Inclusions in Mount St. Helens dacite erupted from 1980 through 1983.

Abstract

Inclusions of plutonic, metavolcanic and volcanic rocks are abundant in dacite pumice and lava from the 1980-1986 eruption sequence at Mount St. Helens. Point counts of inclusions exposed in talus blocks from the dome from 1980 through 1983 show that inclusions form approximately 3.5 vol% of the lava. Eighty-five percent of the inclusions are medium-grained gabbros with an average diameter of 6 cm. Additional rock types include quartz diorite, hornfelsic basalt, dacite, andesite and vein quartz. Disaggregated inclusions are common and define shear planes within the dome. These fragmented inclusions may significantly contaminate analyses of the dacite. The gabbroic inclusions are of four distinct types, all with mineral assemblages consistent with crystallization pressures of less than 9 kb. Textures and major-element compositions indicate that most of the gabbroic inclusions are cumulates. The most abundant inclusion type is laminated gabbronorite, which contains up to 9% interstitial glass, derived from partial melting. The presence of quartz veins and hornblende-bearing veins within sheared zones in the laminated gabbronorite indicates that the source of these inclusions was holocrystalline rock that had been penetrated by water-rich fluids. The gabbronorite contained sufficient water to be susceptible to partial melting when the magma that fed the 1980-1986 eruption sequence was emplaced nearby. Various types of gabbroic inclusions, including the laminated gabbronorite, are common in Mount St. Helens lavas of approximately the last 3000 years. This coincides with the interval in which Mount St. Helens first erupted basalt and basaltic andesite lavas. These observations, together with the fact that the gabbroic inclusions are compositionally unlike any of the Tertiary intrusive rocks in the Mount St. Helens area, strongly suggest that the inclusions are related to the introduction of basalt to the Mount St. Helens magmatic system. The source of the gabbros could be a layered mafic pluton formed through crystal accumulation from multiple batches of basaltic magma emplaced at mid- to upper-crustal depths beneath the volcano. The prevalence of explosive eruptions at Mount St. Helens may play a part in bringing the inclusions to the surface. The eruptive products of the cataclysmic eruption of May 18, 1980 contain notably fewer inclusions than the pyroclastic flows and dome lavas erupted subsequently. This suggests that the May 18 eruption shattered conduit wall rock that was subsequently stoped into the magma and carried to the surface later in the eruption series.

73 samples from this reference