Deciphering the surface environment of the primitive Earth from nitrogen isotopes trapped in Archean sedimentary rocks-Investigation of its trapping forms in the rocks.

在这项研究中，利用从太古宙沉积岩中获得的氮同位素信息来寻找原始地球的地表环境。尽管捕获在岩石样品中的氮同位素有可能提供这样的信息，但我们并不总是清楚如何获得和解释原始数据。我们研究了来自澳大利亚西部(来自Kitty‘s Gap和大理石吧)的3.5Ga古老的硅质岩样品，以及来自印度Dhawar Craton的2.7-2.9Ga的带状铁建造样品。采用分步燃烧法提取样品中的挥发性化合物(N_2、Ar、C和H_2O)，并对其元素和/或同位素组成进行分析，结合不同挥发性化合物的信息可以确定或推断氮的三种捕获位置/形态。在Kitty‘s Gap硅质岩中，氮的浓度与放射性成因的~(40)&gt；Ar的浓度有很好的相关性，表明氮被捕获在含K的粘土矿物中。δ~(15)N值在-4~+10之间变化较大，与Ar/N比值呈正相关，表明粘土矿物中存在两种不同类型的圈闭部位。在大理石条斑硅质岩中，观察到氮的δ^&lt；15&gt；N值在+6.0到+7.3permil之间，与碳共生，表明该样品中的氮是以有机形式捕获的。进一步的研究表明，含氮有机质受到无机矿物的保护。在BIF样品中，观察到N的δ^&lt；15&gt；N值在+10 permil左右。似乎存在两种不同C/N比的含氮碳伴生主相。太古宙岩石中不同化学形态的氮同位素组成的区分，有助于我们破译原始生物圈中氮的同位素演化。

In this study, search of the surface environments of the primitive earth was performed using the information of nitrogen isotopes obtained from Archean sedimentary rocks. Though the nitrogen isotopes trapped in the rock samples have potential of providing such information, it is not always clear how we may obtain and interprete the raw data. We studied 3.5 Ga old chert samples from western Australia (from Kitty's gap and Marble bar), and a 2.7-2.9 Ga old samples of the banded iron formation from Dhawar Craton of India. Volatile compounds (N_2, Ar, C and H_2O) were extracted from samples using a stepwise combustion method, and their elemental and/or isotopic compositions were analysed.We were able to specify or infer three trapping sites/forms of nitrogen by combining the information of different volitile compounds. In Kitty's gap chert, nitrogen concentrations were observed to be correlated well with those of radiogenic ^<40>Ar, suggesting that nitrogen is trapped in K-bearing clay minerals. The δ15N values showed a wide range between-4 to +10 permil, and were correlated with the ^<40>Ar/N ratio, suggesting existence of two different types of trapping sites among the clay minerals. In Marble bar chert, nitrogen with δ^<15>N values between +6.0 to +7.3 permil was observed, associated with carbon, suggesting that nitrogen in this sample is trapped in organic form. Furthur investigations imply that the N-bearing organic matter is protected by inorganic minerals. Among the BIF samples, N with δ^<15>N values around +10 permil was observed. There seem to exist two types of N-bearing C-associated host phases with different C/N ratios. Discrimination of the N isotopic components in different chemical forms among the Archean rocks may largly help us to decipher the isotopic evolution of nitrogen of the primitive biosphere.