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

作者：Watenphul Anke^*; Wunder Bernd; Wirth Richard; Heinrich Wilhelm

来源：Chemical Geology, 2010, 270(1-4): 240-248.

DOI：10.1016/j.chemgeo.2009.12.003

• Ammonium
• Clinopyroxene
• Nitrogen cycle
• High pressure
• IR spectroscopy
• Earth's mantle

In the pseudobinary system CaMgSi(2)O(6)-(NH(4))M(3+)Si(2)O(6), with M = Cr or Al, and NH(4)OH in excess, multi-anvil experiments at 9.5 to 12.8 GPa, 725 to 750 degrees C produced NH(4)-bearing diopside. Incorporation mainly follows the coupled substitution (Ca()M2)(2+) + (Mg(2+))(M1) double left right arrow (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.

• 出版日期2010-2-15