Bromine speciation in hydrous silicate melts at high pressure

Abstract

Br speciation in hydrous silicate melts at high pressure has been investigated up to 7.6GPa using X-ray absorption spectroscopy (XAS) at the Br K-edge in a Paris–Edinburgh press. Br in silicate melts is surrounded by an average of 6 Na cations, a number slightly increasing with pressure (5.8 to 6.6), with a Br–Na distance increasing from 3.49 to 3.72Å. Two oxygens, either from a water, an –OH molecule or from the tetrahedral silicate network, with an average Br–O distance of 1.80Å, form the closest coordination shell around Br ions. The persistence of an alkali shell around Br, in a structure similar to crystalline NaBr, throughout the pressure range investigated shows that Br can be retained in the melt structure at relatively high pressure and supports the idea of its deep recycling. Finally, our results confirm that Br could be efficiently degassed with water at low pressures and that Br may also have been efficiently degassed along with water during the early stages of an oxidized magma ocean.