AGE AND COMPOSITION OF ZIRCON FROM THE MOST RECENT ERUPTIVE PERIODS AT OREGON’S MT. HOOD

Mount Hood in Oregon is an ~500 k.y.-old andesite-dacite stratovolcano, one of the most active and potentially hazardous volcanoes in the Cascade Range. The most recent volcanism at Hood were the Timberline eruptive period (~1.5 ka) and the Old Maid eruptive period (1780-90s CE). These eruptions were preceded by the Polallie eruptive period from 12-30 ka.

Using SHRIMP-RG at the Stanford University/USGS SUMAC lab, we collected ²³⁸U-²³⁰Th -disequilibria ages and trace element data for 114 zircon from juvenile clasts from Old Maid, Timberline, and Polallie eruptions in order to determine the petrochronology of zircon underneath Hood.

The range of ages collected from all samples spans from Old Maid eruption age (220 a) to ²³⁰Th-²³⁸U equilibrium (>350 ka). Old Maid range from eruption age to equilibrium, most between 0.20-127 ka. Timberline range from eruption age (1.5 ka) to 234 ka, most 1.5-61 ka. In contrast, Polallie are older than eruption age (~20 ka), ranging from 29-219 ka, most from 29-122 ka.

Trace element compositions sampled largely overlap. Hf is between 8,005-12,214 ppm for Old Maid, 7,707-13,297 ppm for Timberline, and 8,791-15,279 ppm for Polallie. Yb/Gd is between 6.8-23.1 ppm for Old Maid, 7.4-24.5 ppm Timberline, and 7.4-25.9 ppm for Polallie. Temperatures (Ti-in-zircon) are between 679-868ºC for Old Maid, 635-847ºC for Timberline, and 624-839ºC for Polallie.

These data suggest young zircon growth during Old Maid and Timberline, however there is overlap in zircon age and composition between the three periods. This suggests the existence of zircon-saturated crystal mush at Hood since ~150 ka, with significant zircon nucleation since ~50 ka. This supports the model of Kent et al. (2010) where mantle-derived mafic magma interacts with a “silicic filter” mixing mafic and felsic magma. Extensive young zircon suggest there is currently down-temperature zircon growth associated with recent mixing events that promote eruption of hybrid andesite and dacite.