Fluid loss to the fore-arc controls the recycling efficiency of nitrogen in subduction zones

Nitrogen is a crucial element for life on Earth as it dominates the planet's atmosphere and is present in the deep geological cycle. Previous studies have shown that nitrogen quickly degasses during melting, leaving behind a nitrogen-depleted residue. Nitrogen-rich igneous rocks are rare, whereas diamonds commonly contain appreciable nitrogen: these probably originate from domains enriched with recycled nitrogen from the surface. However, the extent to which the surficial nitrogen cycle interacts with the deeper Earth and the proportion of nitrogen transported into the mantle by subduction zone processes are poorly understood. Previous experimental studies have largely focused on hot subduction zones and suprasolidus conditions, in which siliceous melts are assumed to be the primary carriers of nitrogen. In this study, we present 22 new experiments in the pressure and temperature range of 2–2.7 GPa and 300–800 °C to investigate the partitioning of nitrogen between muscovite mica and a fluid phase in the subsolidus regime of modern, cold to intermediate temperature subduction zones. D_{N(Mica/Fluid)} ranges from 0.1 to 0.2 and shows a weak negative trend with respect to P/T, meaning that nitrogen partitions preferentially into fluid at high pressure and low temperature. This contrasts with D_{N(mica+melt/fluid)} in the suprasolidus regime of subduction zones, which shows a strong positive dependency on P/T, mainly because nitrogen is concentrated in the melt. We determined that the loss of nitrogen to subduction fluids is higher in cold subduction zones at >600 °C (antigorite breakdown), where the slab surface remains subsolidus beyond the volcanic arc front. Increasing D_{N(mica+melt/fluid)} with P/T favours nitrogen retention in the mineral and melt phases under suprasolidus conditions. Recycling of nitrogen to the deeper mantle is probably rare on the modern Earth, whereas the return of nitrogen to the mantle is favoured in hotter subduction zones since melt-rock interaction favours the formation of mica-bearing metasomes that could act as a reservoir for nitrogen. Hence, nitrogen return to the mantle was probably more effective in the young Earth. Additionally, we explored the nitrogen source for diamond-forming fluids using the experimentally determined partition coefficients. While the shift from compatible to incompatible behaviour of NH₄ in mica with increasing P/T limits surface-to-mantle nitrogen cycling to the upper few hundred kilometres, it enables a transport pathway for nitrogen to reach lower cratonic depths and the source regions of diamond-forming fluids. This pathway is favoured by intermediate to hotter subduction zone temperatures that applied in early to intermediate ages of the Earth.