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The Battleship Rock Ignimbrite of the East Fork Member of the Valles Rhyolite (USA) consists of a variably welded compound cooling unit emplaced in the southern part of the Valles Caldera at 74 ka. At the type locality, the unit was deposited in a narrow paleocanyon and is glassy throughout, with little petrographic evidence for post-eruptive alteration. Biotites from different zones in the ignimbrite at this location, and associated lavas and fallout deposits of the East Fork Member, frequently exhibit low analytical totals (< 94 wt%; by electron microprobe) that are correlated with potassium (K) depletion and enriched in lithium (Li). These variations between samples are systematically related to the welding profile of the ignimbrite. Biotites with low totals are accompanied by groundmass glass with low δ¹⁸O, which requires high-temperature interaction with meteoric water. The low totals, K depletion, and Li enrichment …