2,,t
Peter Stille and Graham Shields
The more precipitation that takes place, in the form of rain, snow or hail. the more
negative will be the 8 tsO and 8D values of the remaining water vapour. This
process leads to significant isotopic fractionation. As a result, fresh water is
generally very poor in 180 and D. Conversely, fresh water is relatively enriched in
the lighter isotopes, 160 and H, with respect to seawater. With the help of a ~eat
number of analyses of meteoric waters, collected at various places on the Earth,
Craig (1961) was able to show that the 81sO and 8D values of meteoric waters are
linearly related to each other ("meteoric water line"). The relationship between the
5180 and the 8D values can be represented by the following equation:
c~D = 8c51SO + 10
Let us return to the clay minerals. If the clay minerals were in isotopic
equilibrium with meteoric waters and formed at the same temperature, then their
8tsO and 8D values should come to lie on a straight line, which is parallel to the
meteoric water line (see equation tli, Sect. 1.1). Experiments have shown that this
is indeed the case. 25"C isotherms or weathering lines for illite, kaolinite, smectite
and montmorillonite are shown in a 81sO and 8D diagram (Fig. 2.10). A clay
mineral may form at different temperatures and incorporates water with various
5t80 and 8D values depending on the temperature of formation. Fig. 2. I0 shows
kaolinite which formed at 10, 25, 50 and 100*C.
A study concerning isotopic equilibria in soil was carried out by Lawrence and
Taylor (1971). The authors determined the oxygen and hydrogen isotopic
compositions of clay minerals from 75 Quaternary soils and associated waters of
various regions of the USA (Fig_ 2.1 I). The 81sO and 8D values for meteoric
waters confirm the Craig "meteroic water line". The observed differences in the
isotopic compositions of clay minerals and meteoric waters approach those of
calculated fractionation factors and allow us to assume that isotopic equilibrium
for oxygen and hydrogen can be achieved in the weathering milieu.
2.5 The Sr Isotopic Composition of Fluid Phases in Weathered
Profiles; an Isotopic Model for Rock Weathering
As we have seen, groundwater, circulating surface water and pore water influence
both the course of weathering and the processes that lead to the formation of clay
minerals. Only by the exchange of a large quantity of water with rock can clay
minerals form, their oxygen and hydrogen isotopic compositions being in isotopic
equilibrium with the circulating fluid phases. The information provided by the
preceding discussion allows us to present an isotopic model of rock weathering
that may be comparable with nature. This model will be important in
understanding the behavior of Sr during diagenetic processes as similar
Peter Stille and Graham Shields
The more precipitation that takes place, in the form of rain, snow or hail. the more
negative will be the 8 tsO and 8D values of the remaining water vapour. This
process leads to significant isotopic fractionation. As a result, fresh water is
generally very poor in 180 and D. Conversely, fresh water is relatively enriched in
the lighter isotopes, 160 and H, with respect to seawater. With the help of a ~eat
number of analyses of meteoric waters, collected at various places on the Earth,
Craig (1961) was able to show that the 81sO and 8D values of meteoric waters are
linearly related to each other ("meteoric water line"). The relationship between the
5180 and the 8D values can be represented by the following equation:
c~D = 8c51SO + 10
Let us return to the clay minerals. If the clay minerals were in isotopic
equilibrium with meteoric waters and formed at the same temperature, then their
8tsO and 8D values should come to lie on a straight line, which is parallel to the
meteoric water line (see equation tli, Sect. 1.1). Experiments have shown that this
is indeed the case. 25"C isotherms or weathering lines for illite, kaolinite, smectite
and montmorillonite are shown in a 81sO and 8D diagram (Fig. 2.10). A clay
mineral may form at different temperatures and incorporates water with various
5t80 and 8D values depending on the temperature of formation. Fig. 2. I0 shows
kaolinite which formed at 10, 25, 50 and 100*C.
A study concerning isotopic equilibria in soil was carried out by Lawrence and
Taylor (1971). The authors determined the oxygen and hydrogen isotopic
compositions of clay minerals from 75 Quaternary soils and associated waters of
various regions of the USA (Fig_ 2.1 I). The 81sO and 8D values for meteoric
waters confirm the Craig "meteroic water line". The observed differences in the
isotopic compositions of clay minerals and meteoric waters approach those of
calculated fractionation factors and allow us to assume that isotopic equilibrium
for oxygen and hydrogen can be achieved in the weathering milieu.
2.5 The Sr Isotopic Composition of Fluid Phases in Weathered
Profiles; an Isotopic Model for Rock Weathering
As we have seen, groundwater, circulating surface water and pore water influence
both the course of weathering and the processes that lead to the formation of clay
minerals. Only by the exchange of a large quantity of water with rock can clay
minerals form, their oxygen and hydrogen isotopic compositions being in isotopic
equilibrium with the circulating fluid phases. The information provided by the
preceding discussion allows us to present an isotopic model of rock weathering
that may be comparable with nature. This model will be important in
understanding the behavior of Sr during diagenetic processes as similar