HADEAN

The Earth in Transition: from Earliest Crust to Earliest Preserved Rocks

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 Obiettivo del progetto (Objective)

'Current knowledge of the nature and origin of the earliest crust on Earth comes largely from studies of the mineral zircon. The oldest zircon crystals from Jack Hills in Australia provide a wealth of information and represent a time capsule of what the Earth was like from 4.4 to 4.0 Ga during the Hadean. These zircons are detrital grains preserved in much younger rocks, and no known rocks of this age have survived on Earth. By applying the most up–to-date technologies to various isotopic systems in these zircons, it has been possible to deduce that continental crust and oceans formed early in Earth`s history and that the planet cooled more quickly than was previously realised. The aim of this proposal is to compare the information acquired from these detrital grains with comparable new data to be obtained from the earliest known rocks on Earth. These ancient rocks are preserved in Antarctica, Canada, China, Greenland, Western Australia and Swaziland. Extensive investigation of these rocks will allow us to characterize further the nature of the earliest preserved crust and, more importantly, to document what changes took place from the formation of the earliest zircons (4.4 Ga) to the oldest preserved crust (4.03 Ga) and to evaluate why so few Hadean rocks survived on Earth. This period represents the ‘dark ages’ in terms of Earth history and marks the onset of stable conditions on the Earth that set the stage for a habitable planet.'

Introduzione (Teaser)

An EU study is examining ancient zircons, which reveal conditions of the early Earth and its transition to an environment more conducive to life. The results will provide a better understanding of the processes affecting ancient minerals and could reveal aspects of the origins of life.

Descrizione progetto (Article)

Our current knowledge of the nature and origin of the earliest crust on Earth comes mainly from studies of the mineral zircon (also known as zirconium silicate). Zircon mineral grains are preserved in much younger rocks and represent a time capsule of what the Earth was like during the Hadean Eon. The Hadean was Earth's first geological period, spanning 400 million years, long before life began, when the planet was still largely molten and highly volcanic. Everything we know about the period comes from these zircon mineral grains.

To discover more, the EU funded the project 'The Earth in transition: From earliest crust to earliest preserved rocks' (HADEAN). The study aimed to compare isotope information about the Hadean known from zircons with data now available from our planet's oldest rocks. It revealed the nature of the Earth's crust at the time and helped document the changes from Hadean conditions. This information also accounted for why so few Hadean rocks survive.

A range of analyses were conducted (ion, isotope, electron microscope and synchrotron) on samples obtained from extremely old rocks at various locations. The most significant results concern Antarctic samples, which exhibited a previously unseen patchy distribution of lead. Results indicated an ancient redistribution of lead and were not relics of ancient zircons as previously believed.

A scanning ion imaging tool provided a new approach for testing the validity of age calculations based on samples with complex histories. The results were significant for geochronologists, who determine the age of rocks using signatures inherent in the rocks themselves.

The outcome of the project will be a database of isotopic information for all known locations of old rocks on Earth. Together with information from other work it will be possible to evaluate and propose new theories for what happened during the Hadean Eon.

Studies of the Hadean environment will have major implications for understanding the potential development of life on Earth and the habitability of the planet during this time. Therefore, the project's results will be of great interest to not only geologists but also biologists. Further results are currently being assessed and papers prepared for publication, or under review in leading scientific journals.