Daly gaps at South Sister Volcano, Oregon, USA generated via partial melting

Abstract:

We conducted a petrologic study on six crystal-poor (<12 vol%) dacites from South Sister Volcano (OR) to determine conditions that favor the formation and extraction (eruption) of intermediate magmas (63.2-65.1 wt% SiO₂) in a volcano with prominent Daly Gaps. The dacites are saturated in plag + opx + cpx + ilm + tmt; hbl/bte are not observed. Evaluation of dacite geochemistry shows that mixing cannot produce the dacites; comparison of mineral compositions with those equilibrated in experiments demonstrates that the majority of crystals in the dacites are likely phenocrysts. Maximum temperatures derived from Fe-Ti oxide thermometry and all possible pairs of oxides range from 994-1045°C; temperatures in each sample span a 100-150° range. Opx-thermometry yields maximum temperatures (945-1023°C) that are consistent with Fe-Ti oxide thermometry. Application of a plagioclase hygrometer yields pre-eruptive H₂O contents that range from 1.2-4.0 wt% at the time of plagioclase crystallization. We find that dacites with compositions and temperatures similar to those from South Sister are only found in partial melting experiments on basalts. Partial melting experiments routinely produce dacites at high temperatures due to the peritectic behavior of minerals, which fix the melt composition based on phases present (residual) during melting. Partial melting resolves compositional gaps in at South Sister because it produces eruptible dacites from basalts (1) without requiring the formation of a compositional intermediate, (2) in eruptible melt fractions, (3) that are fluid undersaturated. Fluid undersaturated decompression purges nuclei and crystals from melts, due to the positive Clapeyron slope, which facilitates eruption. Partial melting is viable beneath South Sister because of the elevated regional geothermal gradient and 10⁶y of continuous volcanism preceding South Sister. Lastly, eruption of dacites saturated in two pyroxenes, absent hornblende, may signal partial melting.