The first 500 million years of Earth history, referred to as the Hadean Eon, was a period when the metallic core segregated, the oceans condensed, the atmosphere outgassed and the first rocky crust formed. In some models, it was also a time when plate tectonics started and the first continents grew. Yet, no rocks from the Hadean Eon have ever been found, and so the events occurring in this enigmatic ‘geological dark age’ remain difficult to reconstruct.
This project is to test the exciting possibility that Hadean rocks lie undiscovered in the ancient rock sequences of Western Australia, either in outcrop or as part of the deeper crust.
This project combines traditional geological fieldwork with innovative analytical approaches in the quest to locate the elusive Hadean rocks. Fieldwork is designed to narrow the search through detailed mapping of the exposed geology in the most ancient parts of Western Australia, specifically in the Murchison district (east of Carnarvon) and in the rugged gorges of the Pilbara region. The rationale here is that the venerable Hadean remnants may be recognizable through field and structural relationships.
Analytical work involves two steps – (1) dating zircon crystals in the rock samples by uranium-lead isotope geochronology, and (2) measuring the isotope ratios of the elements oxygen, neodymium and hafnium in the rocks and their constituent minerals by high precision mass spectrometry. The first step provides a direct measure of the time that the host rock solidified from magma, or was affected by metamorphism. This offers the most direct way to establish a Hadean ancestry for rocks and minerals. Data from the second step is used to deduce the age and nature of the lithosphere from which this original magma was derived – such information may signpost the existence of even more ancient rocks residing at depth within the crust, or which have been long since destroyed by erosion.
Ultra-high precision measurements of a shorter-lived neodymium isotope system are also planned. Variations in this ‘extinct’ isotope system can only have arisen in the first 300 million years of planetary evolution, before the parent isotope had decayed away. This signature thus fingerprints the participation of Hadean crustal materials in younger magmatic episodes. The project will utilize world-class mass spectrometry facilities established at The University of Western Australia (the Advanced Geochemical Facility in SEE and the ion microprobes in the CMCA), the John de Laeter Centre for Mass Spectrometry and the Australian National University in Canberra.
- Professor Jeff Vervoort, Washington State University
- Professor Martin Whitehouse, Museum of Natural History