Dolfi, D.

The separate effects of pressure (10 4 and 1.0 GPa), water, CO2, oxygen fugacity and calcium doping on the liquid line of descent of a primitive leucite-basanite magma (SiO2¼ 47.06 wt%, MgO¼ 12.76wt%andMg#¼ 75.1) fromthe Montefiascone Volcanic Complex (Vulsini volcanoes, central Italy) were experimentally investigated in the 1350–1160 C temperature range. Results indicate that low-pressure liquidus temperatures are 1280 C and that the high-pressure Tliquidus is 1350 C under anhydrous conditions; the latter is lowered to 1275 C by the addition of 3 wt% water. Cr-spinel is always the liquidus phase. At comparable fO2 values, high and low pressure runs produced the same phase assemblage (spinel þ olivine þ clinopyroxene) up to 50 % crystallization, although olivine was partially or totally replaced by phlogopite in hydrous experiments. An increase in oxygen fugacity and the addition of CaO determine an increase in both the degree of melt crystallization and the stability field of clinopyroxene. These determine contrasting effects on the composition of residual liquids: the former increases SiO2 content, whereas the latter induces the desilication of melts. The replacement of olivine by phlogopite, induced by increasing amounts of water, leads to the production of glass with lower potassium contents. Comparison of the natural and experimental melts shows that many of major and trace element variations exhibited by high-K primitive (i.e., highMg/Mg þ Fe) magmas at Montefiascone, are consistent with their derivation from a single parental leucite-basanite melt by fractional crystallization of different proportions of mineral phases, plus carbonate assimilation. The changes in phases stability and melt composition caused by carbonate assimilation may also have fundamental implications for the origin of the calcic highmagnesium leucitites and melilitites. In particular, the complex metasomatic interactions that can develop at the interface between potassic magmas and carbonate wall rocks, may lead to melting of calcite. This low-viscosity melt readily mixes with the surrounding magma inducing the crystallization of Ca-Tschermak-rich pyroxene and hercynitic spinel, affecting significantly the SiO2, CaO and alumina composition of the resulting hybrid melt. A key finding of our study is that magmas such as the studied leucite-basanite may be considered parental to the wide spectrum of mafic high-K compositions in the Roman Province, which have been traditionally considered as representing near primary magmas reflecting distinct mantle source compositions and/or processes.

In this paper, we document the evolution of the emergent Panarea dome in the Aeolian islands (Southern Italy), placing particular emphasis on the reconstruction of the explosive events that occurred during the final stage of its evolution. Two main pyroclastic successions exposing fall deposits with different compositions have been studied into detail: the andesitic Palisi succession and the basaltic Punta Falcone succession. The close-in-time deposition of the two successions, the dispersal area and grain-size distribution of the deposits account for their attribution to vents located in the western sector of the present island and erupting almost contemporaneously. Vents could have been aligned along NNE-trending regional fracture systems controlling the western flank of the dome and possibly its collapse. Laboratory analyses have been devoted to the characterization of the products of the two successions that have been ascribed to vulcanianand to strombolian-type eruptions respectively. The vulcanian eruption started with a vent-clearing phase that occurred by sudden decompression of a pressurized magma producing ballistic bombs and a surge blast and the development of a vulcanian plume. Vulcanian activity was almost contemporaneous to stromboliantype fall-out eruptions. The coeval occurrence of basaltic and andesitic eruptions from close vents and the presence of magmatic basaltic enclaves in the final dacitic lava lobe of the dome allow us to speculate that the intrusion of a basaltic dyke played a major role in triggering explosive eruptions. The final explosive episodes may have been caused by extensional tectonics fracturing the roof of a zoned shallow magma chamber or by the intrusion of a new basaltic magma into a more acidic and shallow reservoir. Intrusion most likely occurred through the injection of dykes along the western cliff of the present Panarea Island inducing the collapse of the western sector of the dome.

