Isotope fractionation in silicate melts by thermal diffusion

• Published in: Nature (London) Vol. 464; no. 7287; pp. 396 - 400
• Main Authors: Huang, F., Chakraborty, P., Lundstrom, C. C., Holmden, C., Glessner, J. J. G., Kieffer, S. W., Lesher, C. E.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 18-03-2010; Nature Publishing Group
• Subjects: 639/638/263; 639/638/440/94; 704/2151/209; Crystalline rocks; Diffusion; Earth sciences; Earth, ocean, space; Exact sciences and technology; Experiments; Fractionation; Humanities and Social Sciences; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Isotope fractionation; Isotope geochemistry; Isotope geochemistry. Geochronology; Isotope separation; Isotopes; letter; Magnesium; Melts; Methods; multidisciplinary; Science; Science (multidisciplinary); Silicates; Soret coefficient; Temperature; Temperature gradients; Thermal diffusion; Thermal diffusivity; Thermal properties; Canada; United States; experimental studies; diffusion; steady-state processes; calcium; thermodynamics; magnesium; magmas; silicate melts; composition; isotopes; isotope fractionation; temperature; iron; characterization
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature08840

Thermal diffusion in silicate melts described
The phenomenon known as thermal diffusion, or the Ludwig–Soret effect, has been investigated for over 150 years, yet an understanding of its physics remains elusive. It refers to the mass diffusion observed when a temperature gradient is applied to a fluid mixture. The impression has grown that thermal diffusion, characterized as the Soret coefficient,
S
T
, is markedly sensitive to a wide variety of parameters, making it almost impossible to establish a universal model. Huang
et al
. now report novel experimental results that challenge this belief. In a study of isotope fractionation in a silicate melt, they find that for several elements (iron, calcium and magnesium) the difference in Soret coefficient between diffusing isotopes is independent of composition and temperature. They propose an additive decomposition for the functional form of
S
T
and argue that a theoretical approach based on local thermodynamic equilibrium holds promise for describing thermal diffusion in silicate melts and other complex solutions.
The physics of thermal diffusion — mass diffusion driven by a temperature gradient — is poorly understood. One obstacle has been that the Soret coefficient (
S
T
, which describes the steady-state result of thermal diffusion) is sensitive to many factors. It is now shown that the difference in
S
T
between isotopes of diffusing elements that are network modifiers is independent of composition and temperature. The findings suggest a theoretical approach for describing thermal diffusion in silicate melts and other complex solutions.
The phenomenon of thermal diffusion (mass diffusion driven by a temperature gradient, known as the Ludwig–Soret effect
1
,
2
) has been investigated for over 150 years, but an understanding of its underlying physical basis remains elusive. A significant hurdle in studying thermal diffusion has been the difficulty of characterizing it. Extensive experiments over the past century have established that the Soret coefficient,
S
T
(a single parameter that describes the steady-state result of thermal diffusion), is highly sensitive to many factors
3
,
4
,
5
,
6
,
7
,
8
,
9
. This sensitivity makes it very difficult to obtain a robust characterization of thermal diffusion, even for a single material. Here we show that for thermal diffusion experiments that span a wide range in composition and temperature, the difference in
S
T
between isotopes of diffusing elements that are network modifiers (iron, calcium and magnesium) is independent of the composition and temperature. On the basis of this finding, we propose an additive decomposition for the functional form of
S
T
and argue that a theoretical approach based on local thermodynamic equilibrium
3
,
5
,
10
holds promise for describing thermal diffusion in silicate melts and other complex solutions. Our results lead to a simple and robust framework for characterizing isotope fractionation by thermal diffusion in natural and synthetic systems.