Experimental Constraints on Dacite Magma Storage beneath Volcan Quizapu, Chile

Volcan Quizapu, Chile, is an under-monitored volcano that was the site of two historical eruptions: an effusive eruption in 1846-1847 and a Plinian eruption in 1932, both of which discharged similar to 5 km(3) (dense rock equivalent) of lava and/or tephra. The majority of material erupted in both cases is trachydacite, nearly identical for each event. We present H2O-saturated, phase equilibrium experiments on this end-member dacite magma, using a pumice sample from the 1932 eruption as the main starting material. At an oxygen fugacity (fO(2)) of similar to N NO + 0.2 (where N NO is the nickel-nickel oxide buffer), the phase assemblage of An(25-30) plagioclase + amphibole + orthopyroxene, without biotite, is stable at 865 +/- 10 degrees C and 110 +/- 20 MPa H2O pressure (P-H2O), corresponding to similar to 4 km depth. At these conditions, experiments also reproduce the quenched glass composition of the starting pumice. At slightly higher P-H2O and below 860 degrees C, biotite joins the equilibrium assemblage. Because biotite is not part of the observed Quizapu phase assemblage, its presence places an upper limit on P-H2O. At the determined storage P-H2O of similar to 110 MPa, H2O undersaturation of the zd magma with X-H2O(fluid) = 0.87 would align P-total to mineral-based geobarometry estimates of similar to 130 MPa. However, X-H2O(fluid) < 1 is not required to reproduce the Quizapu dacite phase assemblage and compositions. A second suite of experiments at lower fO(2) shows that the stability fields of the hydrous silicates (amphibole and biotite) are significantly restricted at NNO-2 relative to NNO + 0.2. Additional observations of Quizapu lava and pumice samples support the existing hypothesis that rapid pre-eruptive heating drove the effusive 1846-1847 eruption, with important refinements. We demonstrate that microlites in the end-member dacite lavas are consistent with in situ crystallization (during ascent), rather than transfer from an andesite. In one end-member dacite lava, newly identified reverse zoning in orthopyroxene and incipient destabilization of amphibole are consistent with small degrees of heating. Our work articulates a clear direction for future Quizapu studies, which are warranted given the active nature of the Cerro Azul-Descabezado Grande volcanic axis.