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

Abstract

AbstractNitrogen‐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·H2O), (ii) phase transformation (e.g., H2O, N2, and NH3 release) starts at 550°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.