Hydrothermal 15N15N abundances constrain the origins of mantle nitrogen
Hydrothermal 15N15N abundances constrain the origins of mantle nitrogen
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DOI：
10.1038/s41586-020-2173-4
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发表时间：
2020-04-16
期刊：
影响因子：
64.8
通讯作者：
Young, E. D.
中科院分区：
文献类型：
--
作者：
Labidi, J.;Barry, P. H.;Young, E. D.
氮是地球大气的主要成分，但其在地幔中的来源仍不确定。地球吸积过程中继承的原始氮与从地球表面消减的原始氮的相对贡献尚不清楚(1-6)。在这里，我们表明地幔可能保留了这种原始氮的残余物。我们使用 N-2 的稀有 (NN)-N-15-N-15 同位素异体作为火山气体喷发中空气污染的新示踪剂。通过限制冰岛、埃菲尔（德国）和黄石（美国）气体中的空气污染，我们得出了地幔三角洲 N-15（N-15/N-14 与空气的分数差）、N-2/Ar-36 和 N-2/He-3 的估计值。我们的结果表明，在气体中观察到的负 delta N-15 值（以前被认为表明氮 (7-10) 的地幔起源）实际上代表了主要来自空气的 N-2，在热液系统中经历了 N-15/N-14 分馏。使用二组分混合模型来校正这种效应，(NN)-N-15-N-15 数据允许进行外推来表征地幔端元 delta N-15、N-2/Ar-36 和 N-2/He-3 值。我们表明，相对于洋中脊玄武岩提供的对流地幔的估计值，埃菲尔地区的 delta N-15 和 N-2/Ar-36 值略有增加 (11)，这与添加到地幔源中的俯冲氮一致。相比之下，我们发现，虽然黄石地幔柱的 delta N-15 值远大于对流地幔的 delta N-15 值，类似于表面成分（12-15），但其 N-2/Ar-36 和 N-2/He-3 比率与对流地幔的比率没有区别。这一观察结果提出了羽流中含有原始成分的可能性。我们用两盒模型对俯冲假说进行了检验，描述了地幔和地表氮在地质时期的演化。我们表明，俯冲对深部氮循环的影响可能没有以前的研究表明的那么重要。相反，我们认为洋中脊玄武岩和羽流三角洲 N-15 的高值可能都是主要的原始特征。
Nitrogen is the main constituent of the Earth's atmosphere, but its provenance in the Earth's mantle remains uncertain. The relative contribution of primordial nitrogen inherited during the Earth's accretion versus that subducted from the Earth's surface is unclear(1-6). Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare (NN)-N-15-N-15 isotopologue of N-2 as a new tracer of air contamination in volcanic gas effusions. By constraining air contamination in gases from Iceland, Eifel (Germany) and Yellowstone (USA), we derive estimates of mantle delta N-15 (the fractional difference in N-15/N-14 from air), N-2/Ar-36 and N-2/He-3. Our results show that negative delta N-15 values observed in gases, previously regarded as indicating a mantle origin for nitrogen(7-10), in fact represent dominantly air-derived N-2 that experienced N-15/N-14 fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the (NN)-N-15-N-15 data allow extrapolations that characterize mantle endmember delta N-15, N-2/Ar-36 and N-2/He-3 values. We show that the Eifel region has slightly increased delta N-15 and N-2/Ar-36 values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts(11), consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has delta N-15 values substantially greater than that of the convective mantle, resembling surface components(12-15), its N-2/Ar-36 and N-2/He-3 ratios are indistinguishable from those of the convective mantle. This observation raises the possibility that the plume hosts a primordial component. We provide a test of the subduction hypothesis with a two-box model, describing the evolution of mantle and surface nitrogen through geological time. We show that the effect of subduction on the deep nitrogen cycle may be less important than has been suggested by previous investigations. We propose instead that high mid-ocean-ridge basalt and plume delta N-15 values may both be dominantly primordial features.