In-situ generation of rhyolitic melts in a basaltic crystal mush below Cordón Caulle volcano (Chile) – an experimental study

The Cordón Caulle volcano in the Southern Andean Volcanic Zone in Chile represents an ideal setting to study magmatic processes within an active arc system. Rhyolitic lavas erupted in 2011-2012 host crystal-rich basaltic enclaves which have been interpreted as pieces of an active crystal mush. Interstitial glasses of these mush fragments are compositionally very similar to their host rhyolitic magmas suggesting that a basaltic crystal mush represents the source of the rhyolitic melts, thereby offering a rare possiblity to investigate an active mush system.

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 0.5-1.0 wt.%. Experiments were run at 150 MPa in internally heated pressure vessels (IHPV) at temperatures between 750 and 1000 °C and fO₂ buffered between NNO-1 and NNO. 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 residual melts define distinct differentiation trends and show a close compositional match with bulk rocks as well as groundmass glasses of the rhyolitic lavas strongly supporting a petrogenetical link between basaltic mush enclaves and host ryholites. Consequently, the generation of highly-evolved liquids in a cooling basaltic crystal mush combined with an efficient residual melt extraction mechanism represents a possible differentiation scenario for the Cordón Caulle volcano and similar volcanic systems.