Giulio Ottonello

Irreversible water–rock mass transfer accompanying the generation of the neutral, Mg–HCO3 and high-pH, Ca–OH spring waters of the Genova province, Italy

In a recent survey of the spring waters of the Genova province, many neutral Mg–HCO3 waters and some high-pH, Ca–OH waters were found in association with serpentinites. All the springs are of meteoric origin as indicated by the stable isotopes of water and dissolved N2 and Ar. Interaction of these meteoric waters with serpentinites determines a progressive evolution in the chemistry of the aqueous phase from an immature Mg-rich, SO4–Cl facies of low salinity to an intermediate Mg–HCO3 facies (pH 7.0–8.5, PCO210−3.5–10−2.5 bar, Eh 150–250 mV), and to a mature Ca–OH facies (pH 10–12, PCO2 10−9.4−10−10.6 bar, Eh-390 to-516 mV). The irreversible water–rock mass transfer leading to these chemical changes in the aqueous phase was simulated through reaction path modeling, assuming bulk dissolution of a local serpentinite, and the precipitation of gibbsite, goethite, calcite, hydromagnesite, kaolinite, a montmorillonite solid mixture, a saponite solid mixture, sepiolite, and serpentine. The simulation was carried out in two steps, under open-system and closed-system conditions with respect to CO2, respectively. The calculated concentrations agree with analytical data, indicating that the computed water-rock mass transfer is a realistic simulation of the natural process. Moreover, the simulation elucidates the role of calcite precipitation during closed-system serpentinite dissolution in depleting the aqueous solution of C species, allowing the concurrent increment in Ca and the acquisition of a Ca–OH composition. Calcium–OH waters, due to their high pH, tend to absorb CO2, precipitating calcite. Therefore, these waters might be used to sequester anthropogenic CO2, locally preventing environmental impact to the atmosphere.

Oxidation state of iron in silicate glasses and melts: a thermochemical model

The acid and base dissociation constants of FeO and Fe2O3 components in silicate melts are defined in terms of observed relationships between atomistic properties of dissolved oxides (nephelauxetic parameters, electronegativity, fractional ionic character of the bond) and polymerization constants in simple systems. These constants are obtained from the Toop–Samis model depicting the Gibbs free energy of mixing of binary MO–SiO2 melts, which is coupled with the amphoteric treatment of altervalent dissolved oxides. Model parameterization is carried out on the basis of the extended set of data concerning thermodynamic activity of FeO in melts buffered by equilibrium with pure iron metal and a gaseous phase and on the various measurements of bulk redox state of iron in chemically complex melts at various T, fO2 conditions. Dissociation constants are related to thermodynamic parameters of the main dissolved species (O2−, Fe2+, Fe3+, FeO2−) without any significant error progression. As an ancillary result, thermochemical calculations allow to quantify to some extent the systematic errors in the FeII/FeIII bulk redox ratio arising from the utilization of Mossbauer spectroscopy on quenched melts and glasses.

Petrogenesis of some Ligurian peridotites—II. Rare earth element chemistry

Abstract REE compositions of nine partially serpentinized Iherzolitic rocks from Liguria have been investigated: eight samples are from the Erro-Tobbio thrust sheet of the Gruppo di Voltri (western Liguria); one sample is from an allochthonous eastern Liguria mass within the External Ligurides. As previously described ( Ernst and Piccardo , 1979, Geochim. Cosmochim. Acta43, 219–237), all specimens are spinel (±plagioclase) Iherzolites. They possess major element bulk rock chemistries which are more or less ‘primitive’ or only slightly depleted compared to estimated upper mantle compositions. Mineral chemistries, varying insignificantly from rock to rock, evidently reflect mantle equilibration in the range 1000–1150°C and 14–16 kbar; pervasive retrograde effects testify to later decompression at shallower levels characterized by temperatures exceeding a typical intraplate oceanic geothermal gradient. The investigated lherzolitic samples show concordant REE patterns, LREE-depleted with respect to chondrites. A wide range in values is observed among the LREE: the observed depletion of LREE in most of the samples is accompanied by a Ce-negative anomaly. Inasmuch as the analyzed samples have been affected to a variable extent by serpentinization, REE mobility during hydrous alteration is treated employing both a theoretical approach and natural evidence. Effects of partial melting prior to serpentinization (resulting chiefly in depletion in LREE) can be rationalized by postulating a two-times chondritic REE abundance in the original unaltered peridotitic material and by considering the HREE contents to have been unaffected by hydrous alteration. When comparison is made with REE compositions for residual material after different degrees of partial melting (computed using an equilibrium, non-modal partial melting model and selected REE partition values), the analyzed Ligurian samples show effects of incipient melting on only a small scale, i.e. 1–5%. Judging from their major and REE compositions, the investigated lherzolites cannot have produced substantial amounts of tholeiitic melt, hence they cannot have provided the primary magmas for the oceanic crustal sequences so widespread within the Northern Apennine and Gruppo di Voltri ophiolitic terranes.

