A re-assessment of the nitrogen geochemical behavior in upper oceanic crust from Hole 504B: Implications for subduction budget in Central America
A re-assessment of the nitrogen geochemical behavior in upper oceanic crust from Hole 504B: Implications for subduction budget in Central America
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DOI：
10.1016/j.epsl.2019.115735
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发表时间：
2019-11
影响因子：
5.3
通讯作者：
V. Busigny;P. Cartigny;C. Laverne;D. Teagle;M. Bonifacie;P. Agrinier
中科院分区：
文献类型：
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作者：
V. Busigny;P. Cartigny;C. Laverne;D. Teagle;M. Bonifacie;P. Agrinier
海水-洋壳蚀变过程中N的地球化学行为仍然缺乏约束。然而，它是一个核心参数，以评估N的通量俯冲带。大多数研究认为，热液蚀变玄武岩相对于新鲜玄武岩富含N。然而，从DSDP/ODP孔504 B，一个参考网站的洋壳的组成，公布的数据表明，海水蚀变导致上洋壳的N耗尽。为了更好地解决这个问题，我们分析了21蚀变玄武岩的N浓度和同位素组成的熔岩和席状岩脉采样孔504 B。这些新的分析表明，相对于新鲜脱气的MORB（14.1 ppm），显着的N富集（高达14.1 ppm）。早期和现代数据之间观察到的差异被解释为由于早期使用钼坩埚进行氮提取而导致的分析伪影。此外，我们的新数据显示，随着深度的N浓度逐渐降低，从14.1到1.4 ppm。氮同位素组成显示出很大的变化范围，δ 15 N值从− 0.9到+ 7.3‰，很可能反映了不同成分流体的多阶段蚀变。δ 15 N值在3.0±2.2‰（1 SD）附近振荡，没有全球变化趋势。N含量与岩石全岩δ 18 O值呈正相关，说明蚀变过程中N的行为主要受温度控制。低温（<2 0 0 ° C）时，热液中的N主要以NH3/NH 4形式存在，高温（> 2 0 0 ° C）时，N主要以N2形式存在。这些新的数据被用来重新评估全球输入中美洲俯冲带的N通量，表明上玄武质地壳约占俯冲带埋藏的总N的20%。与以前的结果在火山弧脱气N的比较表明，在“温暖”俯冲带，如中美洲，高达50%的俯冲N可能会转移到地幔深部。这与“冷”俯冲环境形成对比，在“冷”俯冲环境中，> 80%的N输入逃脱了弧下板片的挥发作用，并支持地热梯度在决定俯冲带中N的命运方面起着重要作用。
The geochemical behavior of N during seawater-oceanic crust alteration remains poorly constrained. Yet, it is a central parameter to assess the flux of N to subduction zones. Most studies proposed that hydrothermally altered basaltic rocks are enriched in N relative to fresh basalts. However, published data from DSDP/ODP Hole 504B, a reference site for the composition of the oceanic crust, suggest that seawater alteration leads to the N depletion of the upper ocean crust. To better address this issue, we analyzed N concentration and isotope composition of 21 altered basalts from the lavas and sheeted dikes sampled by Hole 504B. These new analyses show significant N enrichment (up to 14.1 ppm) relative to fresh degassed MORB (∼ 1 ppm). The differences observed between earlier and modern data are interpreted as resulting from analytical artifact due to the earlier use of a molybdenum crucible for N extraction. Furthermore, our new data show a progressive decrease of N concentration with depth, from 14.1 to 1.4 ppm. Nitrogen isotope compositions display a large range, with δ 15 N values from− 0.9 to+ 7.3‰, and most likely reflect multiple stages of alteration with fluids of various compositions. In contrast to N concentration, δ 15 N values do not show a global depth trend but oscillate around a mean value of 3.0±2.2‰(1SD). The N concentration shows a positive correlation with bulk rock δ 18 O values, suggesting that N behavior during alteration process is mainly controlled by temperature. We propose that N speciation in the hydrothermal fluid is dominated by NH 3/NH 4 at low temperature (< 200° C) but is transformed to N 2, associated with H 2, at higher temperature (> 200° C). These new data are used to re-evaluate the global flux of N input into Central American subduction zone, showing that the upper basaltic crust represent about 20% of the total N buried in subduction zone. A comparison with previous results obtained on N degassed in volcanic arc illustrates that, in “warm” subduction zone like Central America, up to 50% of the subducted N may be transferred to the deep mantle. This contrasts with “cold” subduction environments, where> 80% of the N inputs escape sub-arc slab devolatilization and supports that the geothermal gradient plays a major role in determining the N fate in subduction zones.