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

Final Report Summary - HADEAN (The Earth in Transition: from Earliest Crust to Earliest Preserved Rocks)

Project content
Current knowledge of the nature and origin of the earliest crust on Earth comes largely from studies of the mineral zircon (ZrSiO4). 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 the 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 India. 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.
The objective of the HADEAN project was to apply the most up–to-date technologies to various isotopic systems in zircons from the world oldest rocks and evaluate hypotheses on what has happened in the Hadean Earth.

Description of work performed:
Various isotopic systems were analyzed in zircons from key localities around the world where rocks older than 3.8 Ga in age have been reported; selected regions in Antarctica, Greenland Canada and India. By investigating these areas we obtained a unique inventory of the precise age of the samples (U-Pb zircon data), the nature of their protolith (oxygen isotope), the nature of the crust (Lu-Hf geochemistry), geochemistry of zircons (rare earth elements). For selected grains transmission electron microscopy and synchrotron XFM imaging was performed to decipher chemical characteristics of the grains. Additionally an extensive study based on ion imaging was performed on selected zircon grains. This method involved technique development.

Training received:
During the outgoing phase, I undertook the following training: the process of heavy mineral separation and sample preparation at Curtin University; extensive use of different Secondary Electron Microscopes, SEM – JEOL 6400, JEOL 8530F electron microprobe, Philips XL30, Tescan Vega 3 and Zeiss 55 at the University of Western Australia in Perth; different Secondary Ion Microprobes, SHRIMP II A and B at Curtin University in Perth, SHRIMP II and RG at the Australian National University in Canberra, Australian Synchrotron, XRF beamline in Melbourne.
During the incoming phase, I undertook intense training on the Cameca 1280 ion microprobe at the Natural History Museum in Stockholm. Additional training on laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) I took at the Chinese Academy of Sciences in Beijing in China and training on transmission electron microscopy (TEM) I took at the GeoForschungsZentrum in Potsdam.

Description of the main results achieved:
The most significant results concern samples obtained from Antarctica which exhibit previously unknown micrometer-scale patchy distribution of lead that is not related to crystal imperfections (for example cracks). These areas gave elevated 207Pb/206Pb ratios and apparent zircon ages as old as 4.2 Ga (Hadean). These ages were interpreted as artifacts of ancient redistribution of radiogenic Pb, not as relicts of ancient zircon. A scanning ion imaging investigation allowed recognition of this unsupported radiogenic Pb and provides a new method for testing the validity of old ages from grains known to have a long and complicated history. These results are already of widespread interest to geochronologists. Results have already been published in “Geology” and “American Journal of Science”.
Results from samples collected from Aker Peaks in Kemp Land, Antarctica, indicate that Hadean crust was widespread in the Napier Complex and establish it as one of the oldest regions on Earth. This work is prepared for submission in the journal “Precambrian Research”.

Selected zircon grains, were analyzed by transmission electron microscope and results indicate the nature of radiogenic Pb in zircons. This work is under review in “Proceedings of the National Academy of Sciences of the United States of America”. We are now testing other ancient zircons to see if this is a common phenomenon.

The outcome of this work will be that we will now have a database of isotopic information of all known location of old rocks on Earth. In conjunction with information from other workers we will be able to evaluate and propose a new hypothesis for what happened in the Hadean.
Studies of the Hadean environment have profound implications for understanding the potential development of life on Earth and the habitability of the planet during this period, thus making the results of great interest not only to geologists, but also to biologists. Additional results are still under evaluation and scientific papers are being prepared for publications, or are under review, in high-impact international journals.