Numerical Modeling Perspectives on Zircon Crystallization and Magma Reservoir Growth at the Laguna del Maule Volcanic Field, Central Chile

Abstract:

Bridging the divide between petrochronologic data and magma dynamics is a critical step in integrating forensic studies of caldera-forming eruptions with geophysical imaging and monitoring of active silicic systems. The development of large silicic magma reservoirs involves a range of protracted and punctuated processes including repeated shallow magma emplacement, melting of the crust or silicic cumulates, cooling, crystallization, and degassing, magmatic rejuvenation, liquid-crystal separation, and the amalgamation of physically discrete magma bodies. The determination of zircon crystallization ages coupled to trace element compositions and thermometry at the single crystal to intracrystal scale is a powerful and increasingly utilized petrochronologic technique to reconstruct evolving conditions during magma reservoir growth. Yet, due to complex zircon age distributions and disequalibrium trace element partitioning, it is often challenging to make unambiguous interpretations of the magma storage conditions or the dynamics of reservoir growth from the crystallization history of individual zircons or aggregate crystal populations.

In order to improve interpretations of magma dynamics derived from zircon petrochronology, we develop a model of zircon saturation, growth, and dissolution within a numerical simulation of coupled thermal transfer, phase equilibrium, and magma dynamics. We apply this modeling approach to the Laguna del Maule volcanic field (LdM). LdM has erupted at least 40 km³ of rhyolite from 36 vents distributed within a 250 km² lake basin. Ongoing unrest demonstrates that the large, silicic magma system beneath LdM remains active to this day. Zircons from rhyolite erupted between c. 23 and <2 ka produce continuous distributions of ²³⁰Th-²³⁸U ages ranging from eruption to 40 ka, less common crystal domains up to 165 ka, and rare xenocrysts. Many LdM zircons are volumetrically dominated by trace-element-enriched domains suggestive of rapid crystallization. Accordingly, we examine the evolving conditions of zircon crystallization during the growth of the LdM magma reservoir, the development of gradients in zircon saturation and growth, and the potential for open system processes to catalyze episodes of rapid zircon crystallization.