Nitrogen solubility in upper mantle minerals

- First use of SIMS to quantify nitrogen in silicate minerals.
- First data set of nitrogen solubility in upper mantle minerals.
- The upper mantle can store ∼20 times more nitrogen than the present atmosphere.
- The oxidized upper mantle has high nitrogen degassing efficiency upon melting.
- Significant nitrogen may have been retained in the solidified magma ocean.

Nitrogen solubility in the upper mantle minerals forsterite, diopside, enstatite and pyrope has been quantified by SIMS measurements of nitrogen-saturated, synthetic samples. The crystals were grown in a 15N-H-O fluid buffered by Ni-NiO, Co-CoO, and Fe-FeO, at 1000-1300 °C and 15-35 kbar in a piston cylinder apparatus. Nitrogen solubility in minerals is significantly affected by temperature, pressure, mineral composition and, in particular, by oxygen fugacity. Nitrogen in all crystals buffered by Ni-NiO or Co-CoO is below detection limit or at most a few μg/g at very high pressures. Concentrations of 5-24 μg/g nitrogen have been quantified in diopside, enstatite and pyrope buffered by Fe-FeO at 1100 °C/15 kbar. Very high nitrogen solubility up to 100 μg/g is observed at the Fe-FeO buffer in enstatite at high-temperature or in Al-bearing enstatite and diopside. The nitrogen solubility in forsterite at the Fe-FeO buffer also clearly increases with temperature and pressure; a maximum solubility of 10 ppm is obtained at 1300 °C/35 kbar. The strong enhancement of nitrogen solubility under reducing conditions may be related to nitrogen dissolution as either NH+4 or as N3− directly replacing O2−. Both mechanisms require some charge compensation, consistent with the enhancement of nitrogen solubility with Al content in enstatite. Our results demonstrate that the reduced lower part of the upper mantle has a large nitrogen storage capacity, and may store ∼20-50 times more nitrogen than the present atmosphere. Therefore, some 'missing' nitrogen may still be retained in the Earthʼs deep, reduced mantle. The calculated nitrogen partition coefficients between upper mantle minerals and silicate melt reveal that an oxidized mantle source would lose almost its entire nitrogen during partial melting, whereas under reducing conditions a considerable fraction of nitrogen could be retained in the residual solids. The high nitrogen solubility in upper mantle minerals at reducing conditions also suggests that solidification of the magma ocean on the early Earth should have retained significant nitrogen, yielding higher N/Ar and N/C ratios in the young upper mantle as compared to the young atmosphere.