Magmatic N2 degassing dominates nitrogen loss during sediment subduction and granitoid genesis

Abstract

Subducting sediments contain much more abundant nitrogen (N) than the other slab components. The fate of sedimentary N along subduction directly determines the quantity of surficial N that can be released back to the atmosphere/crust or further delivered to the deep mantle, which has a strong impact on the long-term evolution of the N budgets and isotopic signatures of Earth's major reservoirs. Previous studies on the fate of subducted sedimentary N have been intensively focused on assessing the extent of metamorphic N devolatilization from metasedimentary rocks varying from ultrahigh-pressure eclogites facies (subducted to as deep as 90 km) in cold subduction zones to medium-pressure amphibolite facies in extremely hot subduction zones. However, the fate of sedimentary N after these metamorphic stages, particularly during sediment melting and subsequent magmatic crystallization, has not been examined so far. Here, we present the N data of the Cretaceous peraluminous granitoids in the Oman-United Arab Emirates area, which were formed by melting of subducted pelagic sediments followed by intrusion and crystallization in the lithospheric mantle before the entire lithospheric section was obducted to form the Oman-United Arab Emirates ophiolites. The 27 studied samples show surprisingly low N contents (4.0 - 13.7 ppm; mean: 6.9 ± 2.3 ppm; 1σ) and a narrow δ¹⁵N range (mostly in the range of –1.4‰ to +2.2‰; mean: +0.4 ± 1.5‰; 1σ). Data modeling indicates that the N in these granitoids contains little mantle contribution and was predominantly inherited from their source rocks. Compared with potential source rocks, these granitoids display orders of magnitude lower N contents but comparable δ¹⁵N values, indicating that the significant N loss during the genesis of these granitoids was associated with little N isotope fractionation. This suggests that magmatic N₂ degassing (with minor N isotope fractionation) rather than metamorphic N devolatilization (with much larger N isotope fractionation) is the dominant mechanism driving the N loss from subducted sediments.