In-situ rhyolite generation in a basaltic crystal mush - an experimental study of the 2011-2012 Cordón Caulle eruption

The Cordón Caulle volcanic graben system in the Chilean Southern Andean Volcanic Zone represents an ideal setting to study magmatic processes within an active tholeiitic arc system. Rhyolitic lavas erupted in 2011-2012 host crystal-rich basaltic enclaves with interstitial glasses that are compositionally very similar to their host rhyolitic magmas. Thus, these basaltic enclaves have been interpreted as pieces of a crystal mush where the host rhyolites represent late-stage extracted residual melts. This model suggests closed-system, in-situ rhyolite generation at Cordón Caulle, which offers a rare possiblity to investigate the formation of rhyolites in a single differentiation step from a parental basalt.

In this study, we experimentally test this petrogenetic model by performing partial melting experiments on natural rock powders of basaltic enclave samples employing bulk water contents of 0.0 to 1.0 wt.%. Experiments were run in internally heated pressure vessels (IHPV) at 75 and 150 MPa and temperatures between 800 and 1000 °C, corresponding to previously estimated pre-eruptive magma storage conditions for the 2011-2012 Cordón Caulle eruption. Our experimental setup is specifically designed to simulate a crystallisation-driven differentiation mechanism applicable to an in-situ evolving crystal mush, representing a mixture between fractional and equilibrium crystallisation regimes, where the "reactive magmatic system" is continously changing during progressive cooling.

Experimental liquids define distinct differentiation trends and show a close compositional match with the natural rock record. In particular, near-anhydrous runs at 1000 to 900 °C reproduce distinctively best the rhyolites erupted in 2011-2012 inferring rhyolite generation at rather hot and nearly dry magmatic conditions. Consequently, in-situ generation of highly-evolved liquids in a nearly-anhydrous cooling basaltic crystal mush combined with an efficient residual melt extraction mechanism represents a possible differentiation scenario for the Cordón Caulle system. Moreover, we speculate that this mechanism of nearly dry, hot, and shallow magma storage and single step rhyolite generation likely also occurs in similar arc tholeiitic systems worldwide.