Ammonium-bearing clinopyroxene: A potential nitrogen reservoir in the Earth's mantle

In the pseudobinary system CaMgSi2O6–(NH4)M3+Si2O6, with M = Cr or Al, and NH4OH in excess, multi-anvil experiments at 9.5 to 12.8 GPa, 725 to 750 °C produced NH4-bearing diopside. Incorporation mainly follows the coupled substitution (Ca2+)M2 + (Mg2+)M1 ⇔ (NH4+)M2 + (M3+)M1. Ammonium was identified and quantified by IR spectroscopy. In Cr-bearing diopside we found maximum concentrations in the range of 500 to 1000 ppm of NH4.The storage capacity of mantle clinopyroxenes for ammonium turns them to potential candidates for the nitrogen reservoir in the Earth's upper mantle, and this mechanism also contributes to its water budget. While nitrogen is transported into the mantle via cold slabs through NH4 inherited from sedimentary material, and stored in K-bearing minerals and successor high-pressure phases, nitrogen output from the mantle is through degassing of N2. A probable mechanism for that is that nitrogen is kept as NH4 in clinopyroxene in the Earth's mid- and lower mantle, whereas in the upper part, it is lost due to oxidation to molecular nitrogen. It is most likely that clinopyroxene plays a major role in the long-time, large-scale nitrogen cycle between surface and deep mantle of the Earth.