Petrological and Experimental Constraints on the Pre-eruption Conditions of Holocene Dacite from Volcan San Pedro (36 S, Chilean Andes) and the Importance of Sulphur in Silicic Subduction-related Magmas

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 3·5 log units above (NNO + 3·5), and water contents from ∼3 to ∼6 wt %. We also report several experiments where we added sulphur (0·1–1 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 4·5–5·5 wt % H2O in the melt, and at fO2 of NNO + 1·2 ± 0·2, 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 >0·1 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.