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

In the pseudobinary system CaMgSi 2O 6–(NH 4)M 3+Si 2O 6, with M = Cr or Al, and NH 4OH in excess, multi-anvil experiments at 9.5 to 12.8 GPa, 725 to 750 °C produced NH 4-bearing diopside. Incorporation mainly follows the coupled substitution (Ca 2+) M2 + (Mg 2+) M1 ⇔ (NH 4 +) M2 + (M 3+) 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 NH 4. 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 NH 4 inherited from sedimentary material, and stored in K-bearing minerals and successor high-pressure phases, nitrogen output from the mantle is through degassing of N 2. A probable mechanism for that is that nitrogen is kept as NH 4 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.