Thermo-mechanical reactivation of locked crystal mushes: Melting-induced internal fracturing and assimilation processes in magmas

Thermal reactivation of locked crystal mushes in the upper crust is a fundamental step towards volcanic eruptions of crystal-rich magmas. Models of such reactivation events indicate that partial melting of the crystalline framework is energetically costly and lead to average crystallinities that are lower than those observed in many erupted crystal mushes. Here, we show that internal overpressurization of the mush induced by small amounts of melting (10–20%) breaks the crystalline framework by microfracturing and allows for efficient unlocking of the mush. Hence, this melting-induced overpressurization, enhanced by addition of gas in wet magmatic systems, plays an important role in generating volcanic deposits with crystal contents close to the rheological lock-up (∼ 50 vol% crystals) by accelerating the incorporation of highly crystalline parts of the magma chamber (self-assimilation). It can also participate in disintegrating pieces of country rock that are commonly scavenged in magmas, leading to bulk assimilation of crustal lithologies in shallow reservoirs.