Issue 19, 2013 - Balliol College - University of Oxford

features
A lasting legacy
in cosmochemistry
By Alice Lighton (2007)
Grenville Turner (1958), Balliol graduate and Emeritus Professor at Manchester
University, is a 21st-century space scientist, studying the distant cosmos from the
relative comfort of his well-equipped Earth-based laboratory. By analysing
meteorites and other rocks from space he has made contributions to cosmochemistry
which have profoundly informed our history of the solar system and the universe.
Turner arrived at Balliol from St John’s College,
Cambridge, as a graduate student in nuclear
physics. His research at the time had little to do
with his future area of expertise, space: he was
given the task of designing and building a new
kind of mass spectrometer, to determine the
proportions of helium isotopes produced in
nuclear bombardment carried out at Harwell.
Individual chemical elements each have the
same number of protons, but can have different
numbers of neutrons, and are said to be
isotopes of each other (isos/same; topos/place,
i.e. place in the periodic table). Helium, with
two protons, normally has two neutrons (He-
4) but occasionally only one (He-3). A mass
spectrometer can separate them with a magnet
and measure how much there is of each.
Turner spent three years in the Clarendon
Laboratory developing the new techniques of
ultra high vacuum, in the process learning how
to build a complicated mass spectrometer in
stainless steel and avoid the frustrating leaks
which regularly appeared in copper gaskets,
until at the end of three years he could turn to
actually making measurements. He completed
his degree in the fourth year thanks to financial
assistance from Balliol, and took up a post
as Assistant Professor at the University of
California Berkeley, with John Reynolds.
In 1956 Reynolds had built the world’s first
ultra high vacuum mass spectrometer in glass
and, incidentally, had just sold one to the Oxford
Geology Department for their newly formed
Geochronology unit. Turner arrived
at Berkeley in 1962, an exciting time
for the Reynolds group. Reynolds
had recently started measuring
noble gases trapped in meteorites –
rocks from space – in his machine.
Meteorites, which originate in the
asteroid belt between Jupiter and
Mars, were formed at the birth
of the solar system; they are the
debris left over from the creation
of the planets. Reynolds had just
discovered the first example of
an extinct radioactive isotope,
iodine-129. Having a ‘short’ half
life of only 16 million years it no
longer exists in the solar system but
it is produced in exploding stars,
supernovae, and was present four
and a half billion years ago when
the solar system formed. What Reynolds had
observed was in fact not iodine-129 (being
now extinct) but enrichments of xenon-129,
the isotope into which iodine-129 decays. Since
then another ten extinct isotopes have been
discovered and by measuring
their decay relative to
a stable isotope, for
example iodine-127,
While glancing
through some
unexplained argon
measurements
from Berkeley,
Turner had a flash
of inspiration
which led to what
is now one of the
most commonly
used methods for
dating rocks.
they can be used as a kind of ‘stop clock’ to work
out the very precise sequence of events in the
first few million years of solar system history.
In addition to working on ‘iodine-xenon’
dating, Turner discovered a strange component
of xenon in a carbonaceous meteorite, which
23 years later led to the unearthing by Chicago
chemists of the first so-called pre-solar grains,
mineral grains that formed around longdead
stars and which contain a wealth of
information about element building (nucleosynthesis)
in stars.
After a productive two years at Berkeley,
Turner returned to his native Yorkshire, joining
Sheffield University. Cosmochemistry was a
new discipline not yet established there. ‘I was
something of a one-man band,’ he says. He spent
many months setting up equipment, in the
meantime continuing to analyse data from the
experiments he had performed in California.
Dating breakthrough
While glancing through some
unexplained argon measurements
from Berkeley, Turner had a flash
of inspiration which led to what is
now one of the most commonly
used methods for dating rocks.
Potassium-argon dating had
been developed in the late 1950s
and relies on the existence in all
potassium of a tiny amount, 0.01
per cent, of a radioactive isotope,
potassium-40, which decays over
time into the noble gas isotope
argon-40. Potassium-40 is a minor
source of heat in the Earth, while
the argon-40 released from the
Earth’s interior makes up 1 per
cent of the Earth’s atmosphere. The
build-up of argon-40 in a rock such as granite
from the time it solidified and cooled can be
used to date the formation of the rock, but has
some problems. First of all the potassium and
argon are measured by two different techniques
requiring two equivalent samples. Also various
34
floreat domus balliol college news