Timescales of Magmatic Processes Preceding Eruption in a Large, Extraordinarily Restless, Silicic Magma System

Recent investigations of the Laguna del Maule (LdM) volcanic field, central Chile, suggest the presence of a large, shallow, and active rhyolitic magma reservoir. Modest (up to ~1.2 km³) rhyolitic eruptions over the last 20 kyr encircle an area inflating at an average rate of 25 cm/yr since 2007. ⁴⁰Ar/³⁹Ar, ¹⁴C, and tephra stratigraphy indicate that the majority of rhyolitic volcanism was concentrated in two phases (phase 1 and 2) separated by 9 kyr of repose. Here we report new petrological and geochemical results in order to determine if LdM rhyolites were issued from the same reservoir, identify the nature and timescales of processes leading to their eruption, and begin to relate the spectacular signs of unrest to magmatic processes.

All LdM rhyolites are crystal-poor and contain phenocrysts of plagioclase, biotite and rare quartz. Major and trace element contents indicate most plagioclase crystallized in equilibrium with the erupted rhyolitic magma. Incompatible trace element contents (e.g. Ce) delineate distinct crystal populations erupted during phases 1 and 2. Thus, the two magma reservoirs experienced limited physical interaction. A subset of crystal domains from both eruptive phases record melts inconsistent with the whole rock and glass, crystallization-dominated differentiation trend. Plagioclase erupted in the Early Espejos Tephra (phase 1), the largest recent explosive eruption, display the highest An and Mg contents and depletion of Ba and Sr. In contrast, early phase 2 plagioclase contain zones of Ba enrichment. This Mg and Ba enrichment records contrasting responses to the intrusion of mafic magma. The high Mg zones are consistent with an intermediate magma resulting from rhyolite and basalt mixing whereas the Ba enrichment results from melting of Ba-rich phases such as biotite and K-feldspar.

Modeling of Mg, Sr, and Ba diffusion indicates that mixing between these Mg and Ba rich melts and the erupted magma body occurred within a year of eruption. The duration of unrest at LdM has already exceeded the diffusion timescales and thus likely reflects more protracted processes of magma migration and volatile exsolution, potentially associated with the arrival of new magma to the system. However, it probably does not reflect the final stages of mixing prior to eruption recorded by plagioclase crystals.