INTEGRATING PETROLOGIC AND GEOPHYSICAL PERSPECTIVES ON THE RHYOLITIC MAGMA RESERVOIR AT LAGUNA DEL MAULE, CHILE (Invited Presentation)

The Laguna del Maule volcanic field (LdM), central Chile, is among the premier natural laboratories for interdisciplinary investigations of silicic magmatism. Between the last glacial maximum and <2000 years ago, LdM produced >20 explosive and effusive rhyolite eruptions. Since 2007, uplift centered in the ring of rhyolite-producing vents has been sustained at >200 mm/y.

Rhyolite at LdM is generated within a trans-crustal magma system by fractional crystallization and assimilation of juvenile crust. Compositional contrasts between early post-glacial and Holocene rhyolites indicate they were sourced from distinct reservoirs at similar shallow depths. Zircon ages show these reservoirs coexisted for >100 ky, during which time crystals cycled between high- and low-temperature growth. Models of Mg diffusion in plagioclase from Holocene rhyolite suggests mobile magma was extracted from the reservoir only decades prior to eruption; yet there are no signs of pre-eruptive reheating despite ample evidence for the shallow intrusion of mafic magma. Instead, stalled mafic magma contributed CO₂-rich fluid to the overlying rhyolite, potentially catalyzing crystallization, volatile exsolution, and eruption.

Gravity, seismic, and MT imaging at LdM identify a several-hundred km³, largely crystal-rich magma reservoir at 2-10 km depth. The complimentary sensitivities of these methods reveal a more detailed structure including crystal-poor magma and fluid associated with the inflation center and melt-rich lenses distributed throughout the shallow system.

Recent discussions of the duration of mobile magma storage and eruption triggering have contributed to a “warm vs cold” storage framework. That most tomographic studies fail to locate melt-rich bodies is often cited to support “cold” storage. While the LdM magma system is dominantly crystal-rich, the brief storage recorded by plagioclase alongside the long, temperature-variable zircon growth history indicates a temporally and spatially variable thermal regime, that retained mobile rhyolite throughout post-glacial time, obviating a dichotomous “warm vs cold” framework. Moreover, the geophysical results at LdM demonstrate that melt concentrations unresolved by individual methods can be revealed through a multi-disciplinary approach.