Metabasic Rocks as Important Nitrogen Carriers to Forearc Depths: Implications for Deep Nitrogen Cycling
Metabasic Rocks as Important Nitrogen Carriers to Forearc Depths: Implications for Deep Nitrogen Cycling
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DOI：
10.1016/j.gca.2023.10.007
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发表时间：
2023-10
影响因子：
5
通讯作者：
A. Mallik;Anna M Rebaza;Paul Kapp;Long Li;Yifan Du;Ahmed Al Shams;Emily H. G Cooperdock
中科院分区：
文献类型：
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作者：
A. Mallik;Anna M Rebaza;Paul Kapp;Long Li;Yifan Du;Ahmed Al Shams;Emily H. G Cooperdock
要更好地理解深层氮循环，需要研究通过各种岩性向俯冲系统输送氮。与沉积岩相比，通过基性岩输入俯冲带的量更大，更大，这要求对变基性岩中N的行为进行彻底的调查。在这里，我们估计交付的N俯冲带在弧前深度通过调查的斜长角闪岩和绿帘石蓝片岩的地球化学从中央羌塘变质带在西藏的变基性岩可能代表的过渡，从大洋到大陆俯冲。岩石含氮量为21-147 ppm，δ 15 N值为+1.8 ‰ ~+10.0 ‰，其中147 ppm的N值是迄今为止所报道的变质基性岩中最高的。鉴于大多数岩石的N丰度远高于蚀变洋壳（即玄武岩、席状岩脉和辉长岩; 6.0 ± 4.7 ppm），N可能既不是岩浆，也不是在俯冲前的热液蚀变过程中引入岩石的。K2 O/Th对Ba/Th、Th/U对Th和Ba/Rb对K2 O图证实了这一点，这些岩石与变质流体蚀变而不是海底热液蚀变的趋势一致。一个两步的过程导致N收购的变基性岩。在第一步中，变质基性岩在俯冲通道中的变质作用期间从变质沉积物来源的流体中获得了它们的N。第二阶段，部分变质基性岩在俯冲通道内发生脱挥发分作用，导致N的损失和15 N的富集。我们模拟的N通量在弧前深度在55个现代俯冲带通过变质沉积岩和变质基性岩假设其最低，中位数和最大的N浓度，以评估其相对重要性，在交付的N俯冲带。我们发现，变基性岩供应相当的N通量变沉积岩在弧前深度，即使变沉积岩至少有一个数量级更高的N丰度比变基性岩。变基性岩在氮输送中的重要性加强了从全球更多地点调查其氮行为的必要性，以提高我们对深层氮循环的理解。
Understanding deep nitrogen (N) cycling better requires investigating the delivery of N to subduction systems via various lithologies. Input to subduction zones through mafic rocks is more voluminous and massive as compared to sedimentary rocks which calls for a thorough investigation of the behavior of N in metabasic rocks. Here we estimate the delivery of N to subduction zones at forearc depths by investigating the geochemistry of amphibolites and epidote-blueschists from the Central Qiangtang Metamorphic Belt in Tibet where the metabasic rocks likely represent the transition from oceanic to continental subduction. The rocks contain 21–147 ppm N with δ15N values from +1.8 ‰ to +10.0 ‰, and 147 ppm N is the highest that has been reported in a metabasic rock thus far. Given the N abundances for most of the rocks are much higher than those of altered oceanic crust (i.e. basalts, sheeted dikes and gabbros; 6.0 ± 4.7 ppm), the N is likely neither magmatic nor was introduced in the rocks during hydrothermal alteration prior to subduction. This is confirmed by the K2O/Th versus Ba/Th, Th/U versus Th and Ba/Rb versus K2O plots where these rocks align with the trend of metamorphic fluid alteration rather than seafloor hydrothermal alteration. A two-step process led to N acquisition in the metabasic rocks. In the first step, the metabasic rocks acquired their N from metasediment-derived fluids during metamorphism in the subduction channel. In the second step, some of the metabasic rocks underwent N loss and concomitant enrichment in15N due to devolatilization within the subduction channel. We modeled the N fluxes at forearc depths in 55 modern-day subduction zones via metasedimentary and metabasic rocks assuming their minimum, median and maximum N concentrations to assess their relative importance in delivery of N to subduction zones. We find that metabasic rocks supply comparable fluxes of N to metasedimentary rocks at forearc depths, even though metasedimentary rocks have at least an order of magnitude higher N abundance than metabasic rocks. The importance of metabasic rocks in N delivery reinforces the need to investigate the behavior of N in them from more locations globally to improve our understanding of deep N cycling.