Carbon and nitrogen isotope systematics within a sector-growth diamond from the Mir kimberlite, Yakutia
Carbon and nitrogen isotope systematics within a sector-growth diamond from the Mir kimberlite, Yakutia
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DOI：
10.1016/s0009-2541(02)00075-x
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发表时间：
2002-08
期刊：
影响因子：
3.9
通讯作者：
G. Bulanova;D. G. Pearson;E. Hauri;B.J Griffin
中科院分区：
文献类型：
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作者：
G. Bulanova;D. G. Pearson;E. Hauri;B.J Griffin
一个单一的雅库特八面体金刚石，显示惊人的内部生长结构，立方和八面体生长部门相互生长，并包围八面体边缘，已分析了碳和氮同位素组成（二次离子质谱法，西姆斯），氮浓度（西姆斯和傅里叶变换红外光谱法，FTIR）和氮聚集状态（FTIR）。在金刚石中发现的石墨“种子”包裹体富含K、Ca、Ti、Rb和Sr，这证明金刚石可能是从与上地幔岩石相互作用的碳酸盐熔体/流体中生长出来的。碳、氮同位素组成从金刚石核心区（δ 13 C =−7‰ ~ −5‰，δ 15 N =−3‰）向内缘区（δ 13 C =−3‰，δ 15 N =+8.9‰ ~+5‰）逐渐变重。氮浓度和聚集测量显示相应的减少，一般与同位素的变化。这些系统的变化的核心和中间区域的钻石是一致的，它们的形成过程中的钻石生长从富CO2流体作为一个连续的事件，伴随着轻微的进步碳和氮的同位素分馏。然而，所观察到的同位素和氮丰度的趋势不同，从流体-固体平衡在C-N-O-H轴承系统的热力学模型的预测，由于参数的变化，如fO 2。在金刚石八面体边缘区域内，通过逐层机制生长，观察到氮丰度、氮聚集以及氮和碳同位素比的非系统变化。对于这种现象给出了几种解释，包括在不同的条件下从那些核心中间区域的金刚石边缘的生长过程中的动力学效应;或在生长过程中快速变化的流体源;或氮聚集的生长后过程。没有分馏的氮同位素之间的立方和八面体生长区被确定在金刚石，在混合生长的合成金刚石中发现的分馏现象。我们的研究结果说明了潜在的C-N同位素变异在个别钻石，并强调需要更多的这种类型的研究，如果要了解钻石套件中的同位素变异的起源。
A single Yakutian octahedral diamond, displaying striking internal growth structure whereby cubic and octahedral growth sectors are inter-grown and surrounded by an octahedral rim, has been analysed for carbon and nitrogen isotopic compositions (by secondary ion mass spectrometry, SIMS), and for nitrogen concentration (by SIMS and Fourier transform infrared spectroscopy, FTIR) and nitrogen aggregation state (by FTIR). A graphite “seed” inclusion identified within the diamond is enriched in K, Ca, Ti, Rb, and Sr, providing evidence that the diamond may have grown from a carbonate melt/fluid interacting with upper mantle rocks. Carbon and nitrogen isotope compositions become progressively heavier from the core region (δ13C=−7‰ to −5‰ and δ15N=−3‰) towards the inner rim zones (δ13C=−3‰ and δ15N=+8.9‰ to +5‰) of the diamond. Nitrogen concentration and aggregation measurements show corresponding decreases that generally correlate with the isotopic variation. These systematic changes within the core and intermediate regions of the diamond are consistent with their formation during diamond growth from CO2-rich fluids as a continuous event, accompanied by slight progressive isotopic fractionation of carbon and nitrogen. However, the observed isotope and nitrogen abundance trends differ from those predicted from thermodynamic modelling of fluid–solid equilibria in a C–N–O–H-bearing system due to changes in parameters such as fO2. Within the diamond octahedral rim region, grown by a layer by layer mechanism, nonsystematic variations in nitrogen abundance, nitrogen aggregation, and nitrogen and carbon isotope ratios were observed. Several interpretations are given for this phenomenon, including kinetic effects during growth of the diamond rim under different conditions from those of the core-intermediate regions; or rapidly changing fluid sources during the growth; or post-growth processes of nitrogen aggregation. No fractionation of nitrogen isotopes between cubic and octahedral growth zones was identified within the diamond, in contrast to the fractionation phenomena found in synthetic diamonds of mixed growth. Our results illustrate the potential C–N isotopic variation within individual diamonds and highlight the need for more studies of this type if the origin of isotopic variations in diamond suites is to be understood.