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Sources of river water radiogenic strontium: examples from the Achaean granite dominated catchments of Swaziland

AEON-Africa Earth Observatory Network, Department of Geology, University of Stellenbosch, Stellenbosch, 7601, South Africa e-mail: steph{at}sun.ac.za

Maarten J. de Wit

AEON-African Earth Observatory Network, Department of Geological Sciences, University of Cape Town, Rondebosch, 7625, South Africa, e-mail: maarten.dewit{at}uct.ac.za

This study of the geochemistry of rivers draining the Archaean granite-gneissic terrains of Swaziland reports the first anomalous Sr-⁸⁷Sr/⁸⁶Sr data for a river system outside the Himalayas. Both major cation and calcium mass balance approaches suggest that the source of the radiogenic high strontium load of Swaziland rivers is predominantly, or exclusively, silicate weathering. However, a compilation of bedrock compositional data for river catchments in Swaziland demonstrates that the uncertainties associated with these mass balance approaches are large and presently underestimated in global chemical weathering studies. Consideration of the strontium mass balance, based on silicate Na/Sr end member values, demonstrates the potential magnitude of the error involved. Importantly, a compilation of both the Na/Sr and Sr content of silicate rocks in Swaziland demonstrates that the general relationship between (Na/Sr)_sil and Sr content is poor and that high silicate versus carbonate X/Sr end member values cannot be assumed to imply relatively lower Sr weathering yields from silicate rocks.