Drout, Amy

Obsidian Cliffs and Basalt/Rhyolite Mixing

Bimodal basalt-rhyolite volcanism is prevalent throughout the Yellowstone-Snake River Plain volcanic history. The volcanism essentially migrates as a center of explosive silicic rhyolitic volcanism with continuous upwelling of smaller amounts of basaltic lava before and after rhyolite eruptions. The source of basaltic magma is mantle-derived, based on isotopic and chemical signatures, resembling basalt of the eastern SRP. Sr isotope ratios of basalt center around 0.704. Rhyolite magma forms as heat from the mantle plume causes melting in the lower crust; rhyolitic material has a strong crustal Sr signal of 0.709, and has trace and major element chemistry that strongly resembles Precambrian gneiss exposed in the Beartooth mountains to the north of Yellowstone.

The widely different chemistry of rhyolite and basalt indicates that almost no mixing of the two magma types occurs before eruption. Virtually no examples of intermediate-composition volcanic rocks are found in the Yellowstone area. The pronounced viscosity difference between basaltic and rhyolitic magma is likely responsible for the lack of significant mixing between magma types. Laboratory experiments of Campbell and Turner (1985; 1986) demonstrate that during turbulent injection of a low-viscosity fluid into a higher-viscosity host fluid, the miscibility depends upon the viscosity difference between the two fluids to the extent that if the viscosity ratio of the two fluids is >400, no mixing occurs. The criterion for mixing can be described as wd > kV2, where k is a constant, w is flow velocity through input pipe diameter d, and V2 is the viscosity of the host magma.

The results of experiments such as these and of basalt/rhyolite mixing inhibition have relevance for the generation of continental crust. Continental crust, as shown by geochemical and seismic studies, has a bulk andesitic composition. The addition of new continental material of bulk andesitic composition is not well understood; the inability of basaltic and rhyolitic magmas to mix easily and form andesitic melt presents further difficulties to the question of how continental material is formed.

The outcrop at Obsidian Cliffs is an example of the Plateau Rhyolite flows, a group of largely rhyolitic (though bimodal B-R) volcanics that has erupted intermittently over the past 600,000 years since the end of the third eruption cycle in which the Lava Creek Tuff was generated. The Plateau Rhyolites consist of 6 members; the Obsidian Cliffs outcrop is part of the Roaring Mountain Member. Two radiometric dates have been obtained for this member: one of 75 ka, one of 180 ka. Obsidian Cliffs forms the west margin of a rhyolite flow, the vent of which is approximately 1 km to the east. The flow filled a pre-existing valley which has since been excised by Obsidian Creek. The vent fissures from which the rhyolite erupted connect to faults outside the Yellowstone Caldera. It is assumed that rhyolite eruption occurred when the crust above the magma chamber was rigid enough to support the formation of faults that cross the caldera, intersecting the existing ring fractures, and reach the magma body below to produce a subsequent eruption.

The obsidian at Obsidian Cliffs shows columnar jointing at the flow snout, and flow layering higher up in the cliff. Spherulites are abundant (radially oriented acicular crystals of K-spar), which may alter to lithophysae (rounded masses with concentric shells separated by voids). Small-scale fluid structures are apparent. Phenocrysts are very rare in this obsidian, due to the extremely low water content of the magma which inhibits nucleation.

References: Campbell and Turner, 1985. Nature 313, 39-42.

Campbell and Turner, 1986. J. Petrology 27, 1-30.