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Abiotic ammonium formation in the presence of Ni-Fe metals and alloys and its implications for the Hadean nitrogen cycle - PubMed

  • ️Tue Jan 01 2008

Abiotic ammonium formation in the presence of Ni-Fe metals and alloys and its implications for the Hadean nitrogen cycle

Alexander Smirnov et al. Geochem Trans. 2008.

Abstract

Experiments with dinitrogen-, nitrite-, nitrate-containing solutions were conducted without headspace in Ti reactors (200 degrees C), borosilicate septum bottles (70 degrees C) and HDPE tubes (22 degrees C) in the presence of Fe and Ni metal, awaruite (Ni80Fe20) and tetrataenite (Ni50Fe50). In general, metals used in this investigation were more reactive than alloys toward all investigated nitrogen species. Nitrite and nitrate were converted to ammonium more rapidly than dinitrogen, and the reduction process had a strong temperature dependence. We concluded from our experimental observations that Hadean submarine hydrothermal systems could have supplied significant quantities of ammonium for reactions that are generally associated with prebiotic synthesis, especially in localized environments. Several natural meteorites (octahedrites) were found to contain up to 22 ppm Ntot. While the oxidation state of N in the octahedrites was not determined, XPS analysis of metals and alloys used in the study shows that N is likely present as nitride (N3-). This observation may have implications toward the Hadean environment, since, terrestrial (e.g., oceanic) ammonium production may have been supplemented by reduced nitrogen delivered by metal-rich meteorites. This notion is based on the fact that nitrogen dissolves into metallic melts.

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Figures

Figure 1
Figure 1

A ternary diagram of naturally occurring Ni-Fe-M (M = Cr, Sb, Pb, Cu, Co, As, Ag, PGE) alloys and their comparison to synthetic alloys used in this study. Analyses of natural samples were adapted from: [19] [22] [109-115]. Each data point represents an electron microprobe analysis expressed in weight percent.

Figure 2
Figure 2

XRD patterns of metals/alloys used in this study. Note the similarity in patterns of Ni, Ni50Fe50 and Ni81Fe19 stemming from the same space group (Fm-3m). Fe0 possesses Im-3m space group.

Figure 3
Figure 3

SEM photographs of unreacted alloys/metals used in this study. A) Ni metal, b) Ni81Fe19, c) Ni50Fe50 and d) Fe metal.

Figure 4
Figure 4

NH4+ formation from N2 in the presence of Ni, Fe metals and alloys at 200°C. The dashed lines correspond to the Ar blank. The concentrations of KCl in the experiments were 459 μmol.kg-1 in A, B and 476 μmol.kg-1 in C, D. Results are normalized to 1 m2 of surface area.

Figure 5
Figure 5

NH4+ formation from nitrite and nitrate, expressed in terms of % conversion. Panels A, B and C correspond to sets of experiments at 200, 70 and 22°C, respectively.

Figure 6
Figure 6

SEM microphotographs of reacted metals and alloys. A) Fe0 reacted in N2(aq) solution depicting coexisting magnetite and Fe-(oxy)hydroxides; B) Detail of magnetite single crystals formed on Fe0; C) clusters of needle-like Fe-(oxy)hydroxide crystals formed on Fe0; D) pseudomorphoses of magnetite after Fe0 in the H2/N2 solution; E) magnetite crystals formed on Ni50Fe50 in the KNO3 solution; F) Ni0 skeletal crystal revealing no change after reaction.

Figure 7
Figure 7

Normalized XRD patterns of unreacted (lower) reacted (upper) Fe0 showing the presence of magnetite (Mt).

Figure 8
Figure 8

Ni 2p3/2 peaks showing the speciation of Ni on the surface Ni metal powder at various stages of the experiment. Note the presence of residual zerovalent Ni on the surface after 24 hour reaction at 200°C.

Figure 9
Figure 9

N 1 s peaks centered around 400 eV documenting the presence of N in the tested alloys and metals after sputtering with Ar+ (Ni0 400.1 eV, Ni50Fe50 399.7 eV, Fe0 398.9 eV).

Figure 10
Figure 10

Hadean hydrothermal flow as a function of total Earth's heat flow (expressed as multiplicities of present day heat flow – PDHF). Each data line thus represents percentage of the total heat flow released through hydrothermal systems at a given value of Hadean heat flow. The shaded area represents assumed realistic scenarios for the Hadean.

Figure 11
Figure 11

NH4+ formation from N2 in Hadean hydrothermal systems. Fluxes are calculated as a function of N2 conversion between 1 and 10%. NH4+ formation from NO2-/NO3- is not included in these calculations.

Figure 12
Figure 12

Estimated increase in NH4+ concentration of the Hadean Ocean (in μmol.L-1) from the hydrothermal N2 reduction reaction per 1 Ma as a function of N2-to-NH4+ conversion percentages, heat flow and percentage of heat released via hydrothermal systems (5, 20 and 80%).

Figure 13
Figure 13

Estimated annual flux of nitrogen from iron meteorites. Different line styles correspond to different average N content of iron meteorites. Shaded areas represent iron meteorite flux during the period of Late Heavy Bombardment.

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