Equilibrium partitioning and isotopic fractionation of nitrogen between biotite, plagioclase, and K-feldspar during magmatic differentiation
Equilibrium partitioning and isotopic fractionation of nitrogen between biotite, plagioclase, and K-feldspar during magmatic differentiation
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岩浆分异过程中黑云母、斜长石和钾长石之间氮的平衡分配和同位素分馏
DOI：
10.1016/j.gca.2023.07.010
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发表时间：
2023
影响因子：
5
通讯作者：
Boocock T
中科院分区：
文献类型：
--
作者：
Boocock T
大陆地壳的很大一部分是由深成火成岩组成的。然而，在岩浆分异过程中，不同矿物间氮的地球化学行为尚不清楚。为了对地壳形成过程中氮的行为提供新的约束，我们在苏格兰西南部的Doon湖进行了分带钙碱性岩体中氮的地球化学特征描述。我们给出了黑云母、斜长石和钾长石矿物分离的全岩数据的N浓度和N同位素值，并评估了这些数据在岩浆分异过程中保持平衡分配的程度。结果表明，整个岩石的N含量和δ15N特征很可能继承了原始烃源成分，并且在岩体尺度上具有同质性。虽然全岩数据最好解释为地壳来源的N，但在岩体尺度上的均匀化程度与经验N扩散率不一致，排除了同位地壳同化作为N来源的可能性。相反，我们的数据表明，地壳特征继承自与Iapetus俯冲带相关的深度。在矿物尺度上，N优先分布于长石而非云母中，顺序为：k -长石>斜长石≈黑云母>石英，其平均矿物分布系数为：DN斜长石-黑云母= 1.3±0.6，DN斜长石-黑云母= 2.8±0.6。黑云母与两种长石之间存在较大且接近恒定的平衡同位素分馏因子（平均值分别为Δ15NPlag-Biotite= +7.8±1.2‰和Δ15NKspar-Biotite= +7.9±1.0‰）。Δ15NKspar-Plagioclaseclosely近似于0‰，两种矿物δ15N值与大块岩石δ15N值重叠。这些结果表明，云母结晶在深部岩体中形成了一个贫15n储层。此外，我们的数据表明，长石在地球大陆和海洋地壳的火成岩部分可能是比以前认为的更重要的N宿主。
A significant portion of the continental crust is composed of plutonic igneous rocks. However, little is known about the geochemical behaviour of N between the different minerals during magmatic differentiation. To provide new constraints for the behaviour of N during crust formation, we have characterised the geochemistry of nitrogen (N) in the compositionally zoned calc-alkaline pluton at Loch Doon, SW Scotland. We present N concentration and N isotope values for whole-rock data alongside biotite, plagioclase and K-feldspar mineral separates and assess the degree to which these data preserve equilibrium partitioning during magmatic differentiation. We show that whole rock likely inherited its N contents and δ15N signatures from the initial source composition and that this signature is homogenous at a pluton scale. Whilst the whole-rock data are best explained as crust-derived N in the source, the degree of homogenisation across a pluton scale is inconsistent with empirical N diffusivities, ruling out syn-emplacement crustal assimilation as the source of N. Instead, our data suggest a crustal signature inherited from depth associated with the Iapetus subduction zone. At a mineral scale, we find that N preferentially partitions into the feldspars over mica in this system in the order K-feldspar > plagioclase ≈ biotite > quartz, with average mineral–mineral distribution coefficients of DN plagioclase-biotite= 1.3 ± 0.6 and DN Kspar-biotite= 2.8 ± 0.6. Partitioning is accompanied by a large and near constant equilibrium isotope fractionation factor between biotite and both feldspars (averages are Δ15NPlag-Biotite= +7.8 ± 1.2‰ and Δ15NKspar-Biotite= +7.9 ± 1.0‰). In contrast, Δ15NKspar-Plagioclaseclosely approximates 0‰, where both minerals show δ15N values overlapping with the bulk rock δ15N values. These results show that mica crystallisation generates a15N-depleted reservoir within plutonic rocks. Moreover, our dataset suggests that feldspars might be a more significant host of N in the igneous portion of Earth’s continental and oceanic crust than previously thought.
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