Geology Reference
In-Depth Information
quantities of lead found in common igneous rocks. Initially
Patterson was severely hampered by problems with lead
contamination which, he eventually discovered, came from the
atmosphere. But after several years of working with super-clean
micro-chemical and mass spectrometric techniques, Patterson
and his colleagues were finally able to determine accurate and
repeatable lead ages on common igneous rocks. In the early
1950s technology was at last catching up with the dream, and
the possibility of using uranium-lead isotopes as a routine tool
for dating rocks finally came within their grasp. A further result
of this work was Patterson's subsequent campaign to reduce the
amount of lead in petrol, for it was Patterson who first drew the
world's attention to lead pollution of the atmosphere by emis-
sions from car exhausts, when he realised it was causing the
contamination in his lead samples.
Having succeeded in dating common igneous rocks, and as
a natural extension to his interests in astrophysics and
geochronology, Brown next wondered if the age of the solar sys-
tem could be determined by finding the age at which meteorites
had formed. So in 1952, when Brown and Patterson moved to
the California Institute of Technology, Patterson built his first
ultra-clean lead laboratory and began the work of determining
the composition of lead isotopes in iron meteorites.
The advantage of choosing iron meteorites was that the
amount of uranium they contained was negligible, so the
primeval lead they held could never have been contaminated by
radiogenic lead. The disadvantage was that the amount of lead
available for measurement was vanishingly small. Nevertheless,
in 1953 Patterson succeeded in determining the lead content of
the Canyon Diablo meteorite, a huge extraterrestrial body that
had collided with the Earth about fifty thousand years ago, carv-
ing out the Meteor Crater in Arizona, which is half a mile wide
and 200 metres deep. The Canyon Diablo meteorite contained
the lowest lead ratios ever measured. As the enormous
significance of this work dawned on Patterson he suggested that
