First-principles calculations of equilibrium nitrogen isotope fractionations among aqueous ammonium, silicate minerals and salts

Abstract

Nitrogen isotopes are a robust tool to study geological nitrogen cycle between Earth’s reservoirs. However, the application of nitrogen isotope system to understanding geological processes has been limited by the lack of constraints on equilibrium isotope fractionation factors between mineral and fluid and between mineral pairs. Here, we use first-principles methods to calculate the nitrogen isotope fractionations among aqueous ammonium, ammonium- and/or nitrate-bearing salts, and ammonium-bearing silicate minerals that commonly occur in Earth’s lithosphere. Our results show a first-order, large nitrogen isotope fractionations between the nitrate group and the ammonium group, with 15N being more enriched in the nitrate group. In detail, the nitrogen isotope fractionations among nitrate group minerals (NaNO3, KNO3, Ba(NO3)2, NH4NO3) are very small. The nitrogen isotope fractionations are also small among the ammonium group (e.g., aqueous ammonium, ammonium salts, phyllosilicate, and tectosilicate minerals) except inosilicate minerals (e.g., diopside and jadeite), which are however significantly more enriched in 15N than the other ammonium-bearing minerals. These results suggest that nitrogen isotopes may serve as a robust geothermometer only when the rock contains coexisting ammonium and nitrate minerals, or contain diopside and/or jadeite together with other ammonium-bearing silicate minerals. Given that common silicate minerals in crustal rocks (clays, micas, feldspars) do not significantly discriminate nitrogen isotopes, nitrogen isotopes can thus be used as a sensitive tool to trace material source and infer geochemical processes that may cause isotope disequilibrium, such as metamorphic devolatilization, hydrothermal alteration, crust-mantle interaction. Our results also demonstrate that authigenic clay minerals can inherit the nitrogen isotopic signature of organic matter and aqueous ammonium and thus can serve as an environmental proxy. However, it is crucial to distinguish detrital minerals from authigenic minerals in sediments when reconstruct marine and lacustrine environments in the deep time (Archean and Proterozoic).