Carbon isotope anatomy of a single graphite crystal in a metapelitic migmatite revealed by high-spatial resolution SIMS analysis
Carbon isotope anatomy of a single graphite crystal in a metapelitic migmatite revealed by high-spatial resolution SIMS analysis
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DOI：
10.1007/s00410-011-0626-3
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发表时间：
2011-03
影响因子：
3.5
通讯作者：
M. Satish-Kumar;H. Yurimoto;S. Itoh;B. Cesare
中科院分区：
文献类型：
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作者：
M. Satish-Kumar;H. Yurimoto;S. Itoh;B. Cesare
利用二次离子质谱（西姆斯）和常规质谱技术对天然石墨进行高空间分辨率碳同位素分析，证明了从部分熔融的变质岩中沉淀出的单个石墨晶体内的同位素异质性。SIMS ~（13）C/~（12）C测量值用石墨内标进行校正，空间分辨率为2-3 μm，重复性为0.3‰（1σ）。这一分辨率有助于从变质白质体中识别出单个石墨晶体中不寻常的碳同位素分布，显示出显著的核与边缘变化，δ 13 C的变化高达10‰。结果表明，石墨晶体是由一端向另一端生长，形成垂直于轴向的层状结构。这种明显的同位素阶跃表明石墨中存在碳同位素不平衡分带，并可能存在扇形分带。石墨晶内碳同位素不平衡是由于生长介质中12 C和13 C向不同生长区石墨沉淀界面扩散系数的差异造成的。
High-spatial resolution carbon isotope analyses of natural graphite using secondary ion mass spectrometry (SIMS), together with conventional mass spectrometry techniques, demonstrate isotopic heterogeneity within single graphite crystals precipitated from a partially melted metamorphic rock. SIMS13C/12C measurements were calibrated using an internal graphite standard previously analyzed by conventional isotope ratio mass spectrometry, which gave a reproducibility of 0.3‰ (1σ) at a spatial resolution of 2–3 μm. This resolution helped to identify an unusual carbon isotope distribution in a single graphite crystal from a metapelitic leucosome, showing remarkable core to rim variations with sharp δ13C steps up to 10‰. The results suggest that the graphite crystal grew from one edge to other forming layers perpendicular to thec-axis. The sharp isotopic steps indicate the presence of disequilibrium carbon isotope zoning in graphite and points to the possible existence of carbon isotope sector zoning. Intra-crystalline carbon isotope disequilibrium in graphite is believed to have resulted from the difference in diffusivity between12C and13C in the growth medium to the interface of graphite precipitation in different growth sectors.