Very old Australian zircons with a story to tell
The Canadian Shield is the most extensive area of ancient rocks on the earth,
and Canadian geologists are proud to boast that - at 4.03 b.y. - the Acasta
Gneiss, situated to the east of Great Slave Lake, is the world’s oldest
known rock.
Australia also has a large area of ancient
rocks, and Australian geologists, are proud to claim the world’s oldest
minerals, some of which are a lot older than the Acasta Gneiss. These old mineral grains are from the Jack Hills
conglomerate which is situated in Western Australia, not too far from
Perth. The Jack Hills quartz
pebble conglomerate itself is not all that old (3.3 to 3.7 b.y.), but it is
made up of fragments from some very old rocks that either don’t exist any
more, or haven’t yet been found. These
include tiny grains of zircon (ZrSiO4) which range in age from 3.9
b.y. to an astonishing 4.4 b.y.
Zircon
is hard (7½ on the Mohs scale) and very resistant, and it forms as a minor
mineral component in granitic and gneissic rocks.
Because of its chemical and mechanical resistance to weathering, zircon
is commonly found in small amounts in sedimentary rocks.
Zircon is highly prized by geochronologists because of the fact that it
always contains small amounts of uranium and thorium. Isotopic
analysis of the uranium and thorium, and of their daughter lead products, can
give precise estimates of the age of the zircon grains.
Two teams of geologists from Australia,
the US and the UK have recently measured U, Th and Pb isotopes from tiny areas
(ca 30 microns) on zircon grains from the Jack Hills conglomerate.
They have been careful to avoid the weathered surfaces and cracks in
the grains, and they have calculated ages ranging from 3.9 to 4.4 b.y.
These researchers have also measured
oxygen isotopes on zircon grains aged 4.3 to 4.4 b.y., and they both report
that oxygen within the zircons is indicative of formation within rocks which
are continental in character, and in an environment in which liquid water was
present. On the basis of this
evidence it is suggested that water could have existed on the earth’s
surface at this time. 4.4 b.y. is
only a brief 150 m.y. from the presumed time of the origin of the earth and
solar system (the oldest meteorite has been dated at 4.56 b.y.), and most
geologists have assumed that the surface of the earth would not have been cool
enough to allow liquid water at surface this early.
The authors go on to speculate that life
could have evolved on earth at 4.3 to 4.4 b.y., although they have no evidence
of that. The oldest isotopic
evidence of life is dated at 3.8 b.y. It
is unlikely, however, that any life forms which existed before 4 b.y. would
have survived the heavy bombardment of the earth by meteorites at around 3.9
b.y.
References
Wilde, S., Valley, J., Peck W. and Graham, C., Evidence from detrital zircons for the existence of continental crust and oceans on the Earth at 4.4 Gyr ago, Nature, V. 409, p. 175-178, January 2001
Mojzsis, S., Harrison, M. and Pidgeon, R., Oxygen-isotope evidence from ancient zircons for liquid water at the Earth’s surface 4,300 Myr ago, Nature, V. 409, p. 178-181, January 2001
Steven Earle, 2000. Return to Earth Science News