Warm and oxidizing slabs limit ingassing efficiency of nitrogen to the mantle
Warm and oxidizing slabs limit ingassing efficiency of nitrogen to the mantle
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DOI：
10.1016/j.epsl.2020.116615
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发表时间：
2021
影响因子：
5.3
通讯作者：
C. Jackson;E. Cottrell;B. Andrews
中科院分区：
文献类型：
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作者：
C. Jackson;E. Cottrell;B. Andrews
氮是地球大气的主要和基本成分，但相对于其他挥发性元素，对于地球内部和外部之间氮循环的路径，实验上的限制相对较少。我们报道了矿物熔体和矿物流体分配实验，以限制氮在板坯脱水和沉积物熔化过程中的行为。实验将流纹岩熔体与硅酸盐和氧化物矿物反应，反应温度在725-925℃之间，压力在0.2-2.3 Gpa之间。氧逸度介于铁金属饱和度(∼NNO-5)和原始弧玄武岩过量(∼NNO+2)之间。我们的实验表明，与矿物(黑云母、钾长石和角闪石)和流纹岩熔体相比，在所探索的所有条件下，含水流体是氮的首选相。压力(ΔLog(D m e L t−f L u i d N)/Δ(Gpa/K)=76 1±6 8(1σ)，ΔLog(D b i o t t e−f L u i d N)/Δ(Gpa/K)=46 2±16 9)和中等氧逸度效应(ΔL o g(D m e L t−f L u i d N)/Δnno=-0.2 0±0.0 4)、ΔLog(D b i o t−f L u i d N)/Δnno=-0.10±0.0 4对氮的分配有调节作用。我们进一步证明了与含水流体的矿物组成或氯含量有关的分配效应可以忽略不计。在所研究的矿物中，黑云母对N的亲和力最大，应该控制存在的板岩中N的保留。将分配数据应用于板片脱水PT路径，突出了沿较暖和氧化(NNO+1)俯冲地温的板片高度不相容行为(D b i o t e−f−f L u i d N<；0.1)的可能性，而沿还原和较冷的地热进行的脱水将提取适度数量的氮(D b i o t i t e Jet f L u i d N&t；0.1)。我们发现，至少在氧化条件下(NNO+1)，板坯熔化对板坯中氮的提取效果不如流体损失。归根结底，冰块失去液体的条件强烈影响着氮在地球内部和外部的分布。
Nitrogen is a major and essential component of Earth's atmosphere, yet relative to other volatile elements, there are relatively few experimental constraints on the pathways by which nitrogen cycles between Earth's interior and exterior. We report mineral-melt and mineral-fluid partitioning experiments to constrain the behavior of nitrogen during slab dehydration and sediment melting processes. Experiments reacted rhyolitic melts with silicate and oxide minerals, in the presence of excess aqueous fluid, over temperatures between 725-925° C and pressures between 0.2 and 2.3 GPa. Oxygen fugacity ranged between iron metal saturation (∼ NNO-5) to that in excess of primitive arc basalts (∼ NNO+ 2). Our experiments demonstrate that hydrous fluid is the preferred phase for nitrogen over minerals (biotite, K-feldspar, and amphibole) and rhyolitic melts across all conditions explored. Relatively large effects of pressure (Δlog (D m e l t− f l u i d N)/Δ (GPa/K)= 761±68 (1σ), Δlog (D b i o t i t e− f l u i d N)/Δ (GPa/K)= 462±169) and moderate effects of oxygen fugacity (Δ l o g (D m e l t− f l u i d N)/Δ NNO=-0.20±0.04, Δlog (D b i o t i t e− f l u i d N)/Δ NNO=-0.10±0.04) modulate partitioning of nitrogen. We further document negligible partitioning effects related to mineral composition or Cl content of hydrous fluid. Of the minerals investigated, biotite has the largest affinity for N and should control the retention of N in slabs where present. Application of partitioning data to slab dehydration PT paths highlights the potential for highly incompatible behavior (D b i o t i t e− f l u i d N< 0.1) from the slab along warmer and oxidized (NNO+ 1) subduction geotherms, whereas dehydration along reduced and cooler geotherms will extract moderate amounts of nitrogen (D b i o t i t e− f l u i d N> 0.1). We find that slab melting is less effective at extracting N from slabs than fluid loss, at least under oxidized conditions (NNO+ 1). Ultimately, the conditions under which slabs lose fluid strongly affect the distribution of nitrogen between Earth's interior and exterior.