Uranium-Lead Isotopic Variations in Zircons: A Case Study

Abstract

Zircons in a single 250-pound block of Precambrian Johnny Lyon granodiorite from the Dragoon Quadrangle in Cochise County, Arizona, have been concentrated with special attention to yield and nature of impurities. Morphology, zoning, color, inclusions, size distribution, radioactivity, refractive indexes, cell dimensions, and other properties have been compared with the isotopic properties in the U-Pb system (and to a less precise degree in the Th-Pb system). It has been observed: (1) Uranothorite impurities, while less than 1 per cent in abundance, contribute much more than 50 per cent of the activity in conventionally prepared concentrates. The presence of uranothorite drastically affects the apparent ages in the zirons. (2) The uranothorite can be satisfactorily removed by an appropriate acid-washing procedure. (3) The uranothoritefree zircon concentrates are not homogeneous and show systematic variations in radioactivity and various isotopic properties as a function of average crystal size. (4) Individual zircons show internal variation in radioactivity and may differ in specific activity from other individuals by as much as an order of magnitude.