Zircons and early continents no longer to be sneezed at

Dating of detrital zircon grains found in moderately old Archaean sediments from Western Australia first pushed known geological time beyond the previously impenetrable 4 Ga barrier. The record now goes back to around 4.4 Ga, within 95% of the date when the Earth and the Solar System came into being (4.55 Ga).  There has been much written about the oxygen isotopes in this tiny number of resistant minerals regarding whether or not they originated in a crust permeated by liquid water.  Because zircon is a mineral most usually associated with rocks of granitic composition, the very presence of extremely old ones seems to suggest that some degree of fractionation of primitive basaltic magmas must have taken place in the Hadean to form highly evolved magmas.  But did actual continental material arise so early? Processes in island arcs can generate evolved magmas in which zirconium is moderately enriched.  If such a host for the pre-4 Ga zircons was small in volume, it may have been easily recycled back to mantle depths, yet would enough zircons have been eroded from it to yield those preserved in sediments a billion years younger? It is possible to probe the processes involved in zircon formation by using the extremely sluggish radioactive decay of an isotope of the rare-earth element lutetium. The half-life of the 176Lu to 176Hf decay scheme (~37 Ga) is far longer than the time since the Big Bang, so detecting changes in the proportion of 176Hf to other hafnium isotopes is a tough nut to crack, the more so as 176Lu is very rare indeed.

A consortium of geochemists from Australia, the US, France and the UK have used the famous Jack Hills zircons to test the widely believed hypothesis that substantial continental crust has only emerged since 4 Ga ago (Harrison, T.M. et al. 2005. Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 Ga. Science, v. 310, p. 1947-1950). They found that deviations of 176Hf/177Hf from those assumed to characterise the bulk Earth (in fact the proxy of chondritic meteorites) show large variations in the zircons. Some of the deviations are negative, which is consistent with the very early formation of continental crust – perhaps from very soon after the Earth formed. On the other hand, some zircons show positive deviations, a sign that the mantle was depleted, also pointing to crust forming events. The authors boldly suggest that such anomalies refer to a very early geochemical upheaval in the Earth, that likely produced continental material. But the 4 Ga barrier for whole rocks seems clearly to suggest that none remains: either it was all subducted away, or was only a tiny fraction from which the Jack Hills zircons miraculously emerged on their long journey to a final resting place.

Commenting on the paper, Yuri Amelin of the Canadian Geological Survey, points out that no one agrees on the true composition of the bulk Earth (Amelin, Y. 2005. A tale of early Earth told in zircons. Science, v. 310, p. 1914-1915). Other isotopic evidence raises the spectre of our planet having accreted from a mixture of geochemically different meteorite types, and has never mixed thoroughly. Moreover, zircons are notorious for being compositionally zoned, as a result of being able to survive engulfment in later magmas from which new layers of zircon grow. The measurement of 176Hf/177Hf ratios is so difficult that only whole zircons give useful results, but those data hide the variations among the zones. Finally, he points out that studies of the 176Hf/177Hf in post 4 Ga basalts – and therefore the mantle from which they were derived – show that there is a clear divergence from chondritic meteorites that began around 4 Ga, the start of the record of existing continental rocks. In the kindest way, Amelin casts doubt on the sense in studies of such tiny relics of the Earth’s distant past.


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