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

摘要

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 N-2 and NH4+. A negative correlation between the SO42-/Na+ and Ar-40/Ar-16 ratios of the fluid inclusions suggests mixing of two end-members; hydrothermal fluid with low SO42-/Na+ and high Ar-40/Ar-36 ratios, and 3.5 Ga seawater with high SO42-/Na+ and low Ar-40/Ar-36 ratios. Values delta N-15(N2) from the hydrothermal component vary over a considerable range (-3.0 to + 3.7 parts per thousand), and those of the seawater component are well within this range (i.e., -0.7 to -0.2 parts per thousand). This suggests that the isotope ratio of N-2 dissolved in the 3.5 Ga seawater would have been -0.7 to -0.2 parts per thousand. Since isotope fractionation between N-2 in the atmosphere and N-2 dissolved in seawater is minimal, the delta N-15(N2) value of the 3.5 Ga atmosphere would have been within the range -2-0 parts per thousand, which is similar to the delta N-15(N2) value of the present-day atmosphere (delta N-15(N2)=0 parts per thousand). 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. (c) 2006 Elsevier B.V. All rights reserved.