Understanding Upper Crustal Silicic Magmatic Systems Using the Temporal, Compositional, and Thermal Record in Zircon
Claiborne, Lily Lowery
The processes by which magma is transported into, stored within, and expelled (erupted) from the upper crust are critical to understanding volcanism and crustal construction. Recent studies suggest that magmatic systems may be generally long-lived and highly variable in composition, temperature and melt fraction (hence rheology) in both space and time. Petrologic studies of young volcanoes, however, generally suggest short timescales between magma production and eruption. System longevity and spatial and temporal fluctuations are in part a consequence of repeated recharge and can be either cause or consequence of eruption. Understanding how individual systems operate, or drawing general conclusions about and predicting magmatic behavior, hinges on monitoring and comparing the time-temperature-composition histories of both intruded and erupted products. The mineral zircon provides unique opportunities for such monitoring by combining high precision in situ radiometric dating with new methods of analysis and interpretation of elemental zoning. We have tested and applied this new combined methodology (U-Pb and U-Th disequilibria dating and elemental analysis, including Ti-in-zircon thermometry) at the Spirit Mountain Batholith, Nevada and Mount St. Helens volcano, Washington. The Spirit Mountain batholith is a well-characterized intrusive system which we use to better understand the trace element record of zircon in magmas. Zircon from Mount St. Helens provides critical insights into the volcano’s long term history and magmatic plumbing system, aspects of the otherwise well-characterized volcano that remained poorly understood. Current data and models for Mount St. Helens 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 reflect the increasing influence of mafic input into the system through time, recording growth from hotter, less evolved magmas tens of thousands of years prior to appearance of mafic 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 up to hundreds of thousands of years and interact with fresh influxes of magmas that traverse the intrusive reservoir before erupting.