Does the Earth have a chondritic rare-earth element composition? A study using combined cerium - neodymium isotope data to understand the formation of planets in the inner solar system
Final Report Abstract
Ultra high precision analyses of some of the oldest rock samples on Earth provided clear evidence that the planet’s accessible reserves of precious metals are the result of a bombardment of meteorites more than 200 million years after the Earth was formed. During the formation of the Earth, molten iron sank to its centre to make the core. This took with it the vast majority of the planet’s precious metals – such as gold and platinum. In fact, there are enough precious metals in the core to cover the entire surface of the Earth with a three metre thick layer. The removal of precious metals to the core should leave the outer portion of the Earth bereft of them. However, precious metals are 10s to 1000s of times more abundant in the Earth’s silicate mantle than anticipated. It has previously been argued that this serendipitous over-abundance results from a cataclysmic meteorite shower that hit the Earth after the core formed. The full load of meteorite precious metals was thus added to the mantle alone and not lost to the deep interior. To test this theory, I have analysed rocks from Greenland that are nearly four-billion-year-old. These ancient rocks provide a unique window into the composition of our planet shortly after the formation of the core but before the proposed meteorite bombardment. In detail, I determined the tungsten isotopic compositions of these rocks. Tungsten (W) is a very rare element (one gram of rock contains only about one ten millionth of a gram of tungsten) and, like precious metals, it should have entered the core when it formed. Like most elements, tungsten comprises several isotopes, atoms with the same chemical characteristics but slightly different masses. Isotopes provide robust fingerprints of the origin of material and the addition of meteorites to the Earth would leave a diagnostic mark on its W isotope composition. I observed a 15 parts-per-million decrease in the relative abundance of the isotope 182W between the Greenland and modern day rocks observed. This small but significant change is in excellent agreement with that required to explain the excess of accessible precious metals on Earth as the fortunate by-product of meteorite bombardment. Extracting tungsten from the rock samples and analysing its isotopic composition to the precision required was extremely demanding given the small amount of tungsten available in rocks. In fact, this was the first study world-wide that has successfully made such high-quality measurements. The data suggest that a significant amount of the tungsten found on Earth today was added as a socalled ‘late veneer’, a cataclysmic shower of meteorites that hit the Earth after the core formed. These small planetary bodies brought their full cargo of precious metals but avoided losing it to the Earth’s metallic interior. The meteorites would also have added material with lower relative abundances of 182W than found in the silicate Earth. The decrease in 182W I measured thus strikingly confirms the ‘late veneer’ hypothesis. The layer containing most of the precious metals accessible today was then stirred into the silicate portion of the Earth by gigantic convection processes. After the remixing of this layer about three billion years ago, later geological processes then formed the continents and pre-concentrated the precious metals (and tungsten) in ore deposits where they can be mined today. This research was published in the journal Nature. The study has sparked intense media interests including live interviews for BBC Radio 4, BBC Radio Wales, and BBC Radio Bristol, a recorded video interview for CNN International, a program on French television (France 5), recorded podcast interviews for Nature, AAAS Science Update, ARD, and DR (public Danish broadcasting company). Articles were published in the printed versions of The Times, Daily Mirror, Sun, Süddeutsche Zeitung, Le Parisien, in the online versions of BBC News, Discovery Channel, Planet Earth, National Geographic, The Daily Mail, Fox News, NBC, ABC, NASDAQ, The Naked Scientist, International Business Times, Metro, Mirror, Die Zeit, and in the printed versions of the science magazines New Scientist, Discover Magazine, Science News, and Bild der Wissenschaft.
Publications
- (2010). ’Formation of enriched mantle components by recycling of upper and lower continental crust’, Chemical Geology, 276 (3-4), 188-197
Willbold, M., and Stracke, A.
- (2011). ’The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment’, Nature, 477, 195-198
Willbold, M., Elliott, T.R., and Moorbath, S.