Thermo-chemical and thermo-physical properties of the high-pressure phase anhydrous B (Mg14Si5O24): An ab-initio all-electron investigation

Abstract Using the hybrid B3LYP density functional method, we computed the ab-initio thermo-chemical and -physical properties of the mineral anhydrous B (Anh-B), which has been recently suggested as a potential phase responsible for the X-discontinuity in the Earth’s mantle at ~300 km depth through the reaction forsterite + periclase = Anh-B, and also to likely split the 410 km discontinuity within the interior of a cold slab through the reaction wadsleyite/ringwoodite = Anh-B + stishovite. We first conducted an investigation of the static properties through a symmetry-preserving relaxation procedure and then computed, on the equilibrium structure, harmonic vibrational modes at the longwavelength limit corresponding to the center of the Brillouin zone (k → 0). While optic modes are the eigenvectors of the Hessian matrix at Γ point, acoustic modes were obtained by solving the non-zero components of the strain matrix. Following the Kieffer model, acoustic branches were assumed to follow sine wave dispersion when traveling within the Brillouin zone. All thermodynamic properties that depend on vibrational frequencies namely, heat capacities, thermal expansion, thermal derivative of the bulk modulus, thermal correction to internal energy, enthalpy, Gibbs free energy, thermal pressure and Debye temperature, were computed on the basis of quasi-harmonic mode-gamma analysis of the volume effects on vibrational frequencies. Moreover, the strain tensor was used to calculate several thermo-physical properties of geophysical interest (transverse and longitudinal wave velocities, shear modulus, Young’s modulus, and Poisson’s ratio). The ab-initio results derived in this study and the available data on molar volumes were used to calculate the univariant equilibrium forsterite + periclase = Anh-B. The results are in satisfactory agreement with the reversed experimental data of Ganguly and Frost (2006).

Water-rock interaction in the Bisagno valley (Genoa, Italy): application of an inverse approach to model spring water chemistry

Abstract The chemical evolution of the Bisagno valley spring waters, ranging from initial Ca-HCO3 towards final Na-HCO3 composition is reconstructed with a reaction progress scheme, defined by a fractional degree of advancement (ζ) of the irreversible mass-transfer process and attaining the continuum limit during water-rock interaction. The system is solved in terms of a transposed reaction rate vector, introducing the experimental kinetic rate constants and solving for the surface areas of dissolving albite and K-feldspar and precipitating carbonates. This is also done by considering the activities of dissolved silica and Al3+ ion as constrained by the instantaneous equilibrium with quartz and kaolinite, respectively. The results of this inverse geochemical model are fully consistent with those obtained using the EQ3NR-EQ6 forward code, provided this last is run in iterative way, changing progressively the surface areas of solid reactants and fCO2 values. Accounting for the compositional dependence of the kinetic rate constants of plagioclase and alkali feldspar allows explanation of the progressive albitization of the Mt. Antola Formation through a kinetic phenomenon involving nowadays circulating fluids. The comparison of computed surface areas with estimates based on the modal mineralogy, grainsize and intergranular porosity of the arenitic siliciclastic beds outlines the importance of fracture-driven flow in agreement with available Lugeon flow tests.

Polymerization and disproportionation of iron and sulfur in silicate melts: insights from an optical basicity-based approach

Abstract In silicate melts, the speciation state of altervalent elements depends on the extension and distribution of polymeric units in the system. Since polymerization of silicate melts is driven by acid–base properties of constituting oxides depending on their Lux–Flood reactivity, the bulk optical basicity of molten silicates and glasses allows us to compute the distribution of oxygen among three distinct polarization states (O2−, O−, O0) in conjugation with the polymeric model of Toop and Samis. Data available in literature are employed to investigate the mutual interaction between sulfur and iron in silicate melts to constrain the equilibrium between sulfide and sulfate species. Results put in evidence (i) the role of bulk melt chemistry, affecting through polymerization, and hence the transfer of electrons among the three polarization states of oxygens, the oxidation state of altervalent elements such as Fe and S and (ii) the limits of the Temkin approach implicit in chemical equations that consider sulfide and sulfate in melts as free anions. Improvements of the model’s accuracy for sulfur speciation may be attained by accounting for the various dissociation equilibria of molten sulfide M2/νS, oxide M2/ν and sulfate M2/νSO4 species in their standard state of pure component in the melt phase at temperature and pressure of interest instead of that of complete dissociation required by the Temkin model.

