Dumortierite and tourmaline from the Barchi‐Kol diamond‐bearing kyanite gneisses (Kokchetav massif): A Raman spectroscopic study and petrological implications

Abstract

AbstractTwo minerals of boron, dumortierite and tourmaline, have been studied in a diamondiferous kyanite gneiss (Barchi‐Kol area, Kokchetav massif, Northern Kazakhstan) by Raman spectroscopy and electron microprobe (EMP) analysis. Dumortierite and tourmaline (almost pure schorl end‐member) coexist within the sample, with blue/purple dumortierite inclusions hosted by kyanite porphyroblasts and blue tourmaline crystals occurring as inclusions in garnet. Optically, dumortierite appears almost identical to kyanite host and can be easily overlooked in thin sections during optical and scanning electron microscopic observations. Both boron minerals show a very strong anisotropy, and the most prominent Raman spectra can be obtained if the electric field vector E is parallel to the c axis of the dumortierite and tourmaline crystals. The Raman spectra of the investigated dumortierite are very similar to those of the synthetic dumortierite with a pronounced excess of boron (up to 0.26 [4]B), although the EMP examination revealed only one grain of the natural dumortierite studied here to have a very low boron surplus (up to 0.03 [4]B). Neither Raman spectroscopic study nor EMP analysis detected [4]B in tourmaline. The findings of dumortierite and tourmaline in the diamondiferous gneisses provide important implications for the presence of a boron‐rich fluid or melt during subduction/exhumation processes.