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 (7˝ 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