Mineral inclusions in diamonds from the Panda kimberlite, Slave Province, Canada
Mineral inclusions in diamonds from the Panda kimberlite, Slave Province, Canada
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加拿大奴隶省熊猫金伯利岩钻石中的矿物包裹体
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发表时间：
2005
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影响因子：
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通讯作者：
G. Brey
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作者：
R. Tappert;T. Stachel;J. Harris;N. Shimizu;G. Brey
采用电子探针（EPMA）和二次离子质谱（西姆斯）分析了加拿大西北地区Ekati矿始新世熊猫金伯利岩中90颗金刚石矿物包裹体的主量和微量元素组成。此外，使用傅里叶变换红外光谱（FTIRS）测量了基质金刚石的氮聚集特性。在超高压岩石圈内，熊猫金刚石主要来自橄榄岩源（85%），少量榴辉岩金刚石（10%）。含铁方镁石的金刚石（5%）含有铁方镁石与镁铝尖晶石加橄榄石或橄榄石或纯二氧化硅相的组合。这些包裹体的化学特征与含铁方镁石纯橄榄岩的岩石圈成因一致。然而，与CaSiO 3（最有可能最初包含为钙硅酸盐钙钛矿）共存的铁长石被认为是宿主金刚石的下地幔起源的证据。主要元素组成表明橄榄岩金刚石形成于中等贫化环境中，由钙含量通常> 2.5重量% CaO的方辉橄榄岩石榴石包裹体和Mg值（100*Mg/（Mg+Fe））为92-93.5的橄榄石的存在指示。稀土元素（REE）的浓度在橄榄岩石榴石很大程度上遵循细分的基础上的主要元素与二辉橄榄岩石榴石显示中稀土重稀土富集，略正弦模式，而方辉橄榄岩石榴石有明显的正弦REEN。包裹体地质温压法表明橄榄岩金刚石的形成温度范围为1100-1250°C，地热梯度为40-42 mW/m2，与世界各地的类似观测结果一致。接触石榴石-橄榄石和石榴石-斜方辉石包裹体对在1000-1100°C的较低温度下平衡，对应于约37 mW/m2的地热梯度。较高的温度被认为是在金刚石形成期间占主导地位的温度。熊猫钻石中的氮含量变化很大，从低于检测（< 10 ppm）到2700原子ppm。氮的聚集范围从低聚集（IaA型金刚石）到高聚集（IaB型金刚石）。如果从数据集中排除所有在地幔驻留期间显示出塑性变形迹象的金刚石，则金刚石子集变得明显，具有<30% B-中心的总体低氮聚集状态。这一结果可能表明塑性变形促进了熊猫金刚石中氮的聚集。对熊猫金刚石中硫化物包裹体进行早太古代Re-Os等时线定年（Westerlund et al.，2003 b），未变形金刚石的低聚集态可能表明地幔居住在相对较低的温度（< 1100°C）。如果是这样的话，从接触和非接触夹杂物对的比较中推断出的温度下降一定是在金刚石形成后不久发生的。因此，在中央从属层下方的金刚石形成可能仅限于短暂的局部热事件。在金伯利岩喷发时期（上白垩纪至始新世），中央奴隶岩下岩石圈地幔的地热梯度随深度明显增加，这可能有类似的原因，反映了熔体渗透期间深部岩石圈的瞬时加热。
Ninety diamonds from the Eocene Panda kimberlite (Ekati Mine, Northwest Territories, Canada) were analyzed for the major and trace element compositions of their mineral inclusions using electron microprobe techniques (EPMA) and secondary ion mass spectrometry (SIMS). Additionally, nitrogen aggregation characteristics of the host diamonds were measured using Fourier-transform infrared spectroscopy (FTIRS). Within the cratonic lithosphere, Panda diamonds are principally derived from peridotitic sources (85 %) with a minor content of eclogitic diamonds (10 %). Ferropericlase bearing diamonds (5 %) contain combinations of ferropericlase with either Mg-Al spinel plus olivine or with olivine or with a pure silica phase. The chemical char- acteristics of these inclusions are in accordance with a lithospheric origin from ferropericlase-bearing dunites. Ferropericlase coexisting with CaSiO3 (most likely originally included as Ca-silicate perovskite), however, is regarded as evidence for a lower mantle origin of the host diamond. Major element compositions show that the peridotitic diamonds formed in a moderately depleted environment, indicated by the presence of harzburgitic garnet inclusions with calcium contents generally > 2.5 wt% CaO and olivines with Mg numbers (100*Mg/(Mg+Fe)) of 92-93.5. Rare earth element (REE) concentrations in peridotitic garnets largely follow subdivisions based on major elements with lherzolitic garnets showing middle REE to heavy REE enriched, slightly sinusoidal patterns, whilst harzburgitic garnets have distinctly sinusoidal REEN. Inclusion geothermobarometry indicates formation of peridotitic diamonds in the temperature range 1100-1250°C, following a geothermal gradient of 40-42 mW/m2, in accordance with similar observations world-wide. Touching garnet-olivine and garnet-orthopyroxene inclusion pairs equilibrated at lower temperatures of 1000-1100°C, corresponding to a geothermal gradient around 37 mW/m2. The higher temperatures are considered to be those prevailing during diamond formation. Nitrogen contents in Panda diamonds vary strongly from below detection (< 10 ppm) to 2700 atomic ppm. Nitrogen aggrega- tion ranges from poorly aggregated (Type IaA diamond) to highly aggregated (Type IaB diamond). If all diamonds that show signs of plastic deformation during mantle residence are excluded from the dataset, then a diamond subset becomes apparent with an overall low nitrogen aggregation state of < 30 % B-center. This result may indicate that plastic deformation increases the aggre- gation of nitrogen in Panda diamonds. Taking the Early Archean Re-Os isochron date for sulfide inclusions in Panda diamonds (Westerlund et al., 2003b) at face value, the low aggregation states of undeformed diamonds may indicate mantle residence at rela- tively low temperatures (< 1100°C). If this is the case, the decrease in temperature inferred from the comparison of touching and non-touching inclusion pairs must have occurred soon after diamond formation. Thus diamond formation beneath the central Slave may be restricted to short lived and localized thermal events. An apparent increase in geothermal gradient with depth in the litho- spheric mantle beneath the Central Slave for the time of kimberlite eruptions (Upper Cretaceous to Eocene) may have a similar cause and reflect transient heating of the deep lithosphere during melt infiltration.