U-Th fractionation by fluids in K-rich magma genesis: The Vico volcano, Central Italy

The 230Th-238U disequilibrium method has been applied to the potassic volcanic series of Vico (Latium, Central Italy). The 3 main events of the recent volcanic activity have been dated using whole-rock isochrons. The lava flows of the main cone were erupted between 260 and 180 ky. Pyroclastic flows are dated around 90 ky and the final effusive activity (Mte. Venere) around 60 ky. These results are in good agreement with previous K/Ar ages. The intense Th-U fractionation (Th/U varying from 3.2 to 11) is apparently not related to processes of partial melting and crystal fractionation. The Th-U fractionation is the most intense in basic terms of the first episode, and is due to the percolation of a deep hydrothermal fluid which impoverishes U in different batches of primary magmas. More limited Th-U fractionations in highly differentiated magma are due to U (accompanied by Sb and alkalies) enrichments in the shallow magma chambers. A time-integrated model is proposed for the evolution of the (230Th/232Th) ratio in the mantle source of Vico. It shows that Th-U and Th-Ta fractionations are probably related to the bulk K, U and Th enrichment of the mantle of the Roman Province, around 300 ky ago. A metasomatic enrichment by a silicate melt is proposed to explain the anomalously high Th-U and Th-Ta fractionations. (orig.)