UTILITY OF ZIRCON-HOSTED MELT INCLUSIONS FOR CHARACTERIZING THE MILLBRIG K-BENTONITE

The Ordovician Millbrig K-bentonite in eastern North America represents the largest known eruptive deposit in the Phanerozoic. While the bed has been geochemically characterized through correlation work, its petrogenetic history and radiometric age remains poorly constrained. The Millbrig contains a number of phenocrysts, including zircon. Zircon commonly traps silicate melt inclusions during crystallization. These melt inclusions have high potential for recording the magmatic conditions under which they were formed. This study uses zircon-hosted melt inclusions as a tool for characterizing and interpreting the magma body that erupted to form the Millbrig K-bentonite. Backscatter electron (BSE) imaging and cathodoluminescence (CL) imaging are used to contextualize geochemical data for melt inclusions. Cathodoluminescence imaging confirms that the zircon-hosted melt inclusions examined in this study are likely representative of the erupted Millbrig magma, due to their position outside of inherited cores. The volcanic crystal morphologies, volcanic CL zoning patterns, rarity and median size (1 micron) of melt inclusions, abundance and median size (12.5 microns) of clay inclusions, and ubiquity of crystal-through-going cracks of the sample (n=199) are physical evidence for an extremely explosive eruptive style, consistent with previous investigations of the Millbrig (e.g., Kolata et al. 1996). Major element electron microprobe analyses (EMPA) of melt inclusions from this study confirm that the Millbrig K-bentonite likely originated as rhyolitic, calc-alkaline, arc-sourced magma (e.g., SiO₂ 75%, Na₂O 3.5-4.5%, MgO 0.045%, K₂O 1.4-1.8%, TiO₂ 0.089%, ZrO₂ 0.5%, FeO 0.735%). These results are preliminary due to the small EMPA dataset (n=2) but show potential for the continued use of the zircon-hosted melt inclusion technique to obtain more reliable and accurate geochemical data for the Millbrig and other Ordovician K-bentonites. Placing melt inclusions in the context of CL zones (e.g., in inherited cores vs. volcanic rims) may contribute to more precise petrogenetic and geochronological understanding of the Millbrig--and other instances of explosive volcanism-- in future studies.