Mineral Evidence for Generating Compositionally Zoned Rhyolites of the Devine Canyon Tuff, High Lava Plains, Oregon [report]

Erik Shafer
2000 unpublished
Large-volume silicic eruptions are often evacuated from magma reservoirs which display gradients in composition, temperature, crystallinity, and volatile content. The 9.7 Ma Devine Canyon Tuff (DCT) of eastern Oregon represents such an eruption, with >300 km 3 of compositionally zoned pyroclastic material deposited as a variably-welded ignimbrite. The ignimbrite displays homogenous bulk tuff major element compositions with a wide range of trace element compositions, allowing for the
more » ... n of how these magmas were generated, stored, and modified in the magma reservoir by studying pumices which represent the primary magmas composing the DCT. Five pumices ranging from dacite to rhyolite bulk compositions were selected across the range of trace element compositions and were crushed and sieved to measure how crystallinity and mineral abundances change within each pumice at different particle size fractions. Single alkali feldspar and clinopyroxene crystals were analyzed using EMP and LA ICP-MS from each pumice. Physical results yielded a systematic decrease in crystallinity from 22% to 3% going from the dacite to the most evolved rhyolite composition, with the highest crystallinity occurring between <991-425 microns for all pumices analyzed. The dacite pumices displayed a glomerocrystic texture not observed in rhyolite pumices. Two populations of crystals were distinguished using single crystal chemical data, one belonging to the rhyolitic magmas and another belonging to the dacitic magma. Acquired mineral data have relevance for how strongly zoned with regard to trace elements the rhyolitic magmas of the DCT were, how these magmas were generated, and how they were stored within the magma reservoir. Applying melt extraction models to explain ii observed patterns in trace element compositions between pumices is problematic. In this model, the observed range of trace elements in rhyolite pumices would be attributed to two separate melt extraction events from an intermediate crystal mush where the first expulsion of melt from the mush produced the most evolved rhyolite composition and a second expulsion coupled with partial melting produced a second rhyolite with an indistinguishable major element composition but less evolved trace element composition. Mixing of these two rhyolite end members would then be needed for generating the range of intermediate rhyolite compositions. Magma mixing modeled using a mixing equation produced a poor fit for trace elements, suggesting the range of observed trace element compositions cannot be solely generated through the mixing of the extracted rhyolite melts but require processes that subsequently modify the mixed rhyolite compositions.
doi:10.15760/etd.5901 fatcat:f5rserbhnfg3te22wt66ahh4oi