Geoscience Reference
In-Depth Information
Figure 13.5 A single pleochroic
halo developed in biotite in the
Leinster granite fromGarryellen,
County Carlow, Ireland. The
inner dark disc is due to radon
(a gas derivative of radium); the
succeeding inner ring is due to
radiumA alpha decaywhile the
outer darker ring was produced
by radium C alpha decay (from
John Joly, 'Radiant Matter',
Scientific Proceedings of the
Royal Dublin Society, New
Series 13, part 6 (1911), Plate 3,
Figure 4).
PLEOCHROIC HALOS: THE CHRONOLOGICAL RINGS
In 1907 Joly realised that small dark rings, or pleochroic halos as they
came to be known, which he had observed in biotite in some granites
were the products of radioactive decay in zircons enclosed by the
biotite crystals (Figure
13.5
). Previously it had been suggested that
these were due to the presence of organic pigments in the minerals,
but Strutt had earlier demonstrated the radioactive properties of zir-
con, to which Joly attributed the halos. The size of the halo was related
to the type of radioactive decay product and the range of the rays
produced and the intensity, he argued, was due to the duration of
radioactive decay. He observed complex halos with distinctive inner
and outer rings, or corona, in a greisen from Saxony in Germany, and
in 1910 he and his research assistant Arnold Lockhart Fletcher
(d. 1917), who was killed shortly afterwards near Rouen in France
during the First World War, attributed the development of the outer
rings to alpha rays of radium C and the inner rings to radium A.
Subsequently Joly was able to distinguish halos produced by various
radioactive sources including thorium, radium and uranium.
In 1913 Joly and Rutherford developed a unique methodology to
date a rock on the basis of its pleochroic halos, which required knowl-
edge of the mass of the nucleus of the halo and the number of alpha
