Melt extraction in mush zones: The case of crystal-rich enclaves at the Sabatini Volcanic District (central Italy)

A peculiar feature of the Sabatini Volcanic District (SVD, central Italy) is the occurrence of crystal-poor pumices and crystal-rich enclaves within the same eruptive host-deposit. The stratigraphic sequence of pumices and enclaves indicates the tapping of a stratified magma chamber, where a crystal-poor phonolitic magma lay on top of a more primitive crystal-rich magma. The crystal-rich enclaves are genetically related to the pumices and record the evolution of a solidification front, in which a more differentiated melt was produced, extracted and eventually erupted. We collected and analyzed crystal-rich enclaves from one of the largest phonolitic eruptions at the SVD and used their petrological and geochemical features to reconstruct magma differentiation and crystal-melt separation in the solidification front. On this basis, three groups of enclaves have been identified: porphyritic enclaves, holocrystalline enclaves and sanidinites. The mineralogical variability faithfully reproduces the spatial and temporal evolution expected of a solidification front, from early-to-intermediate crystallization conditions (porphyritic and holocrystalline type) to the late stage of solidification (sanidinites), in which the percolation of a more differentiated melt through the crystal mush triggered the instability of the solidification front. Results from numerical models indicate that gravitational instability is the most efficient mechanism to explain melt extraction in mush zones of medium-sized (~10 km3), short-lived (~104 years) magma chambers.