Ocean Origins on Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 27

veneer? The direct evidence is almost pitifully slight. It mostly com-
prises a few zircon crystals, each about a tenth of a millimetre across,
which were once grains washed into a 3 billion-year-old sandstone
that can now be found in the Jack Hills in Australia.
Zircons (crystals of the mineral zirconium silicate) are marvellous
things. When they crystallize (usually deep in the Earth's crust, in a
magma chamber or in the roots of a mountain belt) they can take in
quite a lot of uranium—up to a few per cent—but no lead. Then, the
uranium, being radioactive, breaks down into lead, which is retained
within the molecular structure of the zircon. Zircons are also very
tough, so they can retain these delicate atomic patterns over billions
of years. Find that zircon crystal today, and carefully measure the pro-
portion of uranium to lead using a mass spectrometer, then if one
knows just how quickly uranium decays into lead one can work out
the time since that zircon crystallized.
In the case of a few of the Jack Hills zircons, the ages revealed were
enormous—over 4 billion years old. These did not form in the sand-
stone. Rather, they were washed into the sand that was to later
become the sandstone from the erosion of some yet more ancient
rocks—rocks that have not yet been found, and that have probably
long ago been destroyed. One or two of the zircons revealed ages as
old as 4.4 billion years. 26 What do they tell us, other than the mere
fact of their age?
A surprising amount, it turns out. One pattern in their chemistry
can suggest whether there was significant water in the vicinity when
they crystallized. This is the proportion of the different isotopes of
oxygen—especially that of the heavy isotope 18 O to the normal 'light'
isotope 16 O. This ratio is known to be little affected by melting pro-
cesses in the mantle, but to be changed in rocks that have been altered
in the presence of water. The Jack Hills zircons show such changes in
their oxygen isotope pattern, and thus when they crystallized water is