Nitrogen isotopic compositions in NH₄⁺-mineral-bearing coal: Origin and isotope fractionation

Coals containing ammonium-bearing aluminosilicates usually have highly-elevated concentrations of critical elements, including Ga, Al, U, V, Re, Se, and rare earth elements. Coals from the Moxinpo and Yishan coalfields of southern China and the Daqingshan Coalfield of northern China are unusual in containing abundant ammonium-bearing minerals (buddingtonite and ammonian illite). Understanding the source of N in the ammonium-bearing minerals could provide useful information, not only for the enrichment of the critical elements, but also for the formation of these unusual minerals. A geochemical and nitrogen isotope study was undertaken in order to investigate the origin and behavior of nitrogen in the minerals and associated coals. The N contents decrease as rank increases from low volatile bituminous to semi-anthracite, indicating that substantial N was thermally lost during coalification. The isotopic composition of organic nitrogen (δ¹⁵N_org) ranges from 3.7 to 7.7‰ and increases with coal rank in the studied samples. However, this does not indicate that the isotope fractionation of organic N was due to the coal rank advance; rather, it is largely caused by the depositional environments of peat accumulation and coal-precursor plant assemblages. δ¹⁵N values of inorganic N (δ¹⁵N_ino) in NH₄⁺-minerals generally have a wider range (5.1 to 12.4‰) than those of organic matter (3.7 to 7.5‰) in coal and are commonly higher than those of δ¹⁵N_org in the same coal sample. The N in the ammonium-bearing aluminosilicates was derived from the decomposition of the organic matter of the coals that were subjected to increased geothermal gradient (and fluid flow) as a consequence of igneous intrusion or infiltration of high-temperature hydrothermal fluids. The N released from organic matter was transported by high-T fluids, from which ¹⁴N was preferentially converted into volatile matter and ¹⁵N readily remained in the fluids. Subsequently NH₄⁺ in the high-T fluids substituted for K⁺ in pre-existing minerals or NH₄⁺ − bearing high-T fluids reacted with pre-existing kaolinite in the coals, leading to more positive δ¹⁵N values for the NH₄⁺-bearing minerals than those of the organic matter in these coals. Therefore, the δ¹⁵N_ino values in coal, in a wider context, could be potentially used to estimate the degree of high-T heating on coal metamorphism, to trace the pathway of hydrothermal fluids circulating within the coal basin, and to provide useful information on the sources of critical elements that are enriched in coals.