Earth's patchy late veneer

Abstract

A 'late veneer' of meteoritic material, added after Earth's core had formed, may be the source of our noble metals. Its absence from some parts of Earth's mantle will now force a rethink about this late accretion. See Letter p.195 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.