Osmium in Precipitation is from Refining of Platinum Ores

Mukul Sharma
Radiogenic Isotope Geochemistry Laboratory,
Department of Earth Sciences,
Dartmouth College,
Hanover, NH USA

Osmium (Os) is a platinum group element. Its concentration in the earth’s crust is about 30 picogram/g. Atmospheric sources of Os include cosmic dust, volcanic and continental mineral aerosols. Refining of platinum ores, automobile catalytic converter exhaust, burning of trees, coal and petroleum could also provide Os to the atmosphere. Osmium isotopic composition of these sources is highly variable. Radiogenic 187Os is produced from the ?-decay of 187Re, which has a half-life of 42 billion years. The Os isotopic compositions (ratio of 187Os with respect to stable 188Os) of potential atmospheric sources of Os vary from 0.13 to >13, reflecting their Re/Os ratio and the time elapsed after their formation. On average, the 187Os/188Os ratios of cosmic dust is ~0.13 (with187Re/188Os = 0.36)--about ten times lower than that of continental mineral dust (=1.26 with 187Re/188Os = 48). The isotopic composition of Os associated with platinum bearing sulfide ores from the Merensky Reef, South Africa that produces about 70% of world’s Pt, is mainly between 0.15 and 0.20. Processing of base-metal (mainly Cu, Ni, Zn, and Pb) sulfide ores could also introduce Os into atmosphere. The 187Os/188Os ratios of sulfide deposits are variable. In some cases they are similar to the platinum ores (e.g., Noril’sk, Russia; Yilgarn craton, Australia) and in others they are highly radiogenic due to involvement of continental crust in their formation (e.g., Sudbury, Canada). Finally, the 187Os/188Os for bitumen range from 3.3 to 13.7 and 187Os/188Os values for hydrocarbons are from 1.2 to 4.2 all of which are highly radiogenic. Because of the large isotopic contrast between these sources, Os isotopes provide an easy means to assess the extent of anthropogenic contribution.

The Os isotopic composition in rain and snow samples collected in Hanover, New Hampshire is ~0.2, which is similar to ores supplying the platinum group elements used in automobile catalytic converters. The Os isotopic composition of precipitation from the high Sierra Nevada, California, southern India and Antarctica are similarly low. We argue that human activities have contaminated the global troposphere with highly-reactive OsO4, which is produced as a result of the refining of ores enriched in platinum group elements; automobile emissions are a secondary but minor source.