Protracted Magma Reservoir Growth and Rapid Extraction of Repeatedly Erupted Rhyolite at Laguna del Maule, Chile

Wednesday, 10 January 2018: 11:55
Salon Quinamavida (Hotel Quinamavida)
Nathan L Andersen1, Bradley S Singer2, Fidel Costa3, Jason S Herrin3,4, Gareth Fabbro3 and John Fournelle2, (1)Georgia Institute of Technology Main Campus, Atlanta, GA, United States, (2)University of Wisconsin Madison, Madison, WI, United States, (3)Nanyang Technological University, Earth Observatory of Singapore, Singapore, Singapore, (4)Nanyang Technological University, Facility for Analysis Characterisation Testing and Simulation, Singapore, Singapore
Abstract:
The Laguna del Maule volcanic field (LdM), central Chile, has erupted >40 km3 of rhyolite from 36 vents distributed over 250 km2 since the last glacial retreat. Currently, LdM is in the midst of a decade-long episode of ground uplift at >200 mm/yr. Accompanying this deformation are shallow zones of low seismic velocity, density, and resistivity reflecting the presence of partial melt and fluid. We present petrochronologic data to reconstruct the growth of the active LdM magma reservoir and elucidate the processes leading up to the most recent eruptions.

Subtle contrasts in plagioclase and zircon trace element compositions distinguish rhyolites erupted during early post-glacial time (23-19 ka; EPG) from those erupted during the Holocene. Whereas most plagioclase crystals reflect pre-eruption crystallization within the host magma, in situ 230Th-238U dating of zircons yields continuous age distributions ranging from eruption to 40 ka, less common crystal domains up to 165 ka, and rare xenocrysts. Despite being erupted at different times, the zircon age populations of the EPG and Holocene rhyolites overlap indicating that compositionally distinct magma reservoirs grew simultaneously beneath LdM.

In contrast to this record of protracted magma accumulation, models of Mg diffusion in plagioclase indicate that modest-volume accumulations (<3 km3) of crystal-poor rhyolite, repeatedly erupted during the Holocene, were stored for only decades. Less evolved lavas erupted within a few km of the rhyolites contain abundant evidence of magmatic rejuvenation; yet, this process has imparted only a rare, subtle signature on the rhyolites. We propose fluids derived from mafic to intermediate magmas ascending from depth and the dehydration of hydrous phases are instrumental in catalyzing the rapid segregation of crystal-poor rhyolite from crystalline mush and pressurizing these magma bodies. The dichotomous timescales of plagioclase residence and zircon crystallization indicate that the presence of discrete bodies of crystal-poor, eruptible rhyolite within a long-lived, shallow magmatic system is an ephemeral and potentially unstable state. Thus, the ongoing unrest at LdM may herald a future, potentially explosive, rhyolite eruption.

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