We present an experimental and petrological study aimed at estimating the pre-eruptive conditions of a Holocene dacitic lava from Volcán San Pedro (36°S, Chilean Andes). Phase-equilibrium experiments were performed at temperatures (T) from 800 to 950°C, and mainly at 200 MPa, but also at 55, 150, and 406 MPa. Oxygen fugacity (fO2) ranged from the Ni-NiO buffer (NNO) to 35 log units above (NNO + 35), and water contents from ~3 to ~6 wt %. We also report several experiments where we added sulphur (01-1 wt % S) to the dacite. The main mineral assemblage of the dacite (hornblende + orthopyroxene + plagioclase) is stable at 200 ± 50 MPa, 850 ± 10°C, with 45-55 wt % H2O in the melt, and at fO2 of NNO + 12 ± 02, in accord with the crystallinity, mineral proportions, and T-fO2 determination from Fe-Ti oxides of the lava. However, biotite, which is also present in the dacite, is stable at these same T-fO2 conditions only in experiments with >01 wt % S added. This result is in accord with the occurrence of pyrrhotite in the lava, and with the presence of S in glass inclusions and biotite (~300 ppm, and up 170 ppm, respectively). Moreover, the zoning patterns and compositions of plagioclase phenocrysts together with the presence of high-temperature minerals (e.g. clinopyroxene) in the lava suggest that the petrological history of the dacite is more complex than a single near-equilibrium crystallization stage, and could be explained by short-lived (<100 years) temperature fluctuations (~50°C) in the magma reservoir.
