2 Weathering
_97
This has the consequence that the fluid phases mainly dissolve Sr with relatively
low e'TSr/S6Sr and Rb/Sr ratios, which goes on towards defining pore water
chemistry. The pore waters will be strongly enriched in Sr. The whole rock does
not lose much radiogenic Sr and so, its 87Sr/S6Sr and Rb/Sr ratios must rise. If Rb
and 87Sr were not fractionated by this weathering process, Sr would not dissolve
preferentially relative to Rb, and the whole rock position would move up the
isochron towards even higher 87SrlS6Sr and Rb/Sr ratios (WR1; Fig. 2.12B).
L!
Kf
i /
// \\, //
tl
~,'1
Pt"
'TRb I a~Sr
Fig. 2.13. Sr isotope model of rock weathering. With the onset of weathering of biotite and
alkali feldspar the isotope evolution of Sr in the whole rock begins to change (WR I->WR2
->WR3). Mass balancing tells us that the isotopic composition of the altered whole rocks
WRI.WR2 and WR3 must Lie within the triangles Kf-PI'-B, Kf-Pt"-B and Kf-PI"-B',
respectively.
_97
This has the consequence that the fluid phases mainly dissolve Sr with relatively
low e'TSr/S6Sr and Rb/Sr ratios, which goes on towards defining pore water
chemistry. The pore waters will be strongly enriched in Sr. The whole rock does
not lose much radiogenic Sr and so, its 87Sr/S6Sr and Rb/Sr ratios must rise. If Rb
and 87Sr were not fractionated by this weathering process, Sr would not dissolve
preferentially relative to Rb, and the whole rock position would move up the
isochron towards even higher 87SrlS6Sr and Rb/Sr ratios (WR1; Fig. 2.12B).
L!
Kf
i /
// \\, //
tl
~,'1
Pt"
'TRb I a~Sr
Fig. 2.13. Sr isotope model of rock weathering. With the onset of weathering of biotite and
alkali feldspar the isotope evolution of Sr in the whole rock begins to change (WR I->WR2
->WR3). Mass balancing tells us that the isotopic composition of the altered whole rocks
WRI.WR2 and WR3 must Lie within the triangles Kf-PI'-B, Kf-Pt"-B and Kf-PI"-B',
respectively.