Molybdenum isotope behaviour accompanying weathering and riverine transport in a basaltic terrain

The application of the molybdenum (Mo) isotope system as a proxy for determining changes in the redox state of the oceans is predicated on the assumption that the composition of continental input can be characterised from crustal rock types, and remains constant. However, it has recently been shown that the delta Mo-98/95 composition of global rivers varies between 0.15 parts per thousand and 2.4 parts per thousand and is therefore systematically heavier than the average composition of the continental crust (similar to 0 parts per thousand). In order to understand the processes that control Mo-isotope fractionation during weathering this study presents delta Mo-98/95 and Mo abundance data for rivers (and estuarine samples) from Iceland that drain predominantly basaltic terrains. Resolvable differences are observed in the isotopic composition of the riverine Mo sources; ice (delta Mo-98/95(ice) > 1.8 parts per thousand), basaltic bedrock (delta Mo-98/95(bedrock) similar to 0.0 parts per thousand) and hydrothermal waters (delta Mo-98/95(hydrothermol) similar to-3.4 parts per thousand). Systematic changes in the dissolved Mo-isotope composition are also observed within river catchments, with delta Mo-98/95 values increasing from similar to 0 parts per thousand in glacial rivers (close to the source) to similar to 1 parts per thousand downstream, consistent with Mo-isotopes being fractionated during weathering. Analysis of other riverine phases (bedload, colloids and iron-precipitates) demonstrates that these phases preferentially incorporate light Mo-isotopes, and remain coupled to the dissolved load during riverine transportation. A delta Mo-98/95 profile through the Borgarfjorour estuary exhibits a predominantly conservative mixing behaviour, but suggests that the release of isotopically light Mo from the particulate and/or colloidal phases may occur in the low salinity part of the estuarine mixing zone. (C) 2010 Elsevier B.V. All rights reserved.