The October 1974 basaltic tephra from Fuego volcano: Description and history of the magma body

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The largest of a series of 20 vulcanian eruptions of Fuego volcano between 1944 and 1976 occurred in October 1974 in four distinct 4–17-hour pulses over a 10-day period. The eruption produced more than 0.2 km3 of pyroclastic high-Al2O3 basalt (equivalent to > 0.1 km3 of void-free magma), quenched at a temperature of about 1050°C. Early erupted magma was rich in plagioclase and poor in mafic phenocrysts, magmas erupted next in sequence were poor in all phenocryst phases, while the final magmas were rich in all phenocryst phases. These changes, and variations in 8 major and 21 minor elements can be explained by crystal/liquid fractionation of high-Al2O3 basalt. Crystals in Fuego's magma contain inclusions of glass with an average H2O content of about 3%, an amount that suggests that crystallization began at a depth of ⩾ 5 km. Petrographic data, observed extrusion rates, glass inclusion analyses, regional geological features, and various geophysical considerations collectively suggest that crystallization and differentiation occurred within a vertical, dike-like conduit during a period of a few days to several months. The Fuego parental basalt contained 5% MgO, 10% FeO and about 0.8% K2O3 it began crystallization at 1030 ± 50°C with plagioclase (An95−89), olivine (Fo76−66) and magnetite (4.3–15% TiO2) the principal phases. Plagioclase was apparently rafted upward by bubbles within the magma body. Tidal triggering of crystal growth, magma ascent and eruption are suggested.

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© 1978 Published by Elsevier B.V. Publisher's version of record: https://doi.org/10.1016/0377-0273(78)90027-6

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Journal of Volcanology and Geothermal Research