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10/19/2005
Oldest Specks
In my column of April 13, I mentioned briefly the work of John
Valley, Professor of Geology at the University of Wisconsin-
Madison. Valley was one of a group of workers who found that
a speck of zircon was the oldest “rock” on Earth. I didn’t know
then the details of their work. Now an article by Valley in the
October Scientific American reveals the story of the amazing
measurements he and his colleagues have made on zircons
smaller than the size of the period at the end of this sentence.
After forming 4.5 billion years ago, Earth was an awful place,
fiery hot and molten, bombarded by objects from space. Until
recently, prevailing wisdom was that over 500 million years
passed before Earth’s surface cooled enough for a crust to form
and for water to form oceans. The earliest rocks known to have
formed in an aqueous environment were 3.8 billion years old.
Fortunately, there are regions where very old rocks are exposed,
ripe for the picking by geologists. One such place is the Jack
Hills region of Western Australia, where our tiny zircons were
discovered back in the 1980s. Zircon is a mineral composed of
the elements zirconium, silicon and oxygen.
In the 1980s, Simon Wilde was a member of a group at Curtin
University of Technology in Australia that combined forces with
William Compston and his group at the Australian National
University in Canberra to study the zircons. Compston’s group
had invented an instrument they called SHRIMP (Sensitive
High-Resolution Ion Microprobe). SHRIMP was a special ion
microprobe, an instrument that shoots a beam of ions at a sample.
The ion beam digs a tiny pit in the sample, kicking off the atoms
from the sample to be fed into another instrument called a mass
spectrometer. The mass spectrometer measures the masses and
amounts of the atoms ejected from the sample. If you know the
mass, you know the element.
The Australians were particularly interested in using SHRIMP to
measure traces of uranium and lead in the zircons. Why? The
amounts of uranium and lead indicate the age of the zircon.
When the tiny zircon crystals formed billions of years ago they
typically contained a trace of uranium. Uranium is radioactive
and as it emits particles it is transformed into other elements that
are also radioactive. The net result is that the uranium ends up as
lead, a stable element. A key number in calculating the age of a
zircon is the half-life of uranium. All the uranium doesn’t turn
into lead at once. Otherwise, there wouldn’t be any uranium on
Earth today. If you start with a certain amount of uranium, it will
take some time before half of that uranium has transformed itself
to some other element by emitting a particle. That time is the
half-life. By a curious coincidence, over 99 percent of uranium
is an isotope with a half-life of 4.46 billion years, essentially
identical to the age of Earth, 4.5 billion years.
What does this mean? Let’s pretend we find a 4.46 billion-year-
old zircon. Half of the original trace uranium in the crystal will
have turned to lead. SHRIMP tells us we have equal amounts of
uranium and lead. For zircons not as old, less than half of the
original uranium will have converted to lead. Valley says that a
4 billion-year-old zircon can be dated to within plus or minus 40
million years, an uncertainty of only 1 percent. You’d be hard
pressed to estimate most people’s ages that well!
Well, the Australians in the 1980s found that their oldest zircons
dated as far back as 4.3 billion years, only 200 million years after
Earth was formed. This was exciting but the nature of the parent
rocks was unknown. Zircon is a very stable mineral and the
parent rocks weathered away, leaving the tiny zircon crystals at
the mercy of wind and water, which carried many far from their
source. Some of the zircons are crystals with flat faces and sharp
angles (probably ones that stayed near where they formed) while
others have rounded edges, worn down over the years on their
journeys.
John Valley came into the zircon story when he approached
Wilde to cooperate with one of Valley’s graduate students in his
doctoral research. Wilde agreed and measured over 50 zircon
crystals using an updated SHRIMP. To get these “less-than-a-
period” size zircons they had to crush and sift through hundreds
of pounds of rock. In 1999, he found a zircon in which the lead
was almost equal to the amount of uranium. That zircon was 4.4
billion years old, only 100 million years after Earth was formed.
While Valley and his graduate student, William Peck, must have
been overjoyed to find such ancient zircons, they weren’t
finished. They took the 5 oldest zircons to the University of
Edinburgh in Scotland, where John Craven and Colin Graham
had another ion microprobe that specialized in oxygen isotopes.
Ordinary oxygen is O16 but there’s also a rare isotope O18, with
two more neutrons in its nucleus. The ratio of O18 to O16 in a
crystal depends on the temperature when the crystal forms. The
researchers were “stunned” to find that the O18 to O16 ratios in
the zircons implied that the zircons were formed under low
temperature conditions where water was present.
Hesitant to publish their findings indicating that Earth cooled
down 400 million years sooner than was thought previously, they
held off for a year. Meanwhile, others confirmed their results on
Jack Hills zircons and papers appeared in 2001. But Valley and
Peck still weren’t finished. They also found in the oldest zircons
pieces of other minerals, notably quartz. Quartz and the presence
of certain other trace elements suggest that the zircons were
associated with the formation of a continental crust much earlier
than expected.
How did these researchers get all these data from such tiny
samples? They mounted the crystals, some nearly invisible to
the naked eye, in epoxy and polished them (very carefully!) to
expose a fresh surface. I’ve potted crystals in epoxy and
polished them but they were big crystals. I can’t imagine
polishing a mere speck, not just once but several times. After
each ion probe measurement of uranium and lead, they polished
away the pit to expose a new surface for the oxygen work, and
another for the trace element work. That tiny 4.4 billion-year-old
zircon has traveled all the way from Australia to Wisconsin to
Scotland and, if all went according to plan, it is or will be back in
Australia to be housed on display in a museum there.
About 35 years ago at Bell Labs, we were experimenting with
O18 in gallium phosphide. A colleague and I went to Elmsford,
New York to the Cameca Instruments Company, where they had
an early version of an ion microprobe. We spent a couple of
long, frustrating days there. As I recall, the problem was that we
couldn’t tell the difference between O18 and H2O, normal water
(with O16) and O18 having very nearly the same mass. The
instrument was not sensitive enough to tell the difference. I can
empathize with Valley, who spent 11 sleep-deprived days of
“round-the-clock” analysis on the ion microprobe in Scotland.
Thankfully, he had a vastly better outcome than we did.
Today, labs all over the world are working on zircons and the
number over 4.1 billion years old is in the hundreds! Back in
Wisconsin, Valley now heads up a lab that has acquired a new
Cameca “cutting edge” ion microprobe, which will be used to
look at zircons and other materials ranging from stardust to
cancer cells. I can’t wait to see what comes next.
Allen F. Bortrum
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