VESICLE BANDS IN A DIKE: INVESTIGATING MAGMA DYNAMICS IN A MOUNT TAYLOR DIKE, NEW MEXICO

During magmatic flow, vesicles may grow in response to solidification, decompression, and coalescence, as well as deform in response to shear, thus making vesicle analyses a useful tool for interpreting ephemeral magmatic dynamics. In the region of Mt. Taylor (part of the Jemez Lineament of New Mexico), a radial dike swarm is partially exposed. This study focuses on one well exposed, small (~1m width), trachybasalt dike (35°08’03”N 107°39’41”W), intruded into Cretaceous Mancos Shale. The dike is fine grained, heavily jointed, and hydrothermally altered with amygdules common. Vesicles, in general, coarsen towards the center, which is consistent with slower cooling and more time for growth and coalescence. Vesicles are not uniformly distributed across the dike, rather, vesicles are clustered primarily within six vesicle bands, each a few centimeters wide, symmetrically placed about the center plane, and aligned roughly parallel to the dike contacts. Along the edge of vesicle bands, there is an abrupt shift in vesicle size and aspect ratio. Large, less deformed vesicles within the vesicle bands (average size: 1.5 E-2 cm², average aspect ratio: 1.5) occur directly next to smaller, more deformed vesicles outside of the vesicle bands (average size: 5 E-3 cm², average aspect ratio: 2.1). Because larger bubbles relax more slowly than smaller bubbles, the presence of the band is not reflective of differential relaxation, and rather, is a preserved shear boundary. These findings suggest three major pulses in flow in the Mt. Taylor Dike, each resulting in approximately 30 cm growth in width. Thermal Modelling of dike pulses suggests cooling of the order of ~6 hours per pulse. This timescale is consistent with pulses observed in eruptions at other stratovolcanoes (i.e. Fuego in Guatemala, 1975-1977). The sum of the evidence presented here on vesicles distributed within the Mt. Taylor Dike is consistent with an origin of magmatic pulses.