26
Peter Slille and Graham Shields
@
8J
WR!
pt
'~Rb / "Sr
Fig. 2.12. Sr isotope model of rock weathering. A: Rb-Sr isochron for plagioclase (PI),
whole rock (WR), biotite (B) and alkali feldspar (Kf). B: Leaching of plagioclase
(Ll=leachate; Pl'=plagioclase residue) causes an increase of the Rb/Sr and 87Sr/86Sr ratios
in the whole rock (WR->WRI). C: Selective leaching of Sr out of Ca- and St-rich zones of
the plagioclase (Rb remaining immobile) causes an increase of the Rb/Sr ratio in the
residual plagioclase (Pr) and consequently in the whole rock (WRI). The pore water L I,
which exchanged with the plagioclase has high Sr and low Rb contents and moves away
from the isochron towards lower Rb/Sr ratios.