TESTAMENT TO YOUTH IN ZIRCON: NEAR ERUPTION-AGE ZIRCON CRYSTALLIZATION OVER THE PAST 50,000 YEARS AT MT. HOOD, OREGON (Invited Presentation)

Oregon’s Mt. Hood is one of the most potentially hazardous volcanoes in the Cascade Range. It lies close to the Portland metropolitan area and has erupted multiple times over the past 2,000 years. Andesitic lavas and tephra from Mt. Hood’s Old Maid (0.25 ka) and Timberline (1.2 ka) periods have abundant zircon crystal cargo, which is unexpected as the host lavas are not zircon saturated. SHRIMP-RG U-Th and trace element analyses of zircon from the Old Maid, Timberline and Polallie (12-30 ka) periods at Mt. Hood tell a story of recent zircon crystallization underneath Mt. Hood over the past 50,000 years.

Isotopic and compositional analyses of 85 zircon interiors from the three most recent eruptive periods at Mt. Hood were performed via SHRIMP-RG. All zircon except two are younger than secular equilibrium for U-Th (~350,000 years). Over half of the zircon (n=50) are <25 k.y. old. Of these zircon, between 25-50% are <5 k.y. old. The remaining zircon are dominantly <40 k.y. old while 13 have U-Th model ages >100 k.y. The primary observation for the Hood zircon is their youthful, with a majority within error of the Old Maid and Timberline eruption periods (<1.2 ka).

Zircon exhibit weak zoning in CL, with little-to-no resorption textures or obvious inherited cores. They exhibit more compositional complexity compared to the CL across the crystal populations. Hf concentrations are low (8-11,000 ppm) while Eu/Eu* span from 0.3-0.65. Most zircon have Ti-in-zircon temperatures between 675-825°C, well below the calculated Fe-oxide temperatures from the host lavas.

These zircon data suggest the presence of a high silica, zircon-saturated crystal mush at Mt. Hood. The crystal mush is cool enough to crystallize zircon across the last 25,000 years and is rejuvenated quickly to mobilize these zircon. Only ~1-3.5 km³ of basaltic input is needed to heat a model silicic crystal mush and then mix with the rhyolite melt to produce hybrid andesite, taking the mush’s zircon crystal cargo with it. Mt. Hood-type arc volcanoes appear to have a persistent high-silicic magma at or near the solidus and the hybrid magma is the product of the balance between mixing of the basaltic input and rejuvenated silicic mush.