Petrological and Experimental Constraints on the Pre-eruption Conditions of Holocene Dacite from Volcán 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 200MPa, but also at 55, 150, and 406MPa. Oxygen fugacity ( fO2) ranged from the Ni---NiO buffer (NNO) to 3 5 log units above (NNO þ 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 (hornblendeþ orthopyroxeneþ plagioclase) is stable at 200 50MPa, 850 10 C, with 4 5---5 5 wt % H2O in the melt, and at fO2 of NNOþ 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 with40 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 (5100 years) temperature fluctuations ( 50 C) in the magma reservoir.