MAGMATIC PROCESSES REVEALED BY CRYSTAL TEXTURES, ELDEN MOUNTAIN LAVA DOME, NORTHERN ARIZONA

Textures in extrusive igneous rocks record information about the crystallization history of magma during ascent and surficial cooling. Quantitative textural and chemical analyses of glass and plagioclase microlite populations have been used to provide insights into the crystallization history of the Elden Mountain lava dome in northern Arizona. Major element chemistry indicates that all Elden Mountain lavas were derived from the same parent magma. Glass compositions vary from 70-84 wt% SiO2; however there is no relationship between sample age and SiO2 content. The total crystal content of Elden Mountain lavas varies from ~60-100%. Previous studies have shown that the maximum eruptible crystallinity of dacite magma is 60%, thus a significant amount of post-emplacement crystallization occurred in many of the Elden lavas. The degree of crystallization appears to depend on the position of each sample in relation to the surface of the flow, and the resulting cooling rate. Samples collected near the original flow surface cooled quickly and were less crystalline. Samples further from the surface of the flow were insulated and cooled slower, which resulted in higher crystal contents. All samples contain the crystal assemblage: plagioclase, alkali feldspar, pyroxene, quartz, Fe-Ti oxides, and apatite. In addition, scarce amphibole microphenocrysts were present in about half of the samples. Some amphibole crystals have reaction rims, which form as a result of water loss from the coexisting melt during ascent. Reaction rims thicknesses vary between 0-40 microns. Based on experimental data from Mount St. Helens, amphibole reaction rims of this thickness correlate to a minimum magma ascent rate of 17 meters per hour.