A nitrogen isotope fractionation factor between diamond and its parental fluid derived from detailed SIMS analysis of a gem diamond and theoretical calculations
A nitrogen isotope fractionation factor between diamond and its parental fluid derived from detailed SIMS analysis of a gem diamond and theoretical calculations
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DOI：
10.1016/j.chemgeo.2015.06.020
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发表时间：
2015-09
期刊：
影响因子：
3.9
通讯作者：
D. Petts;T. Chacko;T. Stachel;R. Stern;L. Heaman
中科院分区：
文献类型：
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作者：
D. Petts;T. Chacko;T. Stachel;R. Stern;L. Heaman
为了确定钻石与其母液之间 N 同位素分馏的程度，使用二次离子质谱法对复杂分区的榴辉岩钻石 (JDE-25) 进行了详细的 C 和 N 同位素分析。在四个不同的生长区域进行了组合的 C 和 N 同位素以及 N 丰度测量，并显示了以下值范围：δ13C = − 5.7 至 − 2.1 ‰；δ15N = − 7.0 至 + 5.5 ‰； [N] = 104 至 5420 at。 百万分之一。核心区显示出 δ13C 和 δ15N 值连续向边缘增加，而氮丰度减少，并且被解释为由单次流体脉冲（即封闭系统）中金刚石的分步结晶形成。对来自核心区的同位素和丰度数据进行建模，得到的金刚石-流体氮分配系数 (KN) 为 4.4，N-同位素分馏因子 (Δ15Ndiam-流体) 在 ~ 1100 °C 时为 − 4.0 ± 1.2‰ (2σ)，用于从纯碳酸盐流体中沉淀。如果 JDE-25 由更复杂的流体形成，其中碳酸盐物质仅形成次要成分，则计算出的 KN 和 Δ15Ndiam 流体值将具有更大的量级。与上地幔流体（N2、NH3 或 NH4+）相关的主要 N 物质和 CN− 分子之间的 N 同位素分馏理论计算（作为金刚石中碳氮键的模拟）得出以下 Δ15Ndiam (CN)– 1100 °C 时的流体估计：NH4+ 为 − 3.6 ‰，N 2 为 − 2.1 ‰，NH 3 为 − 1.4 ‰。考虑到金刚石中的 C-N 单键对 15 N 的亲和力比 CN- 分子中更强的 C-N 三键的亲和力低，理论计算仅提供了真实金刚石-流体 N-同位素分馏因子的最小估计值。因此，理论 N 同位素分馏因子与从金刚石 JDE-25 得出的经验分馏因子一致。由于 Δ15Ndiam 流体的量级很大，金刚石中的晶内 N 同位素变化应该为与流体相关的分级结晶过程提供灵敏的测试。此外，较大的 Δ15Ndiam 流体可能反映在天然钻石的 δ15N 值范围较广，以及橄榄岩和榴辉岩钻石的 N 同位素组成缺乏明确定义的模式。
To determine the magnitude of N-isotope fractionation between diamond and its parental fluid, detailed C- and N-isotope analyses of a complexly-zoned, eclogitic diamond (JDE-25) were undertaken using secondary ion mass spectrometry. Combined C- and N-isotope and N-abundance measurements were made across four distinct growth zones and show the following range of values:δ13C = − 5.7 to − 2.1‰;δ15N = − 7.0 to + 5.5‰; [N] = 104 to 5420 at. ppm. The core zone displays a continuous, rimward increase inδ13C andδ15N values and decreases in N-abundance, and is interpreted to have formed by fractional crystallization of diamond from a single pulse of fluid (i.e., closed system). Modelling of the isotopic and abundance data from the core zone yields a diamond–fluid nitrogen partition coefficient (KN) of 4.4 and a N-isotope fractionation factor (∆15Ndiam–fluid) of − 4.0 ± 1.2‰ (2σ) at ~ 1100 °C, for precipitation from a pure carbonate fluid. CalculatedKNand ∆15Ndiam–fluidvalues would have larger magnitudes if JDE-25 formed from a more complex fluid, in which the carbonate species formed only a minor component.Theoretical calculations of N-isotope fractionation between the principal N-species associated with upper mantle fluids (N2, NH3or NH4+) and the CN−molecule, as an analogue for the carbon–nitrogen bond in diamond, yield the following ∆15Ndiam (CN)–fluidestimates at 1100 °C: − 3.6‰ for NH4+, − 2.1‰ for N2and − 1.4‰ for NH3. The theoretical calculations provide only minimum estimates of the true diamond–fluid N-isotope fractionation factor, given that the C–N single bond in diamond would have a lower affinity for15N than the stronger C–N triple bond in the CN−molecule. Accordingly, the theoretical N-isotope fractionation factors are consistent with the empirical fractionation factor derived from diamond JDE-25. As a consequence of the large magnitude of ∆15Ndiam–fluid, intracrystalline N-isotope variations in diamond should provide a sensitive test for fluid-related, fractional crystallization processes. Furthermore, the large magnitude of ∆15Ndiam–fluidcould be reflected in the wide range ofδ15N values for natural diamonds and the absence of clearly defined modes for the N-isotope compositions of peridotitic and eclogitic diamonds.