Reaction between Forsterite and Nitrogen Fluid at High Pressure and High Temperature

Abstract: Behavior of nitrogen in the deep Earth still remains unrevealed. Geochemical data suggest that substantial amount of nitrogen could be stored in the deep Earth. In this study, reactions between forsterite (Mg₂SiO₄) and molecular nitrogen (N₂) were investigated at high pressure and high temperature using laser-heating diamond anvil cells (DACs). Pressure in the DAC was estimated from Raman spectra of nitrogen before heating and the initial pressure was set at 5 GPa. Pelleted sample of powder forsterite or a single crystal of forsterite was loaded in the DAC with N₂ fluid. A carbon dioxide laser (λ = 10.64 μm, <100 W) and a fiber laser (λ = 1.019 μm, <100 W) were used to heat forsterite in the temperature range from 1300 to 3500 K. An SEM image on the surface of the recovered forsterite crystal after the laser heating showed a stepwise texture which strongly suggests the dissolution of forsterite into the N₂ fluid. The EDS chemical mapping showed that Mg-rich area and Si-poor area overlapping each other, which suggests the preferential dissolution of MgO component and its precipitation from the N₂ fluid. X-ray diffraction patterns of the powder and single crystal forsterite samples after the reaction showed reflections assignable to orthopyroxene (MgSiO₃) and periclase (MgO). The present experimental results indicate that Mg₂SiO₄ incongruently melts into MgSiO₃ and MgO in N₂ fluid. Moreover, N1s XPS spectra collected from a single crystal of forsterite after the reaction with N₂ fluid revealed three components assignable to NH4+N₂, and N^3–. The present study provides a new clue to the reaction between forsterite and molecular nitrogen under the upper mantle condition.