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On the saturation surface and oxidation state of C-H-O-S-silicate melt systems
No full textThe equilibrium between a H2O–CO2–SO2–H2S gas phase
and silicate melts is investigated by means of thermochemical
calculations which join homogeneous reactions in the gas
phase and heterogeneous gas–melt saturation modeling based
on classical sub-regular multicomponent mixing and Toop-
Samis polymeric approach. Sulfur in the melt phase is
assumed to be present in two different oxidation states (sulfide
and sulfate ions). The thermodynamic model is an extension
of that presented in Moretti et al. [1] to account for iron
speciation at high pressure with variable dissolved water
contents. The consequences on the equilibrium conditions of
different assumptions on the effective redox buffer in magma
are examined for melts of basaltic and rhyolitic composition,
determining the equilibrium conditions on the basis of i)
constant FeII/FeIII, ii) constant fH2S/fSO2, and iii) constant
relative fO2, expressed as difference in log-units to a solid
buffer. The first two buffers are expected to be effective in
basaltic and andesitic-rhyolitic magmas, respectively,
according to the most abundant reservoir of redox couples.
Furthermore, for each assumed redox buffer the pressure
dependence of phase composition and oxidation state of the
system shows strongly non-linear trends. The largest
compositional differences are shown by sulfur species,
however, the concentrations of H2O and CO2 in the two
phases at equilibrium also show non-negligible dependence on
the redox conditions. For each assumed redox buffer, sulfur
dioxide in the gas phase, and sulfate ions in the liquid phase,
are found to be present in appreciable quantities or represent
the dominating sulfur species. The more reliable redox buffers
represented by constant FeII/FeIII for basalt, and constant
fH2S/fSO2 for rhyolite, show that oxygen fugacity paths
during magma depressurization strongly deviate from those
related to a solid buffer plus or minus a constant.
Results here presented, although not yet accounting for the
separation of S-bearing solid or liquid phases, may furnish
insights on the composition of gases separated from magmas
originated in various geodynamic settings, under different
redox conditions.
Reference
[1] Moretti R., Papale P. and Ottonello G. (2003) In: Volcanic
Degassing (Oppenheimer C., Pyle D. and Barclay J., eds.)
Geol. Soc. London Spec. Publ., 213, 81-101
A NEW METHOD TO COMPUTE FLUIDS SATURATION IN C-H-O-S-SILICATE MELT SYSTEMS
No full textWe developed a method to calculate equilibrium between a
C-O-H-S fluid phase and a silicate melt based on a previous
model for the saturation of H2O-CO2 fluids (Papale, 1999) and
on a thermochemical approach for calculating sulfide and
sulfate solubilities of simple and complex melts. In particular,
this second approach combines the Toop-Samis polymeric
model with the Flood - Grjotheim theoretical treatment of silicate
melts (Ottonello et al., 2001; Moretti, 2002). Moreover,
fugacities in the gaseous phase are computed through the
SUPERFLUID code (Belonoshko et al., 1992). The C-H-O-S
saturation model allows determining the partition of H2O, CO2,
and S between silicate melt and coexisting fluid, and the
composition of the fluid phase in terms of H2O, CO2, SO2, and
H2S, as a function of pressure, temperature, volatile-free liquid
composition, oxygen fugacity, and total amount of volatile
components in the system. For the sake of simplicity, we
assumed that no reduced or oxidized sulfur-saturated solid or
liquid phases nucleate or separate from the liquid-gas system.
Minima in sulfur solubility as a function of oxygen fugacity are
depicted, in good agreement with theory and experiments.
Applications are given for rhyolitic and basaltic melts with
various oxygen fugacities in the range NNO±2, and pressure
from a few hundred MPa to atmospheric. The developed model
accounts for the reciprocal effects of volatiles on their saturation
contents, and the complex relationships between the saturation
surface of a multicomponent fluid and the liquid
composition, volatile abundance, P-T conditions and oxidation
state.
Belonoshko A, Shi PF & Saxena S, Comp. Geosci, 18, 1267-
1269, (1992).
Moretti R, PhD Thesis, University of Pisa
Ottonello G, Moretti R, Marini L& Vetuschi Zuccolini M, Chem.
Geol, 174, 157-179, (2001).
Papale P, Amer. Mineral, 84, 477-492, (1999)
A model for multicomponent fluid saturation in C-O-H-S-silicate melt systems
No full textThe dissolution behavior of volatile components in magmas is essential to model the
volcanic process from the deep regions of magma generation and storage to the shallow
regions of magma eruption and emplacement.
