Natural Silicate Liquids Research Articles

Electrical conductivity of a phonotephrite from Mt. Vesuvius: The importance of chemical composition on the electrical conductivity of silicate melts

The bulk electrical conductivity of the phonotephritic lava from the 1944 eruption of Mt Vesuvius was measured using complex impedance spectroscopy in a multianvil apparatus at 1 GPa and temperatures up to 700 °C. Melting experiments prior to the electrical measurements were also performed on this sample in a piston cylinder apparatus in order to gauge how bulk conductivity varies as a function of its melt fraction. Unlike the behaviour found in basaltic rocks in which conductivity increases with increasing melt fraction, we observe a conductivity decrease of the order of a factor of ten for samples at 700 °C ranging in melt fraction from 32 vol.% to completely molten. We attribute this anomalous behaviour to the progressive loss of highly conductive leucite upon melting. The addition of potassium to the melt phase, however, does not result in an increase of the total alkali concentration due to the melting of other mineral components. We also present an empirical model to predict the electrical conductivity of fully molten silicate liquids as a function of temperature and chemical composition, based on conductivity data for natural silicate liquids found in the literature. The inclusion of compositional terms reduces the error by more than a factor of four with respect to a composition independent, temperature-only parameterization.

Calorimetry of liquids in the system Na2O-Fe2O3-SiO2

Transposed-temperature, drop-calorimetry measurements were performed in the systems Na2SiO3-Fe2O3-SiO2 and Na2SiO3-NaFeSi2O6 at 1373 K to investigate thermodynamic properties of Fe3+-bearing silicate liquids. The results confirm previously measured enthalpy of Na2SiO3 and SiO2 melts. No significant heats of mixing were observed in the Na2SiO3-SiO2 liquids or in peralkaline liquids in the system Na2SiO3-Fe2O3. The enthalpy of mixing of peralkaline liquids in the system Na2SiO3-Fe2O3-SiO2 was less than the standard deviation of drop calorimetry (approx. ±10 kJ/mol). From calorimetric data of Na2SiO3-Fe2O3 liquids extrapolated to hematite composition and published heat capacity, the enthalpy of fusion of hematite is estimated to be 83.3 ± 10.5 kJ/mol at 1373 K and 133.4 ± 10.5 kJ/mol at its melting point 1895 K. The Fe3+/Fe2+ ratio of the liquid was estimated from thermodynamic calculations using the fusion enthalpy of hematite and published data of FeO and O2, and reproduced measured Fe3+/Fe2+ ratios in natural silicate liquids and in CaO-Al2O3-Fe2O3-FeO-SiO2 liquids. No significant heats of mixing were observed in the Na2SiO3-NaFeSi2O6 liquids. The enthalpy of formation of NaFeSi2O6 (acmite) at 1bar and 298 K calculated from calorimetric data is −2546.9 ± 17 kJ/mol. The enthalpy and entropy of fusion of acmite are estimated to be 70.5 ± 9.4 kJ/mol and 51.3 ± 6.8 J/K-mol, respectively, at metastable congruent melting point, 1373 K. The entropy of fusion of acmite is similar to that of NaAlSi2O6 (jadeite), indicating that Fe3+ and Al3+ have analogous structural roles in pyroxene melts.

High pressure melts receive attention at workshop

The physical behavior of natural silicate liquids influences a variety of geodynamic processes including subduction, the generation and chemical evolution of mantle plumes, and the physics of volcanic eruptions. Since most of these processes occur at high pressures, they remain relatively inaccessible to direct observation. Researchers met at a workshop recently to discuss current and future directions of research in natural silicate liquids. At the meeting, sponsored by the Center for High Pressure Research, attendees raised the questions that are necessary to link melt structure research to the physical and thermodynamic properties of silicate liquids that dictate natural igneous processes.

The effect of dissolved water on the oxidation state of iron in natural silicate liquids

The effect of dissolved water on the oxidation state of iron in natural silicate liquids

The effect of dissolved water on the oxidation state of iron in natural silicate liquids

Ferric-ferrous ratios have been measured in 22 experiments on three natural compositions equilibrated at known temperature (950°–1100° C) and oxygen fugacity, and at water-saturated conditions over a pressure range from 0.05 to 0.2 GPa. There does not appear to be any reaction between the melt and the capsule material that affects the redox state of the iron in the melt. An empirical expression for the anhydrous behavior of the redox state of iron in each of these compositions has also been determined at 1 bar as a function of temperature and oxygen fugacity. A direct comparison of the hydrous ferric-ferrous values with the calculated anhydrous values shows that the dissolution of water in a per-alkaline rhyolite, andesite, and an augite minette has no effect on the redox state of the iron in these melts. This result parallels the effect of water on sulfide speciation in basaltic melts, and confirms published results on experimental hydrous basalts.

