30
Peter Stille and Graham Shields
equilibrium with the surrounding porewaters and advanced weathering and dissolution of
plagioclase.
Strictly speaking, the process of weathenng individual minerals does not proceed
stepwise but instead with considerable overlap. The isotopic compositions and the
Sr concentrations of pore waters are products of the mixing of each individual
fluid phase that has come about by the weathering of previous mineral
components. The course of curve A (Fig. 2. t5) represents pore water deriving
from the weathering of plagioclase. Increasing alkali feldspar and biotite
weathering results in curve B with increasing sTSdS6Sr ratios and decreasing Sr
concentrations. Thermodynamic calculations demonstrate that curve C only comes
into play when chemical equilibrium has been reached between biotite, alkali
feldspar and the surrounding fluid phases (Probst et al. 1992). Advanced
weathering and dissolution of plagioclase leads to decreasing S7Sr/S6Sr ratios and
increasing Sr concentrations. Thermodynamic calculations show clearly that the
course of weathering depicted in Fig. 2.15 is greatly accelerated today by the
presence of acid rain (Probst et al. 1992). Clay minerals, which c~stallize out in
the fluid phase, are likely to display highly variable isotopic compositions
depending on when they form.
2.6 References
Blaxland AB !1974) Geochemistry and geochronology or chemical weathering, Butler Hill
granite, Missouri. Geochim Cosmochim Acta, 38:843-852
Clauer N (1979) Relationship belween the isotopic composition of strontium in newly
formed continental clay minerals and their source material. Chem Geol, 27:115-124
Clauer N ( 1981 . ~ Strontium and Argon isotopes in naturally weathered biotites, muscovites
and feldspar. Chem Geol, 31:325-334
Clauer N O'Neil JR, Bonnot-Courtois C (1982) The effect of natural weathering on the
chemical and isotopic compositions of biotites. Geochim Cosmochim Acta, 46:17551762
Craig H ( 1961 ) Isotopic variations in meteoric waters. Science. 133:1702-1703
Dasch EJ (1969) Strontium isotopes in weathering profiles, deep sea sediments and
sedimentary, rocks. Geochim Cosmochim Acta, 33:1521-1552
Fordham AW (1973) The location of iron-55, strontium-86 and iodide-125 sorbed by
kaolinite and dickite particles. Clays Clay Min 21 : 175-184
Fritz B. Tardy Y (1973) Etude thermodynamique du systeme gibbsite-quartz-kaolinite-gaz
carbonique - Application a la genese des podzols et des bauxites. Sci Geol Bull,
Strasbourg, 26:39-367
Garrels RM, Christ CL (1965) Solutions, Minerals and Equilibria. Harper and Row, New
York
Goldich SS (1938) A study in rock-weathenng. J Geol. 46:17-58
Goldich SS, Gast PW (1966) Effects of weathering on the Rb-Sr and K-Ar ages of biotite
from the Morton gneiss, Minnesota. Earth Planet Sci Lett. 1:372-375
Peter Stille and Graham Shields
equilibrium with the surrounding porewaters and advanced weathering and dissolution of
plagioclase.
Strictly speaking, the process of weathenng individual minerals does not proceed
stepwise but instead with considerable overlap. The isotopic compositions and the
Sr concentrations of pore waters are products of the mixing of each individual
fluid phase that has come about by the weathering of previous mineral
components. The course of curve A (Fig. 2. t5) represents pore water deriving
from the weathering of plagioclase. Increasing alkali feldspar and biotite
weathering results in curve B with increasing sTSdS6Sr ratios and decreasing Sr
concentrations. Thermodynamic calculations demonstrate that curve C only comes
into play when chemical equilibrium has been reached between biotite, alkali
feldspar and the surrounding fluid phases (Probst et al. 1992). Advanced
weathering and dissolution of plagioclase leads to decreasing S7Sr/S6Sr ratios and
increasing Sr concentrations. Thermodynamic calculations show clearly that the
course of weathering depicted in Fig. 2.15 is greatly accelerated today by the
presence of acid rain (Probst et al. 1992). Clay minerals, which c~stallize out in
the fluid phase, are likely to display highly variable isotopic compositions
depending on when they form.
2.6 References
Blaxland AB !1974) Geochemistry and geochronology or chemical weathering, Butler Hill
granite, Missouri. Geochim Cosmochim Acta, 38:843-852
Clauer N (1979) Relationship belween the isotopic composition of strontium in newly
formed continental clay minerals and their source material. Chem Geol, 27:115-124
Clauer N ( 1981 . ~ Strontium and Argon isotopes in naturally weathered biotites, muscovites
and feldspar. Chem Geol, 31:325-334
Clauer N O'Neil JR, Bonnot-Courtois C (1982) The effect of natural weathering on the
chemical and isotopic compositions of biotites. Geochim Cosmochim Acta, 46:17551762
Craig H ( 1961 ) Isotopic variations in meteoric waters. Science. 133:1702-1703
Dasch EJ (1969) Strontium isotopes in weathering profiles, deep sea sediments and
sedimentary, rocks. Geochim Cosmochim Acta, 33:1521-1552
Fordham AW (1973) The location of iron-55, strontium-86 and iodide-125 sorbed by
kaolinite and dickite particles. Clays Clay Min 21 : 175-184
Fritz B. Tardy Y (1973) Etude thermodynamique du systeme gibbsite-quartz-kaolinite-gaz
carbonique - Application a la genese des podzols et des bauxites. Sci Geol Bull,
Strasbourg, 26:39-367
Garrels RM, Christ CL (1965) Solutions, Minerals and Equilibria. Harper and Row, New
York
Goldich SS (1938) A study in rock-weathenng. J Geol. 46:17-58
Goldich SS, Gast PW (1966) Effects of weathering on the Rb-Sr and K-Ar ages of biotite
from the Morton gneiss, Minnesota. Earth Planet Sci Lett. 1:372-375