Can the evolution of nitrogen cycle be traced by the N isotopic composition in mica?

A significant portion of nitrogen present in sedimentary rocks has a biological origin, trapped either in organic form, or as ammonium ion substituting potassium in mica. Mica might preserve biological N isotopic signatures (δ¹⁵N) in the geological record, allowing the evolution of the N cycle to be traced. However, diagenetic or metamorphic events can modify the pristine N isotopic signature leading to inaccurate interpretations. For example, devolatilization of the rock leads to a reduction in the N abundance and a contemporary increase of the δ¹⁵N because ¹⁴N escapes faster than ¹⁵N. We measured N isotopic compositions in whole rock, mica and feldspars separates from two Archean suites of cherts: 3.5 Ga Kitty's Gap and North Pole sequences in Pilbara, Western Australia and from the 3.45 Ga Hooggenoeg Fm, Barberton Greenstone Belt, South Africa. N was compared with the argon elemental and isotopic composition, because a relation between NH₄⁺, which replaces K⁺ and radiogenic ⁴⁰Ar*, which is produced by electron capture of K⁺ is expected. Both Pilbara and Barberton cherts show a clear correlation between N and ⁴⁰Ar*, confirming the occurrence of a common speciation. K-Ar dating of the Hooggenoeg Formation mica and feldspars give ages of 2.1 and 1.1 Ga, respectively, indicating that loosely-bounded noble gas ⁴⁰Ar* is lost from the host mineral during known metamorphic events. Observed correlations between ⁴⁰Ar* and N suggests that nitrogen, although more strongly bounded as ammonium is also lost, possibly leading to isotopic fractionation. Measured δ¹⁵N values, however, are relatively constant (+8.1±0.6% for whole rock and +10.9±1.2% for mica) and do not display an inverse correlation with N abundances. This suggests either 1) that isotopic fractionation is not produced during N loss or; 2) that a process other than devolatilization fractionate N isotopes. Measured δ¹⁵N values are at levels far greater than those expected for Early Archean kerogens (0±2%) thus suggesting that fractionation took place but probably is induced by a process other than devolatilization. Step-combustion analyses of N and Ar from Kitty's Gap cherts reveals the presence of an inverse correlation between δ¹⁵N values and the ⁴⁰Ar*/N ratios indicating mixing between two isotopically distinct components. The first, released at temperatures between 400° and 700° C from hydrous minerals, has a δ¹⁵N value close or below 0% and is accompanied by radiogenic Ar. The second, void of radiogenic Ar, is released at temperatures >800° C from anhydrous phases and has a δ¹⁵N value of +6 to +8%. The first component is likely ammonium replacing K in mica while the second is possibly ammonium adsorbed in-between negatively charged layers of clay minerals. Upon dehydration, the inter-layer site will be "closed", and loosely adsorbed cations are finally trapped in the mineral while noble gas Ar is lost. The higher δ¹⁵N in mica is possibly due either to (1) trapping of N representing a later (post-Archean) event, or; (2) fractionation of N with negative δ¹⁵N value due to partial release of N from the adsorption site. Mixing between different aliquots of these two components might possibly explain the observed N isotopic variability among micas in the Archean.