Long Incubation Time and Repeated Chamber Evacuation for Explosive Dacites from Glacier Peak Volcano, Washington

Glacier Peak, Washington State, erupted two voluminous plinian pumice deposits in rapid succession shortly before the end of the Pleistocene (ca. 13.5 cal yr BP; vol. estimates: 1.4-9.4 km³ ea). These widespread G and B tephras are useful stratigraphic markers in the western U.S. and Canada and consist of similar porphyritic dacites with phenocrysts of plagioclase, hornblende, hypersthene, titanomagnetite, minor ilmenite, and microcrysts of apatite and zircon. The second fall deposit (B) differs most strikingly by containing pumices with generally higher Sr concentrations (500-570 ppm) than the first (G, 400-480 ppm), although analyses of FeTi oxide pairs fail to show a difference in pre-eruptive temperature and fO₂, and plagioclase rim and interior compositions are likewise indistinguishable between deposits. Sparse quenched enclaves are andesites but have insufficiently high Sr concentrations to account for the difference between the two pumice deposits. Instead, the high-Sr B dacites are on-trend with other much higher-Sr "adakitic" dacites that also erupt from Glacier Peak. SIMS ²³⁸U-²³⁰Th analyses of zircon interiors (n=39) and surfaces (n=72) are also indistinguishable between the G and B deposits. Model ages span from shortly pre-eruptive to the equiline but can be subdivided into peaks of zircon growth centered (differing peak values depending on use of equiline or 20% ²³⁰Th excess for melt) at 20 or 15 ka, 50 or 40 ka, 130 or 120 ka, and >250 ka. Over the time span recorded by zircons, Glacier Peak erupted andesites and dacites commonly crowded with quenched andesitic enclaves, showing that the G and B dacitic magmas did not then occupy the shallow sub-edifice region. Instead, they may have incubated in the deeper roots of the magmatic system, possibly on the flanks of the more active central magmatic axis. Eventually they accumulated in sufficient abundance to capture that system, ascended to shallow storage levels, and fueled two major pumice eruptions in rapid succession. Andesitic magma mingled into the system and quenched, but the main difference between the deposits was the involvement in the second of higher-Sr dacitic magma too similar in composition and temperature to crystallize, and too close before the second eruption to change T and fO₂ recorded by FeTi oxides or to grow distinct plagioclase rims.