The behaviour of nitrogen during subduction of oceanic crust: insights from in situ SIMS analyses of high-pressure rocks
The behaviour of nitrogen during subduction of oceanic crust: insights from in situ SIMS analyses of high-pressure rocks
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DOI：
10.1016/j.gca.2022.01.018
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发表时间：
2022-01
影响因子：
5
通讯作者：
B. Harris;J. D. De Hoog;R. Halama
中科院分区：
文献类型：
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作者：
B. Harris;J. D. De Hoog;R. Halama
了解地球的地质氮 (N) 循环需要了解俯冲地壳脱水过程中氮的行为。我们首次通过二次离子质谱法对硅酸盐矿物中的氮进行原位测量，重点关注代表俯冲洋壳的高压岩石。我们研究了矿物相之间氮的分布，并将氮与其他微量元素和主量元素的分析结合起来，以限制氮在流体-岩石相互作用过程中的行为。数据证实，白云母（多硅白云母、钠长石）是氮的主要宿主，氮含量高达 320 μg/g，而单斜辉石、角闪石和绿帘石等矿物的氮含量< 5 μg/g。绿泥石也可能含有氮（高达 83 μg/g），并且可能在俯冲带氮循环中发挥着以前未被认识到的作用。根据矿物氮浓度和矿物模式估计的块体岩石氮浓度与通过块体燃烧测量的氮浓度一致，这证实大多数氮存在于硅酸盐矿物中，而不是沿晶界或流体包裹体中。块体岩石N含量与K 2 O相关(N/K 2 O=19.3±2.0)。利用N/K 2 O比率和蚀变洋壳的平均K 2 O，上洋壳俯冲的N通量估计为0.6-2.4× 10 11 g/yr，这与之前的估计一致，但处于较低水平。这些数据还用于研究流体-岩石相互作用过程中氮的行为。开放系统流体-岩石相互作用模型用于模拟石榴子石-多白硅云岩石英岩中多白硅云岩中流体-岩石相互作用过程中 N、B 和 Li 含量的演变。通过与 B 和 Li 的数据进行比较，N 的多硅白云石-流体分配系数估计为 0.1-1.5。另外，蓝片岩中流体-岩石相互作用过程中钠长石的生长被证明可以从多硅白云母中螯合氮，并限制大量氮流失到流体中。因此，白云母在流体-岩石相互作用过程中的稳定性对于控制氮的行为至关重要。沉积物衍生流体中的氮添加似乎是俯冲带岩石中的一个重要过程。如果白云母稳定，镁铁质外壳可以充当该 N 的汇。这项工作首次对俯冲带条件下氮的流体-矿物分配行为提供了自然约束，并强调了俯冲带内氮迁移的复杂性，不同相和岩性之间的重新分配非常重要。
Understanding the Earth’s geological nitrogen (N) cycle requires an understanding of how N behaves during dehydration of subducted crust. We present the first in situ measurements of N in silicate minerals by secondary ion mass spectrometry, focusing on high pressure rocks representing subducted oceanic crust. We investigate the distribution of N between mineral phases, and combine analyses of N with other trace and major elements in order to constrain the behaviour of N during fluid-rock interaction. The data confirm that white mica (phengite, paragonite) is the primary host for N, containing up to 320 μg/g, whereas minerals including clinopyroxene, amphibole and epidote contain< 5 μg/g N. Chlorite can also contain N (up to 83 μg/g) and may play a previously unrecognised role in subduction zone N cycling. Bulk rock N concentrations estimated from mineral N concentrations and mineral modes are consistent with N concentrations measured by bulk combustion, which confirms that most N is hosted within silicate minerals and not along grain boundaries or in fluid inclusions. Bulk rock N contents correlate with K 2 O (N/K 2 O= 19.3±2.0). Using N/K 2 O ratios and the average K 2 O of altered oceanic crust, the flux of N subducted in upper oceanic crust is estimated to be 0.6-2.4× 10 11 g/yr, which is consistent with but at the lower end of previous estimates. The data were also used to investigate the behaviour of N during fluid-rock interaction. Open system fluid-rock interaction modelling was used to model the evolution of N, B and Li contents during fluid-rock interaction in phengite from a garnet-phengite quartzite. By comparison to data for B and Li, the phengite-fluid partition coefficient for N was estimated to be 0.1–1.5. Separately, the growth of paragonite during fluid-rock interaction in a blueschist was shown to sequester N from phengite and limit bulk N loss to the fluid. The stability of white mica during fluid-rock interaction is therefore critical in controlling the behaviour of N. Nitrogen addition from sediment-derived fluids appears to be an important process in subduction zone rocks. Mafic crust can act as a sink for this N if white mica is stable. This work provides the first natural constraints on the fluid-mineral partitioning behaviour of N at subduction zone conditions and emphasises the complexity of N mobility within subduction zones, with redistribution between different phases and lithologies being important.