摘要
西南印度洋脊的ODP 735B岩心中拥有大量富含钛铁氧化物的含氧化物辉长岩,此现象在岩性单元Ⅳ中表现尤为显著。同时,岩性单元Ⅳ中主要硅酸盐矿物亦具有独特的成分变化趋势。以前研究将造成岩性单元Ⅳ中氧化物辉长岩成因和主要造岩矿物成分变化趋势归因于具浮力的、高度分异的富钛铁熔体的作用,这些熔体应形成于735B钻孔之下或其附近,沿剪切带运移至剖面中,并与原岩矿物发生反应。通过对岩性单元Ⅳ氧化物辉长岩进行岩相学特征分析和结构可控的电子探针矿物成分测试,发现当某些因外来物质加入改造而成分进化的硅酸盐矿物形成之时,液相体系中钛铁氧化物并未处于饱和状态。此外,在单斜辉石结晶过程中,单斜辉石内部成分具有从普通辉石—易变辉石—普通辉石的变化,而且,在与某些后期生成的矿物(如钛铁氧化物或斜方辉石等)接触处,单斜辉石则表现出明显进化组成特征。根据岩相学特征和矿物电子探针数据,并结合对前人提出的富钛铁熔体的物理性质的讨论,本文认为在岩性单元Ⅳ形成过程中,更有可能是源自地幔的外来含水流体/含水流体与熔浆的混合体,而非仅仅是极度分异的玄武质残余熔体,参与了岩性单元Ⅳ中氧化物辉长岩的形成。从理论上讲,这些外来含水流体可能与玄武质熔体并非来自同一源区,这意味着其运移方式可能不仅有垂向运移,而且有可能为远距离侧向运移。岩性单元Ⅳ中的氧化物辉长岩可能具有相当独特的同位素特征,这可能会成为判别岩性单元Ⅳ乃至是整个735B岩心中氧化物辉长岩成因的重要线索。
ODP 735B core from the Southwest Indian Ridge is highly abundant in Fe-Ti oxide-rich gabbros,which is particularly significant in lothologic UnitⅣof this core.Meanwhile,the primary silicate minerals in UnitⅣdisplay unique trend of compositional variations.The formation and the unusual compositional variations of the oxide gabbros are commonly considered to have occurred through modifications by highly differentiated Fe-Ti-rich melts forming beneath or adjacent to Hole 735B,which migrated into the section along shear zones and reacted with an upwardly differentiated olivine gabbro protolith.Here,we report the detailed petrographic features and structure-controlled mineral profiles of the UnitⅣoxide gabbros to provide a new perspective on the their formation.It can be seen that when some evolved silicate minerals formed due to the injection of exotic materials,Fe-Ti oxides should have remained unsaturated in the liquid.Additionally,the clinopyroxenes follow the crystallization sequence of augite→pigeonite→augite during crystallization,and near contacts with the later-formed minerals in the oxide gabbros(e.g.,Fe-Ti oxides or orthopyroxene),the clinopyroxenes have significantly evolved compositions.Based on the detailed petrographic features and mineral compositional microprobe data,combining with discussion on Fe-Ti-rich melts by former researchers,we propose exotic mantle-derived hydrous fluids or liquid mixtures of hydrous fluids and basaltic melts,rather than just highly differentiated Fe-Ti-rich melts,as the suitable exotic materials involved within the formation of the oxide gabbros in UnitⅣ.Theoretically,these fluids might not originate from the same source region as the basaltic melt,which means that these fluids might migrate not only vertically,but also laterally over long distances.Therefore,it is reasonable to speculate that oxide gabbros in UnitⅣshould have unique isotopic features,which would be important clues in testing the mechanism of the formation of oxide gabbros in UnitⅣ,even in t
作者
靳野
方念乔
袁晓博
胡克
JIN Ye;FANG Nianqiao;YUAN Xiaobo;HU Ke(Department of Tourism,Resources and Environment,Zaozhuang University,Zaozhuang 277160,China;School of Ocean Sciences,China University of Geosciences(Beijing),Beijing 100083,China;Institute of Marine Science,Hebei Normal University of Science&Technology,Qinhuangdao 066004,China)
出处
《地学前缘》
EI
CAS
CSCD
北大核心
2021年第1期334-352,共19页
Earth Science Frontiers
基金
国家自然科学基金项目(40876029)。
