Polonium halos and geochronology



Pleochroic halos had been known for about a quarter of a century before the geologist, J. Joly, explained that they owed their origin to the newly discovered phenomenon of radioactivity [Joly, 1907]. With the aid of a microscope, such halos are usually seen as small darkened circular spots surrounded by dark or colored rings. Joly proposed that the tiny mineral inclusions frequently seen at the centers of the halos were radioactive and that the passage of a particles from the inclusion through the host crystal produced the discoloration seen. Uranium was known to decay through a whole series of radioactive daughters, a number of which also ejected α particles. The energies of these α particles were characteristic of each emitter, and so each α-emitting radioactive element in the decay chain ejected α's with a characteristic range in a given medium. Thus, said Joly, the various rings in a halo corresponded to the ranges in the mineral of the α particles from particular members of the uranium decay chain. Herein was a beautiful, almost photographic, record of the radioactive process, as it had gone on for hundreds of millions of years. Joly's explanation was soon accepted, and during the next 30 or so years, a number of investigators, including Joly, examined halos in some detail [Holmes, 1931]. The hope that they might be useful for the measurement of the ages of rocks was a strong motivation. The idea was that laboratory-induced discoloration of minerals by known a doses could be compared with the natural discoloration in halos, thereby indicating the natural dose involved in the generation of the halos. This natural dose could then easily be converted into an age, if the uranium content of the radioactive inclusion at the center of the halo were known. The hopes of using pleochroic halos foundered, however, on the great difficulty of quantitatively measuring the degree of discoloration of a mineral. As a time piece, the pleochroic halo was an even bigger disappointment than its contemporary, U-He dating. The classical era for the study of halos came to an end with the outbreak of the Second World War. It was also marked by the appearance in the Proceedings of the Royal Society of London of two striking papers [Henderson, 1939; Henderson and Sparks, 1939] by G. H. Henderson of Dalhousie University, Nova Scotia. These papers form the basis of the remainder of our discussion.