The behaviour of nitrogen during subduction of oceanic crust: insights from in situ SIMS analyses of high-pressure rocks

Abstract

Understanding the Earth’s geological nitrogen (N) cycle requires an understanding of how N behaves during dehydration of subducted crust. We present the first in situ measurements of N in silicate minerals by secondary ion mass spectrometry, focusing on high pressure rocks representing subducted oceanic crust. We investigate the distribution of N between mineral phases, and combine analyses of N with other trace and major elements in order to constrain the behaviour of N during fluid-rock interaction. The data confirm that white mica (phengite, paragonite) is the primary host for N, containing up to 320 µg/g, whereas minerals including clinopyroxene, amphibole and epidote contain < 5 µg/g N. Chlorite can also contain N (up to 83 µg/g) and may play a previously unrecognised role in subduction zone N cycling. Bulk rock N concentrations estimated from mineral N concentrations and mineral modes are consistent with N concentrations measured by bulk combustion, which confirms that most N is hosted within silicate minerals and not along grain boundaries or in fluid inclusions. Bulk rock N contents correlate with K2O (N/K2O = 19.3 ± 2.0). Using N/K2O ratios and the average K2O of altered oceanic crust, the flux of N subducted in oceanic crust is estimated to be 0.6 - 2.4 × 1011 21 g/yr, which is consistent with but at the lower end of previous estimates. The data were also used to investigate the behaviour of N during fluid-rock interaction. Open system fluid-rock interaction modelling was used to model the evolution of N, B and Li contents during fluid-rock interaction in phengite from a garnet-phengite quartzite. By comparison to data for B and Li, the phengite-fluid partition coefficient for N was estimated to be 0.1–1.5. Separately, the growth of paragonite during fluid-rock interaction in a blueschist was shown to sequester N from phengite and limit bulk N loss to the fluid. The stability of white mica during fluid-rock interaction is therefore critical in controlling the behaviour of N. Nitrogen addition from sediment-derived fluids appears to be an important process in subduction zone rocks. Mafic crust can act as a sink for this N if white mica is stable. This work provides the first natural constraints on the fluid-mineral partitioning behaviour of N at subduction zone conditions and emphasises the complexity of N mobility within subduction zones, with redistribution between different phases and lithologies being important.