THE ISOTOPIC COMPOSITION OF LEACHABLE OSMIUM FROM RIVER SEDIMENTS

The Os-187/Os-186 ratios of acidic hydrogen peroxide leaches of river sediments are inferred to be representative of the isotopic compositio n of Os delivered to sea water from the continents. The Os-187/Os-186 values for river sediment leaches from the eastern and central U.S., t he Ganges River, and the Rio Maipo range from 10.1 to 21.5. These resu lts suggest that the O-187/Os-186 of the soluble river load delivered from the major continental areas to the oceans is, on average, more ra diogenic than average currently eroding continental crust (10.5). Sedi ment leaches from three rivers from the western U.S. that drain ophiol itic peridotites have distinctly lower Os-187/Os-186 values, 1.4-7.1; such rivers may be significant contributors to sea water of the relati vely unradiogenic osmium required to yield the contemporary oceanic Os -187/Os-186 of 8.6.