Experimental studies and petrologic constraints suggest that H2O contents of deep, primitive melts in subduction settings may reach up to >15 wt % H2O (e.g., Krawczynski et al. 2012). But curiously, mafic glasses preserved in melt inclusions—commonly the best available tool to analyze H2O contents of melts—seem to be limited to much lower values, mostly <6 wt% (Plank et al. 2013). This apparent conundrum suggests that empirical results defy predictions and challenges our view of H2O in subduction-related magmatism. To address this issue, Gavrilenko et al. (2019) experimentally tested...

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