2 Weathering
19
Muscovite can, however, be broken down through mechanical erosion and
transport to attain clay mineral grain sizes. Investigations carried out by Clauer
(1981) on muscovites from a partly fresh, partly strongly weathered pegmatite of
the Congo region demonstrated that the K-Ar and Rb-Sr ages had not been
changed during the weathering process. He inferred that the isotopic systems
would not be disturbed as long as the crystallographic characteristics did not
change.
2.3 Weathering of Feldspars
Several clay minerals such as illite, kaolinite and gibbsite can be produced by the
continual weathering of feldspars, However, how their isotopic systems behave
has been only scantily studied. Observations made by Goldich (1938) allowed him
to assume that feldspars, like biotite, were particularly sensitive to weathering.
Only in tropical soils are the Rb-Sr and K-Ar isotopic systems in feldspars more
resistent to weathering than those of biotite (Zartman 1964; Clauer 1981).
Rb-Sr isotope data from feldspars investigated by Clauer (1981) from a
weathering profile in the Chad republic are displayed in Fig. 2.7A. The gradient of
the reference line corresponds to the age of intrusion. With the exception of one
sample, the feldpars lie near or on the reference line. They appear therefore to
have remained stable relative to weathering. This is also the case for the K-At
system of the feldspars (Fig, 2.7B). The reference line in this case corresponds to
to the age of intrusion, too.
In the Sr diagram, one sample #F5 displays higher 87Sr/S6Sr ratios relative to
the other samples (Fig. 2.7A). This sample was collected at great depth within the
weathering profile. Several investigations have shown that feldspars taken from
close to the surface in tropical soils are far less weathered than those from greater
depths. Close to the surface, the insides of the feldpars remain relatively well
preserved, protected by a thin clay mineral coating on the crystal surfaces. Deeper
in the weathering profile, the cores of the feldspars are altered with significant
transformation into various clay minerals. This can also be observed in sample
#F5. Mechanical action on the sample whilst in the ultrasonic bath has the
consequence that the feldspars from close to the surface are freed from their clay
mineral coating. These near surface samples help to define the magmatic age.
The strongly transformed feldspars from depth, enriched in clay minerals,
contain a high 87Sr/S6Sr ratio which does not represent the magmatic isotopic
composition any more but reflects more the influence of the surrounding
weathering milieu. It is possible that radiogenic Sr was released during the
weathering of biotite and has been incorporated into the clay minerals.
19
Muscovite can, however, be broken down through mechanical erosion and
transport to attain clay mineral grain sizes. Investigations carried out by Clauer
(1981) on muscovites from a partly fresh, partly strongly weathered pegmatite of
the Congo region demonstrated that the K-Ar and Rb-Sr ages had not been
changed during the weathering process. He inferred that the isotopic systems
would not be disturbed as long as the crystallographic characteristics did not
change.
2.3 Weathering of Feldspars
Several clay minerals such as illite, kaolinite and gibbsite can be produced by the
continual weathering of feldspars, However, how their isotopic systems behave
has been only scantily studied. Observations made by Goldich (1938) allowed him
to assume that feldspars, like biotite, were particularly sensitive to weathering.
Only in tropical soils are the Rb-Sr and K-Ar isotopic systems in feldspars more
resistent to weathering than those of biotite (Zartman 1964; Clauer 1981).
Rb-Sr isotope data from feldspars investigated by Clauer (1981) from a
weathering profile in the Chad republic are displayed in Fig. 2.7A. The gradient of
the reference line corresponds to the age of intrusion. With the exception of one
sample, the feldpars lie near or on the reference line. They appear therefore to
have remained stable relative to weathering. This is also the case for the K-At
system of the feldspars (Fig, 2.7B). The reference line in this case corresponds to
to the age of intrusion, too.
In the Sr diagram, one sample #F5 displays higher 87Sr/S6Sr ratios relative to
the other samples (Fig. 2.7A). This sample was collected at great depth within the
weathering profile. Several investigations have shown that feldspars taken from
close to the surface in tropical soils are far less weathered than those from greater
depths. Close to the surface, the insides of the feldpars remain relatively well
preserved, protected by a thin clay mineral coating on the crystal surfaces. Deeper
in the weathering profile, the cores of the feldspars are altered with significant
transformation into various clay minerals. This can also be observed in sample
#F5. Mechanical action on the sample whilst in the ultrasonic bath has the
consequence that the feldspars from close to the surface are freed from their clay
mineral coating. These near surface samples help to define the magmatic age.
The strongly transformed feldspars from depth, enriched in clay minerals,
contain a high 87Sr/S6Sr ratio which does not represent the magmatic isotopic
composition any more but reflects more the influence of the surrounding
weathering milieu. It is possible that radiogenic Sr was released during the
weathering of biotite and has been incorporated into the clay minerals.