INVESTIGATING THE PETROGENESIS OF LOW-VOLUME BASALTIC VOLCANIC SYSTEMS: THE TABERNACLE HILL FLOW, UTAH

As part of a wider project investigating the details of crystallization and assimilation processes in low-volume basaltic monogenetic eruptions, we are studying the Tabernacle Hill Flow in Utah's Black Rock Desert. This area is a good example of a small volume volcanic field, with just 10 spatially distinct lava flows in the last 1.5 Myrs. The Tabernacle Hill lava is a young (14.5 ka), primitive basaltic (c. 7.5% MgO) flow that is rich in phenocrysts (>20%) of olivine and plagioclase (some plagioclase crystals measure 3.5cm x1cm). This project focuses on information that these minerals can provide about the processes that the magma went through during its transit from the mantle; both from compositional variations, and from melt inclusions trapped as the crystals grew. Olivine compositions measured by electron microprobe at the University of Notre Dame range from Fo 67 to 85. Core-rim pairs indicate that most crystals are normally zoned. High CaO contents (c. 0.2 wt%) indicate that the olivines are magmatic and not xenocrysts from the mantle. Most analyses (57 out of 62) have Fo contents of 83 ± 2, consistent with the composition for olivine predicted to be in equilibrium with the melt, based on the whole rock major element analyses. Plagioclase phenocrysts show a larger diversity in size and textures, indicating a more complex magmatic history. Nomarksi imaging is being used to detail resorption and zoning patterns in the plagioclase crystals that will then be analyzed by electron microprobe in order to assess the possible role of magma replenishment, crystal recycling, and crustal xenolith disaggregation. Olivine-hosted melt inclusions can potentially preserve distinct stages of magmatic evolution not seen in the host lava. Inclusions in hand-picked olivine grains have been homogenized to glass in a 1 atm gas-controlled furnace, and will be analyzed by electron microprobe in the near future.