The behavior of noble gases in silicate liquids: Solution, diffusion, bubbles and surface effects, with applications to natural samples

Solubilities of noble gases in five natural silicate liquids as a function of temperature and partial pressure at 1 atm total pressure were determined and diffusion coefficients of the noble gases were measured in a tholeiite basalt at 1350°C. Solubilities of noble gases in silicate liquids obey Henry's law and are a strong function of composition and/or physical properties of the liquids. Solubility is greatest in less dense, more silica-rich liquids. Solubility is highest for the light gases and is related to the radius of the gas atom according to K i = a exp (− br 2 i ). Temperature dependence of solubility is weak, but in general solubility increases with increasing temperature yielding positive enthalpies of solution. Diffusion coefficients in a basalt liquid at 1350°C show more or less the same linear relationship with r 2 as solubility and are larger than what would be expected from extrapolation of values determined at lower temperatures. A large percentage of samples of andesite composition had bubbles that contained gas which was fractionated from the gas of the experiment. Concentrations of noble gases in samples equilibrated on Pt wire loops correlate with the surface/volume ratio, suggesting that surfaces of silicate liquids can accommodate more noble gases than the liquid proper. Solubility fractionation is a valid process to account for certain patterns in marine basalts. The density of silicate liquids appears to be a good predictor of noble gas solubility in these liquids.

Oxidation-reduction relations in basic magma: a case for homogeneous equilibria

Published analyses of Fe 2O 3 and FeO in basaltic glassy lavas allow their equilibrium oxygen fugacities to be calculated. These values are plotted relative to NNO (nickel-nickel oxide oxygen buffer) at the same temperature (1200°C), so that the relative f O 2 , ΔNNO, becomes essentially independent of temperature. For 61 submarine tholeiitic lavas from the Pacific, ΔNNO lies between FMQ (fayalite-magnetite-quartz oxygen buffer) and NNO, whereas 95 glassy lavas from the Atlantic show a greater range, and can be more oxidised than NNO; in both oceans, alkali types have higher ΔNNO. In contrast, subaerial glassy lavas, dominated by those of Kilauea, are more reduced than the Pacific submarine lavas, and cluster between FMQ and W-Mt (wüstite-magnetite oxygen buffer). This subaerial reduction, first recognised by Anderson and Wright [1], is caused by SO 2 degassing. The iron redox state of Kilauea subaerial and submarine glassy lavas can vary considerably, but as found in other submarine lavas, their sulphur redox state varies more. We conclude that the iron and sulphur redox systems of tholeiitic liquids, in combination, could act as a self-regulating internal oxygen reservoir, constraining a cooling liquid to the constant relative oxygen fugacity observed in well characterised liquid lines of descent. A second experimentally verifiable hypothesis is offered, namely that Fe 2O 3 in natural silicate liquids is very compressible. Thus magmas equilibrating at great depth (∼30–50 kbar) with a solid mantle assemblage of low relative f O 2 (intrinsic measurements) would have a higher iron redox state than those equilibrating with shallow mantle sources at the same relative f O s . The redox states of basic magmas, ascending isochemically and involving Fe and S, could therefore be a depth indicator to their source regions.

Fe2?Mg2 and TiAl2?MgSi2 exchange reactions between clinopyroxenes and silicate melts

