Effects of structural relaxation on cationic tracer diffusion in silicate melts

Dingwell, Donald B.

doi:10.1016/0009-2541(90)90082-i

Cited by 77 publications

(37 citation statements)

References 34 publications

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“…E Ca does, however, appear to be strongly correlated with the ratio D Ca / D Si (Fig. 4), which represents the ratio of the diffusivity of a "solute" component relative to that of the "solvent" The observed range in D i /D Si and its explanation have been described previously (Dingwell, 1990) and it has been shown that D i /D Si generally increases with increasing liquid viscosity g (cf. Dingwell, 2006).…”

Section: Isotope Separation By Diffusion In Silicate Meltsmentioning

confidence: 82%

Influence of liquid structure on diffusive isotope separation in molten silicates and aqueous solutions

Watkins

DePaolo

Ryerson

et al. 2011

Geochimica et Cosmochimica Acta

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Molecular diffusion in natural volcanic liquids discriminates between isotopes of major ions (e.g., Fe, Mg, Ca, and Li). Although isotope separation by diffusion is expected on theoretical grounds, the dependence on mass is highly variable for different elements and in different media. Silicate liquid diffusion experiments using simple liquid compositions were carried out to further probe the compositional dependence of diffusive isotopic discrimination and its relationship to liquid structure. Two diffusion couples consisting of the mineral constituents anorthite (CaAl 2 Si 2 O 8 ; denoted AN), albite (NaAlSi 3 O 8 ; denoted AB), and diopside (CaMgSi 2 O 6 ; denoted DI) were held at 1450°C for 2 h and then quenched to ambient pressure and temperature. Major-element as well as Ca and Mg isotope profiles were measured on the recovered quenched glasses. In both experiments, Ca diffuses rapidly with respect to Si. In the AB-AN experiment, D Ca /D Si ~ 20 and the efficiency of isotope separation for Ca is much greater than in natural liquid experiments where D Ca /D Si ~ 1. In the AB-DI experiment, D Ca /D Si ~ 6 and the efficiency of isotope separation is between that of the natural liquid experiments and the AB-AN experiment. In the AB-DI experiment, D Mg /D Si ~ 1 and the efficiency of isotope separation for Mg is smaller than it is for Ca yet similar to that observed for Mg in natural liquids.The results from the experiments reported here, in combination with results from natural volcanic liquids, show clearly that the efficiency of diffusive separation of Ca isotopes is systematically related to the solvent-normalized diffusivity-the ratio of the diffusivity of the cation (D Ca ) to the diffusivity of silicon (D Si ). The results on Ca isotopes are consistent with available data on Fe, Li, and Mg isotopes in silicate liquids, when considered in terms of the parameter D cation /D Si . Cations diffusing in aqueous solutions display a similar relationship between isotopic separation efficiency and D cation =D H 2 O , although the efficiencies are smaller than in silicate liquids. Our empirical relationship provides a tool for predicting the magnitude of diffusive isotopic effects in many geologic environments and a basis for a more comprehensive theory of isotope separation in liquid solutions. We present a conceptual model for the relationship between diffusivity and liquid structure that is consistent with available data.

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Section: Isotope Separation By Diffusion In Silicate Meltsmentioning

confidence: 82%

Influence of liquid structure on diffusive isotope separation in molten silicates and aqueous solutions

Watkins

DePaolo

Ryerson

et al. 2011

Geochimica et Cosmochimica Acta

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“…The B tracer diffusivity is expected to be faster than, but perhaps within a log unit, of the Si tracer diffusivity at the same composition at the high temperatures of these experiments ( DINGWELL, 1990). The tracer di&sivity of Si in a melt may be related to the viscosity of the melt by the Eyring equation ( GLASSTONE et al, 194 I ) and calculations for a wide variety of melts using reasonable estimates of jump distances (see, for example, SNIMIZU and KUSHIRO, 1984;DINGWELL, 1990) show the relationship to be a useful approximation. The jump distances used in these studies are the diameter of the oxygen ion (2.8 nm).…”

