Barium isotopic fractionation in latosol developed from strongly weathered basalt

Abstract

Weathering is a key process in the transfer of material from continents to the hydrosphere. A latosol profile in Zhanjiang, Guangdong Province, South China, formed through intense weathering of basalt, was studied to improve understanding of Ba isotopic fractionation during basalt weathering. Profile horizons were grouped into Ba-depleted and Ba-enriched layers (D- and E-layers, respectively) according to the mass fraction of Ba lost or gained from the weathered profile relative to bedrock. δ137/134Ba values in the soil profile ranged from −0.22‰ to +0.02‰, lower than those of the parent basaltic rock (0.03‰ ± 0.03‰). In the D-layers, Ba isotopic fractionation can be explained by Rayleigh fractionation, implying that heavy Ba isotopes are preferentially leached. The Rayleigh fractionation model is not applicable to the E-layers because they preferentially acquired isotopically light Ba isotopes during weathering. Results indicate a net loss of heavy Ba isotopes during strong weathering of basalt due to the precipitation of FeMn (oxyhydr)oxides and adsorption on secondary minerals. A mass-balance model indicates that the average δ137/134Ba value of materials leached from the weathered profile is ~0.08‰, slightly higher than that of the bedrock. This suggests a loss of heavy Ba isotopes into the hydrosphere during weathering of basalt, consistent with the enrichment of heavy Ba isotopes in river waters.