Olivine-melt thermometry and hygrometry applied to calk-alkaline mafic lavas from the Cascade arc: agreement with volatile analyses of olivine-hosted melt inclusions

The Cascade arc undergoes subduction of relatively young and warm lithosphere, leading to the expectation that basalts formed in the arc mantle wedge are relatively hotter and less hydrous compared to those in other arcs (e.g., Marianas). This prediction is supported by analyses of olivine-hosted melt inclusions from the literature. Here, we test application of the olivine-melt thermometer and hygrometer, developed by Pu et al. (2017; 2021), to six mafic Cascade lavas (48.4-54.8 wt% SiO2; 10.2-7.3 wt% MgO) from the southern Cascades (northern California), as well as an adjacent LKOT (low-K olivine tholeiite; 0.08 wt% K2O), which has geochemical characteristics like mid-ocean ridge basalts. For four of the six Cascade arc samples, H2O analyses in olivine-hosted melt inclusions are available (1.2-3.0 wt%; LeVoyer et al., 2010; Walowski et al., 2015). The thermometer/hygrometer is based on the partitioning of Mg and Ni between olivine and melt. Because DNiol/melt is independent of dissolved H2O in the melt, the Ni-thermometer gives the actual temperature of olivine crystallization, whereas the Mg-thermometer gives the anhydrous temperature. The difference between the two (∆T = TMg-TNi) gives the depression of the liquidus due to dissolved H2O, and an experimentally calibrated ∆T-H2O relationship gives wt% H2O in the melt. This study also tests the hypothesis that olivine phenocryst growth in the Cascade basalts occurred rapidly during ascent and that the most Mg-rich olivine analyzed in each sample represents the liquidus composition in each sample. When the most Mg-rich olivine is paired with a liquid represented by the whole-rock composition (average Fe3+/FeT of ~0.21; ∆NNO = 0.7), the resulting Fe2+-MgKD (olivine-melt) values are consistent with those predicted by the model of Putirka (2016; with an average of 0.34). In addition, Mn-MgKD values range from 0.19-0.24, which are within 0.03 of those predicted by the equilibrium model of Blundy et al. (2020). Resulting temperatures and dissolved H2O contents at the onset of olivine growth range from 1127-1171 °C and 1.6-3.6 wt% H2O for the calc-alkaline samples, whereas the LKOT was distinctly hotter (1217°C) and less hydrous (0.4 wt% H2O). These results are fully consistent with the direct analyses of H2O in olivine-hosted melt inclusions from the literature.