Long-Term Temperature Cycling in a Shallow Magma Reservoir: Insights from Sanidine Megacrysts at Taápaca Volcano, Central Andes

Abstract

Abstract Hybrid dacite magmas from Taápaca volcano in the Central Andean Volcanic Zone (18°S, northern Chile) contain sanidine crystals of unusual size (1–12 cm) and abundant mafic enclaves of variable composition throughout the entire eruptive history (1·5 Ma to recent) of the volcano. They are rich in mineral inclusions and strongly zoned in Ba with distinct growth bands separated by resorption interfaces. Resorption is followed by a sudden increase in Ba with compositional contrasts up to 2·3 wt% BaO. We argue that resorption and the sharp jumps in Ba concentration reflect distinct heating and melting events, suggesting that different growth zones formed at different temperatures. Amphibole–plagioclase thermobarometry based on mineral inclusions gives variable temperatures of ∼720–820 °C at shallow pressures (0·1–0·3 GPa) for individual growth zones. Using these temperatures for diffusion modelling, Ba profiles from X-ray scanning profiles and grey-scale gradients based on accumulated back-scattered electron images across these interfaces allow us to estimate crystal residence and reactivation times prior to eruption. This temperature control allowed the application of a ‘non-isothermal’ diffusion algorithm to obtain diffusion times for individual diffusive boundaries that range from 0·4 to 490 kyr and add up to total residence times of 9–499 kyr for different crystals from different stages of eruption. A combination of temperatures, pressure, diffusion times and R-MELTS modelling of the parent rhyodacite suggests storage conditions for the Taápaca reservoir at near eutectic composition at shallow depth (4–10 km). Temperatures never fell below the magma solidus but frequently cycled between 720 °C and 820 °C (i.e. between eruptible and non-eruptible state with crystallinity circling around ∼40–50 vol%) for tens to hundreds of thousands of years. We define this as ‘long-term transitional temperature cycling’ or LTTC storage. Frequent recharge events of basaltic andesite magma, as represented by abundant mafic enclaves, orchestrated the temperature cycling, resulted in multiple heating events that caused frequent resorptions and interrupted crystal growth, and kept the reservoir thermally ‘alive’. Recharge events became more frequent only ∼3–11 kyr before the eventual eruption that carried a particular set of sanidine megacrysts to the surface. Thus, after many earlier recharge events that did not result in eruption, a final event involved mixing at a critical recharge rate to mobilize, entrain, and erupt a particular set of megacrysts from the resident rhyodacite in a hybrid dacite host. This process, happening not more than a few centuries before an eruption, has been repeated at similar time-scales at different stratigraphic stages throughout the 1·5 Myr history of Taápaca volcano. The observed mineral zonation patterns and size of sanidine crystals from the resident magma reservoir below Taápaca volcano are identical to those observed in the megacrysts from granite intrusions that also show typical age ranges of zircon crystallization that are comparable with the residence times extracted here from Ba zonation. Taápaca sanidines thus may represent an erupted equivalent and provide ‘smoking gun’ evidence of temperature cycling during the formation of such K-feldspar megacrysts in granites.