In one step: Insights into shallow differentiation from basalt to rhyolite at Cordón Caulle from rhyolite-MELTS simulations

Abstract

Magma mush systems are commonly invoked as the source from which crystal-melt segregation produces rhyolites, but these systems are rarely observed. The 2011–2012 VEI 4 eruption of Cordón Caulle produced rhyolite lavas which scavenged basaltic enclaves. These enclaves contain interstitial glass similar to their host rhyolitic lava, suggesting that these enclaves offer a window into an active, shallow basaltic mush system. This mush was proposed as the source from which crystallization generates high-silica rhyolite in a single step. Here, we use rhyolite-MELTS to determine whether this is thermodynamically realistic. First, we use melt geobarometry to establish that enclave-derived pressures (∼25–200 MPa) are consistent with those previously determined for the lavas. We then simulate isobaric crystallization using a range of initial starting water concentrations to test if it is possible to generate rhyolites of the appropriate composition, assuming a starting composition which is the same as the basaltic enclave whole-rock. We find that it is possible to produce suitable rhyolitic compositions via fractional crystallization. We then explore the simulated physical consequences of crystallization and fluid exsolution. Lower water simulations (0.5–1.0 wt.% H₂O) at pressures of 100–200 MPa produce changes in volume most consistent with pre-eruptive ground deformation signals. We determine the timescales of heat loss from the crystallizing basaltic magma to be ∼8–25 ka, but it is plausible that heat loss occurred on the order of ∼1–10 ka which is broadly consistent with repose times of the system. The application of rhyolite-MELTS to an actively monitored system offers multifaceted insights into the geochemical, thermal, and physical evolution of magmas.