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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Diamond genesis, mantle fractionations and mantle nitrogen content: a study of δ~(13)C-N concentrations in diamonds
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Diamond genesis, mantle fractionations and mantle nitrogen content: a study of δ~(13)C-N concentrations in diamonds

机译：金刚石的成因，地幔分馏和地幔含氮量：金刚石中δ〜（13）C-N浓度的研究

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A compilation of more than 1200 δ~(13)C-N data from well-characterised diamonds show a correlation of the maximum diamond nitrogen content (i.e. a limit sector) with δ~(13)C over the full diamond δ~(13)C range (i.e. more than 30‰). Diamonds with low δ~(13)C values are characterised by rather low N contents (～0 ppm at δ~(13)C < -30‰), whereas diamonds with high δ~(13)C have more variable nitrogen contents, with a much higher upper limit (～3500 ppm at δ~(13)C = -4.5‰). This correlation defines a concave trend that is therefore incompatible with a mixing relationship, such as would be produced by the admixture of subducted and primordial components. The limit sector more likely reflects the evolution of mantle melts (or fluids) during differentiation. Nitrogen uptake is seen as a kinetic process, depending mostly on the diamond rate of growth; at a given δ~(13)C value, as a result of slow growth conditions, diamonds with nitrogen contents lower than the maximum value are interpreted as having fractionated the N/C ratio relative to their growth medium. The limit sector is applicable to every diamond paragenesis (peridotitic, eclogitic and fibrous) suggesting that every diamond type may derive from a similar isotopic source. Assuming a mantle δ~(13)C value of -4.5‰, we deduce that the initial C/N ratio of mantle melts (i.e. the diamond growth medium) from which diamonds crystallise ranges between 200 and 500, which is surprisingly similar to that of mid-ocean ridge basalts. Therefore, in spite of their different context and age, it appears that subcontinental and oceanic mantles give samples with similar δ~(13)C, δ~(15)N and C/N, suggesting an overall homogeneity of volatiles within these parts of the Earth since the Archaean. Diamonds also demonstrate that carbon and nitrogen do not behave similarly during the evolution of the diamond growth medium. Accordingly, mantle nitrogen concentration cannot be deduced in a simple way. If N behaved as an incompatible element during partial melting, a mantle nitrogen concentration of about 2 ppm could be expected, provided that the mantle carbon content is about 400 ppm. However, from several lines of evidence presented in this study, nitrogen is not regarded as a totally incompatible element, and a higher mantle nitrogen concentration (perhaps up to 40 ppm) is preferred.
机译：来自特征明确的钻石的超过1200个δ〜（13）CN数据的汇编显示，在整个钻石δ〜（13）C上，最大钻石氮含量（即极限扇形）与δ〜（13）C的相关性范围（即大于30‰）。 δ〜（13）C值低的钻石的特征是氮含量较低（δ〜（13）C <-30‰时为〜0 ppm），而δ〜（13）C较高的钻石具有可变的氮含量，上限更高（在δ〜（13）C = -4.5‰时约为3500 ppm）。该相关性限定了凹形趋势，因此该凹形趋势与诸如由俯冲的和原始的成分的混合所产生的混合关系不兼容。极限区域更可能反映出在分化过程中地幔熔体（或流体）的演化。氮的吸收被认为是一个动力学过程，主要取决于钻石的生长速度。在给定的δ〜（13）C值下，由于缓慢的生长条件，氮含量低于最大值的钻石被解释为相对于其生长介质而言，N / C比处于分馏状态。极限扇形适用于每种金刚石共生（橄榄岩，岩石和纤维质），表明每种金刚石类型都可能来自相似的同位素来源。假设地幔的δ〜（13）C值为-4.5‰，我们可以推断出地幔融化的初始C / N比（即钻石生长介质），从中结晶出的钻石介于200到500之间，这令人惊讶地类似于中洋脊玄武岩。因此，尽管背景和年龄不同，但次大陆和大洋幔似乎给出了具有相似的δ〜（13）C，δ〜（15）N和C / N的样品，这表明挥发分在这些部分的总体均一性自古以来的地球。钻石还表明，在钻石生长介质的演化过程中，碳和氮的行为不同。因此，无法以简单的方式推断出地幔氮浓度。如果N在部分熔融过程中表现为不相容元素，则只要覆盖层碳含量为约400 ppm，则可以预期覆盖层氮浓度约为2 ppm。但是，从本研究中提供的多种证据来看，氮并不是完全不相容的元素，而较高的地幔氮浓度（可能高达40 ppm）是优选的。

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