MELT INCLUSION RECORD OF LITHIUM-RICH MAGMAS AT MCDERMITT CALDERA, WESTERN USA

The 16.4 Ma McDermitt caldera straddles the Oregon-Nevada border and is an early center of the Yellowstone hotspot track. Lithium clay deposits are found within intracaldera lake sediments and heterogeneously distributed within the caldera basin. Determining the source of Li, whether from magmas or igneous rocks related to the McDermitt caldera system, or an external source, is an unresolved question. We present new melt inclusion analyses from a pre-caldera rhyolite along the northeastern caldera margin to constrain pre-eruptive Li contents. High-spatial resolution geochemical data were acquired for sectioned and polished melt inclusions (n=17) hosted in quartz crystals (n=16) using an electron microprobe and SHRIMP-RG ion microprobe. Obsidian glasses from the same sample were analyzed for comparison. The melt inclusions are rhyolitic (72-75 wt% SiO₂) and have a wide range of Li concentrations (90-1,350 ppm). About 90% of analyzed melt inclusions have >400 ppm Li, which is about an order of magnitude higher than the obsidian glasses (20-70 ppm Li) and unaltered McDermitt Tuff rocks (20-50 ppm Li). Melt inclusions with the highest Li concentrations (1,240-1,350 ppm Li) also have the highest concentrations of incompatible elements Cl (1,580-1,670 ppm), Rb (340-365 ppm), Y (100-110 ppm), Nb (54-56 ppm), Th (30-40 ppm) and U (15-17 ppm), and comparatively low concentrations of light REE (11-12 ppm La) and high concentrations of heavy REE (1.4-1.6 ppm Lu). All melt inclusions have a strongly negative, yet variable, Eu anomaly (0.1-0.7 ppm Eu; 1-10x chondritic). No consistent trends with major or trace elements are observed in melt inclusion chemistry, indicating that they were not trapped in a single (closed system) evolving magma, but rather represent multiple magma batches and compositions. Our study demonstrates that ore-grade lithium concentrations were present in early McDermitt rhyolites and largely lost (degassed) during eruption. Melt inclusions retained in physically and chemically resistant quartz phenocrysts provide the most reliable archive of original magmatic compositions, and support derivation of Li in the McDermitt deposits from magmas (± solidified rock equivalents) related to the caldera-forming system.