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

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 SO₄^2-/Na⁺ and ⁴⁰Ar/³⁶Ar ratios of the fluid inclusions suggests mixing of two end-members; hydrothermal fluid with low SO₄^2-/Na⁺ and high ⁴⁰Ar/³⁶Ar ratios, and 3.5 Ga seawater with high SO₄^2-/Na⁺ and low ⁴⁰Ar/³⁶Ar ratios. Values of δ¹⁵N_N2 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 δ¹⁵N_N2 value of the 3.5 Ga atmosphere would have been within the range - 2~0‰, which is similar to the δ¹⁵N_N2 value of the present-day atmosphere (δ¹⁵N_N2 = 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.