【作者】 汪小妹；

【导师】 曾志刚；

【作者基本信息】 中国科学院研究生院（海洋研究所） ， 海洋地质， 2010， 博士

【摘要】 本文对取自马里亚纳前弧南部的蛇纹石化橄榄岩进行了岩石学、矿物学和元素地球化学研究,其研究目的在于探讨马里亚纳前弧地幔所经历的部分熔融及熔体-地幔相互作用的历史,并试图揭示马里亚纳橄榄岩后期所经历的蛇纹石化作用过程及其影响。部分熔融模拟计算结果表明橄榄岩经历了15-25%的高程度部分熔融。橄榄岩相对较为平坦的PGE配分模式显示其可能遭受了与熔体的相互作用。马里亚纳前弧南部蛇纹石化橄榄岩的组成矿物主要为橄榄石、角闪石和尖晶石,以及蛇纹石、绿泥石和滑石。不完全的蛇纹石化作用导致了磁铁矿的缺乏,后期富SiO₂流体的加入导致了水镁石的缺失和滑石的形成。橄榄岩蛇纹石化初期、干燥、低渗透性和孔隙度,以及低温的环境条件导致了利蛇纹石的形成而纤蛇纹石和叶蛇纹石的缺失。样品中滑石主要有网脉和大脉状两种产出状态,滑石的低摩擦系数和固有稳定性可能对马里亚纳俯冲带逆冲型地震下界限的形成意义重大。铬铁矿具有明显的蚀变分带特征,其蚀变发生在蛇纹石化后期,在此过程中,Al和Mg扩散出铬铁矿颗粒,残留形成了铁铬铁矿（ferrian chromite）。D7和D8两个站位橄榄岩中铬铁矿化学分带的差异可能由流体-橄榄岩反应过程中流体-岩石比值的不同所致。橄榄岩的蛇纹石化会造成岩石中MgO和SiO₂的丢失,但对MgO和SiO₂的影响程度相似。此外,蛇纹石化也会造成橄榄岩中CaO的丢失。蛇纹石化对马里亚纳前弧南部橄榄岩微量元素的影响主要体现在Pb、Sr、Li这几个在流体中较活动（fliud-mobile）的元素上,而橄榄岩中U含量的增加则是橄榄岩与海水相互作用的结果。此外,橄榄岩的稀土元素组成具有明显的负Ce异常,可能指示了俯冲物质对马里亚纳地幔楔熔融源区的物质贡献。更多还原

【Abstract】 Detailed petrology, mineral and geochemistry study was carried out on serpentinized peridotites dredged from the southern Mariana forearc. To study the histories of partial melting and melt-mantle interaction occurred in the Mariana forearc mantle, and to reveal the serpentinization process and its influence on peridotites. Melting model result suggests that all samples have undergone high degrees of melting （15-25%）. The relatively flat PGE patterns indicate these peridotites may suffer interaction with melt.The constituent minerals of these southern Mariana forearc peridotites are olivine, amphibole and spinel, as well as serpentine, chlorite and talc. The absence of magnetite in the serpentinized peridotites is due to the incomplete serpentinization, the adding of later SiO₂-rich fluid leding to the lack of brucite and the occurrence of talc. Scarcity of H₂O, low porosity and permeability, as well as the actual situation of initial serpentinization and the low temperature in our study area, results in the absolute prevalence of lizardite over chrysotile and antigorite.Talcs mainly occur as meshwork veinlets and large veins, the sufficiently low frictional strength and inherently stable sliding behavior may have great significance on downdip limit of subduction thrust earthquakes. Chromites typically exhibit distinct chemical zones, the most reasonable mechanism for alteration of chromites should be the postserpentinization alteration, during which ferrian chromite leaves as chromite diffuse out A1 and Mg. The variability of chromtie zoning charater between sites D7 and D8 may be due to the difference in fluid-rock ratios.Serpentinization of the study peridotites resulted in the lost of MgO and SiO₂, and the influence degree on this two elements are similar. Besides, serpentinization could also lead to the lost of CaO in the peridotites. The influence of serpentinization on trace elements in the study peridotite mainly embodies the addition of fluid-mobile elements Pb, Sr and Li, while the concentration increase of U in the study samples is considered due to the interaction of peridotites with seawater. In addition, the Mariana forearc peridotites have distinct negative Ce anomaly, which may indicate the significant contribution of the subducted material to the melting source region of the Mariana wedge mantle.更多还原