Observations on the Tn-U relationship in zircons from granitic rocks and from kimberlites

Existing data for thorium and uranium in zircons from granitic rocks are discussed, and new data for thorium and uranium in zircons from kimberlites are presented. The new data show that thorium and uranium are at very low levels (from 1.8 to 7.2 ppm for thorium, and from 7.0 to 28.0 ppm for uranium) in kimberlitic zircons, but that the Th:U ratio is just as low (from 0.23 to 0.70) as for granitic zircons. The Th:U ratio in zircons is then discussed by comparison with the ratio in igneous rocks, and possible reasons for the low ratio in zircons are suggested. © 1967.