Uncommon K-foiditic magmas. The case study of Tufo del Palatino (Colli Albani Volcanic District, Italy)

Leucititic rocks, K-foiditic in composition are volumetrically important in the Colli Albani (also known as Alban Hills) volcanic district (Central Italy)especially during the most explosive phases of activity (>200 km 3 ). The Colli Albani tephra in distal (>500 km) deposits indicates that K-foiditic magma chambers fed large explosive eruptions (i.e., tens of km 3 of pyroclastic rocks). Major oxides, trace elements and Raman spectra were measured on the glasses and minerals occurring in the K-foiditic scoria clasts of the~530 kyr-old Tufo del Palatino, erupted in the Colli Albani volcanic district. The Colli Albani pre-eruptive magmatic system is characterized by theaH2O<1andhighCO2activity in the melt, as testified by the CO3in the clinopyroxene melt inclusions, by the early crystallization of CO3-bearing apatite and by the high CO2activity in the free volatile phase that led to crystallization of calcium carbonate in the scoria clast vesicles. The K-foiditic magmas plot on the Cpx + Lc + melt divariant surface of the Ol-Cpx-Lc-MelH2O-CO2, P≥0.2 GPa andT≤1100 °C. The assimilation of cold carbonate by hot magmas is an important opensystem process allowing the establishment ofaH20< 1 condition in the volatile-rich, Colli Albani magma chambers where the stabilityfields of the olivine and phlogopite are reduced infavor of clinopyroxene and leucite. Trace element modelling indicates large amount of carbonate assimilation (~12.4 wt%) involved in the differentiation process that origins the K-foiditic magmas starting from a K-rich, phonotephritic parental magma. The large amount of assimilate carbonate is consistent with the peculiar distribution of the latent heat across the crystallization interval of the phonotephritic parental magma. The isenthalpic assimilation process is very efficient in the phonotephritic magma because the crystallization of clinopyroxene and leucite in equilibrium with a K-foiditic melt proceeds over a relatively large temperature interval (>200 °C) and the K-foiditic melt shows low viscosity (10 4 Pa·s at 1000 °C). Actually, the low melt viscosity, that increases the growth rate, and the large temperature interval of crystallization are intrinsic factors that increase the release ofthe latent heat of crystallization from the phonotephritic parental magma. Extrinsic factors enhancing the assimilation process efficiency are the thickness (>4 km) and the depth (down to 5–7 km) of the carbonate substrate in the Colli Albani volcanic district