The interdependence of the Fe(Mg)−1 (e.g., FeO-MgO in silicate melt; CaFeSi2O6-CaMgSi2O6 in pyroxene) and TiAl2(MgSi2)−1 exchange reactions between silicate melts and coexisting Ca-pyroxene has been examined. High-calcium clinopyroxenes were grown in 1 atmosphere melting and crystallization experiments on rock powders spanning the composition range tholeiite to melilitite (1,092<T<1,150° C: \(f_{{\text{O}}_{\text{2}} } \)∼QFM). Only microprobe analyses of the crystal rims were used in conjunction with those of the quenched glasses. Assuming equilibrium, relationships between apparent distribution coefficients for Fe2+⇄Mg2+ exchange and \((X_{{\text{Ti}}^{{\text{4 + }}} }^{M1} )^{{\text{CPX}}} \) suggest that at given values of \(\mu _{{\text{Fe(Mg)}}_{ - {\text{1}}} } \) extent of Fe(Mg)−1 substitution is strongly coupled with the TiAl2(MgSi2)−1 substitution in pyroxenes near the five-component space CaMg(Si2O6-CaFe(Si)2O6-CaTi(Al)2O6-CaFe‴(Al,Si)2O6-CaAl(Al,Si)2O6. The inferred stabilization of Ti in iron-rich relative to magnesium pyroxene is consistent with the operation of Fe2+⇄Ti4+ intervalence charge transfer interactions (e.g., Rossman 1980) and observations on zoning in natural titanaugites (e.g., Tracy and Robinson 1977). Although the rims of some pyroxenes grown in some melting experiments exhibit prominent zoning in TiAl2(MgSi2)−1, the average values of \((X_{{\text{Ti}}^{{\text{4 + }}} }^{M1} /X_{{\text{Mg}}^{{\text{2 + }}} }^{M1} )\) inferred from the compositions of these pyroxenes, together with those of the relatively homogeneous pyroxenes produced in crystallization experiments, exhibit a 1∶1 correlation with values of \(\mu _{{\text{TiAl}}_{\text{2}} ({\text{MgSi}}_{\text{2}} )_{ - 1} } \) derived from the solution model of Ghiorso et al. (1983) with a standard error of 750 calories. The Ti contents of Ca-rich pyroxenes crystallizing from a wide range of natural silicate liquids can therefore be predicted.

The Gibbs free energy of mixing of natural silicate liquids; an expanded regular solution approximation for the calculation of magmatic intensive variables

The compositions of liquids coexisting with experimentally grown crystals of olivine, plagioclase, clinopyroxene, orthopyroxene, leucite, spinel, rhombohedral oxide, melilite and potassium feldspar are used to define, through mass action expressions of liquid/solid equilibrium, compositional derivatives of the Gibbs free energy of mixing of naturally occuring silicate liquids as a function of temperature, pressure and the fugacity of oxygen. The available experimental data describe these derivatives over a range of compositions which includes basic magmas. Therefore, for silicate liquids in this composition range, the topology of the Gibbs free energy of mixing can be approximated from experimental determinations of its derivatives. The majority of the existing thermodynamic data on the liquid phase is consistent with the application of regular solution theory to model the free energy of mixing. Strictly symmetric, temperature and pressure independent, regular solution interaction parameters are calibrated from this phase equilibrium data using regression techniques which have their basis in inverse theory. These techniques generate numerically stable interaction parameters which incorporate inter-variable correlation and account for experimental uncertainty. The regular solution model fits the available data on anhydrous silicate liquids to within the accuracy of the thermodynamic database +/−550 cals). Extensions to regular solution theory allow water solubility in more silica rich liquids to be modelled somewhat less accurately (+/−750 cals).

The ferric-ferrous ratio of natural silicate liquids equilibrated in air

Results of chemical analyses of glasses produced in 46 melting experiments in air at 1,350° C and 1,450° C on rocks ranging in composition from nephelinite to rhyolite have been combined with other published data to obtain an empirical equation relating in \((X_{{\text{Fe}}_{\text{2}} {\text{O}}_{\text{3}} }^{{\text{liq}}} /X_{{\text{FeO}}}^{{\text{liq}}} )\) to T, \(\ln f_{{\text{O}}_{\text{2}} } \) and bulk composition. The whole set of experimental data range over 1,200–1,450° C and oxygen fugacities of 10−9.00 to 10−0.69 bars, respectively. The standard errors of temperature and \(\log _{10} f_{{\text{O}}_{\text{2}} } \) predictions from this equation are 52° C and 0.5 units, respectively, for 186 experiments.

Ferric-ferrous equilibria in natural silicate liquids at 1 bar

Ferric and ferrous iron concentrations have been measured in 57 silicate liquids equilibrated at temperatures (1,200°–1,330°C) above the liquidus and at oxygen fugacities close to those defined by quartz-fayalite-magnetite. The experimental results reported here span virtually the entire known compositional range of lavas. An empirical equation relating the mole fraction of Fe2O3 and FeO to oxygen fugacity, absolute temperature and liquid composition at 1 bar has been formulated, based on the present experimental results and published data. Extrapolating the proposed empirical relationship over several hundred degrees, for instance below the solidus of the 1965 Makaopuhi tholeiitic lava lake, yields calculated oxygen fugacities which are a little lower than those measured directly in drill holes; at 1,100°C this discrepancy is 0.4 log unit. However, the agreement between the oxygen fugacities calculated from our empirical equation and those indicated by the composition of Fe-Ti oxides in andesites and siliceous obsidians is much closer.

