Evidence for dynamic withdrawal from a layered magma body: The Topopah Spring Tuff, southwestern Nevada

Abstract

The Topopah Spring Tuff is a classic example of a compositionally zoned ash flow sheet resulting from eruption of a compositionally zoned magma body. Geochemical and petrographic analyses of whole rock tuff samples indicate that the base of the ash flow sheet and the predominant volume of erupted material consist of crystal‐poor high‐silica rhyolite, with a gradational transition into overlying crystal‐rich quartz latite. However, major and trace element analyses of glassy pumice lumps and microprobe analyses of their silicate and oxide phenocrysts provide closer approximations of the chemical and thermal gradients within the magma body. The gradients inferred from these data indicate that the transition from high‐silica rhyolitic to quartz latitic magma within the chamber was abrupt, rather than gradational, with a distinct liquid‐liquid interface separating the contrasting magma layers. Compositionally and texturally distinct pumice lumps are present throughout the ash flow sheet. The degree of heterogeneity within and among pumice lumps increases with stratigraphic height, becoming most pronounced in the uppermost quartz latite, where the chemical variability among pumice lumps is as great as that of the entire ash flow sheet. These observations are consistent with fluid dynamic models in which the velocity field developed near the entrance region of the vent(s) results in simultaneous withdrawal of magma from all points of a continuously expanding lateral and vertical region within the chamber. The abrupt transition to chemically bimodal pumice types near the top of the ash flow sheet, dominated by those of quartz latitic composition, implies that the interface between the magma layers remained relatively stable until draw‐down breached the interface and preferentially erupted hotter, more mafic magma along with lesser amounts of the remaining high‐silica rhyolitic magma.