An investigation of symplectite-rimmed olivine and magmatic processes during the 2006 eruption of Augustine Volcano, Alaska

Abstract

The frequency and pattern of eruptions at Augustine volcano makes it an ideal natural laboratory. The 2006 eruption produced deposits that were petrologically and compositionally similar to the 1976 and 1986 eruptions. Olivine phenocrysts have up to a 500-pm thick rim of intergrown orthopyroxene and magnetite. Petrologic data constrained Augustine magmatic conditions, which were then recreated in the laboratory to determine what conditions favor the growth of symplectite reaction rims on olivine. Oxygen fugacity (fO2) of Augustine magmas, recorded by Fe-Ti oxides, is -8.51 to -10.72 log units, which is slightly above the Re-ReO2 (RRO) buffer. Experiments were set up to investigate the effect of high fO2 on the formation of olivine rims. They involved seeding rhyodacite powder with high- to med-Mg olivine and placing it into Rene-style furnaces at 850°C and 150 MPa for 1 to 4 weeks at RRO. The full symplectite was not recreated, but high fO2 changed the rim texture and increased growth rate by nearly a factor of 10, relative to similar experiments run at Ni-NiO. Experiments and petrologic data suggest the natural symplectite rims took years to grow and that they did not result from mixing immediately prior to or during the 2006 eruption.