| Popis: |
The current flux of nitrogen into the mantle in subduction zones is about three times its amount outgassing at mid-ocean ridges, arc and intraplate volcanoes, i.e., some efficient nitrogen hosts and carriers should exist in slabs. The K+ → (NH4+) substitution in silicate minerals is possible only within limited redox-favorable parts of slabs. Whether nitrogen can be transported and immobilized in the mantle as part of solids by some redox-independent mechanisms? The experimental study of the muscovite-NH3-N2-H2O and eclogite+muscovite-NH3-N2-H2O systems at 6.3-7.8 GPa and 1000 to 1200°C shows that NH3- and N2-rich K-cymrite can be stable in metapelite and act as a redox insensitive carrier of nitrogen to mantle depths >200 km in downgoing slabs. This ability is related to its unique clathrate structure that can accommodate three species of nitrogen: N2 and NH3 molecules in cages and (NH4)+ substituting for K+, while imprisoned N2 and NH3 were first discovered in cages of ultra-high pressure minerals. The storage capacity K-cymrite with respect to nitrogen increases from 2.9 to 6.3 wt.% with increase of fO2 from ~IW to ~NNO, at the N2/(NH3+N2) ratio in fluid from 0.1 to 0.9. Comparison of equilibrated muscovite and K-cymrite synthesized at 7.8 GPa, 1070°C, and fO2 ~IW demonstrates that the clathrate mechanism of nitrogen entrapment by aluminosilicates (in the form of N2 and NH3 molecules) is much more efficient than the K+ ® (NH4+) substitution even in strongly reduced conditions. The presence of an N-bearing fluid in the studied systems stabilizes the K-cymrite structure. Muscovite does not convert to K-cymrite in the absence of NH3-N2-bearing fluid within 7.8 GPa and 1070-1120°C. Our estimates of normalized volume per non-hydrogen atom show that N2-bearing cymrite is the densest in the series of K-cymrite with cages filled to different degrees: K-CymNH3 > K-CymH2O > K-CymN2 and is thus the most stable among cymrite-type compounds under high pressure.The research was performed by a grant of the Russian Science Foundation (16-17-10041). |
