Cannibalization Processes in Hotspot Rhyolites as Deduced from the Kimberly Rhyolite, Central Snake River Plain, Idaho, USA

• Main Author: Spencer, Danielle Jeannette
• Format: Others
• Published: BYU ScholarsArchive 2019
• Subjects: Yellowstone; Kimberly Member; Project HOTSPOT; assimilation; cannibalization; rhyolite; lava; Physical Sciences and Mathematics
• Online Access: https://scholarsarchive.byu.edu/etd/7726; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=8726&context=etd

The 7.7 Ma Kimberly Member of the Cassia Formation is part of a succession of A-type rhyolites associated with the Yellowstone hotspot track. It was sampled by the Kimberly core that was drilled on the Snake River Plain as part of Project HOTSPOT (Shervais, et al., 2013). The Kimberly Member is a 170 m thick high-silica rhyolite lava flow containing quartz, plagioclase, anorthoclase, sanidine, augite, pigeonite, magnetite, ilmenite, zircon, and apatite. δ 18O of zircon ranges from 0 to 4.9‰ (Colón et al., 2018), typical low values for the Snake River Plain. Quartz is intensely embayed. Exsolved and resorbed pigeonite cores are mantled by augite. REE-poor apatite cores are resorbed and oscillatory zones truncated by rims with SiO2 as high as 12.8 wt% and LREEtot up to 4.7%. There are three chemically distinct feldspars. Rounded and pitted anorthoclase (Or21 Ab64 An15) mantles plagioclase (An20 to An40) cores. Sanidine (Or47 Ab48 An05) forms thin, subhedral drapes on the outer edges of anorthoclase. Sanidine also fills some of the sieved holes in plagioclase and anorthoclase. There are two chemically distinct glasses, a light glass (~95%) and a dark glass (~5%). Relative to the light glass, the dark is enriched in Al2O3 , CaO, and Na2O and depleted in Fe2O3 and K2O. The dark glass is depleted in Rb and enriched in Sr and Ba, but they have similar concentrations of the high field strength elements (Y, Zr, Nb, Hf, and Ta). LREE are slightly more enriched in the dark glass than in the light glass. Temperatures of 926°C (magnetite-ilmenite thermometry with QUILF), 894°C (pigeonite-augite pairs with QUILF), and 889°C (zircon-saturation) are calculated for the magma. Although Fe-Ti oxides appear to have equilibrated with melt before eruption, most of the other phases preserve strong evidence of disequilibrium. These complex mineral textures also indicate assimilation and mixing processes. We propose a pigeonite-bearing, dry, metasomatized, A-type granite was fragmented and assimilated by the Kimberly member, mantling exsolved pigeonite with augite. Also incorporated into the Kimberly member were volcanic xenocrysts indicative of rhyolite assimilation or magma mixing. These components are embayed volcanic quartz, and composite plagioclase-anorthoclase grains (mantled by sanidine upon assimilation). Complex zircon grains could be sourced from metasomatized rhyolite or intrusion, and complex apatite grains could be due to mixing or assimilation. We propose the distinct glass types are caused by mingling of the Kimberly magma with the melted metasomatized assimilant. This scenario demonstrates the complexity of open system processes involved in some Snake River Plain magmas.