古铜钙长无球粒陨石NWA 14701后成合晶富硫化物结构的成因研究

Study on the genesis of sulfide-rich textures in symplectite of the Howardite NWA 14701

后成合晶在HED族陨石中较为罕见,对其中次生结构的研究有助于了解其母体岩浆期后矿物的变质作用,甚至可以揭示与含挥发分流体有关的交代作用过程。本次工作在Howardite NWA 14701的三相后成合晶中发现存在“富钙普通辉石-陨硫铁-SiO_(2)相(SRT-Ⅰ)”和“紫苏辉石-陨硫铁-少量SiO_(2)相(SRT-Ⅱ)”两种富硫化物结构,而在一些后成合晶相邻的鳞石英中还出现富硫化物交代结构(SRT-Ⅲ)。通过详细分析后成合晶岩屑的岩相学和矿物化学特征,认为NWA 14701后成合晶中SRT-Ⅰ和SRT-Ⅱ两种富硫化物结构分别为原亚稳态富铁辉石和后成合晶中的富铁橄榄石与富S蒸汽发生硫化作用的产物。此外,富S蒸汽亦交代了后成合晶相邻的鳞石英颗粒形成第三种富硫化物结构(SRT-Ⅲ)。本次工作在HED族陨石后成合晶岩屑中发现存在三种矿物硫化作用的痕迹。硫化作用可发生在HED族陨石母体演化过程的不同时间段,其中富铁辉石的硫化作用应是相对早期的过程,而富铁橄榄石的硫化作用则是一个相对晚期的过程。

Symplectites are rare in HED meteorites.The study of secondary textures in them is helpful to understand the metamorphism of minerals after the magmatic stage of its parent body,and even reveal the metasomatic process related to volatile-bearing fluid.In this work,two sulfide-rich textures,namely,the“Ca-rich augite-troilite-SiO_(2)(SRT-Ⅰ)”and the“hypersthene-troilite-a small amount of SiO_(2)(SRT-Ⅱ)”,have been found in the three-phase symplectite of Howardite NWA 14701.Sulfide-rich replacement textures also appear in some tridymites(SRT-Ⅲ)adjacent to the symplectite.Based on the detailed analysis of petrographic and mineralogical characteristics of the symplectite lithic clasts,we suggest that the two sulfide-rich textures of SRT-Ⅰand SRT-Ⅱin the symplectite lithic clasts of NWA 14701 are the products of sulfidation between the S-rich vapor and the primary metastable Fe-rich pyroxene or the Fe-rich olivine in symplectite,respectively.The S-rich vapor also metasomatized the adjacent tridymite grains to form SRT-Ⅲ.In this work,traces of sulfidation of three minerals have been found in the symplectite lithic clasts of the HED meteorites.Sulfidation may occur at different time periods during the evolution of the HED's parent body,in which the sulfidation of Fe-rich pyroxene should be a relatively early process,while the sulfidation of Fe-rich olivine should be a relatively late process.