Noble Gases in Archean Barites: Precise Determination of the Isotopic Fractionation of Atmospheric Xenon 3.48 Ga Ago

IF 2.9 3区 化学 Q2 CHEMISTRY, MULTIDISCIPLINARY
Guillaume Avice*, Helge Mißbach-Karmrodt, Félix Vayrac and Joachim Reitner, 
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Abstract

Following the evolution of the composition of the atmosphere informs on the entire geological evolution of our planet. The discovery that Archean atmospheric xenon was isotopically fractionated compared to modern atmospheric xenon paved the way for using this noble gas as a tracer of hydrogen escape on the primitive Earth. The curve of the evolution of the isotopic composition of atmospheric xenon remains, however, poorly defined. Recent studies proposed that the evolution was discontinuous with brief episodes of escape and fractionation of xenon separated by up to several 100 Ma long pauses. Similarly, some major unknowns remain regarding the progressive depletion of xenon in the atmosphere due to the proposed escape mechanism. In this study, we report the noble gas elemental ratios and isotopic compositions of noble gases released from a ca. 3.48 Ga old barite sample from the Dresser Formation (North Pole, Australia) by stepwise crushing in high vacuum. All samples released xenon enriched in light isotopes relative to heavy isotopes compared to modern atmospheric xenon but with various degrees of isotopic fractionation. Krypton is enriched in heavy isotopes relative to light isotopes in some crushed samples. After correction for Kr and Xe loss, results show that 3.48 Ga ago atmospheric xenon was fractionated by −19.1 ± 1.8‰ per atomic mass unit (u–1). This value is more negative than that reported previously for 3.3 Ga old atmospheric xenon. A new curve for the evolution of the isotopic composition of atmospheric xenon is proposed. Xenon is also enriched relative to Kr in the gas released from the measured samples. While this is consistent with the scenario of a progressive selective escape of xenon from the Archean atmosphere, the exact abundance of Xe in the paleoatmosphere remains elusive.

太古宙重晶石中的稀有气体:3.48 Ga前大气氙同位素分馏的精确测定
随着大气成分的演变,我们的星球的整个地质演化就会发生变化。太古代大气中的氙与现代大气中的氙进行同位素分馏的发现,为利用这种稀有气体作为原始地球上氢逸出的示踪剂铺平了道路。然而,大气中氙同位素组成的演化曲线仍然模糊不清。最近的研究提出,演化是不连续的,氙的逃逸和分离的短暂事件被长达100 Ma的停顿分开。同样,由于提出的逃逸机制,关于大气中氙的逐渐耗尽仍然存在一些主要的未知因素。本文报道了澳大利亚北极Dresser组约3.48 Ga古重晶石样品在高真空中逐步破碎后释放的稀有气体元素比和同位素组成。与现代大气氙相比,所有样品释放的氙相对于重同位素富集于轻同位素,但存在不同程度的同位素分馏。在一些破碎的样品中,氪的重同位素相对于轻同位素富集。在对Kr和Xe损失进行校正后,结果表明,3.48 Ga前大气中的氙每原子质量单位(u-1)的分馏率为- 19.1±1.8‰。这个值比先前报道的3.3 Ga年的大气氙的值更负。提出了一种新的大气氙同位素组成演化曲线。在测量样品释放的气体中,氙也相对于Kr富集。虽然这与太古代大气中氙逐渐选择性逸出的情况是一致的,但古大气中氙的确切丰度仍然难以捉摸。
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来源期刊
ACS Earth and Space Chemistry
ACS Earth and Space Chemistry Earth and Planetary Sciences-Geochemistry and Petrology
CiteScore
5.30
自引率
11.80%
发文量
249
期刊介绍: The scope of ACS Earth and Space Chemistry includes the application of analytical, experimental and theoretical chemistry to investigate research questions relevant to the Earth and Space. The journal encompasses the highly interdisciplinary nature of research in this area, while emphasizing chemistry and chemical research tools as the unifying theme. The journal publishes broadly in the domains of high- and low-temperature geochemistry, atmospheric chemistry, marine chemistry, planetary chemistry, astrochemistry, and analytical geochemistry. ACS Earth and Space Chemistry publishes Articles, Letters, Reviews, and Features to provide flexible formats to readily communicate all aspects of research in these fields.
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