Speciation and isotope ratios of nitrogen in fluid inclusions from seafloor hydrothermal deposits at ∼ 3.5 Ga

Manabu Nishizawa, Yuji Sano, Yuichiro Ueno, Shigenori Maruyama

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Abstract

We report here the results of our study on speciation and isotope ratios of N in fluid inclusions preserved in 3.5 Ga hydrothermal deposits (silica dikes and quartz veins) from the North Pole area of the Pilbara Craton, Western Australia. Crush-leach analysis and Raman microspectrometry revealed that N within the fluid inclusions exists as N2 and NH4+. A negative correlation between the SO42-/Na+ and 40Ar/36Ar ratios of the fluid inclusions suggests mixing of two end-members; hydrothermal fluid with low SO42-/Na+ and high 40Ar/36Ar ratios, and 3.5 Ga seawater with high SO42-/Na+ and low 40Ar/36Ar ratios. Values of δ15NN2 from the hydrothermal component vary over a considerable range (- 3.0 to + 3.7‰), and those of the seawater component are well within this range (i.e., - 0.7 to - 0.2‰). This suggests that the isotope ratio of N2 dissolved in the 3.5 Ga seawater would have been - 0.7 to - 0.2‰. Since isotope fractionation between N2 in the atmosphere and N2 dissolved in seawater is minimal, the δ15NN2 value of the 3.5 Ga atmosphere would have been within the range - 2~0‰, which is similar to the δ15NN2 value of the present-day atmosphere (δ15NN2 = 0‰). This study also suggests that the fluid inclusions contain NH4+ that would have been derived from the seawater and/or mantle at 3.5 Ga. Therefore, kerogens in Archean cherts might have been partly derived from biological assimilation of NH4+ in hydrothermal fluids.

Original languageEnglish
Pages (from-to)332-344
Number of pages13
JournalEarth and Planetary Science Letters
Volume254
Issue number3-4
DOIs
Publication statusPublished - Feb 28 2007
Externally publishedYes

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Keywords

  • Early Archean
  • fluid inclusion
  • N cycle
  • N isotopes
  • N speciation

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics
  • Earth and Planetary Sciences (miscellaneous)

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