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Osmium-187/Osmium-186 in Manganese Nodules and the Cretaceous-Tertiary Boundary

1983; American Association for the Advancement of Science; Volume: 222; Issue: 4624 Linguagem: Inglês

10.1126/science.222.4624.613

1095-9203

J. M. Luck, Karl K. Turekian,

Radioactive element chemistry and processing

As a result of the radioactive decay of rhenium-187 (4.6 × 10 10 years) the osmium-187/osmium-186 ratio changes in planetary systems as a function of time and the rhenium-187/osmium-186 ratio. For a value of the rhenium-187/osmium-186 ratio of about 3.2, typical of meteorites and the earth's mantle, the present-day osmium-187/osmium-186 ratio is about 1. The earth's continental crust has an estimated rhenium-187/osmium-186 ratio of about 400, so that for a mean age of the continent of 2 × 10 9 years, a present-day osmium-187/osmium-186 ratio of about 10 is expected. Marine manganese nodules show values (6 to 8.4) compatible with this expectation if allowance for a 25 percent mantle osmium supply to the oceans is allowed. The Cretaceous-Tertiary boundary iridium-rich layer in the marine section at Stevns Klint, Denmark, yields an osmium-187/osmium-186 ratio of 1.65, and the one in a continental section in the Raton Basin, Colorado, is 1.29. The simplest explanation is that these represent osmium imprints of predominantly meteoritic origin.