Barium Isotope Fractionation In The Strongly Weathered Granites

With the development of high-precision Ba isotope analytical method,Ba isotope data can provide useful constraints on Ba cycle in the rivers and oceans.Ba isotopes can be a tracer for land-sea interaction and ocean mixing process.The Ba isotope compositions of river waters are significantly heavier than that of the average upper continental crust.Weathering of continental silicates is an important process releasing Ba from the continental crust into the rivers and then into the oceans.Therefore,it is crucial to investigate the behavior of Ba isotopes during continental weathering for understanding the variations of Ba isotope composition in soils,rivers,and oceans.Previous study on basalt weathering profile shows that heavy Ba isotopes are preferentially released to the hydrosphere.However,the weathering of granite,one of the most important rock types on the continental crust,has not been reported for Ba isotopes.In order to study the behavior of Ba isotopes during weathering of granite,we analyzed the Ba concentrations and Ba isotope compositions in the granite weathering profile in Luogang from Guangzhou,south China.Furthermore,we conducted sequential-extraction experiments for 10 samples to investigate the mechanism of Ba isotope fractionation during granite weathering.We separated the Ba into exchangeable portion,reducible Fe-Mn oxide/hydroxide portion and residual portion from granitic saprolite,and then determined the Ba isotope composition in these portions.Under the environment of high temperature and humidity,granite suffers strong weathering.A large amount of alkali metal and alkali earth metal elements are lost.Barium concentration in the whole profile is relatively low(10?20 ppm),except for a few enriched layers.The Ba isotope composition of the whole profile varies greatly with ?138/134Ba ranging from-0.95‰ to+0.23‰.The results of sequential-extraction experiments show that Ba concentrations in both exchangeable portion and residual portion are relatively low.Barium concentration in bulk saprolite shows a positive correlation with Ba concentration in reducible Fe-Mn oxide/hydroxide portion,implying that reducible Fe-Mn oxide/hydroxide controls the Ba distribution in the whole profile.Meanwhile,there is a positive correlation between Ba concentration and Mn concentration in reducible Fe-Mn oxide/hydroxide,indicating that Mn oxide/hydroxide plays an important role in the preservation of Ba during granite weathering.The enrichment of light Ba isotopes in the reducible Fe-Mn oxide/hydroxide can account for the enrichment of light Ba isotopes in the profile.