Geoscience Reference
In-Depth Information
2.4 Distillation processes
The distillation of alcohol, whereby we seek to enrich the ethyl-alcohol content of the
condensate resulting from the boiling of wine or any other low-grade fermented beverage,
is a familiar enough example. When a finite quantity of a substance changes state and the
product of the transformation is isolated, the progressive chemical or isotopic fractionation
associated with the progress of this transformation defines the phenomenon of distillation.
This process occurs naturally in various geological settings:
•
fractional crystallization of magmas during which solids, termed cumulates, are succes-
sively isolated from the residual magma;
•
fractional melting of the mantle, producing liquids extracted immediately from the
molten source;
•
progressive condensation of atmospheric water vapor, in the course of which precipita-
tion loses contact with the high atmosphere H
2
O;
•
boiling of hydrothermal solutions.
It can be shown (see box) that the change in concentration
C
res
of an element
i
in the
residual phase during formation of a new phase obeys Rayleigh's law:
D
i
1
dln
f
dln
C
res
=
−
(2.20)
where
D
i
is the bulk partition coefficient between the new phase and the parent residual
phase (the order is essential) and
f
the fraction by mass of the residual phase relative to
the stock of original material. The most common form of this equation is:
f
D
i
C
res
=
C
0
−
1
(2.21)
where
C
0
is the concentration in element
i
of the parent phase of the original material, i.e.
for
f
1.
A first application of this theory is the fractional crystallization of magmas, for which
we obtain:
=
f
D
i
C
liq
=
C
0
−
1
(2.22)
Here, the parameter
f
represents the residual liquid fraction and
C
0
the composition
of the parent magma. The concentration of the solid at equilibrium with the liquid is
obtained by multiplying
(2.22)
by the partition coefficient of the element. For incompati-
ble elements, like Th, Ba, and La, the partition coefficients
D
i
, which measure solid/liquid
fractionation, are low: their concentrations in the residual magma and, consequently, in
the extracted mineral phases are therefore inversely proportional to
f
. Over most of the
range of magma differentiation, liquid is the dominant phase. Parameter
f
is therefore
close to unity and the concentrations of incompatible elements vary little during crystal-
lization. Moreover, the ratios of incompatible elements (e.g. Th/La) in differentiated lavas
are virtually unaffected by fractional crystallization. By contrast, for compatible elements,
such as Ni and Cr, high
D
i
values cause very high variations in concentration in the residual
