April 8, AB20 or room 140 Weeks Hall (near the zircon exhibit), 1215 W. Dayton St., Madison WI, at 3:30 pm
No rocks on Earth are known to have survived the intense meteor bombardment that affected the planet between its formation at ~4,550 million years (Ma) and 4,030 Ma ago, the age of the oldest preserved rocks within the Acasta Gneiss terrane of northwestern Canada. However, detrital and xenocrystic zircons with U-Pb ages significantly older than 4 billion years (Ga) have been found at five localities within the Yilgarn Craton of Western Australia. The oldest is from Jack Hills with an age of 4,404 Ma, making it the most ancient fragment of the Earth's crust so far identified, formed approximately 150 million years after the planet itself. These crystals thus provide our only solid evidence of conditions on Earth during the earliest stages of its history and bring into question some commonly held views about what conditions were like at this time.
Background information on the discovery and initial investigations will be presented and a review given of our recent work that will highlight just how important these minute crystals are in deciphering the early history of our planet.
Simon A. Wilde is from the Department of Applied Geology, Curtin University of Technology, Perth, Western Australia; John Valley and Aaron Cavosi work in the UW-Madison Department of Geology & Geophysics.