The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment
| Autor: | Tim Elliott, Matthias Willbold, Stephen Moorbath |
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| Rok vydání: | 2011 |
| Předmět: | |
| Zdroj: | Nature. 477:195-198 |
| ISSN: | 1476-4687 0028-0836 |
| Popis: | It has long been speculated that a 'late heavy bombardment' of Earth by meteoritic material replenished the mantle's budget of siderophile (iron-loving) elements, such as tungsten, that were largely lost to the core during its segregation. However, evidence for this 'late veneer' remains indirect, and its influence has been much debated. Matthias Willbold and colleagues present high-precision tungsten isotope analyses of ancient Greenland rocks and show that they have significantly higher 182W/184W ratios than modern terrestrial samples. This finding is in good agreement with the expected influence of a meteoritic late heavy bombardment. They speculate that both the tungsten isotope data and the observed decrease in 142Nd/144Nd neodymium ratios can be explained if late meteorite bombardment triggered the onset of the current style of mantle convection. Many precious, ‘iron-loving’ metals, such as gold, are surprisingly abundant in the accessible parts of the Earth, given the efficiency with which core formation should have removed them to the planet’s deep interior1. One explanation of their over-abundance is a ‘late veneer’—a flux of meteorites added to the Earth after core formation as a ‘terminal’ bombardment that culminated in the cratering of the Moon2. Some 3.8 billion-year-old rocks from Isua, Greenland, are derived from sources that retain an isotopic memory of events pre-dating this cataclysmic meteorite shower3,4. These Isua samples thus provide a window on the composition of the Earth before such a late veneer and allow a direct test of its importance in modifying the composition of the planet. Using high-precision (less than 6 parts per million, 2 standard deviations) tungsten isotope analyses of these rocks, here we show that they have a isotopic tungsten ratio 182W/184W that is significantly higher (about 13 parts per million) than modern terrestrial samples. This finding is in good agreement with the expected influence of a late veneer. We also show that alternative interpretations, such as partial remixing of a deep-mantle reservoir formed in the Hadean eon5,6 (more than four billion years ago) or core–mantle interaction7, do not explain the W isotope data well. The decrease in mantle 182W/184W occurs during the Archean eon (about four to three billion years ago), potentially on the same timescale as a notable decrease in 142Nd/144Nd (refs 3 and 6). We speculate that both observations can be explained if late meteorite bombardment triggered the onset of the current style of mantle convection. |
| Databáze: | OpenAIRE |
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