The Alban Hills ultrapotassic volcanic district is one of the main districts emplaced during Quaternary time along the Tyrrhenian margin of Italy. Alban Hills lava flows and scoria clasts are made up essentially of clinopyroxenes and leucites and their chemical composition is mostly K-foiditic. Differentiated products (MgO<3 wt%) are characterized by low SiO2 concentration (<50 wt%) and geochemical features indicate that this unique differentiation trend is driven by crystal fractionation plus carbonate crust interaction. Notably, the Alban Hills Volcanic District was emplaced into thick limestone units. With the aim of constraining the magmatic differentiation, we performed experiments on the Alban Hills parental composition (plagioclase-free phono-tephrite) under anhydrous, hydrous, and hydrous-carbonated conditions. Experiments were carried out at 1 atm, 0.5 GPa and 1 GPa, temperatures ranging from 1050 to 1300 °C, and H2O and CaCO3 in the starting material up to 2 and 7 wt%, respectively. The experiments performed at 0.5 GPa resulted to be the most representative of the Alban Hills plumbing system. Clinopyroxene and leucite are the main phases occurring under all the investigated conditions and the liquidus phases. Nevertheless, our experimental results demonstrate that the occurrence of CaCO3 in the starting material strongly affects phase relations. Experiments performed under hydrous conditions crystallize magnetite and phlogopite at relatively high temperature. This early crystallization drives the glass composition towards a silica enrichment, resulting in a differentiation trend moving from phono-tephritic (Alban Hills parental composition) to phonolitic compositions. This is in contrast with micro-textural evidences showing late crystallization of magnetite and phlogopite in the natural products and with the composition of the juvenile products. On the contrary, in the CaCO3-bearing experiments (i.e., simulating magma-carbonate interaction) the magnetite and phlogopite stability fields are strongly reduced. As a consequence, the melt differentiation is mainly controlled by the cotectic crystallization of clinopyroxene and leucite, resulting in a differentiation trend moving towards K-foiditic compositions. These experimental results are in agreement with micro-textural features and chemical compositions of Alban Hills natural products and with the magmatic differentiation model inferred by geochemical data. Magma-carbonate interaction is not a rare process and its occurrence has been demonstrated for different plumbing systems. However, the uniqueness of the Alban Hills liquid line of descent suggests that the efficacy of the carbonate contamination process is controlled by different factors, the dynamics of the plumbing system being one of the most important.

Abstract We present new results on the velocity structure of the Somma^Vesuvius volcano,obtained by joint inversion of Pand S-wave arrival times from both local earthquakes and shot data collected during the TOMOVES 1994 and 1996 experiments. The use of a large set of earthquakes,recorded over a period of ten years by both temporary and permanent seismic stations,allowed us to enhance the resolution of the structure beneath the Somma^Vesuvius down to 5 km depth. The results obtained show the presence of a high Vp and Vp/Vs anomaly located around the crater axis,between 0 and 5 km depth,involving the volcano edifice and the carbonate basement westward deepening from the adjacent Apenninic belt. The whole available seismic catalogue between 1987 to 2000 (1003 events) has been relocated in the obtained 3-D velocity model. Seismicity appears to be clustered around the anomalous high rigidity body. Laboratory experiments at high temperatures and pressures on 1944 eruption lava samples,taken representative in composition of the magma masses erupted through the cycle 1631^1944,support the interpretation of this anomaly in terms of magma quenching along the main conduit,because of the exsolution of magmatic volatiles. The effect of volatiles from the melt at the eruption onset and through its explosive phases is to increase the solidus temperature well above its eruptive temperature,causing the immediate quenching of the system. This paper shows a good example of how seismic tomography and experimental petrology constrain magmatic models. Results have important implications for the hazard assessment at Somma^Vesuvius,and at other volcanoes worldwide where similar seismological evidence has been recently observed. A 2004 Elsevier B.V. All rights reserved.