Short time leaching experiments (2, 5, and 20 h) have been carried out on less evolved biotite to highly evolved muscovite granites from the Oberpfalz, Fichtelgebirge and Erzgebirge (Germany). The mildly acidic (self-adjusting to pH 3) leaching solution (50 ml bidist. water) reacted with 2 g of powdered rock in a PE-bottle at 70°C. A cation exchange resin was present to collect all cations and to prevent re-adsorption of cations as well as precipitation of secondary minerals. The results revealed extreme differences of the leaching behaviour of Rb, Sr and the Sr isotopes which is mineralogically controlled. The release of 87Sr depends on the leaching of Rb; effects based on lattice damage by radioactive decay have not been detected.
In less evolved granites with highly reactive biotite, the leaching solution contains high amounts of Rb from the biotite that can be easily exchanged for hydrogen within the interlayer sites. The less leachable feldspars contribute only minor amounts of Sr to the solution: in general, Rb is about 10 times more leachable than Sr. Hence, the 87Sr/86Sr ratio of the leachate is controlled by the leached Rb-fraction (and 87Sr) which results in a higher 87Sr/86Sr of the leachate compared to the rock. Plotting the calculated isotopic composition of the leached samples in the Nicolaysen diagram, they shift down along the isochron.
In highly evolved granites with muscovite, K-feldspar, and albite the leachate contains high amounts of Sr leached from highly soluble phosphates and carbonates, e.g. apatite or calcite. The less leachable muscovites and feldspars contribute only minor amounts of Rb and Sr to the solution: in general, Sr is up to 50 times more leachable than Rb. Hence, the 87Sr/86Sr ratio of the leachate is controlled by a Sr-fraction, whose isotopic composition is that of common Sr. It is similar to the initial 87Sr/86Sr ratio of the granite and lower compared to the present day whole-rock ratio. Plotting the calculated isotopic composition of the leached rock, the samples shift away from the isochron.
Investigations of alteration sequences have shown that during most common types of alteration a spread of the Rb/Sr whole-rock ratio occurred due to drastic loss of Sr, whereas Rb increases due to serizitization. Only in case of strong albitization the Rb/Sr whole-rock ratio is lowered due to removal of phyllosilicates and K-feldspar, and the fixation of some Sr in newly formed albite.
With respect to these results, showing both mobilization of Rb and Sr but also their selective re-deposition, we calculated a modified isotopic composition for the leached samples assuming two model cases: mobilization of 87Rb, 87Sr and 86Sr (as in the leaching experiments), but accompanied by selective fixation (e.g. by alteration minerals) of (case a) only 87Sr and 86Sr or (case b) only 87Rb. The new results show that all samples shift away from the isochron, but the degree of the shift depends on the amount of biotite relative to muscovite and K-feldspar: the shift increases with increasing level of geochemical evolution (Fig. 1). A rotation of the isochron is visible, enlarging (model case a) or lowering (model case b) the Rb-Sr whole-rock age.
Fig. 1: Plot of the isotopic composition of fresh (solid) and the leached (open) rock samples (granite of Leuchtenberg/Oberpfalz). The shift of the leached samples results in a rotation of the isochron depending on the theoretical model of alteration (see text).