Mineral chemistry and U-series geochronology reveal timescales of differentiation for late Pleistocene peraluminous rhyolite erupted from Hayes Volcano, Alaska

Abstract:

The Hayes River ignimbrite is a recently recognized deposit from Hayes volcano, the northernmost and easternmost volcano in the Aleutian-Alaskan arc, with unusual whole-rock composition (peraluminous rhyolite; 74.2‒75.5 wt% SiO₂, 1.14 to 1.18 ASI) and phenocryst mineralogy (biotite-sanidine-plagioclase-quartz) compared to the Quaternary arc. The accessory minerals zircon, monazite [(LREE)PO4], and xenotime [(Y,HREE)PO4] are also present. We use ion microprobe ²³⁸U-²³⁰Th ages and trace-element geochemistry of unpolished rims and sectioned interiors of individual zircon and monazite grains to track differentiation of the silicic magma body. Core-to-rim zoning in zircon indicates that the parent melt became progressively enriched with U, HREEs, P, and Sc, and depleted in Th and LREEs due to monazite crystallization. Zircon (²³⁸U/²³²Th) values reach as high as 110 in the most differentiated rims. Monazite rims exhibit similar differentiation trends with lower LREE, higher M–HREEs, and higher U than crystal interiors, which eventually led to co-precipitation of monazite and xenotime. Monazite grains form a curved array on an activity ratio plot, with unpolished rims at the higher end. The unusual abundance of monazite, which can accommodate up to several weight percent Th, in the crystallizing assemblage significantly affected the U-Th ratio of the magma as differentiation progressed. ²³⁸U/²³²Th values ranges from 2.6 for early melt, represented by the whole-rock value, to 7.4 for groundmass glass. Assuming monazite fractionation alone is responsible for this change, it would take ~0.12 wt% monazite crystallization, using partition coefficients of 120 and 1000 for U and Th, respectively. This amount of monazite is consistent with that observed in the samples. An isochron for early melt and low-²³⁸U/²³²Th monazites yields an age of 67.0±2.8 ka, whereas one for late melt and high-²³⁸U/²³²Th monazites yields 42.5±0.9 ka. This younger age is indistinguishable from the laser single crystal ⁴⁰Ar/³⁹Ar age for sanidine of 41.1±2.3 ka. We interpret the apparent 25 k.y. crystallization interval to represent the assembly and differentiation timescale associated with the Hayes magma body.