Phenocrysts versus xenocrysts in the youngest Toba Tuff: Implications for the petrogenesis of 2800 km3 of magma.
James E. Gardner et al. Geophysical Institute and Department of Geology and Geophysics, University of Alaska, Fairbanks, Alaska 99775-7320, USA. Pages 347-350.
Large explosive eruptions of thousands of cubic kilometers of magma represent the most horrific volcanic hazards to man. Understanding how such large bodies of magma accumulate beneath volcanoes could provide valuable insight into such eruptions. The authors have investigated the petrogenesis of the 2800 km3 of erupted magma at the Toba Tuff ~74 ka in Sumatra, Indonesia, using a combination of experimental petrology and 40Ar/39Ar dating of crystals from the tuff. The authors found that some crystals present in the magma do not crystallize experimentally from the magma at known temperatures and pressures. Those same crystals are also not in isotopic equilibrium with others that do grow experimentally. It thus appears that a large population of crystals in the tuff were foreign to the magma. Common grouping of the foreign crystals into clots implies that they came from the same source, most likely the walls of the Toba magma body. Isotopic ages and diffusive modeling suggest that the source of the foreign crystals was at least 1.5 Ma, and that some were entrained as recently as 10 years before the eruption. A new e
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