2 Weathering
29
On the other hand, the assumption that the system remains closed throughout
means that pore water (LI) and both fresh and weathered rock (WR, WRI
respectively) must all lie on one straight line. Continued weathering would lead to
the isotopic evolution (WR-->WRI-->WR2-->WR3; Fig. 2.13). Only with the
onset of weathering of biotite and alkali feldspar does the evolution of Sr in the
whole rock begin to change. Two effects start to influence the isotopic
composition significantly. First. biotite releases radiogenic 87Sr (Sect. 2.2),
Iowering its sTSdS6Sr ratio. Second, biotite will tend to adsorb more and more Sr
from the surrounding pore waters and perhaps lower its Sr concentration. This
pore water will be strongly enriched in only weakly radiogenic Sr due to the
previous weathering of plagioclase (Sect. 2.2). This has the consequence that the
STSr/S6Sr and also the Rb/Sr ratios of the whole rock decrease (evolution of WR2
into WR3: Fig. 2.13). The model helps to explain the positions of the sample
points for the Elberton granite in the isochron diagram (Fig. 2.1) or those observed
from other weathering profiles, e.g. the Butler Hill granite (Blaxland 1974; Fig.
2.14). The course of weathering outlined here results in certain rules of thumb
concerning the isotope geochemistry of pore waters during weathering (and
diagenesis). Pore waters in the early plagioclase weathering stages are likely to
show high Sr contents and less radiogenic isotopic signatures, whereas those of
later stage alkali feldspar and biotite weathering, allow the development of
porewaters with lower Sr contents (due to Sr adsorption of biotites) and strongly
radiogenic isotope characteristics.
9
lo
T i m e
r
Sr (pprn)
Fig. 2.15. Schematic representation of the Sr isotopic evolution of pore waters during
weathering of a granite rich in plagioclase. A: aheralion of plagioclase; B: increasing
alteration of biolite and alkali feldspar; C: biotite and alkali feldspar in chemical
29
On the other hand, the assumption that the system remains closed throughout
means that pore water (LI) and both fresh and weathered rock (WR, WRI
respectively) must all lie on one straight line. Continued weathering would lead to
the isotopic evolution (WR-->WRI-->WR2-->WR3; Fig. 2.13). Only with the
onset of weathering of biotite and alkali feldspar does the evolution of Sr in the
whole rock begin to change. Two effects start to influence the isotopic
composition significantly. First. biotite releases radiogenic 87Sr (Sect. 2.2),
Iowering its sTSdS6Sr ratio. Second, biotite will tend to adsorb more and more Sr
from the surrounding pore waters and perhaps lower its Sr concentration. This
pore water will be strongly enriched in only weakly radiogenic Sr due to the
previous weathering of plagioclase (Sect. 2.2). This has the consequence that the
STSr/S6Sr and also the Rb/Sr ratios of the whole rock decrease (evolution of WR2
into WR3: Fig. 2.13). The model helps to explain the positions of the sample
points for the Elberton granite in the isochron diagram (Fig. 2.1) or those observed
from other weathering profiles, e.g. the Butler Hill granite (Blaxland 1974; Fig.
2.14). The course of weathering outlined here results in certain rules of thumb
concerning the isotope geochemistry of pore waters during weathering (and
diagenesis). Pore waters in the early plagioclase weathering stages are likely to
show high Sr contents and less radiogenic isotopic signatures, whereas those of
later stage alkali feldspar and biotite weathering, allow the development of
porewaters with lower Sr contents (due to Sr adsorption of biotites) and strongly
radiogenic isotope characteristics.
9
lo
T i m e
r
Sr (pprn)
Fig. 2.15. Schematic representation of the Sr isotopic evolution of pore waters during
weathering of a granite rich in plagioclase. A: aheralion of plagioclase; B: increasing
alteration of biolite and alkali feldspar; C: biotite and alkali feldspar in chemical