Hydrogen-isotope systematics in degassing basaltic magma and application to Indonesian arc basalts

We have developed a predictive model for hydrogen-isotope shifts during degassing of basaltic–andesitic magma that takes the variation of water species with water content and temperature into account. OH⁎ (water dissolved as hydroxyl) is the dominant dissolved water species for the range of naturally observed magmatic water contents (0–8 wt.%) and temperatures (>800 °C), and increases in importance with higher temperatures and lower water contents. We used our hydrogen fractionation model and published experimental data to derive values for hydrogen-isotope fractionation factors between melt and H2O vapor of 103 ln αOH⁎ = 34.9 + 16.7 × 106/T2 for OH⁎ and 103 ln αH2O⁎ = − 30.3 + 16.7 × 106/T2 for H2O⁎ (water dissolved as molecular water). A value of zero for the latter, as was assumed in some recent and earlier studies, is incompatible with our model results. As a consequence of the progressively increasing OH⁎/H2O⁎ ratio in the melt during degassing, the bulk hydrogen fractionation factor also increases continuously. In contrast, the influence of temperature on the bulk fractionation factor ΔDvapor–melt is relatively small.

We applied the hydrogen fractionation model to thirty subaerial lavas of basaltic composition from eight volcanoes located in the Indonesian arc system. All samples are extensively degassed, as average contents of residual water in the melt are only ∼ 0.15 wt.%. Whole-rock δD values range from − 109‰ to − 57‰ VSMOW (average = − 89‰). Oxygen isotope compositions of the same samples range from + 5.3‰ to + 6.7‰ VSMOW and are close to MORB values, indicating that crustal contamination or post-eruptive weathering was insignificant. Hydrogen isotopes fractionated markedly during degassing, and our data agree best with dominantly closed-system exsolution of magmatic water during magma ascent and storage, followed by syn-eruptive open-system degassing from depths of less than a few hundred meters. Magmatic water was derived from a mantle source with a typical D-enriched arc isotopic signature. Within-suite systematics suggest variations in initial δD and δ18O values for some volcanic centers.