Sr isotope evolution during chemical weathering of granites - impact of relative weathering rates of minerals

The Sr isotopic systematics in the weathering profiles of biotite granite a nd granite porphyry in southern Jiangxi Province were investigated. The res ults showed that during the chemical weathering of granites, remarked fract ionation occurred between Rb and Sr. During the early stages of chemical we athering of granites, the released Sr/Si and Sr/Ca ratios are larger than t hose of the parent rocks, and the leaching rate of Sr is higher than those of Si, Ca, K, Rb, etc. Dynamic variations in relative weathering rates of t he main Sr-contributing minerals led to fluctuation with time in Sr-87/Sr-6 6 ratios of inherent and released Sr in the weathering crust of granite. Su ccessive weathering of biotite, plagioclase and K-feldspar made Sr-87/Sr-66 ratios in the weathering residues show such a fluctuation trend as to decr ease first, increase, and then decrease again till they maintain stable. Th is work further indicates that when Sr isotopes are used to trace biogeoche mical processes on both the catchment and global scales, one must seriously take account of the preferential release of Sr from dissolving solid phase and the fluctuation of Sr-87/Sr-66 ratios caused by the variations of rela tive weathering rates of Sr-contributing minerals.