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The use of the 187Os/186Os ratio as a diagnostic indicator of cosmic osmium in high osmium (and iridium) layers in sedimentary strata, such as those found at the Cretaceous/Tertiary boundary, is explored. The 187Re/186Os ratio in meteorites is 3.2 and in crustal rocks it is 400, resulting in different 187Os/186Os ratios in the two systems after billions of years have passed. (The radioactive decay constant of 187Re is 1.6 × 10−11 y−1.) Meteorites now have a 187Os/186Os ratio of about one, and the earth’s crust averages (assuming no crustal recycling through the mantle) between 13 and 30. As a result, Os from the crust concentrated in certain sedimentary strata should be readily distinguished from Os from extraterrestrial projectiles.

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