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Chemistry of the lavas of the 1959-60 eruption of Kilauea Volcano, Hawaii

1966; United States Government Publishing Office; Linguagem: Inglês

10.3133/pp537a

2330-7102

K. J. Murata, D.H. Richter,

Geology and Paleoclimatology Research

During the 1959-60 eruption of Kilauea Volcano, the outpouring magma was sampled systematically to obtain detailed information on chronological changes in composition.The temporal variations in composition, when correlated with mineralogy, lava temperature, rate of discharge, and other parameters measured during the eruption, provide new insight into the preeruptional crystallization of basaltic magma, entailing intratelluric depositions of various phenocrysts and migration of rest liquids.The hottest and most mafic lavas were produced in the 1959 summit phase of the eruption.The compositional variation (7 to 19 percent MgO) among most summit lavas is ascribable solely to fractional crystallization of olivine (Fata), the first silicate mineral to separate from the primitive magma.A direct relationship between the olivine content of summit lavas and the rate of lava discharge suggests that strong currents of magma erode beds of previously sedimented olivine crystals lying on the uottom of the magma chamber.The coolest, olivine-poor summit lavas contained some small phenocrysts of clinopyroxene and plagioclase as well as olivine.The compositions of these lavas indicated a slight overall accumulation of clinopyroxene, the second mineral to start separating from the cooling magma.The 1960 flank phase of the eruption involved two magmas of different vintage, the cooler, early-1960 magma representing old material that had not been expelled in the previous flank eruption of 1955 and the hotter, late-1960 magma that had probably just migrated down from the summit region along conduits in the east rift zone.Compositional variations (6.1 to 6.6 percent MgO) among the early-1960 lavas indicated an advanced magma that was fractionally crystallizing plagioclase (Anou), clinopyroxene (En4r.sFS12.2W04o.o),and olivine (Fa23.5) in the weight ratio of 11.0:10.0:2.3.This magma had cooled sufficiently to crystallize plagioclase Anou in some abundance.Its composition lay along the principal line of liquid descent for this stage of basaltic differentation.The late-1960 lavas (7 to 13 percent MgO) represented variable amounts of total phenocrysts, consisting of olivine (Fa1~) and clinopyroxene (En4r.sFS12.2Wo,o.o) in a weight ratio of 20:0: 3.0, suspended in a rather uniform, moderately differentiated melt.In accord with the very clear petrographic evidence, lava compositions of the intermediate period of the 1960 eruption indicated a mixing of the early-and late-1960 magmas.Thus, the 1959 summit, the late-1960 flank, and the early-1960 flank magmas clearly showed trends of differentiation resulting from the separation of various proportions of three major min-eral~olivine, clinopyroxene, and plagioclase--at progressively lower temperature.No lavas representing the stage of chemical transition from the 1959 summit magma to the 1960 flank magmas were obtained.Presumably, the transition took place largely during the subterranean migration of the magma away from the summit region.A plausible mechanism for the transition would be the separation, in an olivine-depleted summit magma, of clinopyroxene (EI4o.sFSlo.7WOao.5),plagioclase (Ane4), olivine (Fa1s), and magnetite in the weight ratio of 6.00: 2.25: 0.25: 0.50.The weighted-average composition of the lavas, expelled during the first phase of the 1959 summit eruption, shows 13.9 percent magnesia.The most mafic glass obtained from one of the hottest summit lavas contains 10.0 percent magnesia.These compositions suggest that the primitive Kilauea magma holds at least 10 percent magnesia, but whether it could be even more mafic remains unknown.