Volatiles and magmas: about the chemical effects on the H+ and D+ in silicate melts and their effects on the ?D isotopic ratio

The speciation of water, dissolved as silicate-bonded OH- groups and water molecules H2Omol in magmas, influences its impact on magmas’ physico-chemical properties. By studying hydrous binary silicate glasses M2O-SiO2 (with M = Li, Na, K), we show that, in addition to temperature, the chemical composition of the melt influences the OH-/H2Omol partitioning as well as the environment of OH- groups. This seems related to the effect of the M+ ionic field strength on the dissociation of H2Omol as OH- groups. Accordingly, changing the ionic field strength of H+, by substituting it with D+, produces similar effects on the proton environment. As a result, it appears that the interpretation of ?D isotopic data, for tracking the source of subduction fluids for instance, must considerate the possible occurrence of chemical effects affecting the ?D. Furthermore, this study suggests that alkali-rich and potentially alkaline-earth rich magmas can present interesting degassing and viscous behavior, because of ionic field strength effects on water speciation and environments.