FROTHY MAGMA: A POSSIBLE CAUSE FOR FLOW BANDING IN SOUTH MOUNTAIN RHYOLITE UNIT, VALLES CALDERA NM.

Flow banding in the South Mountain rhyolite unit in Valles Caldera, New Mexico, likely formed due to variation in bubble content and density within a single magma, not from the mixing of two distinct magmas. Valles Caldera is part of the Jemez volcanic field, which has experienced continuous volcanism for the past 13 million years, and occurs on the west shoulder of the Rio Grande Rift at its intersection with the Jemez lineament. Studying this collapsed caldera has been crucial to the understanding of major cataclysmic caldera-forming eruptions andmechanics of volcanism in the Western United States. South Mountain exhibits distinctive black and white flow banding and this research centered on analyzing mineral grains within each colored band using a JEOL Superprobe. Microprobe analysis of samples from the light and dark bands of this volcanic dome show no compositional variation in grains of biotite, sanidine or glass, therefore magma mixing could not be the cause of flow banding in this rhyolite unit. Instead of inferring that the mixture of basaltic magma with silicic continental crust caused the flow banding, as would have been the case for magma mixing, an alternative mechanism for the banding phenomenon may simply be density variations within a single magma. A variable amount of bubbles in the magma could explain this proposed density variation without inducing mineral heterogeneity. White “frothier” bands with more bubbles and less density could have resided in the top of the magma chamber. As these “frothy” bands were likely degassing, they erupted from the magma chamber and mixed with denser, less “frothy” dark bands. Variation in bubble content within a single magma could plausibly have played a role in forming the light and dark flow banding of this rhyolite unit.