Impure marbles of the Lesser Himalaya: another source of continental radiogenic osmium

The marked rise of the 187Os/188Os ratio of seawater over the Cenozoic Era is generally attributed to increased continental input, and particularly to increased erosion of black shales. Thus Os isotopes are thought to trace the weathering of organic carbon. Here we present evidence that impure marbles, lacking organic carbon, provide an important source of radiogenic Os to the bedload of Himalayan rivers. These marbles provide a critical component necessary for balancing the Os isotopic budget of the bedloads. The analyzed marbles have 187Os/188Os ratios as high as 194 and often contain as much 187Os as typical black shales. Most of this radiogenic Os is not contained in the carbonate itself, but rather in associated phases. These phases are often of hydrothermal origin, and their Os may ultimately be provided by fluids circulating through black shales. While these results complicate the relationship between the Os isotopic ratio of seawater and erosion of organic-rich rocks, they also provide new information necessary for understanding the marine Os isotopic record.