Experimental Phase Relations of Water- and Sulfur-Saturated Arc Magmas and the 1982 Eruptions of El Chich n Volcano

The equilibrium phase relations of two volcanic rocks from the subduction-related Mexican Volcanic Belt have been determined with an argon-pressurized internally heated vessel. One rock is the trachyandesite erupted from El Chichón Volcano in 1982; the other is a primitive basalt erupted from Jorullo Volcano in 1759. A simplified synthetic equivalent to the trachyandesite was also investigated in lesser detail. All charges were saturated with hydrous vapor and a sulfur-bearing mineral. Temperature ranged from 800 to 1000°C, pressure from 1 to 4 kb, and fo2 was controlled by four different solid oxygen buffers in a double gold capsule configuration: fayalite-magnetite-quartz (FMQ), Ni-NiO (NNO), manganosite-hausmanite (MNH), and magnetite-hematite (MTH). Pyrrhotite was the only sulfur-bearing mineral observed in charges buffered under FMQ and NNO, whereas anhydrite crystallized under the more oxidizing MNH or MTH; both of these observations are consistent with those of earlier workers. With increasing temperature and pressure, SiO2 and K2O decreased in the experimental melts, whereas Al2O3 and CaO increased. Sulfur solubility in silicate melts was low (<0·1 wt% equivalent SOt3) for pyrrhotite-saturated charges, but significantly greater (to 1·3 wt.% SOt3) when anhydrite was present. Sulfur solubility in anhydrite-saturated melts showed strong positive dependence on both temperature and Pvapor. Sulfur amounted to some 2·5 wt.% (SOt3) of the total ejecta during the 1982 El Chichón eruptions, and the original magmatic sulfur content was in the range 1·25–2–5 wt% SOt3. Extrapolations of experimental temperature and pressure dependences for sulfur solubility indicate that such concentrations could be contained in a hydrous, oxidized, basaltic parent melt generated under Benioff zone conditions. During ascent through the upper mantle and crust, the sulfur solubility limit of the melt would continuously decrease; in response, most of the sulfur would be transferred from the melt to anhydrite crystals and a separate gas phase. Trachyandesite pumices erupted from El Chichón in 1982 contained both pyrrhotite and anhydrite at a temperature of ∼800°C. The composition of the natural pyrrhotite yielded an fo2 estimate ∼1 log unit above the NNO buffer. Based on compositional variations in the experimental melts with temperature and pressure, the composition of the matrix glass in the 1982 pumices indicates equilibration of the magmatic liquid at about Ptotal=Pvapor=2 kb just before eruption. At that time, sulfur in El Chichón trachyandesite was about equally partitioned between anhydrite microphenocrysts and some 20 vol.% gas phase in which H2S was probably the dominant sulfur-bearing species. The melt then contained only 0·05 wt.% SOt3, consistent with experimental solubility limits at 800°C and Pvapor=2 kb.