UNVEILING HOSTS FOR NITROGEN TRANSPORTATION AND STORAGE IN META-SERPENTINITES

Nitrogen (N) is the most abundant element in the atmosphere and is ubiquitous in Earth’s surface environments from oceans to soils to the bases that make up DNA. Subduction of oceanic crust and serpentinized lithosphere delivers N into the mantle, which is volumetrically the largest N reservoir on Earth. However, the mechanisms of N delivery and the amount present in lithospheric reservoirs are uncertain. Previous studies reveal that N concentrations and δ¹⁵N of oceanic serpentinites overlap with those of subducted low- to high-grade meta-serpentinites, suggesting significant N retention during prograde metamorphism. This retained N may then be transported into the mantle. While these data support bulk-rock serpentinites as a N host, the mineral residency of N in these rocks is unknown and has implications for how deeply it is recycled during subduction. Serpentine minerals are a likely host due to their phyllosilicate structure, where N can be adsorbed onto the mineral structure. In this study, we explore whether common phyllosilicates in serpentinites (i.e., serpentine, talc, and chlorite) are the primary hosts of N. Seven bulk rock samples and 18 mineral separates from ultramafic mélange matrix units in Syros, Greece, and New Caledonia were characterized by petrography and X-ray diffraction (XRD). Mineral separates are variably intergrown with differing proportions of serpentine (antigorite and lizardite), talc, chlorite and, in one sample, carbonate. Bulk-rock N concentrations range from 25 to 102 ppm and δ¹⁵N_air ranges from -0.2 to +6.9‰. Phyllosilicate mineral separates have N concentrations from 8 ppm to 176 ppm and δ¹⁵N from -1.2 to + 7.0‰. On an individual sample basis, the mineral separates show varying N and δ¹⁵N enrichment or depletion relative to the bulk-rock. These data indicate that, for the majority of our samples, N must be hosted in minerals or sites within the bulk rock other than the sheet silicates that are present. These results suggest that the stability and sites for N of minerals other than phyllosilicates may be important to consider for deep N cycling at subduction zones.