Volcanic Processes in Volatile-Rich Magmas: Constraints From Phase Equilibria and the Role of the Fluid Phase

Brown University, Providence RI

Investigators

Abstract

Volcanic processes in volatile-rich magmas: constraints from phase equilibria and the role of the fluid phase. P.I. Malcolm J. Rutherford, Geology Department, Brown University ABSTRACT The research proposed here involves three projects, all directed at developing a more complete understanding of how processes such as magma mixing, crystallization, crystal-melt separation, and degassing occur in subvolcanic gas-rich magma systems. The tasks involve a range of analytical work on samples collected during field study, and on samples prepared experimentally under controlled conditions. The first project is an investigation of the pressure effect on hornblende-plagioclase-melt equilibria with a major objective being an improved geobarometer for hornblende-bearing volcanic rocks. A second objective is to better understand magmatic processes and the role of a magma storage zone at Mount St Helens. This volcano is one of the most active in the USA, and study of this center represents a great opportunity to gain an understanding of how volcanic processes work in such magma systems, given the variety and number of eruptions at this center. The second research task in this proposal involves experiments to investigate a theory regarding the possible role of a volcanic fluid phase transfer as an explanation for the origin of oxidation and sulfur enrichment in silica-rich magma systems. This task is one in which fundamental data will be obtained on the composition of volcanic gases as a function of magmatic conditions and the composition of coexisting crystals and melt. Data such as this is fundamental in one of the main methodologies of volcanic eruption prediction, that of monitoring the composition of the gases released. The task is focused on how this process occurred at Mount Pinatubo in 1991, but similar injections of basaltic magma into pre-existing silica-rich magma storage zones appear to happen regularly, and it is often theorized that the mafic intrusion triggers the eruption of the silicic magma. The process of two magma interactions in a subvolcanic magma storage zone is one of fundamental processes that might control the changes in behavior of an erupting volcano. The final task is somewhat exploratory, building on observations that have recently been made in studies of the Soufriere Hills eruption and the much older and larger Fish CanyonTuff eruption. Cyclically zoned hornblende crystals in the eruption products appear to record a recurring process in the pre-eruption magmas. The composition zones carry information about the processes involved in their formation and, potentially about the time span of their growth. The research proposed is a preliminary attempt to extract this information using a detailed compositional and modeling study of these crystals. The project will involve the P.I., and three graduate students, two of whom are women.