Progressive mixing of meteoritic veneer into the early Earth’s deep mantle

Mixing the platinum veneer Maier et al. have sampled a class of volcanic rocks called komatiites, which are indirect samples of the deepest portions of the Earth's mantle mainly from the Archaean era. It is generally thought that the mantle lost essentially all of its platinum-group elements to the core shortly after the Earth's accretion, and these elements were subsequently replenished during an episode of influx of meteorites, particularly the Late Heavy Bombardment about 4 billion years ago. The new komatiite data, gathered from rocks 3.5 to 2.7 billion years old, show that the 'late veneer' produced by the bombardment was gradually mixed into the mantle over the subsequent billion years, and the mantle attained its present relatively homogenous platinum-group elements content only by 2.7 billion years ago. This finding overturns common assumptions about rapid incorporation of the late veneer, and constrains models of the convection, stirring and mixing of the terrestrial mantle.