Experimental determination of H₂O loss from melt inclusions during laboratory heating: Evidence from Raman spectroscopy

Experiments have been conducted to quantify H₂O loss from melt inclusions during laboratory heating using Raman spectroscopy. Quartz-hosted melt inclusions from the early-erupted plinian stage of the Bishop Tuff were heated to 800 °C and 1 kbar for 4 to 1512 h (63 days). Previous studies had shown that unheated melt inclusions from this unit of the Bishop Tuff contain 4.8-6.5 wt.% H₂O. Many Bishop Tuff melt inclusions fluoresce under visible (514 nm) Raman excitation. To minimize fluorescence interference, a method was developed to analyze silicate glasses and melt inclusions using an UV (244 nm) excitation source. The Raman microprobe was calibrated using silicate glass standards with known H₂O contents, and the results indicate that the UV excitation data produce a calibration line with a different slope compared to that produced using visible excitation. The inclusions show insignificant H₂O loss when heated for less than 12 h, while approximately 75% of the original H₂O was lost after 1512 h. The rate of H₂O loss decreases after a few hundred hours, suggesting either a change in H₂O speciation or in the mechanism of H₂O loss. Our results suggest that most silicic melt inclusions maintain their original H₂O concentration if they are not heated for more than about 12 h during laboratory studies.