Supplementary files for "Metabasic Rocks as Important Nitrogen Carriers to Forearc Depths: Implications for Deep Nitrogen Cycling"

Understanding deep nitrogen (N) cycling better requires investigating the inputs 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 constrain the input of N to subduction zones 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 d¹⁵N 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 input fluxes of N 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 at forearc depths to metasedimentary rocks, even though metasedimentary rocks have at least an order of magnitude higher N abundance than metabasic rocks. This reinforces the need to investigate the behavior of N in metabasic rocks from more locations globally to improve our understanding of deep N cycling.

Data files:

• SI Table 1: Calculation of fluxes of nitrogen through oceanic crust and sediments in subduction zones worldwide.
• Supplementary Document: Petrography of samples analyzed in this study. The image captions are the sample numbers referenced in the main text of the publication. Also included is the devolatilization modeling of the metabasic rocks from this study.
• Table 1: Spreadsheet of data in Table 1 in the main text of the publication.