Water, carbon dioxide, and sulfur compounds are the main volatile components in natural
magmas, constituting in most cases more than 99% of the volcanic gases released
before, during, and after eruption.We have developed a method to calculate the chemical
equilibrium between a fluid phase in the C-O-H-S system and a silicate melt with
composition defined by ten major oxides. The method is based on a previous model
for the saturation of H2O-CO2 fluids [1] and on a sulfur solubility model [2] in silicate
liquids. For the computation of the fugacities of components in fluids with complex
composition we used the SUPERFLUID code [3]. The model allows determining the
partition of H2O, CO2, and S between the silicate liquid and the coexisting fluid, and
the composition of the fluid phase in terms of H2O, CO2, SO2, and H2S, as a function
of pressure, temperature, volatile-free liquid composition, oxygen fugacity, and
total amount of each volatile component in the system. App lications are presented
to several silicate liquids with rhyolitic and basaltic composition, oxygen fugacities
in the range NNO ± 2, and pressure from a few hundred MPa to atmospheric, with
the simplifying assumption that no reduced or oxidized sulfur-saturated solid or liquid
phases nucleate or separate from the liquid-gas system. Results show the well-known
minima in sulfur saturation contents as a function of oxygen fugacity, the reciprocal
effects of volatiles on their saturation contents, and the complex relationships between
saturation surface of a multicomponent fluid, liquid composition, volatile abundance,
P-T conditions, and oxidation state. The method represents therefore a new powerful
tool for the prediction of multicomponent gas-melt equilibria in magmas.
REFERENCES
[1] Papale P. (1999) Am. Mineral., 84, 477-492
[2] Moretti R. and Ottonello G. This issue
[3] Belonoshko A.B., Shi P. and Saxena S,K, (1992) Comp. Geosci., 18, 1267-1269.
The compositional dependence of the saturation surface of H2O+CO2 fluids in silicate melts
No full textThe volatile saturation surface in H2O-CO2-silicate melt systems is modeled by applying thermodynamic equilibrium between gaseous and liquid volatile components. The whole database of existing saturation data in the C-O-H-silicate liquid systems has allowed us to re-calibrate a previously developed fully multicomponent H2O-CO2 saturation model [Papale, P., 1999. Modeling of the solubility of a two-component H2O + CO2 fluid in silicate liquid. Am. Mineral., 84, 477-492]. The new database nearly doubles the previous one, greatly improving the performances of the whole model, which now adopts a significantly lower number of model parameters with respect to the previous calibration. The multicomponent H2O + CO2 saturation model is fully non-ideal, the only assumption being that the excess Gibbs free energy of the silicate mixture can be represented by an expansion of first-order symmetric interaction terms. No a-priori assumption is made on the P-T dependence of the volatile-oxide interaction terms, meaning that no assumption is made on the partial molar volume and enthalpy of the dissolved volatiles. The whole treatment is evaluated by restrictive statistical algorithms, which confirm the model validity on an extended database. The model allows to investigate extensively the dependence of the complex volatile saturation surface on composition. In order to explore the non-linear behaviors implicit in the physics of the dissolution process, the model is employed in a series of calculations aimed at illustrating some of the compositional features of the volatile saturation surface in both one-component and two-component volatile conditions. The results show compositional-dependent minima and maxima, some of which are known from the experiments. Non-ideal behavior is enhanced in two-component fluid phase conditions and pressures above a few hundreds MPa, where calculated isobaric H2O-CO2 saturation curves reveal the possible existence of a maximum in CO2 saturation at non-zero H2O contents. Due to the compositional dependence of the volatile saturation surface, it is outlined the important role played by redox conditions, especially in iron-rich melt systems like basalts
Volatile solubility and melt reactivity in the C-H-O-S-silicate liquid system: the role of redox variables
No full textMODELING THE COMPLEX SATURATION SURFACE OF CO2-H2O-SO2-H2S-SILICATE MELT SYSTEMS: TWO-WAY LINK BETWEEN EXPERIMENTS AND THEORY
No full textThermodynamic equilibrium in multicomponent gas-melt systems is
characterized by complex non-linear distributions of species concentrations,
especially when different oxidation states are possible. Such distributions affect
all the physico-chemical properties and largely drive the dynamics of magmatic
systems. In order to simulate the saturation surface of
CO2-H2O-SO2-H2S-silicate melt systems, we have combined different modeling
approaches based on classical Gibbs thermodynamics and Toop-Samis polymeric
treatment of silicate melts. The model is developed according to more than
2,500 experimental data from the literature on saturation contents of H2O, CO2,
S, and iron oxidation state, in silicate melts with compositions from
two-component synthetic to natural, and in wide P-T ranges. Model applications
to natural systems reveal that simple trends characterizing one-component gas
phases can be deeply modified due to the multicomponent nature of the
equilibrium. The theoretical S-solubility minimum commonly observed in
laboratory experiments at fixed fSO2 can be totally hidden in the
multicomponent system, a feature which has puzzled the interpretation of
natural samples. Maxima and minima in the concentration of volatile
components in the melt and gas phases can characterize depressurization paths,
depending on the specific redox state of the system. Critical experiments aimed
at revealing such complex patterns can be defined with the aid of
multicomponent modeling. In this way, an efficient feedback between
experimental and theoretical investigations provides the means to understand
and reproduce the complex behaviors of volatiles in natural magmas
ON THE EVOLUTION OF VOLATILES AND OXIDATION STATE IN MULTI-COMPONENT DEGASSING MAGMAS
No full textThe study of volcanic degassing requires the definition of gas-melt equilibria in order
to relate observed gas compositions to magma PTX conditions. This knowledge,
unless prevented by scrubbing of magmatic gases by hydrothermal systems at
shallower levels, rests on the constitution of saturation models of the fluid phase in
melts, which should also account for the evolution of both volatiles and oxidation
state in a multicomponent degassing magma.
