马里亚纳弧前蛇纹岩泥火山角砾蛇纹岩流体流动性元素富集特征及影响因素  

Enrichment and influencing factors of fluid-mobile elements in breccia serpentinite from serpentinite mud volcanoes in Mariana Forearc

作　　者：崔彩英 佟宏鹏 陈琳莹 陈多福[1,2] CUI Caiying;TONG Hongpeng;CHEN Linying;CHEN Duofu(College of Marine Science,Shanghai Ocean University,Shanghai 201306,China;Shanghai Engineering Research Center of Hadal Science and Technology,Shanghai 201306,China)

机构地区：[1]上海海洋大学海洋科学学院,上海201306 [2]上海深渊科学工程与技术研究中心,上海201306

出　　处：《海洋地质前沿》2023年第5期12-24,共13页Marine Geology Frontiers

基　　金：国家自然科学基金(41776080,41776050)。

摘　　要：马里亚纳弧前蛇纹岩泥火山被认为是认识俯冲作用的直接窗口,其发育的角砾状蛇纹岩富含的流体流动性元素(FMEs)记录了引起蛇纹石化作用的流体和俯冲带的流体-岩石相互作用及元素循环等信息。本文整理了马里亚纳弧前物源深度逐渐增加的5座蛇纹岩泥火山(Yinazao、Fantangis?a、Asùt Tesoru、South Chamorro和Conical)发育角砾蛇纹岩的FMEs数据,通过不同泥火山、同一泥火山不同深度样品及同一块状样微区子样品间的FMEs特征对比,分析了蛇纹岩泥火山引起的蛇纹石化作用的流体性质、来源和形成机制。5座蛇纹岩泥火山角砾蛇纹岩的FMEs均较亏损地幔(DM)显示富集特征:B、Cs、As强烈富集(常>100×DM),Li、Rb、Sb中等富集(>10×DM),Ba、Sr、Pb弱富集(<10×DM)。浅源泥火山角砾蛇纹岩中B、Sr、Ba、Pb含量最高,且由浅源至深源泥火山其含量呈逐渐减少的趋势。而Li、Rb、Cs、As、Sb含量显示相反特征,浅源泥火山的角砾蛇纹岩中含量最低,且从浅源到深源泥火山其含量逐渐增加。不同泥火山角砾蛇纹岩FMEs具有系统性变化特征,反映了俯冲板片衍生流体是其发生蛇纹石化作用的主要流体。离海沟较近的泥火山下部俯冲板片成岩作用以蛋白石脱水为主,形成的板片衍生流体极富B,略富Li、Ba、Sr、Pb;离海沟较远的泥火山下部俯冲板片成岩或进变质作用主要为碳酸盐矿物分解和少量蚀变洋壳脱水及黏土矿物转化,板片衍生流体富Ba、Sr、Li、Rb、Cs;居于上述2类泥火山之间的泥火山,下部俯冲板片主要成岩作用为黏土矿物脱水及转化,板片衍生流体以富集B、Li、Rb、Cs、Ba为特征。海底以下深度<50 m的样品FMEs含量最高,指示海水风化对FMEs含量有一定影响,尤其是B、Sr。同一块角砾蛇纹岩不同微区结构的FMEs含量不同,纯蛇纹石区域其含量最高,指示蛇纹石化产物对FMEs有一定影响;绢石结构中Li、Rb、CThe serpentinite mud volcano is considered as a direct window into subduction in Mariana Forearc.Serpentinites in serpentinite mud volcano are rich in water and fluid-mobile elements,and an archive of information about serpentinized fluids,fluid/rock interactions,and element cycles in subduction zones.We reviewed the FME(fluid-mobile element)behaviors of breccia serpentinite collected from five serpentinite mud volcanoes that erupted from the Mariana subduction zone in gradually elevated depths(Yinazao,Fantangisña,Asut Tesoru,South Chamorro,and Conical).We summarized and compared the FME characteristics of breccia serpentinite samples from different mud volcanoes at different depths of the same mud volcano and from microsamples in the same block,aiming at uncovering the features and sources of the serpentinized fluids,and the formation mechanisms of the Mariana forearc serpentine mud volcanoes.The FME characteristics of breccia serpentinite from all five serpentinite mud volcanoes show enrichments compared with those in depleted mantle(DM):strong enrichments of B,Cs,and As(often over 100×DM(depleted mantle)),moderate enrichments of Li,Rb,and Sb(>10×DM),and weak enrichments of Ba,Sr,and Pb(<10×DM).The contents of B,Sr,Ba,and Pb are the highest in breccia serpentinite samples from shallow mud volcano and show a decreasing trend from shallow mud volcano to deep mud volcano.On the contrary,the contents of Li,Rb,Cs,As,and Sb show the opposite trend,and these elements are lowest in breccia serpentinite samples from the shallow mud volcano and increase gradually from shallow mud volcano to deep mud volcano.The systematic variation of FMEs of breccia serpentinite in the five different mud volcanoes indicates that subducted plate-derived fluid is the main serpentinized fluid of the breccia serpentinite.The plate-derived fluid and main serpentinized fluid of Yinazao that is close to the trench,are extremely rich in B and slightly rich in Li,Ba,Sr,and Pb that are originated from opal dehydration processes at subduction

关 键 词：流体流动性元素 蛇纹岩角砾 蛇纹岩泥火山 马里亚纳弧前 

分 类 号：P583[天文地球—岩石学] P736[天文地球—地质学]