| Wu Tuoyu,Tian Liyan,Gao Jinwei,Dong Yanhui. 2019. Petrology and geochemistry of serpentinized peridotites from Hahajima Seamount in Izu-Bonin forearc region. Acta Oceanologica Sinica, 38(11):99－110 |
| Petrology and geochemistry of serpentinized peridotites from Hahajima Seamount in Izu-Bonin forearc region |
| 伊豆-小笠原前弧地区Hahajima海山蛇纹石化橄榄岩岩石学和地球化学研究 |
| Received:November 07, 2018 |
| DOI：10.1007/s13131-019-1469-x |
| Key words:Hahajima Seamount|serpentinized peridotites|IBM forearc seamounts|fluid-mobile elements |
| 中文关键词: Hahajima海山 蛇纹石化橄榄岩 伊豆-小笠原-马里亚纳前弧海山 流体活动元素 |
| 基金项目:The National Natural Science Foundation of China under contract Nos 41506047, 41876044 and 91858214; the Chinese Academy of Sciences’ Strategic Priority Research Program Grant under contract Nos XDB06030103 and XDB06030204. |
| Author Name | Affiliation | E-mail | | Wu Tuoyu | Beijing Maiqin Nengyuan Jishu Fuwu Co. Ltd, Beijing 100004, China | | | Tian Liyan | Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China
Laboratory for Marine Geology, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266237, China | lytian@idsse.ac.cn | | Gao Jinwei | Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China
Laboratory for Marine Geology, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266237, China | | | Dong Yanhui | Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China | |
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| Abstract: |
| Serpentinites, which contain up to 13 wt% of water, are important reservoirs for chemical recycling in subduction zones. In the past two decades, forearc mantle serpentinites were identified in different locations around the world. Here, we present petrology and whole rock chemistry of ultramafic and mafic rocks dredged from the Hahajima Seamount, which is located 24–40 km west to the junction of the Izu-Bonin Trench and the Mariana Trench. Nearly all the collected samples are extensively hydrated, and olivine grains in ultramafic rocks are replaced by serpentine minerals, with only one sample preserving remaining trace of orthopyroxene. Our new results show that the Hahajima serpentinized peridotite samples are all MgO-rich (~42 wt%), but have low contents in Al2O3, CaO, rare earth and high field strength elements, which is consistent with the overall depleted character of their mantle protoliths. Model calculations indicate that these Hahajima peridotite samples were derived from 10%–25% partial melting of the presumed fertile mantle source, which is generally lower than those of peridotites from Torishima Forearc Seamount, Conical Seamount and South Chamorro Seamount (mostly >25%). All the serpentinites from these four forearc seamounts show strong enrichment in fluid-mobile and lithophile elements (Li, Sr, Pb and U). In details, Hahajima Seamount serpentinites do not have obvious enrichment in Cs and Rb, and display remarkably high abundances of U. These observations indicate that the serpentinization of Hahajima peridotites occurred by addition of seawater or low temperature seawater-derived hydrothermal fluid, without or with little contribution from slab-derived fluids. The geochemical signature of serpentinites from Hahajima Seamount could be interpreted as the result of the combination of extensive partial melting and subsequent percolation of seawater through the mantle wedge. |
| 中文摘要: |
| 含水量高达13%的蛇纹岩是俯冲带化学循环的重要储库。在过去二十年间，前弧地幔蛇纹岩在许多地方被相继发现。本文针对采自伊豆-小笠原海沟和马里亚纳海沟交界处Hahajima海山的超基性和基性岩石样品进行了岩石学和地球化学研究。研究结果表明超基性岩石样品蛇纹石化明显，橄榄石矿物都被蛇纹石类矿物所替代。全岩地球化学分析结果表明Hahajima海山蛇纹石化橄榄岩富集MgO （~42%），但其Al2O3、CaO、稀土元素和高场强元素含量极低，与其原岩的亏损特性相对应。模式计算显示Hahajima海山橄榄岩是经过15-25%的地幔部分熔融的残余体，比其他伊豆-小笠原-马里亚纳前弧海山（Torishima，Conical，South Chamorro海山）橄榄岩的熔融程度低（> 25%）。尽管这四座前弧海山的蛇纹岩都富集流体活动元素（Li，Sr，Pb，U），但Hahajima海山的样品并不富集Cs，Rb，且U含量更高，这表明是Hahajima海山蛇纹石化橄榄岩的地球化学特征是地幔楔高度部分熔融及海水渗入至地幔楔中综合作用的结果。 |
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