Depth-related carbon isotope and nitrogen concentration variability in the mantle below the Orapa kimberlite, Botswana, Africa
Depth-related carbon isotope and nitrogen concentration variability in the mantle below the Orapa kimberlite, Botswana, Africa
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DOI：
10.1016/0016-7037(93)90390-i
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发表时间：
1993-06
影响因子：
5
通讯作者：
P. Deines;J. Harris;J. Gurney
中科院分区：
文献类型：
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作者：
P. Deines;J. Harris;J. Gurney
来自Orapa金伯利岩的立方金刚石具有非常有限的δ 13 C范围，平均值为− 5.98±0.57‰，相对较高且恒定的氮含量为897±171 ppm，氮聚集态较低。它们被认为是来自浅地幔深度。含有橄榄岩包裹体（P型）的钻石的δ 13 C范围在− 4.2和− 18.9‰之间，主模式在− 5和− 7‰之间。它们的氮含量低，聚集状态高。压力-温度（P-T）平衡条件估计其包裹体接近湿橄榄岩固相线。Orapa金刚石（E型）中榴辉岩包裹体的P-T平衡条件比P型矿物高。其寄主的δ 13 C值在− 2.6%至− 18.0%之间变化。主振型在-13 ‰到-15 ‰之间。E型金刚石的含氮量明显高于P型金刚石。Orapa金刚石榴辉岩的矿物和碳同位素组成与E型金刚石的矿物和碳同位素组成的比较表明，只有部分E型金刚石可能来自金刚石榴辉岩的物理裂解。贫13 C的E型金刚石和低δ 13 C金刚石榴辉岩捕虏体形成于相似的有限温压区。包裹体矿物共生组合的橄榄岩和榴辉岩矿物之间的过渡组成（websteritic，W型）已被确认。其特征在于相对高的Chrome和Fe含量的独特组合。W型金刚石的δ 13 C值除了一个例外（-6.9%），在-15.2到-22.4 ‰之间。这些金刚石的氮含量明显低于E型金刚石，并且与P型金刚石无法区分。它们的P-T平衡条件与橄榄岩共生的P-T平衡条件相似。集体的数据表明，更高的频率出现的13 C-贫金刚石，金刚石的N含量的减少，和增加的N聚集状态与地幔深度的增加。然而，N含量的下降不是单调的，因为P型金刚石倾向于具有比E型金刚石更低的N含量。镁橄榄石金刚石包裹体的高Chrome和Fe含量很难与低δ 13 C寄主的俯冲起源相一致，并表明13 C亏损的C可能是一个主要的地幔特征。
Cubic diamonds from the Orapa kimberlite have a very restricted δ 13 C range with a mean of− 5.98±0.57‰, a relatively high and constant nitrogen content 897±171 ppm and low nitrogen aggregation state. They are thought to have been derived from shallow mantle depth. Diamonds with peridotitic inclusions (P-Type) range in δ 13 C between− 4.2 and− 18.9‰ with a major mode between− 5 and− 7‰. Their nitrogen content is low and state of aggregation high. Pressure-temperature (P-T) equilibration conditions estimated for their inclusions are close to the wet peridotite solidus. Eclogitic inclusions in Orapa diamonds (E-Type) indicate higherP-T equilibration conditions than P-Type minerals. Theδ 13 C values of their hosts vary from− 2.6 to− 18.0%. with a major mode between− 13 and− 15‰. The nitrogen content of E-type diamonds is significantly higher than that of P-Type diamonds. Comparison of the mineral and carbon isotopic composition of Orapa diamond eclogites with those of E-Type diamonds indicates that only part of the E-Type diamonds could have been derived from a physical breakup of diamond eclogites. 13 C-depleted E-Type diamonds and eclogite xenoliths with low δ 13 C diamonds were formed in a similar and restricted P-T range. An inclusion mineral paragenesis with compositions transitional between those of peridotitic and eclogitic minerals (websteritic, W-Type) has been recognized. It is characterized by a unique combination of a relatively high chrome and Fe content. The δ 13 C values of W-Type diamonds lie, with one exception (− 6.9%.), in the range of− 15.2 to− 22.4‰. The nitrogen content of these diamonds is significantly lower than that of E-Type and is indistinguishable from that of P-Type diamonds. The P-T equilibration conditions estimated for them are similar to those of the peridotitic paragenesis. The collective data indicate a higher frequency of occurrence of 13 C-depleted diamonds, a decrease in the N content of diamonds, and an increase in the N aggregation state with increasing mantle depth. The decline in N content is not monotonic however, because P-Type diamonds tend to have lower N contents than E-Type diamonds. The combined high chrome and Fe content of the websteritic diamond inclusions are very difficult to reconcile with a subduction origin of the low δ 13 C host and indicate that 13 C-depleted C may be a primary mantle feature.