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 uranothorite-free 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. (5) It is possible to strip outer layers experimentally from an aggregate of zircons to determine variations in composite internal isotopic properties. (6) The family of uranium-lead systems distinguished in the inhomogeneous zircon suite may be utilized to establish patterns of isotopic ratio variations that yield much more useful geochronological information than any single system. (7) All systems in this rock appear to have formed 1,655 million years ago and to have been profoundly disturbed by an event 90 million years ago. There is no evidence of any type of disturbance other than this simple episodic pattern. (8) While the mechanism of disturbance is not directly established, it is evident that radioactivity and radiation damage strongly influence susceptibility of the systems to disturbance. (9) It is possible to offer reasonable explanations for some of the puzzling discrepancies between uranium-lead and thorium-lead ages determined on a single mineral concentrate. (10) Recognition of the existence of families of uranium-lead systems among the variable members of a single mineral species, or in associated mineral species, in a single typical granitic rock provides a powerful tool for investigation of the processes and conditions that have influenced the age-dating systems. (11) The systematic variations in U and Th provide interesting information on the role of some trace elements and accessory minerals in the crystallization history of the rock. (12) The Johnny Lyon granodiorite is the oldest igneous rock dated in Arizona thus far and places a minimum age of 1,655 ± 20 million years on the orogeny called Mazatzal Revolution.

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