To try and solve the problem, Murakami and colleagues performed a series of laboratory measurements by replicating the lower mantle's environment. They pushed sound waves through different minerals at densities, pressures and temperatures matching the conditions that exist more than 660 kilometres below the surface. After comparing their results with seismic data, they conclude that the lower mantle is far more highly enriched in silicon than the upper mantle. They speculate this may have been caused by fractional crystallisation of the magma ocean extending into the lower mantle during Earth's early history Their findings also show that there is little movement of materials between the lower and upper mantle. The new study by researchers including Dr Motohiko Murakami from Japan's Tohoku University, found that 93 per cent of the lower mantle is composed of a silicate perovskite mineral. Murakami and colleagues say their new data is consistent with the chondritic Earth model. These models rely on the composition of chondritic meteorites, which are thought to have formed in the same region as the Earth more than four billion years ago.
Murakami and colleagues conclude that this primordial chemical stratification may have been preserved until the present day.
In an accompanying commentary in the same issue of Nature, Professor Ian Jackson from the Australian National University, says Murakami and colleagues' work breaks new ground.
But he warns that the story is unlikely to be over yet.
"Inevitably in such pioneering work, there are still a few technical issues that need to be resolved," says Jackson.
"These tests are done over extreme pressures to more than a million atmospheres, and so the issue of how the pressure is calibrated becomes a key factor."
"What's needed now is more follow up work that would allow these very high pressure and temperature results to be compared with the same tests carried out under more precisely known conditions found at lower to moderate pressures," says Jackson.
"One line of attack would involve using new computer modelling of mineral properties. This is an increasingly powerful way of assessing the behaviour of rocks under extreme conditions."
But Jackson says there is still the possibility the entire chondritic Earth model is wrong.
Some scientists believe the movement of planets during the early formation of the solar system, known as planetary migration, could have mchanged the distribution and mix of asteroids and meteors.
"I think we haven't yet heard the final word," says Jackson.