摘要:
华北克拉通山东金刚石晶体生长的多阶段性及碳同位素组成已被人们关注,但很少有人对不同阶段金刚石及其内部的碳同位素组成变化进行分析。本文利用二次离子质谱(SIMS)微区分析技术对山东金刚石晶体内部不同阶段生长的δ13C进行了原位测试分析,得出山东金刚石δ13C的变化范围在-5.6‰~-2.01‰,平均值为-3.63‰,与前人对华北克拉通金刚石碳同位素组成测试值(δ13C在-14.71‰~-0.46‰,主要集中在-9‰~-4‰之间)大致相同,位于全球橄榄岩型金刚石及幔源碳同位素组成主要范围(-8‰~-2‰)内。结合金刚石生长环带阴极发光图像,认为金刚石自核心部位至边缘,在同一期生长环带内,其δ13C值呈变重趋势;在多期复杂生长环带之间,δ13C值因结晶时的地质环境不同而存在差异。δ13C值的这种变化揭示了该地区金刚石复杂的生长环境,为解析华北克拉通地幔碳循环提供了定量数据。
Abstract:
Multi-stage growth and its significance in Shandong diamonds, China has been comprehensively studied, whereas microanalysis of δ13C is absent for diamonds at different stage and inner variation. 10 gem-quality Shandong diamond plates were investigated by Second Ion Mass Spectrometry (SIMS) and cathodoluminescence. It was found that the carbon isotopic composition of Shandong diamonds varies over a narrow range, from -5.6‰ to -2.01‰ with an average δ13C of -3.63‰. The range was generally in accordance with the previously studied carbon isotope compositions (δ13C values from -14.71‰ to -0.46‰, mainly lying between -9‰ and -4‰) of diamonds in the North China Croton, which was also within the range of δ13C values of global peridotitic diamonds and mantle(-8‰--2‰). Associated with cathodoluminescence imaged growth zones, variations of δ13C within a diamond from core to rim were concluded. δ13C trends to heavier from core to rim within the same growth zone, while the variations are different between multi-phase growth zones. The variations of δ13C values in diamonds reveal complication of growth environment and conditions in the region, also provide quantitative data to study the mantle carbon cycle in the North China Croton.
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