Daly Gaps at South Sister, Oregon, USA, generated via partial melting

Abstract

We conducted a detailed petrologic study on six crystal-poor (< 12 vol%) dacites from South Sister volcano (OR) to determine the conditions that favor the formation and extraction (i.e., eruption) of intermediate magmas in a system that features prominent Daly Gaps. We present whole rock compositions for six dacites (63.2–65.1 wt% SiO₂) that form the base of South Sister and six mafic flows (51.3–56.1 wt% SiO₂) that erupted in the peripheral volcanic field. The dacites are saturated in plagioclase + orthopyroxene + clinopyroxene + ilmenite + titanomagnetite; hornblende is not observed. Evaluation of trace element concentrations in whole rock analyses reveals that end member mixing (basalt with rhyolite) is not the primary petrogenetic mechanism for generating the dacites, and comparison of compositions of minerals in the dacites with those equilibrated in experiments suggests that the majority of crystals in the dacites are plausible phenocrysts. Application of multiple thermometers to pyroxene and Fe–Ti oxide compositions in the dacites shows that crystallization in the dacites initiated at elevated temperatures (980–1045 °C) and that modest zonation in the phenocryst assemblage can be attributed to the effects of minor closed system crystallization ± degassing ± cooling. When pre-eruptive temperatures, plagioclase and whole rock compositions from the dacites are incorporated into the plagioclase hygrometer, we obtain maximum pre-eruptive H₂O contents that range from 2.1 to 3.3 wt%; these H₂O contents are consistent with those associated with plagioclase-in curves determined from phase equilibrium experiments. Through review of glasses equilibrated in experimental run products in the literature, we find that dacitic melt compositions with temperatures similar to the South Sister dacites (i.e., 980–1045 °C) are restricted to partial melting experiments on MgO-rich (> 5 wt% MgO) basalts and that dacitic melt compositions most common in partial melting experiments conducted under low H₂O fugacities. Partial melting of basalt resolves the compositional gaps between the basalts and dacites in the South Sister volcanic record because, during partial melting, dacites (1) can be generated without requiring the formation of a compositional intermediate, (2) are produced in eruptible melt fractions (e.g., 20–25%), (3) are favorably erupted if they occur in fluid undersaturated conditions (owing to the positive Clapeyron slope in the absence of a fluid phase). Though the efficiency of partial melting is debated because of the amount of heat required to generate melt, it is a viable process beneath South Sister because of the elevated geothermal gradient, high pre-eruptive temperatures of primitive basalts erupting in the surrounding areas and the recent history of volcanism in the region. Our results suggest that eruption of dacites saturated in two pyroxenes, absent hornblende, may signal partial melting.