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

Abstract

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 • C produced NH 4 -bearing diopside. Incorporation mainly follows the coupled substitution (Ca. 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.

Nitrogen isotopes in ophiolitic metagabbros: A re-evaluation of modern nitrogen fluxes in subduction zones and implication for the early Earth atmosphere

“…Accordingly, available evidence is consistent with N occurring mainly as NH 4 þ in Ca-Na minerals in low-strain metagabbros and in phengite in some high-pressure veins. The occurrence of NH 4 þ in mafic and ultramafic rocks is in full agreement with the recent conclusions from high-pressure experiments on clinopyroxene (Watenphul et al, 2009(Watenphul et al, , 2010) and a study on mantle peridotite xenoliths (Yokochi et al, 2009).…”

The relationship between mantle pH and the deep nitrogen cycle

“…For example, molecular nitrogen 69 (N 2 0 ) and ammonia (NH 3 0 ) are both neutrally charged, and therefore highly incompatible in most 70 mineral phases (Brooker et al, 2003 (Li et al, 2013;Watenphul et al, 2010). Therefore, the behavior of nitrogen 76 is predictably governed by the speciation of nitrogen where neutrally charged compounds are -77 thermodynamically speaking -highly incompatible (Blundy & Wood, 2003), an assertion that has 78 been demonstrated experimentally (Brooker et al, 2003).…”

Modeling ammonia–ammonium aqueous chemistries in the Solar System’s icy bodies

“…Ammonium silicates on Earth are primarily associated with siliceous hydrothermal and meta-sedimentary rocks. Ammonium silicates in the mantle are thought to have supplied N 2 and NH 3 to the primordial atmosphere (Eugster and Munoz, 1966;Andersen et al, 1995;Watenphul et al, 2010). Direct evidence for ammonium silicates in the modern mantle includes ammonium in lamprophyres (Wu et al, 2004).…”