Applied Physics/Physics Colloquium: Nuclear Geochronology and the Age of the Earth

Nuclear Geochronology and the Age of the Earth
Tuesday, September 25, 2018 - 4:30pm
Hewlett 200
Mark Harrison (Distinguished Professor, Dept. of Earth, Planetary and Space Sciences, UCLA)
Abstract / Description: 

How Old Is Earth? Because Earth formed by protracted accretion of planetestimals, asking the age of our planet is in some ways akin to asking your friends theirs and in other ways different. While your pals are unlikely to date themselves from the moment of conception, we can use U-Pb dating to pinpoint the arrival of our solar system to the formation of the first solids in primitive meteorites that condensed from the circumstellar disk at 4,567.2±0.5 million years (Ma). The timing of volatile loss from parent bodies constrains Earth to have accreted most of its mass by 4,550 Ma from impactors broadly similar in composition to meteorites but, surprisingly, of a class not yet recognized. Whereas your pals almost certainly know the day they emerged from the womb, the continuous mobility of our planet has erased any vestige of its origin. Clues remain nonetheless. Sixty years ago, Clair Patterson argued that the similarity of Pb isotopes between terrestrial rocks and meteorites established Earth's age as 4550 Ma. While his age was approximately correct, he was right for the wrong reason. Recently, two approaches have more clearly constrained an upper age bound to Earth formation; 182Hf-182W dating of core formation at ca. 4,540 Ma and 176Lu-176Hf data from terrestrial and lunar zircons as old as 4,380 Ma that require primary differentiation on both bodies to have ended by 4,510 Ma. The question of Earth's age remains of societal import as about half the population of our country believes that it is less than 10,000 years old and arose in the fashion described in Genesis. But creationists are not alone in promulgating origin myths. In the absence of any empirical evidence, the scientific community long coalesced around the view that the first (~500 Ma) of Earth history saw a continuously molten surface disrupted only by extraterrestrial impacts. Those ancient zircons noted above are seriously challenging that longstanding paradigm.