Raman spectroscopic study of the transformation of nitrogen-bearing K-cymrite during heating experiments: Origin of kokchetavite in high-pressure metamorphic rocks

Nitrogen-bearing K-cymrite was synthesized in experiments at high pressures but so far has not been found/identified as inclusions in minerals subducted and subsequently exhumed from mantle depths. K-cymrite is a potential carrier of nitrogen; it can store and transport up to 4 wt% of nitrogen in the deep mantle, thus contributing to the global nitrogen cycle. The stability of nitrogen-bearing K-cymrite synthesized at pressure of 6.3 GPa, upon exhumation to the Earth's surface, remains unknown. Here, we report an in situ Raman spectroscopic study of nitrogen-bearing K-cymrite, using Linkam heating and freezing microscope stage (THMS600). The transition induced in this experiment corresponds to the retrograde PT-path of deeply subducted rocks. Our results demonstrate that (i) nitrogen-bearing K-cymrite structure is more stable upon heating than pure H2O bearing specimens (KAlSi3O8 center dot H2O), (ii) phase transformation (e.g., H2O, N-2, and NH3 release) starts at 550 degrees C leading to the formation of kokchetavite (an anhydrous hexagonal polymorph of KAlSi3O8) frequently observed in high-pressure metamorphic rocks, and (iii) Raman spectra of kokchetavite obtained from different precursors (e.g., nitrogen-bearing and H2O-bearing K-cymrite) are identical. Thus, nitrogen-bearing K-cymrite can be considered a potential host phase for nitrogen in subduction-related environments, and it can be preserved as inclusions in refractory minerals during the exhumation of deeply subducted rocks.

Automated summary

Nitrogen-bearing K-cymrite has been synthesized at high pressures, but it has not yet been discovered/identified as inclusions in subducted minerals that have been subsequently exhumed. This study demonstrates the stability and transformation of nitrogen-bearing K-cymrite upon heating, as well as its potential role as a host phase for nitrogen in subduction-related environments.