Nitrogen mineral hosts in subduction zone metamorphosed ultramafic rocks: Implications for deep Forearc (to Subarc) transfer

Subduction of serpentinized mantle lithosphere delivers nitrogen (N) into the mantle, but the residency and speciation of N therein remain largely unknown. Serpentine, talc, and chlorite have been proposed as likely hosts in K-poor ultramafic rocks due to the minerals' sheet-like structures and the ability for N to reside in interlayer sites. In this study, we explore whether these three minerals are the primary hosts of N in subduction-related mantle lithosphere and metasomatized hybrid rocks (serpentinites, talc schists, and chlorite schists) by analyzing the N concentrations and isotope compositions of paired whole-rock and phyllosilicate mineral separates. Mineralogy of nine samples and sixteen mineral separates from ultramafic units in Syros, Greece, and Pam Peninsula, New Caledonia, were characterized by petrography and X-ray diffraction (XRD) prior to N analyses. Whole-rock N concentrations are from 25 to 102 μg/g and N-isotopes compositions ( δ 15 N air ) range from −0.2 to +6.9‰, whereas mineral separates contain 8 μg/g to 176 μg/g N with δ 15 N of −1.2 to +7.0‰. These results show that ultramafic rocks and phyllosilicate minerals from subduction zone settings contain 10's to 100's of μg/g of N, and have δ 15 N consistent with mixing with a sedimentary-derived fluid. However, mineral separates show variable N and δ 15 N enrichment or depletion relative to their respective whole-rocks, with most separates containing ≤40% of the whole-rock N and systematically lower δ 15 N values. These data indicate that, for the majority of the samples, a considerable fraction of the N is hosted in minerals or sites within the whole-rock not captured in the mineral separates. We combine textural observations and geochemical correlations to propose that the discrepancy between the whole-rocks and their mineral separates can be explained by heterogeneous N distribution among different generations of phyllosilicate minerals within single samples and/or significant N hosted in other minerals and sites (e.g., accessory phases or interstitial phases). Further investigation is required to distinguish between these possibilities, with implications for N speciation and stability during subduction zone metamorphism from the forearc to depths beneath arcs and beyond. • Rock and mineral N concentrations are from 8 to 172 μg/g and δ 15 N from −1.2 to +7.0‰. • Phyllosilicate minerals alone cannot account for whole-rock N-budget in most cases. • Nitrogen composition of the same phyllosilicate mineral may vary by generation and texture. • Nitrogen may reside in accessory phases, complicating N mobility during subduction. • Significant amounts of N could be subducted in such rocks to deep forearcs and beyond.