1,721,039 research outputs found
Experimental constraints on pre-eruption conditions of the 1631 Vesuvius eruption
No full textWe established the phase equilibria of a representative tephriphonolitic sample of the 1631 eruption of Vesuvius (Italy). Experiments were conducted at 100 MPa, in the temperature range 950–1050 °C for melt water content ranging from 1.3 to 3.2 wt%, and at an oxygen fugacity (fO2) of NNO+1 to NNO+3 (one to three log unit above the fO2 of the Ni-NiO solid redox buffer). Results show that clinopyroxene, biotite and leucite dominate the crystallizing phase assemblage, with minor proportions of plagioclase and amphibole, in agreement with the petrological attributes of the tephra. Comparison between the phase proportions and compositions obtained in experiments and those observed in the rock indicates a pre-eruptive temperature of 950 ± 30° and a melt water content H2Omelt = 2.3 ± 0.3 wt%, for an oxygen fugacity around NNO+1. These T-H2O estimates are confirmed by empirical geothermometers based on experimental clinopyroxene and melt compositional trends. As for other Vesuvius eruptions, the most felsic part of the 1631 reservoir appears to have reached pre-eruptive leucite saturation, although a large amount of leucite microcrystals in the studied samples likely grew syn-eruptively. Our results confirm that magma storage conditions beneath Vesuvius became hotter, shallower, and more CO2-rich after the AD 79 Pompeii Plinian event
Low-P hydrous phase equilibria of a pantellerite melt: constraints on pre-eruptive conditions of recent felsic explosive volcanism at Pantelleria.
No full text
Eruptive paroxysms at Stromboli: an experimental simulation of pre-eruptive conditions of "yellow pumice"
No full textReactive solubility approach to the dissolution of halogens in basaltic melts
No full textPartitioning of halogens between gas and basaltic melts at 1200-1265°C and 10-2000
bar (Alletti, 2008) has been analyzed in detail by means of the CTSFG model (Moretti
and Ottonello, 2005) in order to: 1) evaluate redox conditions consistent with related
experimental information such as PH2 and S2−/S6+ ratio, 2) estimate the relative
abundance of H2O, CO2, KCl, NaCl and HCl of the gas phase, 3) investigate
the solubility mechanisms of chlorine and fluorine in (basaltic) melts. For chlorinebased
runs, it was found that for input [Na+K]/Cl atom ratios > 0.45 the proportion
of gaseous NaCl and KCl exceeds that of HCl. In terms of solubility mechanisms,
halogen chemical reactivity can be assessed in terms of dissolution into the melt as
chloride or fluoride ion, i.e, Cl− and F−. A complementary solubility mechanism can
be invoked to improve the precision and explain somehow unexpected features, such
as i) a P-independent background values of halogen solubility, and, for chlorine only,
ii) a XCl,melt-squareddependence shown by Cl dissolution. In this additional mechanism,
undissociated alkali-halogenides dissolve in the melt up to a solubility limit
which depends on structural conditions, i.e. on melt composition for the same T. Although
this could be a fascinating hypothesis, somehow recalling the concentration
limits of Henry’s law behavior for trace element dissolution into minerals, it is then
questioned whether this may reflect or not mixing of a molten salt-like component
with the silicate melt.
Partition coefficients derived from equilibrium constants of reactions involving Cl−
and F− show well the role of water vapor as stripping agent of chlorine. Therefore,
the more the CO2 in the system, the more the amount of chlorine getting dissolved
into the melt phase
A model of sulphur solubility for hydrous mafic melts: application to the determination of magmatic fluid compositions of Italian volcanoes
No full textWe present an empirical model of sulphur solubility that allows us to calculate f S2 if P, T, fO2 and the melt composition,
including H2O and S, are known. The model is calibrated against three main experimental data bases consisting
in both dry and hydrous silicate melts. Its prime goal is to calculate the f S2 of hydrous basalts that currently lack
experimental constraints of their sulphur solubility behaviour. Application of the model to Stromboli, Vesuvius, Vulcano
and Etna eruptive products shows that the primitive magmas found at these volcanoes record f S2 in the range
0.1-1 bar. In contrast, at all volcanoes the magmatic evolution is marked by dramatic variations in f S2 that spreads
over up to 9 orders of magnitude. The f S2 can either increase during differentiation or decrease during decompression
to shallow reservoirs, and seems to be related to closed versus open conduit conditions, respectively. The calculated
f S2 shows that the Italian magmas are undersaturated in a FeS melt, except during closed conduit conditions,
in which case differentiation may eventually reach conditions of sulphide melt saturation. The knowledge of f S2, fO2
and fH2O allows us to calculate the fluid phase composition coexisting with magmas at depth in the C-O-H-S system.
Calculated fluids show a wide range in composition, with CO2 mole fractions of up to 0.97. Except at shallow
levels, the fluid phase is generally dominated by CO2 and H2O species, the mole fractions of SO2 and H2S rarely exceeding
0.05 each. The comparison between calculated fluid compositions and volcanic gases shows that such an
approach should provide constraints on both the depth and mode of degassing, as well as on the amount of free fluid
in magma reservoirs. Under the assumption of a single step separation of the gas phase in a closed-system condition,
the application to Stromboli and Etna suggests that the main reservoirs feeding the eruptions and persistent volcanic
plumes at these volcanoes might contain as much as 5 wt% of a free fluid phase. Consideration of the magma
budget needed to balance the amounts of volatiles emitted in the light of these results shows that the amount of nonerupted
magma could be overestimated by as much as one order of magnitude.PublishedJCR Journalope
- …
