THE ROLE OF ANHYDRITE AND PYRITE IN IGNEOUS SYSTEMS

The viscosity of magma is linked to the cause of volcanic eruptions. Volatiles, such as sulfur, have an important role in influencing the eruptivity of volcanoes. Anhydrite is most commonly an evaporite mineral, however it recently has been found in igneous rocks. The purpose of the project was to determine the role of sulfur as a contributor to volcanic eruptions, and to try to stabilize igneous anhydrite in the melt. In the experiments, 6 wt% anhydrite was added to granodiorite and obsidian compositions to determine the effects of S in the melt at 900°C, 200 to 240 MPa for 5 to 7 days. Hematite/magnetite (HM) buffers were attached to the capsule in order to attempt to increase the oxygen fugacity of the capsule; without the buffers the oxygen fugacity was near NNO. Under both of these conditions anhydrite was not stabilized. Samples from the buffered runs are highly vesicular and have little sulfur in the glass, even though the initial compositions contained 1.5 wt% S. The buffered capsules had pyrite crystallized in vesicles. Since little sulfur was dissolved in glass, the vesicles probably were filled with a sulfur-rich gas, which combined with Fe from the melt to form pyrite. The partial pressure of sulfur gas in the system is greatly reduced by the precipitation of pyrite. Therefore, under these conditions, the pressure from sulfur gas has less of a role in causing a volcano to erupt. Additional experimental results will be discussed from coupled experiments and shorter-duration, supplementary fO₂(HM) buffered experiments that were conducted and are now being examined.