室, 西安 710069,青岛海洋科学与技术试点国家实验室, 海洋地质过程与环境功能实验室, 青岛 266061;中国科学院海洋研究所, 深海研究中心, 青岛 266071 |
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| 基金项目： | 本文受国家自然科学基金项目（41903009、41427804、41421002）和第64批中国博士后科学基金（2018M643711）联合资助. |
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| 摘 要： | 蛇纹石是大洋岩石圈和俯冲带内水和流体活动性元素最重要的载体之一。研究蛇纹石化和蛇纹岩变质脱水过程中流体活动性元素的行为是认识俯冲带元素地球化学循环的关键。蛇纹岩是指主要由蛇纹石类矿物构成的岩石,包括利蛇纹石、纤蛇纹石和叶蛇纹石。蛇纹石化过程中会造成流体活动性元素(B、Li、As、Sb、Pb、Cs、U、Sr和Ba等)的显著富集,并且由于原岩性质、流体成分和氧逸度等条件的不同,大洋岩石圈蛇纹岩和弧前蛇纹岩的特征也略有不同。例如,弧前蛇纹岩具有相对高的As、Sb、B和相对低的U,这反映了俯冲沉积物来源流体的贡献。在俯冲带蛇纹岩的变质脱水过程中,利蛇纹石向叶蛇纹石的转变伴随着矿物内超过50%F和Cl的释放,以及一些流体活动性元素(如B和Li)的迁出;此外,蛇纹石分解形成的变质橄榄石中的流体包裹体指示,蛇纹石脱水分解所产生的流体具有高于原始地幔几个数量级的Cl、Cs、Pb、As、Sb、Ba、Rb、B、Sr、Li和U含量。由于利蛇纹石中的Fe~(3+)含量较叶蛇纹石高,这种矿物相转变过程中也伴随着俯冲通道内的一系列氧化还原过程,从而影响流体性质和新形成的叶蛇纹石的成分。蛇纹岩与岛弧岩浆在流体活动性元素富集规律上的相似性说明蛇纹岩在俯冲带元素循环中扮演着重要的角色。此外,蛇纹石矿物相转变过程中F、Cl、B等元素的释放,可能对于斑岩型金矿、蛇绿岩中的金矿和某些蛇纹岩作为赋矿围岩的硼矿的形成起到重要的作用。
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| 关 键 词： | 俯冲带 流体活动性元素 元素分异 蛇纹岩 蛇纹石化 |
| 收稿时间： | 2019/4/25 0:00:00 |
| 修稿时间： | 2019/7/25 0:00:00 |
The behavior of fluid mobile elements during serpentinization and dehydration of serpentinites in subduction zones |
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| WU Kai,YUAN HongLin,LYU Nan and ZHANG LiPeng.The behavior of fluid mobile elements during serpentinization and dehydration of serpentinites in subduction zones[J].Acta Petrologica Sinica,2020,36(1):141-153. |
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| Authors: | WU Kai YUAN HongLin LYU Nan and ZHANG LiPeng |
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| Institution: | State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China,State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China,State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China and Laboratory for Marine Geology, Pilot National Laboratory for Marine Science and Technology(Qingdao), Qingdao 266061, China;Center of Deep Sea Research, Institute of Oceanography, Chinese Academy of Sciences, Qingdao 266071, China |
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| Abstract: | Serpentine is one of the most important carriers of water and fluid mobile elements in the oceanic lithosphere and subduction zones. The behavior of fluid mobile elements during serpentinization and dehydration of serpentinites is crucial for understanding the geochemical cycle in subduction zones. Serpentinite consists mostly of the serpentine-group minerals including lizardite, chrysotile and antigorite. Fluid mobile elements (B, Li, As, Sb, Pb, Cs, U, Sr, Ba, etc.) are remarkably enriched during serpentinization, and there exist some geochemical differences between abyssal serpentinites and forearc serpentinites due to different protoliths, fluid compositions and oxygen fugacity. For example, forearc serpentinites are relatively more enriched in As, Sb and B, but less enriched in U, indicating of the contributions from subducting sediments. The transition from lizardite to antigorite during dehydration of serpentinite in subduction zones will release more than 50% F and Cl, as well as some fluid mobile elements (e.g. B and Li); formation of metamorphic olivine at the expense of antigorite will release fluids with high fluid mobile element concentrations with Cl, Cs, Pb, As, Sb, Ba, Rb, B, Sr, Li and U up to several orders of magnitude higher than that of primitive mantle. Due to the higher Fe3+ proportions in lizardite compared to antigorite, the transition from lizardite to antigorite will be accompanied by a series of redox reactions in subduction channels, which may affect the nature of the dehydrated fluids and the composition of antigorite formed during this stage. The similarity in enrichments of fluid mobile elements between serpentinites and arc magmas indicates that serpentine minerals play a critical role in element cycling in subduction zones. In addition, the release of F, Cl and B during the phase transition from lizardite to antigorite may play a vital role in formation of porphyry gold deposits, gold deposits in ophiolite and boron deposits reserved in serpentinites. |
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| Keywords: | Subduction zones Fluid mobile elements Element fractionation Serpentinite Serpentinization |
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