Pre-eruption compositional gradients and mixing of andesite and dacite magma erupted from Nevado del Ruiz Volcano, Colombia in 1985

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A small volume of mixed andesite and dacite magma was ejected as pumice fall, pyroclastic flow and pyroclastic surge during the 13 November 1985 explosive eruption of Nevada del Ruiz volcano in Colombia. Whole-rock compositions range from 59 to 63% SiO2 and are classified as medium- to high-K andesites and dacites. Three types of pumices occur in the fall deposit: (1) homogeneous, crystal-rich dacite; (2) hetereogeneously mixed dacite and andesite; and (3) homogeneous andesite with abundant microlites (volumetrically dominant). Matrix glasses show a distinct compositional gap between 68 and 71% SiO2. The gap is similar within heterogeneous pumices of type (2) and between homogeneous pumices of types (1) and (3). In contrast, melt inclusions in phenocrysts preserve a continuous compositional spectrum between the two end-member matrix glass compositions of the homogeneous andesite and dacite. Modelling of fractionational crystallization involving removal of plagioclase, clinopyroxene, orthopyroxene and Fe-Ti oxides can produce the liquid trend defined by melt inclusions. A compositional gap in the Ruiz pumice matrix glasses is attributed to the establishment of a two-layer zonation of the magma chamber prior to eruption. Collection of a fractionated liquid (dacite) at top of the chamber resulted from side-wall crystallization and fractionation of an andesitic magma body. Independent convection in each layer led to two thermal regimes which stabilized two dominant melt compositions. The large contrast in SiO2 of these liquids reflects the shallow slope of cotectic boundaries in composition/temperature space of calc-alkaline magmas of these compositions. Intermediate liquids trapped by melt inclusions represent volumetrically small, random sampling of the fractionation process occuring at the margin of the andesite magma. Mixing of andesite and dacite magma occurred during the explosive eruption as draw-up of denser andesite magma through the overlying dacite took place. Fluid-dynamic calculations indicate that discharge conditions of the Ruiz eruption and the viscosity ratio of the magmas was suitable for the production of mixed pumices even though the flow was in the laminar regime. These observations corroborate experimental work by Blake and Campbell (1986) on the mixing of magmas with contrasting viscosites. Sulfur yield from the eruption, manifested by a large SO2 plume detected by satellite, and high adsorbed sulfur content on tephra fallout, indicate total sulfur emission of 4.7 × 108 kg S, or about an order of magnitude larger than can be accounted for by degassing of the erupted magma. If a separate sulfur-rich vapor phase existed in the magma reservoir, as implied by our results, then its pre-eruption volume was of the order of 4 to 7 vol. % of the erupted magma. © 1990.

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Journal of Volcanology and Geothermal Research