Section: Tracer Diffusivity Of Boronmentioning

confidence: 99%

Chemical diffusivity of boron in melts of haplogranitic composition

Chakraborty

Dingwell

Chaussidon

1993

Geochimica et Cosmochimica Acta

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A~~~-~erni~Asiatic of B in synthetic melts of hapl~~tic composition have been measured by the diffusion couple technique at 1 atm between 1200-1600°C. The compositional profiles were measured by ion microprobe and modelled using the Boltzmann-Matano formalism to retrieve compositionally dependent interdiffision coefficients.At the experimental conditions, &OS is found to exchange primarily with SiO, and the interdiffusion coefficient increases with increasing replacement of Si by B in the melt. No isotopic fractionation of boron was observed in the diffusion zone at the experimental conditions. The compositional dependence of di~sivity increases with decreasing temperature. The activation energy of Dillon ( -70 kcal) is similar to that for viscous flow in melts of the same composition and is relatively insensitive to B content between l-10 w-t% B203 in the melt. However, the addition of the initial 1 wt% B203 to a haplogranitic melt appears to dramatically lower the activation energy for these processes from -100 kCa1 to -70 kCal. Thus, common geochemical concentrations of B may affect petrogenesis of granitic rocks by their influence on these transport properties. Some implications of these results for crystal growth and dissolution in B-bearing melts and boron isotopic variation of granitic melts have been discussed. If diffusion is the mte-limiting process, boron isotopic heterogeneity may be maintained in granitic melts at magmatic temperatures on time scales of millions of years on a millimeter scale. The influence of small amounts of B on transport properties may also contribute toward resolution of an enigma regarding emplacement mechanisms of peraluminous granites. I~ODU~ONTHE GEOCHEMICAL SIGNIFICANCE of B has been receiving increasing attention in recent years. It has been shown that the B content of late-stage fluids during the evolution of granite-pegmatite systems may be as high as 5-10 wt% ( LON-DON, 1986a). A body of experimenta data now exists on some ~uilib~um properties of B-bearing melts of geological relevance. For example, experiments have shown that the addition of B lowers both the solidus and liquidus temperatures of water-saturated granitic melts (CHORLTON and MARTIN, 1978; PICHAVANT, 198 1, 1987), shifts the composition of minimum melting in the haplogranite system to more albite-rich compositions ( PICHAVANT, 1987), and increases the solubility of water in melts in the haplogranitic system ( HOLTZ et al., 1993). The influence of B on the differentiation trends of granite-pegmatite systems has been investigated by LONDON and coworkers (LONDON, 1987; LON-DON et al., 1988.Boron also plays a role in the evolution of geological systems via its influence on transport processes. In particular, because of strong partitioning of B into the fluid phase under many geological conditions, B and its isotopes are used as indicators of fluid-rock interaction and to test for open-system behavior. As an example, VAN BERGEN ( 1980) considers me~irnen~~ ~ndidie~te to be produced by reaction between Al mine...

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“…For silicate melts the ot relaxation appears to be the Si-O bond exchange that occurs as silicon is exchanged among silicate species. High-temperature nuclear magnetic resonance (NMR) data Stebbins, 1990, 1994 Dingwell [1990] for Na20-3SIO2 melt, the rate of the faster [3 relaxation associated with the motion of alkalies in this melt can be related to the time required for tracer diffusion, electrical modulus relaxation, electrical conductivity relaxation, and lowtemperature (mechanical) internal friction relaxation. If the Si-O bond exchange structural relaxation occurring in a silicate melt does cause the observed relaxation of volume, shear, and enthalpy and contributes at the same rate to the equilibration of each of these properties, then the relaxation times and the rate-dependent glass transition temperatures determined for these three physical properties will be the same.…”

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confidence: 99%

Silicate melts: Relaxation, rheology, and the glass transition

Webb¹

1997

Reviews of Geophysics

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Abstract. Movement of magma within the Earth depends upon the density, compressibility, thermal expansion, viscosity, diffusivity, heat capacity, and thermal conductivity of silicate melts as a function of temperature, pressure, and composition. These physical properties are controlled by the atomic coordination and bond strengths of the melt. To contribute more to our understanding of geological processes than numerous measurements of physical properties of individual magmas of local interest, the relationship between melt structure and physical properties needs to be understood. In geological settings where the viscosity of the molten component is high (108-10 TM Pa s), the melt structure requires a long time (seconds to weeks) to equilibrate in response to changes in pressure, temperature, and composition. The effects of this structural equilibration upon the viscosity, density, and compressibility of magmas and the time required for equilibration need to be included in discussions of magma ascent and emplacement and of crystal nucleation, growth, and segregation within mag- There is an increasing body of data on the velocity of compressional waves propagating through silicate melts.The ultrasonic and shock wave data in the literature have been plagued by a "relaxation" or equilibration of the melt structure occurring at a seemingly unpredictable temperature and viscosity [see Bockris and Kojonen,

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Effects of structural relaxation on cationic tracer diffusion in silicate melts

Cited by 77 publications

References 34 publications

Influence of liquid structure on diffusive isotope separation in molten silicates and aqueous solutions

Influence of liquid structure on diffusive isotope separation in molten silicates and aqueous solutions

Chemical diffusivity of boron in melts of haplogranitic composition

Silicate melts: Relaxation, rheology, and the glass transition

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