Partial molar volumes of oxide components in silicate liquids

Densities of 21 silicate liquids have been determined from 1,000 ° to 1,600 ° C. The compositions studied contain from two to eight oxide components and have the following ranges in composition (mole %): SiO2, 35–79%; TiO2, 4–36%; Al2O3, 5–25%; FeO, 11–41%; MgO, 7–28%; CaO, 7–35%; Na2O, 5–50%; and K2O, 4–20%. The compositions thus cover the upper range observed in magmas for each oxide. Precision for each determination of liquid density is always better than ±1%. Volumes/gfw (gram formula weight) calculated from the density measurements and the chemical compositions of the analyzed liquids have been combined with data on 96 silicate liquids reported in the literature. From this data set we derive, by using multiple linear regression, partial molar volumes of the components SiO2, TiO2, A12O3, FeO, MgO, CaO, Na2O, and K2O at five temperatures. The standard deviation of the multiple regression is 1.8% of the molar volumes, which is considered about equal to the total errors due to compositional and instrumental uncertainties. These derived partial molar volumes have been used to calculate volumes/gfw of natural silicate liquids which are found to agree within 1% of the measured values. No compositional dependence of the partial molar volumes can be detected within the error considered to be typical of the measurements. This is further supported by the close agreement between the calculated volumes of CaMgSi2O6 and Fe2SiO2 liquids derived from the initial slopes of their fusion curves and their heats of fusion, and the volumes obtained by summing the respective partial molar volumes. The experimental data indicate that silicate liquids mix ideally with respect to volume, over the temperature and composition range of this data set.

Partitioning of iron and magnesium between crystals and partial melts in peridotite upper mantle

The iron-magnesium distribution coefficient, $$K'_D = (X_{\Sigma {\text{FeO}}} /X_{{\text{MgO}}} )^{{\text{olivine}}} (X_{{\text{MgO}}} /X_{\Sigma {\text{FeO}}} )^{{\text{liquid}}} ,$$ has frequently been used as a means of testing whether experimental and natural silicate liquids could have been in equilibrium with olivine of mantle composition. It is shown here that this K′ D decreases with increasing oxygen fugacity (xxx) for a hydrous partial melt in equilibrium with a natural spinel peridotite assemblage under pressure and temperature conditions corresponding to those of the upper mantle (from 0.52 at the xxx of the iron-wustite buffer to 0.04 at the xxx of the magnetite-hematite buffer). K′ D also increases with increasing pressure, with decreasing temperature, and probably with increasing Mg/(Mg+∑ Fe) of the parental peridotite, suggesting that $$K_D = (X_{{\text{FeO}}} /X_{{\text{MgO}}} )^{{\text{olivine}}} (X_{{\text{MgO}}} /X_{{\text{FeO}}} )^{{\text{liquid}}}$$ also increases with increasing pressure and decreasing temperature. Thus, unless these four variables (P, T, xxx, silicate composition) are known for a natural magma, K′ D and probably K D are variables, and the Mg/(Mg+∑ Fe) of such a magma cannot be correlated to that of the parent. The K D determined at 1 atm pressure by Roeder and Emslie has frequently been used to test whether the Mg/(Mg+∑ Fe) ratios of experimentally formed liquids at high pressure in equilibrium with olivine of known Fo content represent the equilibrium Mg/(Mg+Fe2+) of this liquid, assuming that ∑Fe=Fe2+ and that K′ D does not vary with P, T, and composition of the system. Published data demonstrate that the oxygen fugacities of the experimental designs employed by different laboratories vary between those of the magnetite-hematite and magnetite-wustite buffers (6 orders of magnitude), resulting in K′ D between 0.04 and 0.31 at 1050° C and 15 kbar, for example. Thus, published arguments as to whether the quenched liquids represent equilibrium compositions based on iron-magnesium partitioning are inadequate. The effects of P, T, xxx, and the composition of the starting material must also be considered.