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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Evidence for a mantle component shown by rare gases, C and N isotopes in polycrystalline diamonds from Orapa (Botswana)
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Evidence for a mantle component shown by rare gases, C and N isotopes in polycrystalline diamonds from Orapa (Botswana)

机译：来自Orapa（博茨瓦纳）的多晶钻石中稀有气体，C和N同位素显示的地幔成分的证据

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In an attempt to constrain the origin of polycrystalline diamond, combined analyses of rare gases and carbon and nitrogen isotopes were performed on six such diamonds from Orapa (Botswana). Helium shows radiogenic isotopic ratios of R/Ra = 0.14-1.29, while the neon ratios (Ne-21/Ne-22 of up to 0.0534) reflect a component from mantle, nucleogenic and atmospheric sources. Ar-40/Ar-36 ratios of between 477 and 6056 are consistent with this interpretation. The (Xe-129/Xe-130) isotopic ratios range between 6.54 and 6.91 and the lower values indicate an atmospheric component. The He, Ne, Ar and Xe isotopic compositions and the Xe isotopic pattern are clear evidence for a mantle component rather than a crustal one in the source of the polycrystalline diamonds from Orapa. The delta(13)C and delta(15)N isotopic values of - 1.04 to - 9.79 parts per thousand and +4.5 to + 15.5 parts per thousand respectively, lie within the range of values obtained from the monocrystalline diamonds at that mine. Additionally, this work reveals that polycrystalline diamonds may not be the most appropriate samples to study if the aim is to consider the compositional evolution of rare gases through time. Our data shows that after crystallization, the polycrystalline diamonds undergo both gas loss (that is more significant for the lighter rare gases such as He and Ne) and secondary processes (such as radiogenic, nucleogenic and fissiogenic, as well as atmospheric contamination). Finally, if polycrystalline diamonds sampled an old mantle (1-3.2 Ga), the determined Xe isotopic signatures, which are similar to present MORB mantle - no fissiogenic Xe from fission of U-238 being detectable - imply either that Xe isotopic ratios have not evolved within the convective mantle since diamond crystallization, or that these diamonds are actually much younger. (C) 2005 Elsevier B.V. All rights reserved.
机译：为了限制多晶钻石的起源，对六种来自Orapa（博茨瓦纳）的钻石进行了稀有气体以及碳和氮同位素的联合分析。氦气的放射同位素比为R / Ra = 0.14-1.29，而氖气比（Ne-21 / Ne-22最高为0.0534）反映了地幔，核源和大气源中的一种成分。 Ar-40 / Ar-36的比值介于477和6056之间与该解释一致。 （Xe-129 / Xe-130）同位素比在6.54和6.91之间，并且较低的值表示大气成分。 He，Ne，Ar和Xe同位素组成以及Xe同位素模式是Orapa多晶金刚石来源中地幔成分而不是地壳成分的明确证据。 δ（13）C和δ（15）N同位素值分别为-1.04至-9.79千分之和+4.5至+ 15.5千分之千，位于该矿山的单晶钻石获得的值范围内。另外，这项工作表明，如果目的是考虑稀有气体随时间的成分演变，则多晶金刚石可能不是最适合研究的样品。我们的数据表明，结晶后，多晶金刚石既经历气体损失（对于氦和氖等较稀有稀有气体而言更重要），又经历二次过程（例如放射源，核基因和裂变源以及大气污染）。最后，如果多晶钻石采样了一个旧地幔（1-3.2 Ga），则确定的Xe同位素特征与当前的MORB地幔相似-无法检测到U-238裂变引起的裂变Xe-暗示Xe同位素比率没有自钻石结晶以来，在对流幔中演化，或者这些钻石实际上更年轻。 （C）2005 Elsevier B.V.保留所有权利。

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