Carbon, Nitrogen, and Noble Gases in the Diamond Fractions of the Novo Urei Ureilite
Carbon, Nitrogen, and Noble Gases in the Diamond Fractions of the Novo Urei Ureilite
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Novo Urei Ureilite 钻石馏分中的碳、氮和稀有气体
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发表时间：
2004
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通讯作者：
C. Pillinger
中科院分区：
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作者：
A. Fisenko;A. Verchovsky;L. Semjonova;C. Pillinger
我们测量了在重液体（ρ = 2.9 g/cm3）中分离的金刚石馏分中C、N和稀有气体的含量和同位素组成，这些重液体是从Novo Urei ureilite富集金刚石的样品中分离出来的。结果表明，从上清液中分离出的金刚石馏分（命名为DNU-1）中的氮和惰性气体浓度比从残渣中分离出的金刚石馏分（命名为DNU-2）中的氮和惰性气体浓度高出1.5倍以上。这种差异可能是由较小的晶粒尺寸和（或）较小的晶粒簇以及与DNU-2相比，DNU-1分数中的金刚石晶格缺陷较大造成的。两个馏分在碳和氮的同位素组成和捕获的化学元素的比例上是相似的。所得的结果和已发表的关于其他矿物中不同馏分的C、N和惰性气体的数据使我们可以得出以下结论。(1)石墨与细晶（或半无定形）碳质相在母体中发生激波转变而形成的菱铁矿极有可能是由石墨和碳质相形成的。(2)在南极未受冲击的uilite ALH 78019 （Rai et al., 2002）中，氮同位素轻组分（δ15N约为-100‰）的寻找结果为阴性，这给解释撞击过程中母体中uilite diamond的起源带来了严重困难，这很可能是由于陆地风化过程中碳质物质对大气氮的吸收所致。(3)轻氮（δ15N ~ -100‰）的来源很可能是原始铀母体碳质物质中的前太阳金刚石，因为该金刚石中的杂质元素，包括氮（δ15N < -350‰），可以被碳质物质困在岩浆过程中，成为冲击事件中铀金刚石的前驱体。
We measured the contents and isotopic compositions for C, N, and noble gases in the diamond fractions separated in a heavy liquid (ρ = 2.9 g/cm3) from a sample enriched with diamond from the Novo Urei ureilite. The results show that the concentrations of nitrogen and noble gases in the diamond fraction isolated from the supernatant (the fraction is named DNU-1) are more than a factor of 1.5 higher than those in the diamond fraction from the residue (DNU-2). This difference is probably caused by smaller sizes of grains and (or) clusters of smaller grains as well as by larger defectiveness of the crystal lattice of the diamond in the DNU-1 fraction as compared to DNU-2. Both fractions are similar in the isotopic composition of C and N and in the ratios of trapped chemical elements. The results obtained and the published data concerning C, N, and noble gases in different fractions of other ureilites allow us to conclude the following. (1) The ureilite diamond was most likely formed from graphite and the fine-grained crystalline (or semiamorphous) carbonaceous phase as a result of shock transformation in the parent bodies. (2) The negative result in the search for the isotopically light component of nitrogen (δ15N is about –100‰) in the Antarctic unshocked ureilite ALH 78019 (Rai et al., 2002), which introduced serious difficulties for explaining the origin of the ureilite diamond in the parent bodies during the impact, is most likely caused by the absorption of atmospheric nitrogen by the carbonaceous material in the processes of terrestrial weathering. (3) The source of light nitrogen (δ15N ∼ –100‰) in the ureilite diamond was probably the presolar diamond in the initial carbonaceous material of the ureilite parent bodies, because the impurity elements, including nitrogen (δ15N < –350‰), in this diamond could be trapped in the magmatic processes by the carbonaceous material, which became a precursor of the ureilite diamond in the shock event.