I-1.
Peter Stille and Graham Shields
The fact that the weathered samples also lie on the isochron would suggest in the
case of recent weathering that radiogenic 87Sr and Rb of particular relative
amounts had been either lost from or added to the system. The sample points
would, in this case, move along the isochron during the course of weathering.
Although such a process is conceivable, it does appear rather improbable. In the
case of an ancient phase of weathering, it would have to be assumed that it
followed closely after intrusion, as a consequence of hydrothermal activity or
surface weathering, and after rapid excavation of the pluton. In such a case, the
mineral's isotopic systems would not yet have developed in significantly different
ways, and in the case of a pluton where the system has remained a closed system,
the weathering products would have taken on a similar Sr isotopic composition_
In an isochron diagram, which displays the data at the time of the intrusion or
shortly after, all the data points would plot on a near horizontal line, the exact
location on the line depending on the Rb/Sr ratio. Such a model implies, however,
that the weathering profile has remained a closed system since its origination. In
this case, the weathering would be datable.
Let us delve a little deeper into the processes of weathering and discuss the
behaviour of isotopic systems of single mineral components during weathering
and during the formation of, for example, clay minerals.
2,2 W e a t h e r i n g o f B i o t i t e a n d M u s c o v i t e M i c a
As biotite is such an important mineral in Rb-Sr and K-Ar age determinations, its
behaviour during weathering was already intensively studied in the sixties
(Zartman 1964; Goldich and Gast 1966). A general conclusion to be drawn from
all these studies was that the decrease in age was a result of the relatively weaker
bonds in the crystal lattice formed by the radiogenic isotopes 8"Sr and "~~
compared with those of the parent isotopes STRb and a~
The weathering process significantly controls the migration of pore waters
through the rock. In a combined mineralogic and isotope study, Clauer et al.
(1982) were able to observe and quantify the natural, progressive weathering of
biotite in the soil profile lying above a migmatite. Samples were collected from
variously weathered basement rocks as well as the soils themselves. Biotite and its
weathering product kaolinite were investigated for their Rb-Sr, K-Ar, O and H
isotopes. Mineralogic studies were carried out as a,n additional control.
The early stages of biotite weathering is marked by oxidation of Fe-'+, transport
of K, Ti, Mn and Rb, and a strong increase in water and calcium contents.
Radiogenic 87Sr and ~Ar were both quickly dissolved out of the biotite particles,
Peter Stille and Graham Shields
The fact that the weathered samples also lie on the isochron would suggest in the
case of recent weathering that radiogenic 87Sr and Rb of particular relative
amounts had been either lost from or added to the system. The sample points
would, in this case, move along the isochron during the course of weathering.
Although such a process is conceivable, it does appear rather improbable. In the
case of an ancient phase of weathering, it would have to be assumed that it
followed closely after intrusion, as a consequence of hydrothermal activity or
surface weathering, and after rapid excavation of the pluton. In such a case, the
mineral's isotopic systems would not yet have developed in significantly different
ways, and in the case of a pluton where the system has remained a closed system,
the weathering products would have taken on a similar Sr isotopic composition_
In an isochron diagram, which displays the data at the time of the intrusion or
shortly after, all the data points would plot on a near horizontal line, the exact
location on the line depending on the Rb/Sr ratio. Such a model implies, however,
that the weathering profile has remained a closed system since its origination. In
this case, the weathering would be datable.
Let us delve a little deeper into the processes of weathering and discuss the
behaviour of isotopic systems of single mineral components during weathering
and during the formation of, for example, clay minerals.
2,2 W e a t h e r i n g o f B i o t i t e a n d M u s c o v i t e M i c a
As biotite is such an important mineral in Rb-Sr and K-Ar age determinations, its
behaviour during weathering was already intensively studied in the sixties
(Zartman 1964; Goldich and Gast 1966). A general conclusion to be drawn from
all these studies was that the decrease in age was a result of the relatively weaker
bonds in the crystal lattice formed by the radiogenic isotopes 8"Sr and "~~
compared with those of the parent isotopes STRb and a~
The weathering process significantly controls the migration of pore waters
through the rock. In a combined mineralogic and isotope study, Clauer et al.
(1982) were able to observe and quantify the natural, progressive weathering of
biotite in the soil profile lying above a migmatite. Samples were collected from
variously weathered basement rocks as well as the soils themselves. Biotite and its
weathering product kaolinite were investigated for their Rb-Sr, K-Ar, O and H
isotopes. Mineralogic studies were carried out as a,n additional control.
The early stages of biotite weathering is marked by oxidation of Fe-'+, transport
of K, Ti, Mn and Rb, and a strong increase in water and calcium contents.
Radiogenic 87Sr and ~Ar were both quickly dissolved out of the biotite particles,