火星陨石NWA 11288岩石矿物学和冲击变质特征  

Petrography,mineral chemistry and shock metamorphism of the NWA 11288basaltic shergottite

作　　者：闫艺洪 胡森[1] 张婷[1,2] 计江龙 YAN Yihong;HU Sen;ZHANG Ting;JI Jianglong(Key Laboratory of Earth and Planetary Physics,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;University of Chinese Academy of Sciences,Beijing 100049,China)

机构地区：[1]中国科学院地质与地球物理研究所,地球与行星物理重点实验室,北京100029 [2]中国科学院大学,北京100049

出　　处：《地质学报》2021年第9期2866-2877,共12页Acta Geologica Sinica

基　　金：民用航天预先研究项目(编号D020201,D020203);北京市科技计划项目(编号Z191100004319001);国家自然科学基金项目(编号41973062);中国科学院地质与地球物理研究所重点部署项目(编号IGGCAS-201904)联合资助的成果。

摘　　要：NWA 11288是一块2015年发现的玄武岩质辉玻无球粒陨石,具有典型的次辉绿结构,主要由辉石(面积比68.6%)和熔长石(面积比28.1%)组成,含少量磷酸盐(面积比1.35%)、不透明矿物(面积比1.45%)和二氧化硅等副矿物。通过扫描电子显微镜、电子探针和拉曼光谱等实验分析方法,对NWA 11288进行了岩相学、矿物学及冲击变质作用研究。基于辉石核部的Ti/Al比值,推测其结晶压力为930 MPa,结晶深度约为50 km,接近火星壳幔边界。NWA 11288经历过强烈的冲击变质作用,其中长石全部熔长石化,发育冲击熔脉及熔融囊并出现重结晶的辉石和柯石英。柯石英具有一种前人未报道的新产状:以柯石英-石英-二氧化硅玻璃三相集合体共生,柯石英和石英主要分布在集合体的边部,包裹着核部的二氧化硅玻璃,指示柯石英是在卸压阶段从熔体中结晶形成,且在后续的卸压阶段部分柯石英转变为石英。此外,熔融囊中可见自形的斯石英形态的二氧化硅玻璃;粒间区域可见具有斯石英/赛石英典型特征的二氧化硅颗粒。这些冲击变质特征说明NWA 11288经历的峰值温度至少为1600℃,峰值压力或可高达15~20 GPa。通过对比NWA 11288和NWA 8657/8656的岩石结构、矿物化学成分以及冲击变质特征,本文认为NWA 11288很可能是NWA 8657/8656的成对陨石。The NWA 11288 is a new basaltic shergottite found in 2015 with a typical subophitic texture, mainly composed of pyroxene(68.6% in area) and maskelynite(28.1% in area) with accessory phosphate(1.35% in area), opaque minerals(1.45% in area), and silica. Petrographic feature, mineralogy and shock metamorphism of the NWA 11288 were studied by scanning electron microscopy(SEM), electron probe(EPMA), and Raman spectroscopy. Based on the Ti/Al ratios of pyroxene cores, we estimate that the pressure and depth when the pigeonite cores formed are about 930 MPa and 50 km, respectively, corresponding to the boundary of the Martian crust and mantle.The NWA 11288 has undergone strong shock metamorphism supported by maskelynization of plagioclase, formation of shock induced melt veins and pockets, recrystallization of pyroxene in shock-induced melt areas, and the presence of high pressure polymorph of silica, coesite, in the shock-induced melt region. A new occurrence of coesite was identified in this study: aggregates of coesite, quartz, and silica glass were identified in the shock-induced melt pockets. The coesite and quartz mainly locate at the rim of the aggregates with silica glass in the central area, indicating that coesite recrystallized from silica melt during decompression, and some coesite partially transformed to quartz before quenching of silica glass. In addition, we also identified abundant acicular silica glass in shock-induced melt pockets and a typical stishovite/seifertite texture of silica grain in the interstitial area. These shock metamorphism features suggest that the impact peak pressure and temperature may reach up to 15~20 GPa and 1600℃, respectively. By comparing the petrography, mineral chemistry, and shock metamorphism features of the NWA 11288 and the NWA 8657/8656, we propose that the NWA 11288 is likely paired with the NWA 8657/8656.

关 键 词：火星陨石 玄武岩质辉玻无球粒陨石 岩相特征 冲击变质作用 柯石英 

分 类 号：P185.3[天文地球—天文学] P185.83