Plagioclase as archive of magma ascent dynamics on “open conduit” volcanoes: The 2001-2006 eruptive period at Mt. Etna

Plagioclase stability models indicate that: (1) H2O strongly influences the plagioclase-melt equilibrium allowing the crystallizations of more calcic compositions only at shallow levels; (2) patchy cores form at high pressure (up to 350 MPa) and low water content (< 1.7 wt.%); (3) clear dissolved cores form at lower pressure (150 MPa) and higher water content (1.5-2.8 wt.%); (4) dusty rims form at even lower pressure straddling the H2O-saturation curve and, (5) melt alignments form during degassing. According to experimental works each of these textures can be related to a different process within the feeding system, such as multiple magma inputs (patchy core), volatile addition or increase in T (clear core), mixing (dusty rims) and rapid decompression and degassing (melt inclusion alignment at rims). These inferences were successfully compared with the eruptive evolution of each event as deduced from direct observations, and geophysical and petrological data. The overall picture shows that plagioclase crystallizes under polybaric conditions in a vertically extended and continuous feeding system in which at least two magma crystallization levels were identified. Plagioclase stability also indicates that a large variability in water content characterizes the magma within the feeding system.