Petrology and geochemistry of serpentinized peridotites from Hahajima Seamount in Izu-Bonin forearc region  

作　　者：Tuoyu Wu Liyan Tian Jinwei Gao Yanhui Dong 

机构地区：[1]Beijing Maiqin Nengyuan Jishu Fuwu Co.Ltd,Beijing 100004,China [2]Institute of Deep-sea Science and Engineering,Chinese Academy of Sciences,Sanya 572000,China [3]Laboratory for Marine Geology,Pilot National Laboratory for Marine Science and Technology(Qingdao),Qingdao 266237,China [4]Key Laboratory of Submarine Geosciences,Second Institute of Oceanography,Ministry of Natural Resources,Hangzhou 310012,China

出　　处：《Acta Oceanologica Sinica》2019年第11期99-110,共12页海洋学报（英文版）

基　　金：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

摘　　要：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.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.

关 键 词：Hahajima SEAMOUNT serpentinized PERIDOTITES IBM FOREARC SEAMOUNTS fluid-mobile elements 

分 类 号：P58[天文地球—岩石学]