Magma recharge and crystal mush rejuvenation associated with early post-collapse Upper Basin Member rhyolites, Yellowstone caldera, Wyoming

Abstract

The Upper Basin Member rhyolites are the oldest known post-collapse rhyolites of the third Yellowstone caldera. They were erupted near the caldera's resurgent domes between 516 ± 7 and 473 ± 9 ka and at 257 ± 13 ka. An unusual characteristic is their low δ18O signature. Few data are available on their mineralogy and glass geochemistry, and this study fills an important gap in understanding their petrogenesis. We report new mineralogical observations and plagioclase, whole-rock and glass compositional data. Based on our observations, we describe a new lava flow for which we propose the name East Biscuit Basin flow. This unit is a quartz- and sanidine-free low-silica rhyolite (71-72% SiO2 in the whole-rock) in which the dominant mineral, plagioclase, comprises two populations: (1) small fresh euhedral crystals of An20-48 (average of An31) composition, commonly part of aggregates with pyroxenes and Fe-Ti oxides; (2) large sieve-textured isolated crystals, which are slightly more sodic in composition (An19-34, average of An27). Plagioclase compositions in most other Upper Basin Member rhyolites are similar. The range of compositions for trace elements such as Rb, Th, Y and the rare earth elements is small (e.g. 158-189 ppm Rb, 20-25 ppm Th, 52-63 ppm Y, 60-82 ppm La in the whole-rock), and there is no systematic variation of these elements as a function of SiO2 content or mineralogy. Certain trace element signatures and ratios are specific to each of these rhyolites, allowing us to propose that the Upper Basin Member rhyolites originate from six independent magma batches. The coexistence of the two types of plagioclase and their progressive disappearance in the more evolved rhyolites suggest the following petrogenetic model for each magma batch. A low-δ18O rhyolitic protolith is heated by replenishing magmas, which initiate melting, forming a crystal mush. Replenishment by buoyant silicic magma may enhance melting and cause mixing with the mush material. As a consequence, crystal-poor eruptible magma batches are formed, which contain small, more calcic plagioclase crystals (aggregates) formed during cooling and mixing of the replenishing silicic melt, and larger, lower temperature crystals exhibiting dissolution features inherited from the protolith. © The Author 2009. Published by Oxford University Press. All rights reserved.