Temperature, composition, and fO2 effects on intersite distribution of Mg and Fe2+ in olivines

Single-crystal X-ray structure refinement of natural olivines equilibrated at high temperature under controlled oxygen fugacity (fO2) conditions, coupled with a structure-energy model were used to establish the influence of T, fO2 and bulk chemistry on intracrystalline disorder.The results are: 1) The kD (kD = [(Fe M1·Mg M2)/(Fe M2·Mg M1)]) factor describing site population on M1, M2 polyhedra increases from values lower than 1 at T below 400–600° C (depending on composition) to values higher than 1 at higher temperature. 2) The increase of kD with T is quite regular. 3) At constant temperature and pressure, kD increases with increasing fayalite content in the mixture; 4) Contrary to previous observations (Will and Nover 1979; Nover and Will 1981) varying fO2, within the stability range of the substance, has a negligible influence on intracrystalline disorder. 5) As ancillary results, the model confirms the defect scheme of Nakamura and Schmalzried (1983) for the investigated solids. Moreover, it shows that cationic vacancies are always created on M 1 site at the expense of Mg ions, while trivalent iron is always stabilized on M2 site. This explains the marked anisotropies observed in Fe-Mg interdiffusion (Buening and Busek 1973; Misener 1974; Schock et al. 1989).

Thermodynamic constraints arising from the polymeric approach to silicate slags: the system CaO–FeO–SiO2 as an example

Abstract Systematization of mixing properties in the system CaO–FeO–SiO2 in the liquid state is carried out in the framework of some simplifying assumptions arising from polymer theory. The Gibbs free energy of mixing is conceived as composed of a chemical interaction term and a mechanical strain energy term arising from chain elasticity of polymeric units. The chemical interaction is resolved adopting a modified Toop–Samis approach. The polymerization path is shown to be compositionally dependent upon the local polymerization constant, Kp that, in its turn is exponentially related to the values attained at the limiting binaries, i.e.: K p , CaO–FeO–SiO 2 =K p , CaO–SiO 2 X Ca X Ca +X Fe K p , FeO–SiO 2 X Fe X Ca +X Fe . The mechanical strain energy is shown to be satisfactorily reproduced by the equation G strain = 3 RT 2 ν Si x a 2 , where ν Si is the local mean polymerization extent and x/a is a bending term taking into account the relative arrangement of monomers in the chain. The model reproduces fairly well the experimentally observed thermodynamic activities of the components along the limiting binaries and within the ternary field.

Water-rock interaction on Zabargad Island, Red Sea—A case study: I. Application of the concept of local equilibrium

Abstract Zabargad Island is an uplifted fragment of Red Sea lithosphere. The main lithospheerc unit of the island is constituted by exceptionally fresh peridotite. Fractured portions of the peridotite bodies, however, locally underwent complex water-rock interactions. We apply a nonlinear minimization routine to study this complex event in the framework of the postulate of local equilibrium in disequilibrium processes (Prigogine, 1955). Such a study allows a rough estimate of the chemistry of interacting fluids (and of P, T conditions at equilibrium). The process took place at an average pressure of 1 kbar and, after an initial hydration stage, a thermal maximum was reached at about 500°C, corresponding to the zone of retrograde formation of olivine gems, after which the temperature progressively fell to about 200°C. Fluids were hypersaline and extremely aggressive (pH = 2–3.5). The composition of the fluids plots in the three-phase region of the H2OCO2NaCl system (i.e., an “aqueous” liquid coexisting with a CO2-rich gas phase and halite).

Energy and long-range disorder in simple spinels

Interionic potential calculations conducted on simple spinels in the framework of the Born model indicate that, within experimental uncertainties, observed control of P, T intensive variables on long-range disorder may be completely due to static effects. Parameterization of repulsive potentials allows us to rationalize the existing experimental data and quantify the variation of static energy, configuration entropy and static Gibbs free energy with temperature. The static energy of the investigated phases cannot be expressed in simple quadratic terms of the inversion parameter, as commonly assumed, but requires higher order terms. Knowledge of the longrange disorder at various temperatures allows us to express configurational entropy as a Maier-Kelley type polynomial expansion on temperature. As configurational entropy simply depends on the static part of the internal energy, the integration of the obtained functions in d T allows rapid evaluation of the effect of long-range disorder on the static part of energy.