Raffaello Trigila

The Villa Senni Eruption (Alban Hills, central Italy): the role of H2O and CO2 on the magma chamber evolution and on the eruptive scenario

Abstract The Villa Senni Eruption Unit (VSEU) belongs to the Tuscolano-Artemisio phase of volcanic activity in the Alban Hills Volcanic District, the closest to Rome of the recent or active volcanoes of central Italy. The most important products of this eruption are represented by pyroclastic flow deposits, named lower and upper flow unit (LFU and UFU, respectively). Three main rock types form VSEU as follows: (1) juvenile K-foiditic scoria clasts of the LFU; (2) juvenile phonotephritic scoria clasts of the UFU; and (3) holocrystalline phonotephritic lithic inclusions (Italites) in the UFU. On the basis of the chemistry, mineralogy and petrography of the three studied rock types their phase relations have been discussed. Other petrologic constraints from laboratory melting experiments are presented and used to investigate the role of volatiles on the evolution of the magma chamber system. Some broad implications on withdrawal pattern are also presented. It has been verified the LFU rock type can be obtained from a parental melt of UFU composition by a CO2-controlled crystal-liquid fractionation of a solid assemblage close in composition to that of Italites. Because it can be proved that the storage of magma occurred at shallow depth within the Mesozoic carbonate country rocks, it is proposed that CO2 diffusion, originating from thermal decomposition of wall-rock carbonates, controlled the evolution trend of the melt at the periphery of the magma chamber, whereas the inner part of the magma body retained the volatile component (essentially H2O) of the original melt. The corresponding eruption model is therefore comprehensive of an early eruptive phase (LFU rock type) involving the more differentiated central magma bulb, whereas the late eruptive phase (UFU rock type) tapped the more mafic peripheral magma. The Italite xenoliths are believed to represent the chilled margins of the magma chamber.

Experimental determination of aluminous clinopyroxene-melt partition coefficients for potassic liquids, with application to the evolution of the Roman Province potassic magmas

Abstract We have experimentally determined the partition coefficients of rare earth elements (REE), HFSE (Ti, Zr, Hf, Nb, Ta) and other trace elements between clinopyroxene and potassic silicate melts at 0.1–200 MPa and 1042–1140°C. The major and trace element contents of clinopyroxene and liquid were determined by electron probe and ion microprobe, respectively. The liquidus clinopyroxenes are extremely rich in Al2O3 (up to 12%) and Fe2O3, which enables us to test suggestions that partition coefficients of HFSE and REE depend on the Al content of the clinopyroxene (e.g., Lundstrom, C.C., Shaw, H.F., Ryerson, F.J., Williams, Q., Gill, J., 1998. Crystal chemical control of clinopyroxene–melt partitioning in the Di–Ab–An system: implications for elemental fractionations in the depleted mantle. Geochim. Cosmochim. Acta 62, 2849–2862.). When compared with previous data for low-alumina clinopyroxene crystallised under similar P–T conditions, the increase in weight partition coefficient Di (=[I]xtl/[I]liq) for all the HFSE and REE is dramatic. For Ti and the REE, partition coefficients increase by about one order of magnitude as Aliv concentration of clinopyroxene increases from 0.02 to 0.5. For Nb, Ta and Zr, the effect is even more pronounced, a two order of magnitude increase being found to correspond to this increase in Aliv. We applied the data to model the fractionation of historic lavas from Vesuvius. Previous work using the major elements (Belkin, H.E., Kilburn, C.R.J., DeVivo, B., 1993. Sampling and major element chemistry of the recent (AD 1631–1944) Vesuvius activity. J. Volcanol. Geotherm. Res. 58, 273–290.) indicates a liquid line of descent from trachybasalt to leucite tephrite dominated by clinopyroxene crystallisation. The REE and HFSE concentrations in the lavas are consistent with 40–50% fractional crystallisation of clinopyroxene (accompanied by minor leucite) between the two magmatic endmembers, with partition coefficients for the REE appropriate for clinopyroxenes containing ≤7.5 wt.% Al2O3. Pyroxenes of the latter composition are frequent phenocrysts in the lavas and previous experimental work (Dolfi, D., Trigila, R., 1988. Chemical relations between clinopyroxenes and coexisting glasses obtained from melting experiments on alkaline basic lavas. Rend. Soc. Ital. Mineral. Petrol. 43, 1101–1110; Trigila, R., De Benedetti, A.A., 1993. Petrogenesis of Vesuvius historical lavas constrained by Pearce element ratio analysis and experimental phase equilibria. J. Volcanol. Geotherm. Res. 58, 315–343.) implies that they precipitate under very low pressure low water activity conditions, i.e. within the magma chamber at

Petrogenesis of Vesuvius historical lavas constrained by Pearce element ratios analysis and experimental phase equilibria

Petrogenesis of Vesuvius historical lavas erupted in the interval 1631–1944 was investigated by petrological methods and experimental phase equilibria. Lavas are strongly porphyritic with principal phenocrysts of clinopyroxene, leucite and calcic plagioclase. Olivine, biotite, FeTi oxides and apatite are present occasionally. Their bulk composition defines a chemical group spanning the leucite-tephrite and phonolite-tephrite regions of the TAS (total alkalies vs. silica) diagram. Through PERs (Pearce element ratios) analysis it has been possible to determine the lava differentiation relationships (controlled by cpx joined by minor lc, plag and Ti-mt) and to estimate the extent of differentiation processes (about 25% of initial mass). Chemistry of cpx indicates a multistage crystallization process in the upper crust. The crystallization environmental parameters deduced from the experimental cpx and liquids are consistent with the existence of a shallow magma chamber within the carbonate rocks of the Triassic basement where the nearly dry Vesuvius magma becomes CO2 saturated. The generally small volume of the products from single eruptions, and Vesuvius lavas geochemical and petrographic features suggest magma reservoir is zoned with respect to temperature and phenocrysts abundance. The dynamics of magma withdrawal controls mixing of the melt fractions proceeding from different areas of the magma chamber and, therefore, it produces the within-flow chemical differences. Chemical variations among the lavas of different eruptive cycles are instead believed to be controlled essentially by the speed of magma ascent through the upper crust allowing differential removals of cpx phenocrysts crystallized from the magma on its way to the surface.

Seismicity and 3-D substructure at Somma^Vesuvius volcano:evidence for magma quenching

Abstract We present new results on the velocity structure of the Somma–Vesuvius volcano, obtained by joint inversion of P- and 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.

The role of water in the 1944 Vesuvius eruption

AbstractThe P-T path of magma associated with the 1944 Vesuvius eruption has been outlined on the basis of probe mineralogy and the relationships between the crystallising phases. Equilibrium P-T values, obtained from the reactions:1.CaMgSi2O6(liq) = CaMgSi2O6(cpx)2.NaAlSi3O8 (liq) = NaAlSi3O8 (plag)3.CaAl2Si2O8 (plag)=CaAl2SiO6(cpx)+SiO2(liq) have been established for three intracrustal crystallisation stages: I) 8.0 kbar and 1255 °C; II) 4.0 kbar and 1178 °C; III) 0.5 kbar and 1105 °C. The H2O content in the magma has been estimated from an experimental calibration of

Investigation of the He solubility in H2O–CO2 bearing silicate liquids at moderate pressure: a new experimental method

a_{^{CaMgSi_2 O_6 } }^{liq}

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