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The Cordón Caulle volcano, part of the Puyehue-Cordón Caulle volcanic complex (PCC) located within the Southern Andean Volcanic Zone in Chile, represents an ideal volcanic setting to study magma differentiation and storage processes within an active arc system. Rhyolitic lavas erupted in 2011 to 2012 host crystal-rich basaltic enclaves which have been interpreted as pieces of the active crystal mush system below the Cordón Caulle volcano. Interstitial glasses of these basaltic mush fragments are compositionally very similar to their host rhyolitic magmas suggesting that the basaltic crystal mush represents the source of the rhyolitic melts, thereby offering a rare possiblity to directly investigate an active mafic 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 the Leibniz University Hannover. Run temperatures were varied between 750 and 1000 °C with fO2 buffered close to NNO using an Ar-H2 gas mixture as pressure medium. Our experimental setup is specifically designed to simulate a crystallisation-driven differentiation mechanism applicable to an in-situ evolving crystal mush system, representing a mixture between perfect fractional and equilibrium crystallisation regimes, where the "reactive magmatic system" is continously changing during progressive cooling.

Residual melts of first experiments define distinct differentiation trends and show a very close compositional match with bulk rocks as well as groundmass glasses of the rhyolitic lavas strongly supporting the postulated petrogenetical link between basaltic mush enclaves and erupted host ryholites. Consequently, the generation of highly-evolved liquids in a cooling primitive basaltic crystal mush combined with an efficient residual melt extraction mechanism represents a likely differentiation scenario for the Cordón Caulle volcano and similar volcanic systems.