Victor C. Kress

The compressibility of silicate liquids containing Fe2O3 and the effect of composition, temperature, oxygen fugacity and pressure on their redox states

Ultrasonic longitudinal acoustic velocities in oxidized silicate liquids indicate that the pressure derivative of the partial-molar volume of Fe2O3 is the same in iron-rich alkali-, alkaline earth- and natural silicate melt compositions at 1 bar. The dV/dP for multicomponent silicate liquids can be expressed as a linear combination of partial-molar constants plus a positive excess term for Na2O−Al2O3 mixing. Partial-molar properties for FeO and Fe2O3 components allow extension of the empirical expression of Sack et al. (1980) to permit the calculation of Fe-redox equilibrium in a natural silicate liquid as a function of composition, temperature, fo2 and pressure; a more formal thermodynamic expression is presented in the Appendix. The predicted equilibrium fo2 of natural silicate melts, of fixed oxygen content, closely parallels that defined by the metastable assemblage fayalite+magnetite+β-quartz (FMQ), in pressure-temperature space. A silicate melt initially equilibrated at 3 GPa and FMQ, will remain within approximately 0.5 log10 units of FMQ during its closed-system ascent. Thus, for magmas closed to oxygen, iron-redox equilibrium in crystal-poor pristine glassy lavas represents an excellent probe of the relative oxidation state of their source regions.

Ultrasonic investigation of melts in the system Na2OAl2O3SiO2

Longitudinal acoustic velocities have been measured for liquids in the system Na2OAl2O3SiO2 spanning much of the composition range Na2SiO3NaAlSiO4SiO2. Measurements were made at temperatures between 1185°C and 1620°C, using the technique of ultrasonic interferometry. Sound speeds, and therefore bulk moduli, show a general increase with increasing alumina content. This trend is explained qualitatively in terms of a molecular model of the effect of sodium and aluminum on the structure and dominant compression mechanism of silicate liquids. We present a computational model for estimation of both sound velocities and isothermal compressibilities within much of the compositional ternary Na2SiO3NaAlSiO4SiO2. Implications of these results for the high pressure anhydrous melting curve of albite (NaAlSi3O8) are also discussed. Acoustic velocities are presented for liquid sodium chloride in order to provide a basis for comparison with data from other laboratories.

The lime-iron-silicate melt system: Redox and volume systematics

Abstract Fifty experiments are presented which explore ferric ferrous redox equilibrium in melts in the CaO-FeO-FeO 1.5 -SiO 2 system. Plots of ln( FeO 1.5 FeO ) against ln(ƒ o 2 ) for compositions equilibrated at a single temperature have slopes of 0.21. This result is consistent with observations in a wide variety of silicate liquid compositions, including compositions corresponding to those found in nature, but contrasts with the slope expected if ferric-ferrous mixing were ideal (0.25). Compositional trends for redox equilibrium in the lime-iron-silicate system contrast with those found in the Na 2 O-FeO-FeO 1.5 -SiO 2 system. It is shown that minor amounts of quench crystals in the glasses significantly increase ferric ferrous ratios from their equilibrium values. The results of quench crystal-free ferric ferrous equilibration experiments are used to constrain a simple empirical equation by which one can estimate the distribution of iron between the ferric and ferrous species at a given composition, temperature, and oxygen fugacity. This predictive model reproduces measured ferric ferrous distribution within analytical error. The simplest thermodynamically valid model by which the novel systematics of the iron redox reaction in silicate melts can be rationalized involves partial association of FeO and FeO 1.5 to a hypothetical melt species with FeO 1.3 stoichiometry. We apply our empirical ferric ferrous estimation model to recalculate the ferric ferrous distribution in the liquid-density experiments of Dingwell and Brearley (1988) and Mo et al. (1982). A simple (ideal) volume of mixing model is derived by which the volumes of CaO-FeO-Fe 2 O 3 -SiO 2 melts with more than 20 wt% silica can be estimated to within ±0.16 cc/gfw ( 2σ

When is a silicate melt not a liquid

0.7% ). This average residual in our linear volume model is well below the reported experimental uncertainty ( Dingwell and Brearley , 1988), indicating that partial molar volumes can be considered independent of composition within the experimental resolution in melts in this composition range. Density data in melts with very low silica ( CaO SiO 2 less than 0.3) suggest a negative volume of mixing between CaO and SiO 2 components.

Stoichiometry of the iron oxidation reaction in silicate melts

Abstract Recent evidence from static compression, spectroscopy and relaxational studies all support the conclusion that there are no significant structural relaxation mechanisms with mean durations longer than the order of microseconds (frequencies below the MHz range) at superliquidus temperatures in silicate compositions. Evidence suggests that volume and shear relaxation times are similar, and are distributed over a narrow range of frequencies. These relaxation processes probably represent high-frequency high-temperature manifestations of the glass transition. The results of this study support earlier assertions that relaxed longitudinal ultrasonic velocities provide an accurate means of determining the compressibility of silicate melts. The observed discrepancy between ultrasonically derived compressibilities and those derived from high-pressure falling-sphere experiments cannot be attributed to relaxation effects.