德国矿物学研究所A.N. Martin的研究团队，最新研究探明了加拿大2.7Ga年Abitibi绿岩带古代“黑烟囱”氮同位素分馏的机制 。相关论文于2024年1月3日发表在《地质学》杂志上。

研究人员对来自加拿大Abitibi绿岩带的2.7 Ga的变质沉积物中报道的异常¹⁵N富集进行了详细探究，据称与富含¹⁵N的太古代大气有关。鉴于该地区的沉积物与大规模火山成因块状硫化物矿床同生，研究利用Cu和Zn含量来追踪热液循环对氮同位素分馏的影响。高达+23‰的高δ¹⁵N_bulk值与Cu-Zn成矿有关，而未矿化的富有机质页岩显示出低得多的δ¹⁵N_bulk值和δ¹⁵N_kerogen值。

此外，研究还发现δ¹⁵N_bulk与δ¹⁵N_kerogen之间存在一个高达17‰的偏差，这与富有机质脉体晚期侵位过程中有机质结合氮的加入有关。研究表明，以前报道的高δ¹⁵N值可以用生物和非生物机制来简单解释，而不是富含¹⁵N的大气。至关重要的是，这两种机制都需要热液流体中存在NH₄⁺，这支持了热液排放是新太古代海洋生物重要营养来源的假设。

据悉，由于来自太古代(元-)沉积物的有限数据和原生生物特征的潜在变化，早期地球上的生物氮(N)循环是一个谜。

附：英文原文

Title: Mechanisms of nitrogen isotope fractionation at an ancient black smoker in the 2.7 Ga Abitibi greenstone belt, Canada

Author: A.N. Martin, E.E. Stüeken, J.A.-S. Michaud, C. Münker, S. Weyer, E.H.P. van Hees, M.M. Gehringer

Issue&Volume: 2024-01-03

Abstract: The biological nitrogen (N) cycle on early Earth is enigmatic because of limited data from Archean (meta-)sediments and the potential alteration of primary biotic signatures. Here we further investigate unusual ¹⁵N enrichments reported in 2.7 Ga meta-sediments from the Abitibi greenstone belt, Canada, purportedly related to a ¹⁵N-enriched Archean atmosphere. Given that sediments from this region are contemporaneous with large-scale volcanogenic massive sulfide deposits, we utilize Cu and Zn contents to trace the effects of hydrothermal circulation on N isotope fractionation. We show that high δ¹⁵N_bulk values as high as +23‰ are associated with Cu-Zn mineralization, whereas unmineralized organic-rich shales exhibit much lower δ¹⁵N_bulk and δ¹⁵N_kerogen values. Moreover, we find a large offset between δ¹⁵N_bulk and δ¹⁵N_kerogen of as much as 17‰ and relate this to the addition of organic-bound N during the late-stage emplacement of organic-rich veins. We conclude that the previously reported high δ¹⁵N values are most parsimoniously explained by biotic and abiotic mechanisms rather than a ¹⁵N-enriched atmosphere. Crucially, both mechanisms require the presence of NH₄⁺ in hydrothermal fluids, supporting the hypothesis that hydrothermal discharge was an important nutrient source for Neoarchean marine life.

DOI: 10.1130/G51689.1

Source: https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G51689.1/632075/Mechanisms-of-nitrogen-isotope-fractionation-at-an