Metabasic rocks as important nitrogen carriers to forearc depths: Implications for deep nitrogen cycling

Understanding deep nitrogen (N) cycling better requires investigating the delivery of N to subduction systems via various lithologies. Input to subduction zones through mafic rocks is more voluminous and massive as compared to sedimentary rocks which calls for a thorough investigation of the behavior of N in metabasic rocks. Here we estimate the delivery of N to subduction zones at forearc depths by investigating the geochemistry of amphibolites and epidote-blueschists from the Central Qiangtang Metamorphic Belt in Tibet where the metabasic rocks likely represent the transition from oceanic to continental subduction. The rocks contain 21–147 ppm N with δ¹⁵N values from +1.8 ‰ to +10.0 ‰, and 147 ppm N is the highest that has been reported in a metabasic rock thus far. Given the N abundances for most of the rocks are much higher than those of altered oceanic crust (i.e. basalts, sheeted dikes and gabbros; 6.0 ± 4.7 ppm), the N is likely neither magmatic nor was introduced in the rocks during hydrothermal alteration prior to subduction. This is confirmed by the K₂O/Th versus Ba/Th, Th/U versus Th and Ba/Rb versus K₂O plots where these rocks align with the trend of metamorphic fluid alteration rather than seafloor hydrothermal alteration. A two-step process led to N acquisition in the metabasic rocks. In the first step, the metabasic rocks acquired their N from metasediment-derived fluids during metamorphism in the subduction channel. In the second step, some of the metabasic rocks underwent N loss and concomitant enrichment in ¹⁵N due to devolatilization within the subduction channel. We modeled the N fluxes at forearc depths in 55 modern-day subduction zones via metasedimentary and metabasic rocks assuming their minimum, median and maximum N concentrations to assess their relative importance in delivery of N to subduction zones. We find that metabasic rocks supply comparable fluxes of N to metasedimentary rocks at forearc depths, even though metasedimentary rocks have at least an order of magnitude higher N abundance than metabasic rocks. The importance of metabasic rocks in N delivery reinforces the need to investigate the behavior of N in them from more locations globally to improve our understanding of deep N cycling.