陨石中的太阳系外物质及其同位素异常

PRESOLAR GRAINS AND THEIR ISOTOPIC ANORMALIES IN METEORITES

到目前为止从陨石中分离出的太阳系外物质有金刚石、碳化硅、石墨、Si3 N4 、刚玉及尖晶石等。除金刚石为纳米级大小外 ,其他为微米和次微米级颗粒。这些太阳系外物质主要存在于原始的球粒陨石的基质中 ,并通过化学分离的方法获得。金刚石携带分别由 p 过程和r 过程产生的Xe同位素组分 (Xe HL) ,其源区可能是超新星。绝大部分碳化硅相对于太阳系物质富2 9,3 0 Si和13 C ,贫15N ,并携带s 过程产生的各种同位素组分 ,与AGB星的理论模型十分一致。少量碳化硅具有更大程度的同位素异常 ,并划分为不同类型 (X、Y、Z、AB群等 )。其中 ,X 群碳化硅以富2 8Si和各种短寿命核素体为特征 ,表明形成于超新星。石墨可划分为四种比重类型并具有不同的同位素组成 ,分别来自AGB星、新星和超新星等。Si3 N4 的同位素组成与X群碳化硅基本相同。刚玉可根据氧同位素组成划分为四种类型 ,分别来自GB星和红巨星等。对陨石中太阳系外物质的深入研究 ,特别是其他太阳系外成因氧化物和硅酸盐的寻找、次微米级颗粒的同位素分析、以及太阳系外物质在不同化学群和不同岩石类型球粒陨石中的分布特征等研究 ,将为揭示元素起源、恒星演化以及太阳系形成等重大问题提供新的实验依据。

Study on presolar grains including diamond, silicon carbide, graphite, nierite (Si 3N 4), corundum and spinel isolated from meteorites is summarized in this paper. Except for nanometer sized diamond, the other grains are micrometers to submicrometers in size. The presolar grains survived mainly in the fine grained matrix of primitive chondrites and were isolated by chemical treatments. Diamond contains Xe isotopes (Xe HL) typically produced in p and r processes, probably formed in supernovae. Mainstream silicon carbides are enriched in 29,30 Si and 13 C, but depleted in 15 N. They also contain various s process products, consistent with calculations of AGB stars. Other silicon carbides exhibit much larger isotopic anomalies and are classified as groups X, Y, Z and AB. Among them, group X of SiC is characterized by enrichment of 29 Si and daughter isotopes of various short lived nuclides, suggesting an origin from supernovae. Graphite can be divided into four density fractions with distinct isotopic compositions. They may form in AGB stars, novae and supernovae, respectively. Si 3N 4 is similar to X SiC in isotopic composition. Corundum is classified as four groups based on their oxygen isotopic composition. AGB and red giant stars are possible sources for the oxide. More comprehensive study of presolar grains, especially, discovery of the other types of oxides and silicates, isotopic analyses of individual submicrometer sized grains and distribution of presolar grains among various chemical groups and petrographic types of chondrites will supply with new information on nucleosynthesis, stellar evolution and formation of the solar nebula.