In the geochemical literature of fluid systems, the oxidation state is often carefully
evaluated in terms of ratio of moles of species of altervalent elements, such as
CO/CO2, H2/H2O and H2S/SO2 with oxygen fugacity as by-product. On the other
hand, in the petrological literature, the magmatic oxidation state is often given as
fO2, usually expressed as difference in log-units to a solid buffer assemblage, such
as QFM or NNO. Many petrological calculations then assume oxygen fugacity as the
parameter controlling the oxidation state of magmas, implying that redox couples
furnished by abundant altervalent elements such as iron and sulfur are governed
by the fugacity of molecular oxygen, a quantity which is actually undetectable.
The composition of gas released in the C-H-O-S-silicate melt system is obviously
influenced by this assumption of constant oxygen fugacity along the PTX pathways
of system evolution. Also the interpretation of surface data, such as those resulting
from remote-sensing of plume emissions or in-situ sampling of exhaling vents, may
be affected by this assumption when recasting petrological information on the feeding
magmatic system.
In this study we show that application of FeO/Fe2O3 and H2S/SO2 buffers results in
PTX paths of gas exsolution which differ sensibly from those obtained at constant
oxygen fugacity. These complex pathways of gas release and oxidation state evolution
cannot be simply calculated or extrapolated, but must be carefully modeled, being
strongly related to the non-linearity intrinsic in calculations whenever confident
solubility models are joined in a unique saturation algorithm.
The study is based on the model of Moretti et al. (2003), implemented for the calculation
of the ferric to ferrous iron ratio under hydrous conditions in the framework of
an ionic-polymeric approach which resolves the controversies of literature about the
oxidation state of hydrated magmas.
REFERENCES:
Moretti et al. (2003) in Volcanic degassing (Oppenheimer et al., eds.), Geol. Soc.
London Spec. Publ., 213, 8
Arienzo I., MORETTI R., Civetta L., Orsi G., Papale P. (2008) The feeding system of Agnano-Monte Spina eruption (Campi Flegrei, Italy): dragging the past into present activity and future scenarios
No full textThe Agnano Monte Spina (Campi Flegrei, Italy ( 4100 years BP) eruption is a reference scenario for a next large scale eruption at Campi Flegrei caldera, and is here selected to investigate the physico-chemical conditions of the pre-eruptive magmatic system as well as to gain insights into the source processes responsible of the huge hydrothermal-magmatic activity observed at surface nowadays. Isotope data on whole rocks and glasses and melt inclusions studies suggest that two chemically and isotopically distinct magmas, with different volatile signature mixed before the eruption. Our new data reveal that one of the magmas involved in the mixing process is similar to the less differentiated shoshonitic magma erupted at around 10 ka BP, whereas the second represents a residual of the magma discharged during the Neapolitan Yellow Tuff caldera forming eruption. Hence, the mixing process is driven by an abundant gas phase which sustains the ascent of magma blobs of deep provenance. The H2O and CO2 contents in melt inclusions give entrapment pressures between 60 and 150 MPa, corresponding to depths between 2.5 and 6 km. Degassing trends show the presence of two extreme patterns, one likely to represent the volatile signature of magma ascending from depth > 7 km; the other one related to a gas-dominated magma residing at shallow depth and developed upon flushing by deep CO2-rich gas. We suggest that volatile- rich blobs of deep shoshonitic magma periodically ascended and mixed with trachy-phonolitic magma at shallower depths. Our model is consistent with the bulk of geophysical and petrological observations at Campi Flegrei, and allows us to outline the role of magma mixing as a primary feature at Campi Flegrei caldera, as supported by the results of previous investigation of other eruptions in the area A major outcome of this study is the conceptual frame it deserves for recent unrest crises at Campi Flegrei, including the 1982-84 bradyseism. Uplift phases associated to bradyseismic crises are related to major episodes of closed-system ascent of magma blobs from depth > 7 km, followed by single-step volatile release upon their emplacement at shallow levels (3-4 km). This leads both the shallow magmatic and geothermal systems to store and progressively release important amounts of gas, hence energy. In this view, eruptive episodes are strongly conditioned by the critical achievement of an upper limit of gas storage, and by the crustal stress state and the fracturing state of the overlying cap of rocks
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