MAGMA STORAGE TIMESCALES OF RECENTLY-ERUPTED MAGMAS AT MT HOOD VOLCANO, OREGON

Magma storage and mixing processes play major roles in the geochemical evolution and eruptive activity of most volcanoes, yet timescales of these processes are often poorly understood. Uranium-series disequilibrium dating (²³⁸U-²³⁰Th-²²⁶Ra) of phenocryst phases can be used to quantify ages of crystallization prior to mixing, and to calculate minimum magma storage time. Mt Hood is a ~500 ka stratovolcano, located in northern Oregon, that has consistently erupted lavas that show considerable evidence for magma mixing. Moreover, major-element geochemical homogeneity (~58 ± 4 wt % SiO₂) contrasts with oscillatory zoning, sieve textures, and dissolution rims observed in phenocryst phases that are inconsistent with simple fractional crystallization models. Quenched mafic inclusions have also been observed in nearly all lavas. Kinked crystal size distributions indicate the presence of two plagioclase populations separated at ~500 μm. In-situ LA-ICP-MS trace element analyses have been used to link the plagioclase populations to crystallization from mafic (<500μm) and silicic (>500μm) liquids (Darr, 2006. M.S., Oregon State Univ.). We will present new U-series crystal ages which will be combined with trace element diffusion profiles to define both pre- and post-mixing timescales.

We have isolated and analyzed plagioclase phenocrysts from Old Maid (~215 ybp) and Timberline (~1500 ybp) lavas for textural and geochemical analysis. [U], [Th], and [Ba] concentrations by isotope dilution measurements of plagioclase separates analyzed with MC-ICP-MS vary systematically with plagioclase size (likely due to variation in the proportion of impurities in the mineral separates); [U] ranges from 0.040-0.030 ppm, [Th] from 0.120-0.090 ppm, and [Ba] from 184.77-182.45 ppm with lower concentrations in larger size fractions. U-Th-Ra ages will be determined separately for the <500 μm (mafic) and >500 μm (silicic) plagioclase fractions, and will be used to calculate minimum storage timescales of the respective liquids prior to mixing. Based on crystal size distributions (assuming constant growth rate), we predict the <500 μm fraction will be younger than the >500 μm fraction. If ages of mafic crystals are very young (decades to centuries), it would imply that mafic magmas are transient within the Mt Hood system.