Zircon reveals protracted magma storage and recycling beneath Mount St. Helens

Abstract

Current data and models for Mount St. Helens volcano (Washington, United States) suggest relatively rapid transport from magma genesis to eruption, with no evidence for protracted storage or recycling of magmas. However, we show here that complex zircon age populations extending back hundreds of thousands of years from eruption age indicate that magmas regularly stall in the crust, cool and crystallize beneath the volcano, and are then rejuvenated and incorporated by hotter, young magmas on their way to the surface. Estimated dissolution times suggest that entrained zircon generally resided in rejuvenating magmas for no more than about a century. Zircon elemental compositions refl ect the increasing infl uence of mafi c input into the system through time, recording growth from hotter, less evolved magmas tens of thousands of years prior to the appearance of mafi c magmas at the surface, or changes in whole-rock geochemistry and petrology, and providing a new, time-correlated record of this evolution independent of the eruption history. Zircon data thus reveal the history of the hidden, long-lived intrusive portion of the Mount St. Helens system, where melt and crystals are stored for as long as hundreds of thousands of years and interact with fresh infl uxes of magmas that traverse the intrusive reservoir before erupting.on November 6, 2010 geology.gsapubs.org Downloaded from

Timescales of crustal magma reservoir processes: insights from U-series crystal ages

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“…Eruption ages are used to calculate hazard parameters such as recurrence rates and repose periods, as well as to identify vent migration patterns that are crucial to eruption forecasting (Tanaka et al, 1986;Connor and Hill, 1995;Condit and Connor, 1996;Heizler et al, 1999). Timescales of crystallization, typically calculated from U/Th or U/Pb zircon dating, determine the duration of magma assembly, sources of melting, role of assimilation, and transport of magma through the crust (Miller et al, 2007;Claiborne et al, 2010). Both eruption and crystallization ages provide a temporal framework to assess related geothermal systems and to evaluate the implications of low-velocity zones (i.e., crustal melts) and related seismic events beneath volcanic fields.…”

Petrogenesis of Ordovician Magmatism in the Pyrenees (Albera and Canigó Massifs) Determined on the Basis of Zircon Minor and Trace Element Composition

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