The impact of igneous bedrock weathering on the Mo isotopic composition of stream waters: Natural samples and laboratory experiments

Abstract

River waters have been shown to be systematically enriched in the heavy molybdenum (Mo) isotopes when compared to typical granites and basalts, which generally possess Mo isotopic compositions (δ98/95Mo) of around 0‰. This inconsistency has been used to argue against weathering of crustal rocks as the cause for heavy riverine δ98/95Mo signatures. Incongruent dissolution of primary bedrock, however, may be an important process by which the anomalous Mo signatures of the river dissolved load are produced. This study therefore investigates the effect of igneous crustal rock weathering on the aquatic δ98/95Mo signal by comparing stream water and bedrock Mo isotope data to results of bulk rock leach experiments. For this purpose, stream water and bedrock (orthogneiss, granite, basalt), as well as soil and vegetation samples were collected in a small catchment in the French Massif Central. In accordance with the results of earlier studies on riverine Mo, both streams are isotopically heavier (δ98/95Mo=0.5–1.1‰) than the typical granites and basalts. The excellent agreement of these data with those of Mo released during experimental leaching of the basalt bedrock (0.6–1.0‰) identifies a predominance of basalt weathering over the stream water Mo geochemistry, while other processes (i.e. soil formation, secondary mineral precipitation and adsorption) are subordinate in this catchment. Given that the basalt bulk rock δ98/95Mo reflects a value typical for crustal magmatic rocks (ca. 0.1‰), Mo isotope fractionation during the incongruent dissolution of basalt can explain the observed isotopically heavy aquatic Mo signatures. Laser ablation analyses demonstrate that the volumetrically minor magmatic sulfides can be highly enriched in Mo and mass balance calculations identify the sulfide melt inclusions as the principal Mo source for the leach solutions. These data suggest that the magmatic sulfides possess a distinctly heavier δ98/95Mo signature than the coexisting silicate melt. In this case, Mo would behave like Fe by showing a detectable isotope fractionation at magmatic temperatures. Incongruent crustal bedrock weathering may thus cause a preferential release of heavy Mo isotopes. This effect, however, is highly dependent on the primary bedrock mineralogy. Consequently, the average continental runoff may have been significantly affected by incongruent weathering during periods when the Earth system was exceptionally far from steady state, e.g., large glaciations with enhanced physical weathering or large subaerial basalt eruptions such as the Deccan and the Siberian plateau.