Kr environment in feldspathic glass and melt: A high pressure, high temperature X-ray absorption study

Noble gases are used to trace the evolution of the Earth's atmosphere and magmatic processes. However little is known about their solubility mechanisms in silicate melts or glasses, whereas it could be fundamental to interpret these data. We report here the first in situ X-ray absorption spectroscopy data at the Kr K-edge on a feldspathic Kr-bearing glass and melt (0.5 wt% Kr) up to 2.7 GPa and 1086 degrees C. Kr fitting of the EXAFS signal shows that Kr is surrounded by oxygen atoms. Two Kr-O distances are identified: 2.49 +/- 0.1 A and 3.32 +/- 0.1 A. Results can be interpreted as two Kr populations. Kr surrounded by O atoms at 3.32 +/- 0.1 A suggests a filling of the large cavities available in the silicate network, in agreement with previous studies. On the contrary, Kr surrounded by O atoms at 2.49 +/- 0.1 A, and the observed +0.6 eV shift of the edge position at high pressure, suggest Kr bonding to O atoms and Kr oxidation inside cages formed by the largest aluminosilicate rings (i.e. similar to 12-membered-rings). Present results show that heavy noble gases incorporation in silicate melts can no longer be considered as a passive filling of the voids.