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