『Abstract
　This study presents Os isotope and comprehensive major and trace element data for the dissolved load, suspended particulates and bedload for Icelandic rivers, draining predominantly basaltic catchments that range in age from historic to ca. 12 Ma. Hydrothermal waters and precipitation have also been analysed. Both Os and Re concentrations are greater in the suspended load than the bedload, while Re/Os ratios are lower, suggesting that both elements are concentrated in weathering resistant minerals. Despite this elemental fractionation the suspended particulates and bedload for each river yield indistinguishable ¹⁸⁷Os/¹⁸⁸Os isotope compositions that range from 0.136 to 0.292. In contrast, the dissolved load (＜0.2μm filtered) often possesses a significantly more radiogenic Os isotope composition than the corresponding suspended or bed load with ¹⁸⁷Os/¹⁸⁸Os ratios ranging from 0.15 to 1.04. The isotope and elemental data for the dissolved load can be explained in terms of an unradiogenic contribution from congruent basalt weathering (and/or hydrothermal input) and a radiogenic contribution that arises from two distinct processes. For the glacier-fed rivers there is a covariation between ¹⁸⁷Os/¹⁸⁸Os and the extent of glacial cover in the catchment, and this is most readily explained by the entrainment of seawater aerosols into precipitation and subsequent glacial melting. While for direct-runoff (and spring-fed rivers) there is a covariation between ¹⁸⁷Os/¹⁸⁸Os and the age of the bedrock in the catchment, that cannot be explained by congruent weathering of old basalt. Calculations indicate that those direct-runoff rivers with radiogenic ¹⁸⁷Os/¹⁸⁸Os values are also undersaturated with respect to the primary basalt minerals olivine, pyroxene and plagioclase, indicating that these phases are unstable and prone to preferential dissolution. Published Re-Os isotope data indicate that the same phases possess exceptionally high ¹⁸⁷Os/¹⁸⁸Os ratios and thus evolve to radiogenic ¹⁸⁷Os/¹⁸⁸Os compositions in very short time intervals. Taken together, these results indicate that incongruent (preferential) weathering of certain primary basalt minerals can impart a radiogenic Os isotope composition to the dissolved riverine load. Nevertheless, overall the Os isotope signal to the Oceans from Icelandic rivers is little affected because rivers with unradiogenic ¹⁸⁷Os/¹⁸⁸Os values and a high discharge dominate the Os flux.

Keywords: Re-Os isotopes; basaltic rivers; Iceland; weathering susceptibility; saturation state; incongruent weathering』

1. Introduction
2. Climate, geology and hydrology
3. Field and analytical techniques
　3.1. Sampling and storage
　3.2. Anions, cations and trace elements
　3.3. Re-Os analysis
　　3.3.1. Filtered river, rain and hot spring water
　　3.3.2. Oxidation duration
　　3.3.3. Suspended and bed load
4. Results
　4.1. Suspended and bed load
　4.2. Dissolved phase
　4.3. Precipitation and geothermal water
5. Discussion
　5.1. Re-Os isotope and elemental variations
　5.2. Osmium isotope and elemental variations in the dissolved load
　5.3. Weathering susceptibility and saturation state
　5.4. The Os isotope signature of primary basaltic minerals
　5.5. The riverine osmium yield from basaltic terrains
6. Concluding remarks
Acknowledgements
Appendix A
Appendix B. Supplementary data
References