10
A. Theoretical and Instrumental Background
Diffusion processes also have importance in geology on a wide scale.
Diffusional separation processes for gaseous and isotopic mixtures are of
two general types: Those that do not require an additional gas as a
separating agent; and those in which separation of the mixture occurs by
diffusion through an added gas component [carrier diffusion, CRAIG
(1967)]. The first type includes the process of barrier diffusion or effusion, characterized by molecular flow through a porous barrier (e.g., the
migration of methane through porous rocks) and the process of thermal
diffusion across a temperature gradient.
For two isotopic species, their diffusion coefficients D and D* are
related by D/ D* = V M*/ M, where M is the mass of species. In any
natural diffusive process, isotopic fractionations may arise from this
relationship. The theory of isotopic fractionation by liquid or solid diffusion on geologic systems has been discussed by SENFTLE and BRACKEN
(1955), who conclude that probably no large geologic body exists in
which a major constituent is isotopically enriched by these diffusion
processes.
In considering diffusion as a geologic process, it is important to
distinguish between solid-solvent and liquid-solvent phases. The term
"solid-state diffusion" generally includes the processes in which the solute diffuses through a solid matrix, along grain boundaries or over
surfaces. Liquid-state diffusion refers to the diffusion of a solute through
a liquid. In both types of diffusion, isotopic fractionation can occur, but
to different extents. The diffusion coefficients in liquids are larger than
the solid-state diffusion coefficients by several orders of magnitude. Consequently, solid-state diffusion can apply only to shortdistance phenomena. Geologic examples of isotopic fractionation caused by liquid-state
or solid-state diffusion have been described (see potassium).
IV. Basic Principles of Mass Spectrometry
A number of different methods may be used for measuring the
abundance of stable isotopes. The methods used are usually physical,
including analysis by neutron activation, analysis of electromagnetic
spectra, determination of the specific gravity of fluids, and analysis of
mass spectra.
Moderately accurate determinations of the specific gravity (density)
of a liquid have been described, which in favorable circumstances, such
as in water, allow the determination of the isotopic composition from
differences in the specific gravity of the fluids. Methods based on the
characteristic electromagnetic spectra of isotopes include optical, atomic
and molecular, microwave, and nuclear resonance spectrography. Meth-
A. Theoretical and Instrumental Background
Diffusion processes also have importance in geology on a wide scale.
Diffusional separation processes for gaseous and isotopic mixtures are of
two general types: Those that do not require an additional gas as a
separating agent; and those in which separation of the mixture occurs by
diffusion through an added gas component [carrier diffusion, CRAIG
(1967)]. The first type includes the process of barrier diffusion or effusion, characterized by molecular flow through a porous barrier (e.g., the
migration of methane through porous rocks) and the process of thermal
diffusion across a temperature gradient.
For two isotopic species, their diffusion coefficients D and D* are
related by D/ D* = V M*/ M, where M is the mass of species. In any
natural diffusive process, isotopic fractionations may arise from this
relationship. The theory of isotopic fractionation by liquid or solid diffusion on geologic systems has been discussed by SENFTLE and BRACKEN
(1955), who conclude that probably no large geologic body exists in
which a major constituent is isotopically enriched by these diffusion
processes.
In considering diffusion as a geologic process, it is important to
distinguish between solid-solvent and liquid-solvent phases. The term
"solid-state diffusion" generally includes the processes in which the solute diffuses through a solid matrix, along grain boundaries or over
surfaces. Liquid-state diffusion refers to the diffusion of a solute through
a liquid. In both types of diffusion, isotopic fractionation can occur, but
to different extents. The diffusion coefficients in liquids are larger than
the solid-state diffusion coefficients by several orders of magnitude. Consequently, solid-state diffusion can apply only to shortdistance phenomena. Geologic examples of isotopic fractionation caused by liquid-state
or solid-state diffusion have been described (see potassium).
IV. Basic Principles of Mass Spectrometry
A number of different methods may be used for measuring the
abundance of stable isotopes. The methods used are usually physical,
including analysis by neutron activation, analysis of electromagnetic
spectra, determination of the specific gravity of fluids, and analysis of
mass spectra.
Moderately accurate determinations of the specific gravity (density)
of a liquid have been described, which in favorable circumstances, such
as in water, allow the determination of the isotopic composition from
differences in the specific gravity of the fluids. Methods based on the
characteristic electromagnetic spectra of isotopes include optical, atomic
and molecular, microwave, and nuclear resonance spectrography. Meth-