Continuous mixing of crystal mush and replenished magma in the ongoing Unzen eruption

Zoning profiles of magnetite phenocrysts in the successively effused dacite of the ongoing Unzen eruption from 1991 were analyzed to estimate the time scale of magma mixing. The dacite was formed by mixing of relatively high- and low-temperature (T) end-member magmas. The magnetite phenocrysts derived from the low-T magma are reversely zoned by the mixing with high-T magma. A diffusion calculation for reequilibration of the reverse zonings gives the time interval from magma mixing to quenching. The observations lead to a model wherein the highly crystallized remnant magma of the preceding activity has been mixed with the newly injected hot magma of similar bulk composition just prior to the effusion. The proposed mechanism implies that this type of magma mixing is an inevitable process in periodically erupting polygenetic volcanoes. -from Author