Isotopic Constraints on Earth System Processes. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения:
Год издания: 0
isbn: 9781119594963
Скачать книгу

      Onsager (1945) showed that a way to deal with transport in a multi‐component system is to express the diffusive flux of a component as a linear combination of the concentration gradients of all the independent components in the system. The one‐dimensional diffusive flux of component i in an n‐component liquid is then given by

      (1.4)

      when SiO2 is taken as the dependent component. The flux of the SiO2 can be calculated using

or depending on the reference frame, by requiring that the sum of all the volume fluxes ν i Ji ( ν i is the molar volume of species i) must add up to zero (see de Groot & Mazur, 1962).

      (1.5)

      where the Li is called the phenomenological coefficient for diffusion and

is now identified as the diffusion coefficient for a non‐ideal binary system. The extension of this to a multi‐component system governed by a phenomenological diffusion matrix Li,j was studied experimentally by Liang et al. (1966a; 1966b; 1967) with the non‐ideal molten CaO‐Al2O3‐SiO2 system.

      1.2.2. Effective Binary Diffusion Coefficients

for component i. Cooper (1968) showed how the effective binary diffusion coefficient of a given component is related to the full diffusion matrix of the system, and he also pointed out that effective binary diffusion coefficients depend not only on the local composition and thermodynamic state (e.g., temperature and pressure) but also on the direction of the diffusive flux in composition space. It follows from this that in order to determine the appropriate effective binary diffusion coefficient for modeling a natural diffusion profile, one needs to run an experiment with a diffusion couple juxtaposing compositions that are as close as possible to those of the far‐field values of the natural system.

      1.2.3. Self‐Diffusion Coefficients

of elements in silicate liquids can be very different from each other. For example, Liang et al. (1996a) reported a large set of experimentally determined self‐diffusion coefficients as function of composition in molten CaO‐Al2O3‐SiO2 showing that
~ 10×
,
~2×
, and
~1 to 2×
. These differences in self‐diffusion are in marked contrast to the very similar magnitude of the effective binary diffusion coefficients in a molten rhyolite‐basalt diffusion couple. Fig. 1.1 shows this by overlaying diffusion profiles measured by Richter et al. (2003) in a diffusion couple in which natural rhyolite liquid and a natural basaltic liquid were juxtaposed and annealed in a piston cylinder experiment. The remarkably similar diffusive behavior the major oxides