Crystallization conditions of leucite-bearing magmas and their implications on the magmatological evolution of ultrapotassic magmas: the Vico Volcano, Central Italy

Mineral compositions in leucite-bearing and leucite-free rocks from Vico volcano are reported. FeO/MgO partitioning (Kd(ol/liq)) between olivine and latite (0.14-0.22), and between olivine and trachyte (0.06-0.10) indicates a lack of equilibrium between mineral and host rock. This suggests that mingling and/or mixing between magmas was a leading process during magmatic differentiation. In addition, a phono-tephrite olivine population with high (0.84) and equilibrium (0.23-0.29) Kd(ol/liq) values has been produced by the interaction of differently evolved magmas. Zoning in clinopyroxene and plagioclase from these rocks recorded the same processes. In addition, resorbed quartz xenocrysts with coronas of clinopyroxene microlites indicate that digestion of crustal rocks occurred during the residence of magma in a shallow level reservoir. Increasing Fe coupled with decreasing Ca in diopside crystals from some phonolites, together with the petrographic and trace element data, indicate that polybaric fractional crystallisation also may be involved in the genesis of magmas of the second period of Vico activity. Leucite-free trachybasalts erupted in a late stage contain highly forsteritic olivine phenocrysts (forsterite 84-88 mol.%) in-equilibrium (Kd(ol/liq) = 0.24-0.35) with the host rock, which indicate that they did not suffer chemical modification at low pressure.