Isotope Fractionation Processes
9
show a characteristic trend in isotopic composition. Examples of this
type of process are the progressive formation and removal of raindrops
from a cloud, and the formation of crystals from a solution too cool to
allow diffusive equilibrium between the crystal interior and the liquid.
The isotopic composition of residual water vapor (or of the residual
liquid) is a function of the fractionation factor between vapor and water
droplets or between the liquid and surface layer of the growing solid. At
any instant the vapor over the liquid is assumed to be in isotopic equilibrium with the liquid which remains, according to the relation
r = rJ.oR'.
The ideal fractionation factor as represented by rJ. o , which, for a twocomponent system, is equal to the ratio of the respective vapor pressures.
R' refers to the molecular ratio in the liquid remaining, r to that in the
vapor phase. R' changes as the distillation proceeds. A natural example
of a Rayleigh distillation is the fractionation between the oxygen isotopes in the water vapor of a cloud and the raindrops released from that
cloud. The resulting depletion of the 180rO ratio in the residual water
vapor is given as a function of the fraction of original vapor remaining in
the cloud (see Fig.4).
Fraction of Remaining Vapor
Q75
OSO
0.25
a
;? -15
0.()
-20
-25
-30~----~--~--~-L~
20
Cloud Temperature (OC)
Fig.4. £5 18 0 in cloud vapor and condensate plotted as a function of the fraction of
remaining vapor in the cloud for a Rayleigh process. The temperature of the cloud
is shown on the lower axis. The increase in fractionation with decreasing temperature taken into account. (After DANSGAARD, 1964). For definition of £5 lBO, see p. 15.
9
show a characteristic trend in isotopic composition. Examples of this
type of process are the progressive formation and removal of raindrops
from a cloud, and the formation of crystals from a solution too cool to
allow diffusive equilibrium between the crystal interior and the liquid.
The isotopic composition of residual water vapor (or of the residual
liquid) is a function of the fractionation factor between vapor and water
droplets or between the liquid and surface layer of the growing solid. At
any instant the vapor over the liquid is assumed to be in isotopic equilibrium with the liquid which remains, according to the relation
r = rJ.oR'.
The ideal fractionation factor as represented by rJ. o , which, for a twocomponent system, is equal to the ratio of the respective vapor pressures.
R' refers to the molecular ratio in the liquid remaining, r to that in the
vapor phase. R' changes as the distillation proceeds. A natural example
of a Rayleigh distillation is the fractionation between the oxygen isotopes in the water vapor of a cloud and the raindrops released from that
cloud. The resulting depletion of the 180rO ratio in the residual water
vapor is given as a function of the fraction of original vapor remaining in
the cloud (see Fig.4).
Fraction of Remaining Vapor
Q75
OSO
0.25
a
;? -15
0.()
-20
-25
-30~----~--~--~-L~
20
Cloud Temperature (OC)
Fig.4. £5 18 0 in cloud vapor and condensate plotted as a function of the fraction of
remaining vapor in the cloud for a Rayleigh process. The temperature of the cloud
is shown on the lower axis. The increase in fractionation with decreasing temperature taken into account. (After DANSGAARD, 1964). For definition of £5 lBO, see p. 15.