CRYSTAL MUSH STORAGE CONDITIONS ACROSS THE CASCADE RANGE INFERRED FROM ZIRCON AGE AND TRACE ELEMENT COMPOSITIONS

Zircon is an ideal mineral phase in silicic magma to be used for understanding the storage conditions of crystals underneath active continental arc volcanoes. U-Th and U-Pb dating techniques, when combined with trace element compositions of the zircon, can detail the changing thermal-compositional conditions within a crystal mush over 10³-10⁵ years. These changes can be both in down- and up-temperature paths as the mush cools and is remobilized by new intrusions of basalt, a typical occurrence at continental arc volcanoes.

We present zircon age and compositional data from four major Cascade Range volcanoes: California’s Lassen Volcanic Center (LVC), Oregon’s Mt. Hood and South Sister and Washington’s Mount St. Helens. In all examined locations, zircon predates the eruption age of the host lava by 10³-10⁵ years. Some systematic trace element patterns also emerge. In all cases, as Hf (ppm) decreases, Eu/Eu* increases. The bulk of zircon at St. Helens have 0.4-0.65 Eu/Eu* with 9800-14000 ppm Hf, the LVC from 0.25-0.45 Eu/Eu* and 9000-12000 ppm Hf and South Sister 0.2-0.4 Eu/Eu* and 8000-10000 ppm Hf. These likely represent each systems’ “cold storage” conditions for the crystal mush. St. Helens has the lowest Th/U (0.1-1.0) at a given Yb/Gd and the LVC has the highest (0.35-1.9). This pattern may reflect changing composition of basaltic input.

Crystal mush must be remobilized in order to liberate zircon into younger host magma. This remobilization is likely caused by heating of the mush by new injections of magma that allow for melt to form. At St. Helens and the LVC, zircon record multiple thermal perturbations to the mush over the last >100,000 years. Ti-in-zircon temperature distributions at the LVC and St. Helens have similar distributions, with a large lower T peak and a subordinate higher T peak, likely reflecting rejuvenation of the mush from “cold storage”. At South Sister, the temperature distribution is more uniform. This may reflect the lack of long-lived silicic progenitors at South Sister or repeated rejuvenation. Understanding the trace element and temperature distribution in zircon across an active continental arc will allow for better interpretation in ancient volcanic rocks and a clear understanding of the t-T-x (time-temperature-composition) conditions of crystal storage in such systems.