7. Hadean Jack Hills Zircon Geochemistry

Geochemical analysis of zircons older than 4 billion years, found in Early Archean metasediments at Jack Hills, Western Australia, provide insights into the nature of Hadean Earth. Oxygen isotopes have been interpreted as indicating that protoliths of magmas from which Hadean zircons crystallized were formed in the presence of water at or near Earth’s surface. Apparent crystallization temperatures of Hadean zircons cluster at 680 °C. Given the low porosity expected in rocks under anatectic conditions, dehydration melting of micas as the principal source of the melts from which these zircons crystallized can be ruled out. Instead, a regulated mechanism producing near minimum-melting conditions during the Hadean is inferred. Combined, these results have been interpreted to reflect chemical weathering and sediment cycling in the presence of liquid water shortly after Earth accretion. ¹⁷⁶Hf/¹⁷⁷Hf ratios of Hadean Jack Hills zircons show large heterogeneities indicating a major differentiation of the silicate Earth by 4.50 Ga. A possible consequence of this differentiation is the formation of continental crust of similar order to the present. Studies of mineral inclusions within Hadean zircons indicate their crystallization from hydrous, granitoid magmas at pressures greater than 6 kbars, implying low near-surface geothermal gradients which in turn suggests their origin in underthrust environments. Given general agreement that life could not have emerged until liquid water appeared at or near Earth’s surface, a significant implication is that our planet may have been habitable as much as 500 Ma earlier than previously thought. Indeed, carbon isotopic evidence obtained from inclusions in a Hadean zircon is consistent with life having emerged by 4.1 Ga, or several 100 million years earlier that the hypothesized lunar cataclysm. Trace element analyses of aluminum, halogens, sulfur, phosphorus, rare earth elements in Hadean zircons are consistent with their origin in a range of granitoid magma types and redox conditions. Although some of the above interpretations remain subject to debate, there is now a widespread consensus that molecular water was present at or near Earth’s surface since at least 4.3 Ga. Perhaps the most remarkable feature of inferences drawn from investigations of these ancient zircons is that none were predicted from theory, underscoring the importance of observations in testing models of early Earth.