Abstract: Volatile species analysis of fluid inclusions in metamorphic quartz veins from the Llanbedr Formation, North Wales, show nitrogen to be an important component. Microthermometric and Laser Raman analysis indicates that the N₂ is resident in a generation of very N₂-rich inclusions and demonstrates the presence of both N₂-rich and aqueous fluids during metamorphism of the Llanbedr Formation. N₂-rich fluids do not appear to have been present in adjacent lithologies. Isotopic analysis of N₂ in fluid inclusions and in the slates indicates that the N₂ in the slates (thought to be present as NH4+ substituting for K⁺ in muscovite and for Na⁺ in albite) was probably originally derived from organic matter in the sediment and subsequently released to the fluid phase during metamorphism.

Mineral-fluid and fluid-phase equilibrium calculations show that the mineral assemblage in the slates could be in equilibrium with either N₂-rich or aqueous fluid depending on redox conditions. The N₂-rich and aqueous fluids in the veins could, therefore, have been trapped at different times under different conditions, though their coexistence as immiscible fluids is a possibility.